Rolling Road - Follow - Up

After the not-too-successful rolling road session, I had a friend round and we tried to work out what was going on. One of the things suggested was to make sure the timing mark was at TDC so we pulled the plugs out and turned the engine over by hand. As I rotated the harmonic balancer we noticed (with associated groan of realisation) that there are two white marks, we were using the wrong one! I don't know what the second mark equated to, but it explains why the timing was all messed up. We started from scratch and verified TDC and scratched off the second mark on the damper.

We set that straight (after having to go out and buy a new timing light as my ebay one was broken after only a couple of uses). Went out for a drive and surprise surprise, no detonation. The car sounds better, pulls harder and runs smoother at low revs although its still far from perfect below 2000rpm. There's also still a bit of a bog when going for full throttle from low revs, possibly the jet change we made on the RR wasn't necessary now we've got the timing right.

Now that's sorted I'm booked in for another RR session in 2 weeks to eliminate the bog and try and recheck  the idle mixture.

We drove from Cambridge to Letchworth to visit a friend and it was extremely civilised. We used a quarter of a tank which is about 18 litres and did just over 70 miles which equals 17.6mpg, which is frankly astounding!

Rolling Road

Spent the afternoon on the rolling road at Revivals Garage in Thriplow near Duxford yesterday afternoon.

Was very interesting, turns out we were having some detonation problems on higher loads at low-mid revs. We put the timing back from 34° all in to about 25° all in which reduced it but didn't cure it entirely. Either the air/fuel mixture is incredibly lean or the TDC mark on the harmonic balancer is in the wrong place. We looked at the mixture first and changed the standard rods (0.113 main jet and 071x047 rod) to stage 1 rich +4% - calibration point 10 for edelbrock #1407 carb (0.116 main jet and 073x047 rod).

This helped a lot but still didn't entirely eliminate the problem but sadly we ran out of time. We learned an awful lot though. The road was set up to hold 80mph which is about 3800 in 4th and we then observed what was happening throughout the full throttle range. In the end we had it running nicely up to about 2/3 throttle as the secondaries start to come on then it detonates. Not perfect but a lot better than it was. I thought I had a rattle in the exhaust where I put some screws in to hold the collector on but it turns out it was pinking! Whoops!

Next jobs are to check the timing mark is actually at TDC as it was a bit odd we had to back the timing off so much. Then after that we'll change the standard orange rod spring for the stiffer pink spring from the carb calibration kit to keep the rod up and get more fuel in (assuming its a lean problem).

The Hunt for the Clonk

I've noticed (and mentioned on here previously, I'm sure) a clonk from somewhere in the drivetrain when coming sharply off the throttle and got under the car today to have a look. Initially I thought it was diff backlash, the diff is reconditioned and supplied by Dax. Following advice from the forum I jacked the car up, put it in gear to lock the prop and moved the wheels, carefully inspecting each joint in the drivetrain to see where the problem was.

Turns out there is practically no backlash in the diff at all. All the play is in the CV joints. These are off a BMW M5 and in the rolling chassis kit supplied by Dax they are not new parts. If  I had realised at the time then I would have just put new ones in, seems daft and is annoying as it's a big job to disassemble the entire back end to replace them, but at least I've identified the clonk.

Brakes Fixed, Engine Almost Fixed!

I gave my new Eezibleed a go and got a whole load of air out the brakes that I didn't know was there. Works pretty well, if you don't know what one is basicaly you have a bottle with two hoses, one connects to your brake reservoir, the other to a tyre at 20psi, then you just go round and open the bleed nipples and the fluid comes out as if someone is stood on the brakes.

Unfortunately I left the rear passenger caliper off the disc when I connected the eezibleed so the piston popped out and fluid everywhere. Whoops!

Reassembled and had to start all over again with the bleeding. I put about 5 litres (the same litre, 5 times!) through the rear brakes before I got rid of all the trapped air, it takes a long long time!

I was a bit concerned that the brakes didn't feel the same as in our "normal" car and got some advice of the cobraclub forum, the pedal should go hard after a few pumps with the engine off but this wasn;t the case on the cobra. There's a non-return valve from the servo to the inlet manifold and I was advised to check this was the right way round. I was pretty sure of this but I had a look anyway, turns out out the jubilee clip on the hose wasn't done up so I think I had a vacuum leak. Nipped it up, finished the brakes off and started the car, would you believe it's now a lot smoother! Here's the offending bit of hose coming from the brake servo:

I had my friend Max round and we went for a drive to warm it up before having a play with the idle mixture and immediately it was obvious the car was running a lot better. It still bucks a bit but now only below about 1800 revs whereas before anything below 2200 it was kicking and surging. We had it cruising at 1800rpm in 5th nice and steady. I had a quick refresher in carb operation from Max, at normal road speeds to maintain 50 or 60mph in 5th is such a small throttle opening its completely dominated by the idle circuit. 

