Longest Drive Ever! (so far)

At the weekend we decided to embark on a 320 mile round trip over two days, 2 1/2 hours and 140 miles each way (plus some extra local mileage at our destination) which is by far the longest journey we've done in the cobra so far. It took about half an hour to settle down and stop tensing up and desperately trying to work out what every small sound was - e.g. driving over a dry twig which snapped so loudly we could hear it over the exhaust made me nearly poo myself that something had fallen off!


We waited and waited for it to stop raining and eventually set off with very wet road. Naomi was brave and drove the first bit, flooring it on a slip road to join traffic was interesting - "slip" road is definitely the right term! The fuel gauge had me worried too, It's never been completely full apart from the IVA and I had forgotten that it doesn't drop in a linear fashion. Full to 1/2 is only about 20 litres so that was gobbled up in the first 70 miles and I was getting quite worried but then the needle barely moved for the next 50 miles. Range on a full tank I reckon is about 240 miles but that's pushing it a bit unless you know exactly where the petrol stations are!

Got into the swing of things but 2 hours on the motorway at 70mph must be reaching anyone's limit of fun in a cobra. We were togged up with ear plugs, hats gloves etc. We swapped drivers halfway but I was definitely ready to jump out and have a cup of tea by the end. Sunday morning was fun though, visited a number of friends and family and gave them all a go in it. I was especially pleased to go for a drive with my grandad who was an AA man for about 40 years and has been in the motor trade all his life. For him to say "not bad" was practically the highlight of my life so far. He nodded in approval as we thumped through the villages in 5th gear 30mph, 900rpm, then did the classic muscle car trick of 0 to a nice round number entirely in top gear.Rest of the journey home was ok. Slow traffic on the A1 southbound in the evening and we were crawling at walking pace for about half an hour, car was fine, fan working well and was actually quite pleasant with the heat wafting up from the footwells to combat the cold as it got dark. MPG for whole weekend was about 17, I reckon I can get a little more out, maybe 18 or 19 but not that bad really considering almost 40 miles or so were "demonstration" miles. The whole journey used about 150-200ml of oil but I know I've got a small leak from just behind the dizzy which is on my winter list to sort out so wasn't really concerned by that, everything is nicely rust proofed by oil now!
Just got to clean it all up and do a few cursory checks: header bolts need snugging down again as I could hear it chattering towards the end of the trip and I suspect the tracking is slightly off after catching a massive pothole.

HEI Distributor Maximum Advance Limit Mod

Getting ready for a very brave 300 mile round trip to see some friends this afternoon so went for a little shakedown drive this morning. Had a final check of the timing after doing a slight modification to the distributor.

There is a method for limiting the mechanical advance which includes welding up the slots in the dizzy plate but this sounded a bit dodgy but I happened upon another method which I prefer as its completely reversible, and I did it without even removing the distributor from the car.

In the metal strip that the rotor screws on are a some holes punched at the factory. By chance they are just over 5mm in diameter so can easily be tapped to take a homemade M6 grub screw. All I did to make a grub screw was take a regular M6 hex head machine screw and chop the top off. You can see in the photo below all you have to do is leave enough of the grub screw protruding so that when the rotor moves, the screw will hit up against the centre plate - thus limiting the mechanical advance. Lots of people have experimented with grinding the head of the screw down to various shapes and sizes to limit the travel so I did the same, then verified with the timing light.

I got the dremel out and made a slot in to get a flat head screwdriver. You can then do further grinding to the head of the grub screw to get your adjustability. I cut half the top off, retaining some of the original grooved cut so you can still get a screwdriver on it. Rotating the grub screw then gets the flat edge of the screw either closer or further from the baseplate depending on its position.

You can twist the plate that the rotor fits to by hand to see how the sweep of travel is affected and then by rotating the grub screw its easy to see how the position of the screw can alter the maximum rotation of the rotor.

