stag top struts

Lister Jaguar XJS

Spotted this in Cambridge today:

Quick Fuel 750DP Tuning Part 3

Continuation of this thread




Quick recap, I've done a fair bit of tuning with my Quick Fuel 750 mech secondary, annular booster carb.

Stock settings

Primary Jet: 72
Secondary Jet: 82
IFR: Unknown - suspect its .032 or .033
IAB: 72
MAB: 33
PVCR Unknown - estimate somewhere around 0.055" to 0.060"


Revised Settings

Primary Jet: 68
Secondary Jet: 82 - reduced to 78 after very short test drive
IFR: .030
IAB/MAB unchanged
PVCR 0.040"


I was hoping that the revised settings would be perfect however after a few miles I think I either went too aggressive on the IFR or need to look at the accelerator pump. On very light acceleration, there is a brief lean spike and a slight stumble. It sounds like classic accelerator pump but I'm not entirely sure as it certainly wasn't a problem when I first got the carb (before I started making changes).

I suspect that because the original settings were so incredibly rich it was covering the brief lean hole so I have gone back to 0.032 IFR to see if it makes a difference short term and have a larger accel pump shooter on order which will deliver a faster squirt on acceleration.

Below is a snippet of data - the yellow line is manifold vacuum (0 is full acceleration) and the white is AFR.

Just for interest, here's a similar plot of gear changes with my Edelbrock carb which had nasty lean hesitation on quick throttle openings that I could never tune out:

Anyway, back to the top graph. the dips in the yellow line are gear changes. The data shows me accelerating at about half throttle in 2nd to begin with (on the left) then foot flat in 3rd, 4th and 5th after that. I was exiting a roundabout hence part throttle in 2nd and rolled fairly gently into 3rd gear to be super safe in a straight line before nailing it properly. You can see from the later gear changes (shifting to 4th then 5th) that there's clearly nothing wrong with the accelerator pump, the pump volume is adequate and there is no lean spike on shifting. If anything the pump shot is a tad too much. You can also see the AFR leans out at high RPM. I haven't got RPM plotting working yet as I think I messed up the tacho feed wire but I was shifting at around 5000rpm. An adjustment to the high speed air bleed should help reduce the top end lean out.

 The red highlighted area is a lean spike going into 3rd. It wasn't lean enough to feel a hesitation so I've never noticed it before until I looked in detail at the data. I'm a little confused as already discussed - the other gear changes the pump shot is definitely sufficient. More learning required!

Cobra has a new friend

The final (hopefully) carb parts have arrived and I now need to find time to fit them and then an excuse to drive somewhere to test (as if I need an excuse!). Unfortunately my spare time has been taken over by servicing the newest arrival to the family, a Nissan Stagea:

If you don't know what one is, its very closely related to the Nissan Skyline, just an estate and slightly stretched floor plan. They are extremely rare in the UK, being import only (from Japan). Engine is the same RB25DET in the R32 Skyline, producing somewhere around 240-250bhp. The Stagea also has the same ATESSA AWD system which is permanent RWD then some fancy electronic controlled transfer box thingy that brings the front wheels in when you are about to die. Because of this it's a heavy old beast at around 1650kg but still manages 0-60 in 6.5 seconds.

A part of me (which I am ignoring at the moment) suspects that on a bumpy b road this car would probably be faster than the cobra.

The craziest bit - have a look in here:

No carburettor, its witchcraft!

Quick Fuel 750DP Tuning Part 2

Continuation of this thread

Had a chance to go for a good 80 mile round trip which was an excellent test of the revised carb settings - here are the results.

To recap, the car was originally extremely rich in all circuits:

Stock settings

Primary Jet: 72
Secondary Jet: 82
IFR: Unknown - suspect its .032 or .033
IAB: 72
MAB: 33
PVCR Unknown - estimate somewhere around 0.055" to 0.060"


Revised Settings

Primary Jet: 68
Secondary Jet: 82 - reduced to 78 after very short test drive
IFR: .030
IAB/MAB unchanged
PVCR 0.040"


I was hoping that the reduced IFR, primary jet and PVCR would see more reasonable AFR in light throttle cruise at low rpm, medium rpm and medium acceleration. As per the previous post, I only had chance for a very short test drive and on very light throttle it was very lean and misfiring. Heavier throttle it would clear up.