We checked the idle mixture after our warm-up drive and couldn't get it any better than the settings I already had but we surmised that it needed a few miles for the plugs to clean up after probably being fouled by the previous mega-rich mixture before the performance would improve. My last vacuum reading was 11in.hg and the needle was waving around almost +/- 1in.hg. After disconnecting and reconnecting all the vacuum hoses, it now read about 11.5 and was just bobbing gently +/- 0.25in.hg. Much, much smoother! 

Another quick job, we checked the timing. I knew it was at about 32deg all-in (3000rpm) but I wanted to know for reference what the idle timing was, its 14deg. The car won't start with any more initial advance, it kicks back against the starter.

So, the engine is running a lot better, not quite perfect as I'm sure it could be smoother at really low revs but smooth running down to 1800 is good enough for now, at least now I can do 30 in 3rd, 40 in 4th and not get thrown out of my seat.

Going back to the original topic, the brakes are now unbelievably good. The first mile or so of our initial warm-up drive were down a very quiet dead end back road where I did some hard stops to check braking performance. It will quite happily lock the front wheels from 50mph with a good hard stab at the pedal, the front right locked first but I expect a tweak to tyre pressure will balance things out. I've never had a car with such good brakes so its going to take some getting used to but my collarbones hurt from pulling against the harness!

All in all, a very good day, saw a few other nice cars around, corvette c5 gave us a thumbs up, saw a very nice Aston DB6 at the petrol station, had a brief chat with the owner. I think I saw an exige pass the other way but he was driving quite *ahem* enthusiastically (and so were we!) so it was a very brief meeting.

Also, the roads were dry enough to get a fair bit of full throttle and finally managed to get up to proper operating temp. Oil was just over 90deg instead of the 80odd that I was all I managed in the ice/frost, managed to get the exhausts properly hot for the first time too, passenger side was popping a bit on the overrun so it may still be a tiny tiny bit rich on that side.

Also also, may have set another car alarm off simply by driving by hehe : )

The Tweaking Begins

It's surprising how quickly 600 miles have been racked up in the cobra, just from a few trips around the local villages on the back roads, pootled to work in the frost/ice (that was interesting!) and a few other short journeys when the weather has allowed. 

Following the Christmas drive when it was running quite badly, I bought a vacuum gauge and adjustable timing light. The engine info from BAE said the engine should have 36deg "all in" timing at 3000rpm. It was set purposely retarded for run in. I measured this at approx 24deg all-in, static timing was about 4deg so total centrifugal advance is around 22deg. The engine came with no vac advance fitted, dizzy is a newer style HEI with module built into the cap. 

I went for 20min drive to get everything warmed up then set the timing to the 36deg all-in figure, which would give static timing at about 14deg but I forgot to measure and check this. The car re-started easily when warm but the morning after it kicked back against the starter so I had to back off the timing to 32deg all in and it seems fine now.

Using the vac gauge at idle I was getting 9-10in.hg at idle which is pretty low but my cam is pretty big, comp cams xe284h with following specs:

Advertised Duration		Duration @ 0.050		Valve Lift W/1.5 Rocker		Lobe Sep. Angle	RPM Operating Range
Int.	Exh.	Int.	Exh.	Int.	Exh.
284	296	240	246	.507	.510	110º	2300 - 6500

Knowing that my idle set-up was pretty crap, I set it back to "stock" according to the Edelbrock carb manual. It advises that for a reasonable setup when trying to get a new carb to run, you wind the two mixture screws in until they bottom out, then out 1 1/2 turns, then this is your base setting. I did this and immediately saw better vac and the revs rose. I backed off the idle speed screw to get the revs down to about 850 but its hard to tell exactly what revs as the needle isnt steady. I ended up with the mixture screws out roughly an extra turn which gave peak idle vacuum of almost 13in.hg but again this value wasn't steady but probably due to the cam and varied by ±0.5in.hg.

Went out for another drive and it was a little bit better but still lurches and kicks a bit below 2200rpm so I'm wondering if the idle throttle position is still slightly too large which will uncover transfer slots etc that I'm still learning about and trying to understand.

More fiddling required, next stage is wideband O2 sensor to see if I can home in on a better idle mixture setting.