It's probably best just to experiment and have a guess, then check the timing and make further adjustments. I found it quite satisfying to do it this way actually as it proved to me that I knew what I was doing!

The angle of the grub screw required for a certain amount of total advance may change depending what shape weights and centre plate you have because the starting point of the sweep of rotation of the rotor may be different. Best to experiment yourself. As long as you haven't  changed the base timing position you can't hurt your engine if you start it up and run the revs up to 3000 to check the "all-in" figure, there's no load on the engine so it's no problem.

The inside of the plastic rotor housing needed a tiny bit of fettling to clear the screw but all was well. I'm now confident that the max timing is definitely the maximum and there is no extra bonus timing. We briefly ran the car up to 4000, 4500 and 5000rpm and the timing mark on the balancer was steady so I'm happy that's the true maximum.

We went for a short drive to get some fuel and to do the timing in the quiet industrial estate behind our house so as not to annoy the neighbours and it was fine.

We checked the timing: roughly 16-17 static, 30-31 all in at about 2700rpm so thats 15ish of mech advance. A quick adjustment on the grub screw and my mechanical advance was opened up a tiny bit to 16°. The final setting was 18° static and 34° all-in. This allowed me to close the idle speed screw a bit and I've now locked the dizzy down and will probably leave it there.

One important thing to note - if you limit the total advance so you can raise your static advance, you may need to swap to stronger springs because your "all-in" figure will come in sooner.

The Moroso advance kit gives the following information:

RPM	500	1000	1500	2000	2500	3000	3500	4000	4500	5000
light	0°	10°	20°	22.5°	23°	23°	23°	23°	23°	23°
medium	0°	0°	6°	13°	19°	23°	23°	23°	23°	23°
heavy	0°	0°	0°	6°	10°	16°	21°	23°	23°	23°

The values in the table above are added to your static timing to get the total timing for each rpm interval.

An example: say you originally had 12° initial and a a pair of medium springs. By the chart above, you would get a total of 12° + 23° = 35° advance in at 3000rpm.  At 2000rpm you would have 12° + 6° = 18° mechanical advance. If you whack your static up to 18° at idle, at 2000rpm now you have 18° + 13° = 31° mechanical advance. If you already had an aggressive timing curve with light springs, you may suddenly find you have some pinging issues when accelerating from low rpm as the mechanical timing is now higher than it was before.

What I did was to check the timing at 500rpm intervals and swapped the springs for heavier ones to bring the timing curve back to what it should be. Surprisingly I ended up using two heavy springs. The top row is my original timing curve, without the mech limit grub screw, static set to 12° and two medium springs. The bottom row is my new timing curve with 18° initial, total mech advance limited to 16° and two heavy springs.

Total timing:

RPM	500	1000	1500	2000	2500	3000	3500	4000	4500	5000
Original	12°	12°	18°	25°	31°	34°	34°	34°	34°	34°
heavy/heavy	18°	18°	18°	24°	28°	34°	34°	34°	34°	34°

I felt that this was the best match. I tried one medium spring and one heavy spring but the advance rate was too fast and was giving me 34° timing at about 2400rpm and I was getting some light pinging when accelerating from lower revs.

Went for a drive and it felt good. Didn't notice the loss of a couple of degrees of advance in the 2500-3000rpm range. Perhaps on a dyno there might be a small difference, or maybe in a really heavy car you might possibly detect a small change but in the real world with this engine in such a light car you just can't tell. Idle screw settings don't seem to affect the cruise over 2000rpm so that suggests the idle and transition circuit is working how it should (closing down the idle speed means the transition slot is being used properly as designed) but I still get a little lean surge below 2000rpm so I wonder if the idle/transition circuit can supply enough fuel. It's on the list for things to research. EDIT - see post Idle Channel Restrictor for the solution to this! Over 2000rpm AFR's of 12.8 to 13.0:1 for cruise does seem a little rich to me but anything more than that and the lean surging gets worse so I've decided its best to give the engine what it wants, not just what the internet says it should have (mid 13's)

New carb continued....