Before setting off on the 80 mile round trip, I drove a few miles locally armed with carb tweaking tools. WOT was still incredibly rich so I changed from 82 secondary jet to 78 before we set off on our longer journey. I also very quickly got to the bottom of the light throttle lean misfire - its the dreaded Holley lean tip in that is well documented. This is because I went too small on the IFR. I need a 0.031" IFR but don't have one, so as a temporary fix I compensated for this by sticking a bit of guitar wire (B string 0.016") in the primary idle air bleeds (0.071" stock) and setting all four idle mixture screws a quarter turn richer than they needed to be. This got rid of the lean stumble at light throttle but obviously is not a permanent solution as it causes the engine to load up rich at idle and almost die, and on the overrun there's way to much fuel and it was popping and banging.

The solution should be straightforward - I went too lean on the IFRs so I need to go to 0.031", so they are now on order. Good news though is that with my wire and idle screw bodge I was getting consistent light throttle cruise AFR of about 14:1. The reduced primary jet means that high rpm cruise (above 2500rpm) is now almost sensible, hovering around 13.3 or 13.4:1. Medium acceleration staying on primaries only, but getting the vacuum low enough to bring the PVCR in was showing about 12.5:1 which was an incredible improvement on the 11:1 I was seeing with the stock carb settings. This shows that the PVCR change was the right thing to do. Acceleration was now nice and crisp, not blubbery and sluggish. One jet leaner on primary will hopefully nudge the high rpm cruise towards the 13.5:1 to 14:1 range and maybe nudge the medium acceleration figure up a notch, which shouldn't be detrimental to performance. To be honest, I don't cruise much above 2500rpm so the car won't spend much time here so its not so critical.

Similar good news is to be found on heavy acceleration when getting into the secondaries. Hard acceleration ( between 3/4 throttle and WOT) amazingly showed 12.4:1 flooring it from 2000rpm, leaning out to 12.7:1 by 5000rpm which I was very pleased with. The plan here is to go one step leaner on the secondary jet which should raise that 12.4 up to around 12.6 or 12.7: 1, then I can trim the fuel curve with the high speed air bleeds to keep it flat and reduce the high rpm leanout. No lean spike on fast gear changes.

So in summary:

From stock settings

Lean out primary jets 4 jet sizes
Lean out secondary jets 4 sizes
PVCR reduced from unkown size (somewhere between 0.055 and 0.060") to 0.040"
IFR reduced from unknown size (somewhere close to 0.033 or 0.034" to 0.030"

Low rpm cruise - improved from 12:1 to 14:1 but needs slightly larger IFR to cure lean tip in.
Medium rpm cruise (2500rpm and above) imrpoved from 12:1 (or even lower!) to approx 13.4:1
Light accel (primaries but above power valve vacuum) improved from around 12:1 to 12.5:1
Medium accel (primaries only but power valve activated) improved from 11:1 to 12.5:1
WOT improved from around 12:1 or even richer, to between 12.5 and 12.7:1

MPG with my old Edelbrock Thunder 650 was about 18mpg. With the stock calibration on the Quick Fuel, that dived to around 10mpg. With the current revised setup I am now getting around 15mpg but I was enjoying the car so much that quite a lot of our 80 mile journey was with my foot buried. I think perhaps 16 or even approaching 17 is possible with one more step lean on primary and secondary jet and a lighter right foot.

Quick Fuel 750DP Tuning Part 1

Continuation of this THIS THREAD

To recap, the first drive with the new carb was incredibly rich and obviously needed some work, but I wasn't really surprised, and wasn't too worried as everything is adjustable.

Based on the fact that low rpm and mid rpm cruise was incredibly rich, with WOT also definitely on the rich side I leaned out everything.

