24 replies to this topic

[ACDodd]

Timing does have a marked effect. Its related the time of the end of the burning. Hot gasses ignite incoming charge, start the burn earlier finish the burn earlier ita that simple. When you say 9 degrees what rpm? Was the vacuum connected at this point when you set it?
Running rich is a band aid this will wear the engine.

Ac

Edited by ACDodd, 15 June 2016 - 04:27 PM.

[KernowCooper]

You mention adjusting the mixture to chase either weak running or a rich idle, your needle is wrong if your doing this, what needle is in it now?

[skoughi]

It currently has a BDK in it, that's what was in it when I got it, I've serviced it including fitting a new jet. I bought a new BDK needle to replace like for like in case the old needle had been altered, however the difference on the AFR gauge seemed very little. I've fitted a yellow spring, it had a red spring which I still have, dashpot has 20/50 oil in it and I've reduced the "top hat" at the end of the damper rod to a dia of 7mm. I'm having a go at filling the old needle to see if I can get a more rich mixture  at 2000rpm whilst driving. If it doesn't work then nothing lost as I still have the new untouched one, hoping to get a few more hundred miles on her then hopefully (if the wife lets me) will go to a rolling road to get set up properly. I've set the AFR to be about 14.5-14.7 on idle and slowed down the idle speed as much as I can and she seems a lot better now. Stopped her a few times and she didn't run on. The only problem now is to get her to run a bit more rich at 2000rpm whilst driving, usually she's about 15.8 - 16.2, everything else is fine. She goes down to 12.5 when accelerating and there's no hesitation.

Edited by skoughi, 15 June 2016 - 08:18 PM.

[ACDodd]

You need to adjust the needle not the idle screw.

Ac

[tiger99]

I seem to recall that the humble 998 City E (economy), which was tweaked for maximum torque by having high compression, to cope with the 2.95 final drive, needed an anti run on valve. I agree with others and recommend fitting one. They work.

The City E and HLE had other tweaks like wider than standard spark plug gaps, and certain brands such as Bosch had a very short life. The high compression ratio and consequently higher combustion temperature and pressure were to blame. The engines were almost but not quite standard A+. A standard A+ head gave noticeably less performance due to the lower compression. I know from direct experience, but it will likely be disputed as the modern consensus is that the A+ engines were all the same.

I only mention all this as it helps to put the effects of high compression, possibly overheating plugs, and running on into context and helps to confirm the advice already given.

[carbon]

Timing does have a marked effect. Its related the time of the end of the burning. Hot gasses ignite incoming charge, start the burn earlier finish the burn earlier ita that simple. When you say 9 degrees what rpm? Was the vacuum connected at this point when you set it?
Running rich is a band aid this will wear the engine.

Ac

Skoughi,

 

I'm running a MD266 cam in fast road 1293, about 9.75:1 compression. This has about 12-14 degrees static timing and probably nearer 16-18 at idle to get it running smoothly. Using total 20 deg mechanical advance all in by 3000rpm on Shell 99 octane.

 

Would also suggest running compression checks, you should be getting about 195-200psi on all cylinders with a fresh motor.

[skoughi]

I'm thinking about doing a first time compression test on her as she's done about 450 road miles plus running in the garage, so would be handy to see if all four cylinders are similar. I've kinda sorted the running on issue, idle at about 800rpm with an AFR reading of as close to 14.7 as I can get and she'll stop dead when the key turns off. Any more rich or lean and she tends to run on. That's the positives! I'm also trying to teach myself how to profile a needle, mixed results but only on my second attempt, and still fretting if I can hear pinking. Set her at 10 deg and she drives really good, no hesitation. Turned the dizzy a good bit on through the advance but couldn't notice any real difference on the engine note which makes me think she's pinking but I don't know what to listen for! A second opinion will be sought. Very close to getting her at a good running setting but still feels far away! Found out last night of a very good local engineer who's setting up a garage with a rolling road for the growing classic car population up here so will be getting in touch with him very shortly to getting her sorted.

[Spider]

Hey mate!

 

Just picked up on this thread. Everyone else has had a say, so I'll add to the confusion!

 

Most of the time, running on I've found is from a poor state of tune or a manifold leak. IMO, yr static CR is fine.

 

The other thing is hot spots in the head (eg, a bur left from having the deck skimmed). I doubt it would be from plugs and in any case, you can look at the plugs themselves for that story, just compare them to one of the many on line plug charts.

 

Given what your engine is, you shouldn't need an anti-run on valve and IMO by fitting one, you are merely masking an underlying, potentially serious problem. Bad idea in my books.

 

With your AFR meter / gauge, where is the sensor located?  Is this a 'permanent' one or just one you use in the workshop?

[skoughi]

Hi bud! The running on issue is kinda sussed, found that if I can get a reading on the gauge at 14.7 or as near as along with a slow idle then she's ok. Anything either side of this value and she seems to run on. The AFR sensor is just to the rear of where the gear change is, only place where I could get enough space.

[skoughi]

Little update on the "issues" I had; running on seems to be sorted with getting the idle mixture as close to 14.7 as I can get along with an idle speed about 800 - 850, no run on and that's with no6 plugs. The misfire was fixed once I fitted another Acuspark electronic points kit wich also came with a red rotor arm. She seems to run quite well at 10 deg timing, set her up to 12 deg but there was no improvement on the slight hunting, in fact it was a little worse so put her back to 10 deg, also there was no pinking at 12 deg. Did a compression test when she was nice and hot and got 190 psi give or take a couple of psi across the four cylinders, so well happy with that seeing as she's only done about 500 odd miles since I got her MOT'd. I'm still using my second attempt at tweeking a BDK needle and she's a little on the rich side going with the AFR gauge and the plugs are slightly on the black side so I ordered a couple of BDK needles so I think I'll have a another go at one to see if I can get a slightly more lean mixture. She's driving really well with no hesitation when the throttle is floored, good pick up but may be improved with a slightly less rich mixture, all in all very happy with her and will put on a lot more miles on her in the next few weeks to see if she can be improved on