A few other things I've noticed now I'm running the Eddy Thunder AVS 1805 carb (650cfm) over the 1406 Performer (750cfm).

I checked it over before I installed it to adjust the floats and idle speed to standard (floats a little off out the box) and checked what rods and jets were in. Compared to the 1406 Performer carb its a little more difficult to change the rods. Not a lot, but things are a little more cramped at the top of the carb so its more fiddly. The piston that the rods fit into doesn't quite clear the screw of the little screw-down cover plate without a bit of care taken which turns a 2 minute job into a 10 minute job.

Here's what I mean:

1706 Performer, loads of room here to swing the cover plate out the way and get the rod and piston assembly out easily.

1805 Thunder AVS has a different shape surrounding the choke, almost reversed. There is a high lip that goes across the centre which means to get the rod and piston out you have to completely remove the cover plate and screw which adds a bit of time, and adds to the risk of dropping bits down the carb. I much preferred the simplicity of undoing the screw a couple of turns and swinging the plate out the way.

You can also see above the Thunder has got dual fuel feed (one on either side) unlike the performer which is UK driver side only. Seeing that I already had the fuel line plumbed in for the drivers side this didn't bother me but is useful to know. I don't yet know how the car will behave in hard turns but with the previous carb it would bog rich on very hard turns and hard stops. 

One benefit of running an AFR in the 13's at cruise instead of waving around between 14.5:1 and as high as 16:1 is that the engine runs 5 degrees cooler. Previously I was seeing that at prolonged periods of 30 or even at 40mph when I'm forced to do 2000rpm to get a smooth ride the temp gauge needle would creep over 90°C to approx 95°C. Its hard to tell exactly as there are very few increment marks on the gauge. It now sits just below 90°C in all conditions, dropping a little more if cruising over 60mph.

We brought the throttle pedal end stop up a couple of mm to prevent the throttle cable being stretched and the butterflies being damaged. This is a bit awkward, its just an M12 bolt through the front of the footwell but needs someone in the footwell and a helper with their hands stuck between the headers and the brake servo in the engine bay. The travel required on the cable on the 1805 for some reason seems to be about 3 or 4mm less than on the 1406. Not quite sure if this is actually a difference between the carbs or if something has changed on the cable route somewhere. Might look into this a little more.

Still a couple of other things to give attention to - I happened to look down the primaries with the engine running at idle and I could see a couple of drips coming from the primary nozzles which means the throttle is open a little too much at idle. The transition slot should appear square when the throttle is closed but mine isn't perfect as it needs a more throttle to idle because of the cam.

The engine runs on (diesels) ever so slightly, just one or two turns after switching off the key so these two things are likely related to the idle throttle position just being a hair too much. I think there is some leeway on the idle speed - now that the carb responds properly to changes in tune, I can revisit my timing and then coming back to the carb again. Perhaps I could add another degree of timing at idle which would allow me to close the throttle a little more. I'm not currently 100% confident in my cheap timing light so I've borrowed a good snap-on one from a friend to verify the timing.

Carbs - dont go too big! (running problems solved)

After being sent away for work and a few other things it's taken ages to get round to giving my 650cfm eddy thunder carb a whirl. This weekend I finally had chance to see if it solves the running problems at light throttle that have plagued us from the very start.

Short answer: Yes! The engine now runs a lot more smoothly at lower revs. It's still not great below about 1400rpm but the improvement elsewhere is night and day. I may need to look at the timing to squeeze the last bit of improvement but for now I'm happy.

Long answer: The carb came with 6847 rods and 095 jets (primary). I set the throttle so the transfer slots were square (as the manual). Floats were a little off but they are easy to adjust. Idle screws starting point is 1.5 turns out.