Stock settings

Primary Jet:       72
Secondary Jet:   82
IFR:                   Unknown - suspect its .032 or .033
IAB:                   72
MAB:                33
Pump cams:       Pink/Pink

Getting fuel when needed clearly wasn't a problem so I left the pump cams alone. Revised setup was as follows:

Revised Settings

Primary Jet:       68
Secondary Jet:   82
IFR:                   .030
IAB/MAB         unchanged

After several days of being busy, I finally got the chance for a drive. The result - idle screws were now functioning in the 3/4 to 1 turn range, an improvement from 1/2 turn as before. My low speed cruise below 2000rpm was now mid 13's, with very very light acceleration it would shift to the low 14:1 range momentarily and then settle back down to mid 13's with a constant throttle position. This is great!

Harder acceleration, (but still above PV vacuum level) got the mains on which lowered things a little towards the 13:1 area. Harder acceleration (PV active) and it nosedived towards 11:1. WOT was still incredibly rich at between 11:1 and 11.5:1, sometimes dipping as low as 10.8:1 but I had left the secondary jet unchanged so this wasn't too surprising. My aim was to get cruise/light accel sorted first.

This all suggests that the power valve channel restrictor is too large. When the PV is active below 6.5" vacuum, a shed load of fuel is getting dumped in so these were the next target. I stripped the front bowl off and took a look a the PV. I was a little worried as the Quick Fuel SS series does not come with adjustable PVCRs so I would have to find some suitable wire to stick in to reduce the area. Imagine my surprise and happiness when I removed the PV and saw these:

Lurking underneath the PV valve orifice (the big hole in the middle) are two adjustable jets, the same 6-32 size as the other adjustable restrictions in the rest of the metering block! I was slightly suspicious of this carb from the start, as SS series should have red anodised metering blocks and this one is clear. I think what has happened is someone has specified a carb for a particular application then for some reason decided not to buy it, but the shop had already made it up, so they sold it on (to me!). I tried to find out what PVCR size a 750-AN and found anywhere between .055 and .062 depending on the source on the internet. I found some handy bits of wire of various diamters and determined that these PVCR's were approx .055", or somewhere around 1.5mm. 

I already had some unused restrictors that were the original IFR's (.032") that I replaced, so I drilled these out to .040" (1.0mm) and used them as new PVCR's. I figured if I started small it would be obvious what the effect was - I'm hoping the effect will be leaner on part throttle accleration, then I can always drill them progressively larger if I went too far lean.

I reluctantly left the secondary jets at the stock 82 size so I could properly evaluate the effect of the PVCR change. Only make one change at a time! The next test drive will be interesting.

As an extra note, at the end of my test drive yesterday for the last 1 mile or so, on very light throttle cruise the AFR was off the scale lean and it was misfiring all over the place. On application of more throttle it would clear up and run normally so I had to do the last mile nailing it then coasting. I'm slightly confused as to what the cause could be. I pulled all the plugs and they are all a little dark from the rich mixtures I've been running but otherwise ok. All plug leads check out fine, timing is locked down and unchanged, vac can is working normally. Can't think of anything else at the moment other than some crap managed to get in the carb and blocked a channel. I've stitched it all back together ready for the next test drive so I'll see if the problem has gone away.

Later edit - this was resolved with fresh fuel,  it got to the bottom of the tank of fairly old fuel. 

Americana International + local show

Went to a school fair in the neighbouring village last weekend. A new car club has been formed in our area and this was the first event. A nice selection of cars, including a Pontiac Firebird, an Ultima GTR, a Colin Mcrae special edition Impreza, Lancia Delta HF Integrale, Maserati Quattroporte and a very nice Alfra 75 amongst others.

As soon as we had finished our tea and cake, and annhialated the coconut shy, it was off to Loughborough for the Americana International show. We met some friends who have a Triumph Herald and both stayed in the same B&B which thankfully had a big workshop so we could store the cars out of the rain overnight.

The show itself was a little disappointing, not as big as we thought it would be but there was a fair turnout of interesting American cars, something from every corner of American car history ranging from rat rods, a Hudson "lead sled" and a Camaro IROC-Z for novelty factor amongst the highlights. the weather wasn't too great in the morning and the Cobra spent most of the morning under a cover until the rain stopped. The brave little Herald looked pretty funny next to a massive Cadillac.

Quick Fuel SS 750 fitting and first use

Started fitting the new carb last night. Quick Fuel SS 750cfm, annular boosters, mechanical secondaries.

