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