Put it on the car and fired up, once the float bowls had filled it started easily but wouldn't idle, we had to open the idle speed up quite a bit which concerned me a little as I knew this means the idle transition would be less effective. 10 minutes fiddling with idle speed and mixture we got it idling at 1000rpm but the AFR gauge said 18:1 (max reading) at idle. It would drop if the throttle was blipped but stayed at 18:1 at idle. Even with the idle screws out 4 turns it still said 18:1. I decided this had to be wrong as the engine was idling comfortably with 10" vacuum at 950rpm. It wouldn't be able to do that if it was as lean as the gauge said.

After some headscratching I thought "stuff this. Lets see how it drives". As soon as I reversed off the driveway I knew it was better and once I turned onto the main road I was very very happy. I cruised along in 4th gear, 30mph 1400rpm and it was nice and smooth. The exhaust sounded ridiculous of course, burbling and chuffing away but finally the sub-2k rpm surge was gone!

There's an industrial estate down a quiet road behind our house so I stopped down there for a further fiddle to reduce neighbour annoyance and quickly got the idle to 900. I then noticed that the AFR gauge suddenly started reading something sensible - 12.8 at idle.  I have no idea what it was playing at in the garage with the erroneous 18:1 but its fine now.

Chugging around below 2000rpm and the AFR is about 13 but when accelerating very gently it just rises up to 14:1. It's just on the very edge of a lean surge so a tiny bit more tweaking is required but its so much better than it was. Over 2k and its low 13's (as the influence of the idle circuit diminishes and you get full on cruise) then mid 13's on power when the vacuum is below 5". I haven't tried full throttle yet as the throttle travel seems to be slightly less on this carb so I didnt want to stretch the cable. I can adjust the throttle end stop a couple of mm.

I think I can go at least one stage leaner on cruise and possibly lean off the idle a quarter of a turn but it seems to like a rich idle probably due to the cam overlap. When I do that, and sort the throttle stop out I can see what WOT is like and see if there is any bogging. EDIT - I later discovered there is a bog issue. See my Secondary Air Flap post for how to eliminate a WOT bog with this carb.

Over all I'm very pleased. This 1805 eddy thunder carb has worked pretty much straight out the box with the minimum tuning which is explained very clearly in the manual. The smaller primaries compared with the 750cfm mean that the air velocity is much higher and the fuel mixes better ( at least that's how I understand it) and the fuel metering at small throttle openings is much more reliable and stable. It starts to struggle below about 1400rpm but at the moment I don't mind that at all. I can now chug along at 35 in 5th gear and not be thrown out of my seat. Lovely!

Of course this now exaggerates the fact that my intake and cam are designed for higher rpm so flooring it from 1500rpm is a tad disappointing, as mentioned earlier I think there is more to be had from looking at the timing but generally speaking its so much more pleasant to drive now I feel I can finally enjoy the car instead of constantly worrying about never being able to get it running properly.

Thanks to all on the cobra forum who suggested a smaller carb, you were spot on!

Nice little run out

Now I've been really bad recently for constantly bemoaning the poor running of the car and I haven't really driven it much because its never running right.

Well this afternoon after the F1 was finished I decided to stop being mr grumpy and Naomi and I went to Welwyn to see some friends who helped us build the car. It was about 1hr 10mim away and gave a very good opportunity for some good lomg steady state cruise data gathering.

Drilling out the idle jets has made no noticeable difference, its still terribly lean below 2000rpm. Interestingly cruising at 2400rpm (70 in 5th) the AFR dropped to about 12.5:1. It would appear that all my rod and jet changes for cruise mixture have been pointless because all the time I was evaluating the changes I was never truly on the cruise circuit, I was still in the troublesome lean idle/transition circuit!

I think I've just about had enough with this carb. It simply will not meter the fuel correctly at light throttle.  Even with my fairly hot cam it should be better than this, anything below 2000rpm and its bucking and surging at 15:1 or worse AFR regardless of what we do to the mixture screws or idle revs. Time to procure a smaller carb!