The first thing that ruined my evening was that the manifold vacuum port on my Torker II intake is right underneath the rear fuel bowl on a Holley-type carb. I had a 1/2" carb spacer but this wasn't enough to give clearance so I had to remove the right angled fitting from the manifold. To my horror, the thread on the fitting was completely stripped and knackered. Fearing I had ruined the thread in the manifold I carefully inspected it luckily it was ok. I found a few brass 3/8NPT plumbing fittings and ran them gently in hand tight just to clean up the threads in the manifold. On inspection the fitting I used was a blue anodised aluminium no-name "speed shop" fitting. That's the last time I don't use quality fittings from a named manufacturer.

Another few minutes rummaging in my many boxes of spares and I found a replacement right angled fitting that was a little shorter than the one I removed and was chrome-on-brass, not aluminium. I was able to wind it in far enough to give about 2mm clearance to the rear fuel bowl.

Another thing to note, the PCV port is at the rear on this carb, as opposed to the front on Eddy carbs so I had to swap my valve covers round so the PCV hose would reach. This means my oil filler is now on the left hand bank, shown below:

Before fitting the carb I checked the closed throttle positions. They were a bit off out the box. The secondary throttles were completely closed which I believe may be the standard setting but for my fairly large cam (240° duration at 0.050" lift) I followed the instructions and opened them up.  Both primary and secondary throttles are set to show 0.030" (0.76mm) of transition slot. I checked my imperial allen key set and had one small one which was 0.028" across the flats which was a very handy gauge for setting the slot. The secondary throttle stop is adjusted by a set screw underneath the baseplate so can only be done with the carb off the car. Thankfully the tapped hole for this goes all the way through the baseplate so you can remove it and refit from the top and - hey presto adjustable secondary throttle without having the remove the carb! I put a blob of silver model paint on to mark the 0.030" baseline position so I had a reference point if any changes were needed. Here's the secondary throttle stop handily indicated with an arrow:

Note that you have to open the throttle to adjust it as normally the throttle mech will rest on the screw. Here;s the screw now marked with paint to give a baseline reference.

The transition slots appear square at this position. Here's the primary throttles set at the correct position with my 0.028" allen key in position for reference:

I marked and photographed the idle screw so again like the secondary stop I had a baseline reference for the 0.030" transition slot position. As long as I keep accurate notes of all adjustments it should be easy when making idle speed changes to open or close primaries and secondaries by the same amount. I gave each screw half a turn and re-measured the transition slot length. Half a turn takes me to approx 0.045-0.050" exposed transition slot, which is right at the top end of what's acceptable - or even slighty over. I know now for when I start the car that I don't want to wind the screws out more than half a turn if I need more idle RPM otherwise I'm going to screw up the transition slot operation.

Here's the carb fitted with the hard fuel line. My throttle return spring bracket needs modifying as it places the spring right through the front of the fuel line. Small change to vacuum hose routing and of course now requires studs rather than hex head screws to get through the carb spacer but all is well.

I then turned the fuel pump on and checked for leaks. During my research stage I had read a few complaints about the supplied fittings not sealing properly but didn't have any problems. The float levels were a bit high and needed adjusting down. Out of the box the fuel level was about 3/4 up the sight window so I wound both floats down and got the fuel level at the recommended 1/2 way up the sight window. 

I then removed the electric choke. Complete removal of the choke flap doesn't look like a straightforward and easily reversible job so I simply removed the electronic bit off the side and used some stiff wire to lock the choke flap open.

It was then time to start the car. All four idle screws wound 1.5 turns out as per setup instructions. The car started first time after one small squirt of throttle. Straight away though it was clear something was wrong as the manifold vacuum gauge read zero and would flicker when the engine was revved. Turned out I had the vacuum hooked up to ported instead of manifold. The very poor quality grainy picture on the instruction sheet that came with the carb is the wrong way round. 

Refitted it to the correct port and everything was happy, engine was running at exactly 1000rpm with NO adjustments to the idle speed screw necessary. All I had done was set it to show the transfer slots as square on both primary and secondary as per instructions so things were going well so far. This goes to show how spending a bit of time at the start to get things correct and not rushing ahead paid off. I truly can't believe how many forum posts I've read about people putting carbs straight on engines out the box and expecting them to be perfect without checking anything then proceeding immediately to the internet to whine about how bad carb xyz is. What did people do before the internet?! I bet they damn well read the instructions properly first.

Back to the car, I gave it a couple of minutes to warm up to normal operating temperature and looked at the AFR and could see it was incredibly rich, wandering around between 11.8 and 12:1 at idle, vacuum down at 7-8in.hg. I wound all four idle screws in to lean out the mixture. At 0.5 turns out, vacuum peaked at 12in.hg but the revs had risen to 1100rpm. I set this back down to about 950. Vacuum 11in.hg. AFR was now in the 13.8 - 14:1 range. All this took about 5 minutes so I was quite pleased. Remember that I set the mixture/speed based on vacuum first - I've learned to GIVE THE ENGINE WHAT IT WANTS - not some arbitrary AFR target. The AFR check was just a nice secondary indication that everything was operating normally. Interestingly, I had matched the best value I got out of both my Eddy carbs and spent almost 2 years working on those! (although in fairness that includes the steep learning curve).

Check for leaks again, all fine. Re-check float levels now the car was running, they were a tad low so gave them both a quarter of a turn and they were back in the middle of the sight glass. The instructions do say to check the float with the engine running. 

Time to go for a drive - went to a local show and then drove 150 mile round trip to Loughborough for an American car show. Apart from being incredibly rich across the board (12:1 on cruise !!!!) it behaved very well. I didn't need an AFR gauge to tell how rich it was, when rich, the exhaust sounds different, sort of blubbery and fat. When its lean it sounds more crisp and crackly. The AFR tracked throttle position very well with no rich or lean spikes. When cruising below about 2000rpm it was clear that I was on the idle/transition circuit and AFR was around or just below 13:1 which was just about acceptable but still quite rich. The car showed no signs of lean surge so could easily cope with being a little more lean. If the previous carb was anything to go by then somewhere around 13.7:1 to 14:1 would be ok. As far as I understand, the fact that I'm only 0.5 turns out on the idle screws suggests that my idle feed restrictors are a bit too big (explaining the rich low rpm cruise) and need reducing to get the idle screws in their proper range of about 0.75 to 1.5 turns. At the moment they are incredible sensitive. 1/16 of a turn either way and the engine just falls apart so I was very lucky that my first adjustment just happened to be spot on. If I cruise over 2000rpm the main circuit becomes active and the AFR suddenly steps very rich and dives to around 12:1. If I accelerate lightly, staying about the power valve vacuum rating of 6.5in.hg this activates the mains too, and I get the same rich step. Heavier acceleration on the primary side is also very rich, low 11:1s even dropping to around 10.8:1. Note that this is with the throttle pedal held steady - not during transition while the accelerator pump is active. When I get on the secondaries things lean up a bit to around 12:1. WOT is also somewhere in the 12.:1 range. My gut feeling is that the primary side is very rich and the secondary side may actually be a touch lean.

The true test was flooring it sharply and smashing up through the gears. This is where the old Edelbrock carbs were somewhat lacking. I could never get the air flap thingy to work properly with such a quick revving, short geared lightweight manual trans car. I'm happy to report that the double pumper even with the standard pink pump shot cams has no such problems, its like someone heaving a paving slab into your chest. Next step is to lean down the primaries and the idle feed restriction. Its easy to see where the Holley-style double pumpers get their gas guzzler reputation from. Without an AFR gauge and a very loud exhaust right next to my ear I may not have realised how incredibly rich the carb is on cruise but would be very happy at the WOT performance so would just shrug off the poor mileage but of course from my steep learning curve of carb tuning I know 2 things = 1) it doesn't have to be like this! and 2) its not that difficult!

Part of me suspects a large percentage of cars with Holley DP's are like chugging around incredibly rich but because they give such blistering immediate WOT performance compared to whatever piece of old crap they replaced but the owners are happy and unaware of the easy gains they can make if they get the spanners out.

Carb tuning adventure continued HERE

Another New Carb

Another new carb is on its way! The old Edelbrock Performer 1407 750cfm is on ebay and the 1805 Thunder AVS 650 will go the same way. I have tried for several months now to eliminate the secondary bog, only to create a new problem where the excessive air door tension means that if the revs are too low then the door doesn't open and the engine bogs rich until the air flow picks up, or I lift my foot out of the throttle.

Currently in the post from the States is a new Quick Fuel SS series 750cfm mechanical secondary carb. Its a Holley double pumper clone with annular boosters and changeable jets and air bleeds - all the bells and whistles! After a lot of deliberation and research I've decided to try my luck with a mechanical secondary carb which should suit a very light, short geared manual transmission car. The Quick Fuel SS has annular boosters which from what I understand should tame the 750 to make it more driveable at lower rpm. I had the option of a 650cfm but I went for the 750 as I expect the annular boosters to help and on my 750 Eddy the idle circuit is widely known for being insufficient and I expect the Quick Fuel to be better in this respect.

Updates will be coming as I fit it and see what its like.

Brands Hatch American Speedfest III

Sunday 7th June was American Speedfest III at Brands Hatch. We were treated to two rounds of the European NASCAR championship and a variety of support races of F5000, miscellaneous classic V8's and silhouette racers, plus demonstrations and parades of anything else American that you could imagine.

At the end of it all we were all allowed to go on track for a nice steady parade, that in keeping with the parade rules absolutely did not include a rather cheeky burnout down the start finish straight when a little gap opened up hehe :) 

Here's the view looking back downhill from Druids hairpin:

Me standing awkwardly next to my own car, looking like a tourist:

And a few other photos of cars which caught my eye. Unfortunately the camera on my phone is a bit scratched and it diffuses the light a bit to make the images slightly hazy in bright sunshine.

For some reason I both love and hate this age of Mustang in equal amounts. Either way this was a very nice example and did some massive smoking burnouts on the parade lap.

Tom Petty #43, and my best friend Mater :)

Here's our cobra parked up, enjoying the view.

Queueing for the NASCAR grid walk was worth it, really interesting to see some of the cars up close

Naomi meeting a member of the Earnhardt family who was racing today, Jeffrey Earnhardt, no less. Eddie Cheever III was also there.

Number 21 Gabriela Arantes Prado, didn't have a pen handy to sign our programme and I could not for the life of me remember what Portuguese for good luck is. Later remembered its boa sorte!

Vacuum Advance 2

Had another play with the vacuum advance and have switched from ported to full manifold. I mentioned modifying the Crane adjustable vacuum canister to move the limiter to the end of the mechanisms movement rather than the start. This was fairly straightforward with a strip of aluminium. I gave it a curved edge so it was adjustable by rotating it.

My mechanical advance setup was initial timing approx 17°, total timing 34° at just under 3000rpm which is fairly conservative on the rate but is nice and safe. With a car as light as this I have found that increasing the rate and getting the timing in by 2500rpm, as many recommend, makes little difference to the "seat of the pants" dyno.

I know that my idle vacuum on purely mechanical advance is somewhere around 8-9" mercury so if I add vacuum advance from the manifold source, in order to avoid hunting at idle it needs to be fully deployed at 8". If the vacuum cannister is in the middle of it's range at idle it will wander around and give an unsteady idle as the timing is always changing.

I set the vac unit to 0 turns (fully clockwise) on the adjuster. This was loose enough for the vac advance to start coming in at 5.5 and be fully in by 7.5. 

I expect that the extra advance at idle will make it run better, with a higher vacuum which means that I can probably bump up the vac setting to come in higher. I want to avoid any crossover at light acceleration when the vacuum advance is still slightly activated but the mech advance is coming in with rising revs.

I started the car and immediately the idle vacuum jumped up to 11". This was great news. I gave the vac cannister 3 turns on the adjuster. This now means that the vacuum advance comes in at 7" and is fully deployed by around 9 to 9.5". My idle timing with vacuum connected was 26°. This means I'm getting roughly 9° vacuum advance which is fine for now. It takes a bit of fiddling to get the limit adjuster in just the right place and is largely trial and error. Having the vacuum "start" at higher vac levels means that the vacuum advance will definitely have completely dropped out when I accelerate lightly so there is no risk of pinging due to too much timing when the engine is under load.

I was able to lean off the idle mixture screws which gained another 1" of idle vac to give a total of 12" but due to the fact the engine was now running much better, the idle speed had come up to 1050rpm. One quick adjustment on the idle screw and it was back down to about 850/900rpm with 11.5" vacuum. This is the best its ever been.

Out of interest, to make sure I fully understood everything, I lowered the idle speed and as soon as the idle vacuum dropped below about 10" the idle speed started to vary and wobble around a bit. I got a timing light on the damper and could see the timing changing, due to the problem described above - the vac can isn't fully deployed and the timing varies, giving an unsteady idle. As soon as I bumped the idle speed back up again, the manifold vacuum returned to above 11" and the timing was then steady and all was well - the vac can was now fully deployed and the timing wasn't changing. The engine will idle steadily with the vacuum advance fully deployed at 800rpm for 10.5" idle vac but when the fan comes on the extra load drops the idle just a little and then the vacuum advance starts to drop off and leads to an unsteady idle. That's why the final idle speed setting took this into account and was set a little higher to give some margin. The final value that I settled on was as follows:

Idle speed: 900rpm

Idle vac 11.5 in.hg 

Base timing: 17°

Vac timing: 9° fully deployed at 9.5-10 in.hg. Starts at 7.5 in.hg

As an extra bonus job, I removed the steering column bracket and welded some triangulation onto it to make it stiffer as the steering wheel had been vibrating a bit at certain revs (about 1600rpm). Only took a few mins, didn't bother painting it as I wanted to drive it first to see if it needed any more welding first.

Went for a drive and it was really very nice. Smoother down low although it doesn't like accelerating hard from below 1500rpm as its out of the operating window of the cam. The vac at cruise has raised from 13" to 15" which would indicate that the increased cruise advance is well received and is allowing the mixture to burn more completely. The steering wheel also wobbled a lot less so I will remove the new bracket, paint it and refit. The car has always been rich on cruise to prevent surging, as suspected I think I can go another stage leaner (4%) on the mixture rods for cruise because the vac advance allows the leaner mixture to burn properly and save maybe 1mpg which is nice. No surging except at extreme driving conditions e.g below 1200rpm in 3rd and it bucks slightly but good heavens if I think back to how it used to be its like a different car. All these small continuous improvements have added up to make a massive difference.

Next problem to solve - rich bog during hard cornering and braking.

Vacuum Advance

Today I decided to fit the Crane adjustable vacuum advance kit (P/N 99600-1) that I bought a few weeks ago. Now, the internet is chock-a-block full of people getting their knickers in a twist over whether vacuum advance has any place on a performance engine.

I used to be in the "no" camp and told myself that the cobra was basically a street legal racing car and doesn't need any fuel saving nonsense but after doing a bit of reading I've decided there's nothing to lose so may as well fit it. I highly recommend reading David Vizard's book "How to build max-performance Chevy Small Blocks". If vac advance is good enough for someone of Vizard's experience and knowledge, then it will do for me.

Vacuum advance is load sensitive as opposed to RPM sensitive (centrifugal advance) so it won't affect all of the good careful work I've done on my mechanical advance curve. Vac advance will not affect the timing at heavy and wide-open throttle because there is insufficient vacuum to activate the mechanism. It only comes in at light loads when you are cruising along. Just a quick refresher before I get in to fitting it - the basic premise is that at light loads the spark needs to occur earlier because the lean mixture and partially filled cylinder need more time to burn fully. The way I imagine it is that the fuel is more spread out so the flame front travels slower, hence you need to give the engine more time to burn the mixture in each cycle. There, that made perfect sense didn't it!

Well anyway, a whole load of people a lot cleverer and with a lot more experience than me have written reams and reams on the topic and its all out there for the inquisitive mind to digest. I would politely suggest ACTUAL BOOKS or articles by people with proven experience and knowledge as a source for information rather than what some completely unknown person on an internet forum (or blog for that matter!) says.

Right, on with the fitting. My dizzy has a plastic jobbie where the vac advance canister goes so that was removed and the advance can fitted in its place. There is an arm which hooks into a tab on the rotor. This is what pulls the timing in/out.

Here's my distributor before any mods (ignore the completely different advance weights/springs). The black plastic piece screwed in at the bottom of the picture is the blanking piece for the vac advance.

 Here is the advance fitted:

I removed the rotor housing thingy for a better view.

 The kit comes with a fancy looking plate that you can just about see in dark grey below the top fixing screw of the vac advance unit. It looks a bit like a cog-wheel and has an off-centre hole. This is the limiter plate. I set it to give the smallest amount of vac advance possible so I could start safe and increase if required.

When I got the advance kit I also bought a hand vacuum pump so I could determine the vacuum at which the advance comes in, when its fully deployed and also apply known amounts of vacuum with the engine running and measure the advance.

First of all I hooked up the hand vac pump with a spare bit of hose and applied vacuum until the mechanism moved. This was at whatever setting was straight out the box with no adjustment. The mechanism moved at approximately 7 inches of mercury.

I then kept applying vacuum until the vaccum mech stopped moving and the maximum vac advance had been deployed.

You can see in the right hand picture that the arm has moved to the bottom of the slot when fully deployed (just above the Made in Mexico sticker). The limit plate adjusts the start position. This highlights the slight drawback of the limit plate - if you rotate the plate to limit the advance, the spring in the diaphragm is compressed and it will require more vacuum to stat moving. It would make more sense to have the limit plate on the other side of the arm so the mechanism moves into the limiter and the vacuum at which the advance starts moving remains the same.

I can see a potential future modification on the horizon - move the limt plate to the other side of the arm, but that's a task for a rainy day. Back to the task at hand though, I noted down the values and found that the vacuum advance started at 7.5in.hg and finished at 13.5in.hg. I know from the vac gauge on the dashboard that when I'm cruising along at around 2000rpm I get 15in.hg vac which means this setting is ok - all the vac advance will be in so it will be steady and not wandering around (I assume that's the aim) so I may as well go for a drive and see what its like.

The only thing left to check was exactly how much vacuum advance I had. The limiter plate just has a bunch of nobbles in it, it doesn't say what the value is so you have to measure it. With the vacuum pump still attached I got my timing light hooked up and started the car. I let it warm up for 5 minutes then checked the initial timing  which was 17°. I pumped 15in.hg vac on the hand pump and the engine picked up a few revs which showed that the vacuum advance was working. Checked the timing and it was 32°. This meant that I had 15° vacuum advance.

I found a bit of hose and attached it to the vac can, then ran the other end round to the front of the carb. Now here is a dilemma. Ported vac or manifold vac? Ported vacuum on a carb comes from above the throttle and is only present above idle. At idle there is no vacuum on the ported source. Full manifold vacuum is present all the time (apart from wide open throttle of course) as it comes from below the throttle.

Full manifold vacuum is present all the time. After thinking about it I put it in the ported vac. The reason - my idle vacuum is quite low because of the cam, approx 9in.hg. If I hooked up the vacuum advance to manifold vacuum the vac canister would be partially deployed at idle and may lead to an unsteady idle which hunts around because the timing is constantly changing. The weight of opinion is that full manifold vac is the "correct" way to do it - you set your base initial to stock at around 12° or so then you get your extra timing from the vac advance to take it up to around 25° so you get a good clean idle, but this needs to be fully deployed at whatever your idle vacuum is. That means I need a vac advance can that is limited to around 8-10° and is fully deployed at idle vacuum i.e. somewhere around 9in.hg. I may be able to set up this adjustable can to get that setting in the future when I have done some more adjustment but for now I'm just interested with measuring the total vac advance in degrees and seeing what its like when running - hence I went for ported vacuum.

Ported vacuum on an Edelbrock carb is on the left looking from the front. (UK drivers side).

Went for a drive, about 40 minutes or so with lots of cruising but also some varied acceleration to see if I could detect any pinging. Everything seemed fine and things were noticeably smoother at lower revs. I didn't look much at the AFR but will have a look at the data later on. Next thing to do is to lean out the carb in cruise mode and see what I can get away with. I think at least one step leaner (4%).