Poor running /misfire

[slewthy]

Still having problems achieving smooth running on my S3 N/A.

Symptoms as follows.

Starts up cold, no problem. Idles at around 1000rpm but wanders a little, tending to end up at 1500 when warm.

Stutters and almost certainly misfires at at idle and I think, at higher rpms.

Had first proper backfire today when I stopped the engine.

I have rebuilt carbs, cleaned with carb cleaner, replaced seals, o rings, springs, washers filters.

I have tried to balance them using carbtune but I am finding it impossible to balance the rear carb - vacuum on 3 much higher than 4.

O rings at inlet manifold replaced and bolts tightened according to manual.

Compression tested, but with engine cold, gave around 165mmHg for cylinders 1,2 & 3 and 150 for 4.

Recently replaced coil.

New HT leads and correct plugs from SJs. Gap set at 0.9mm as per manual.

All leads produce a spark. TDC matches perfectly with marks on cam sprockets.

Tested timing and adjusted advance to around 8deg at 'idle' and 25degs reached at 2-3000 rpm.

It failed its emmisions MOT due to high Hydrocarbons - 1385ppm (max allowed 1200) Passed CO - 0.14%. Passed smoke test.

Where do I go from here?

[jonwat]

Hi Simon,

First step is to get your carbs set up correctly so it can pass it's MOT.

You can either get them done professionaly or do it yourself by getting a Colour Tune from Halfords to set the correct mixture. It's like a glass spark plug which alows you to see the colour of the flame in the cylinder.

To balance the carbs I found a mercury manometer best, bikers use them & they're advertised in the back of bike magazines.

Hope this helps.

Cheers,

John W

[slewthy]

Thanks John,

Trevor - Greenman - was kind enough to lend me his colourtune and his carbtune for this exact purpose. The carbtune was the easiest to use in that it gave precise measurements but I found it impossible to balance 3 and 4. 1 and 2 were dead easy.

Its odd though - when I adjust No 4 idle mixture, it has the most effect on idle speed. When I adjust 1 and 2, there is virtually no effect at all. Same in that if I remove No 4 HT lead, the engine almost stalls but 1 and 2 leads dont make much difference.

I'm confused.

Oh, I've also checked carb float levels and adjusted as accurately as I can.

[jonwat]

when I adjust No 4 idle mixture, it has the most effect on idle speed. When I adjust 1 and 2, there is virtually no effect at all. Same in that if I remove No 4 HT lead, the engine almost stalls but 1 and 2 leads dont make much difference.

This is because the engine is only running on cylinders 3 & 4 as the butterflies for cylinders 1 & 2 are closed.

You need to balance the airflow for cylinders 2 & 3 using the balance screw between the carbs.

The will get all cylinders flowing more or less the same volume & the engine running on all 4 clylinders.

Cheers,

John W

[fenton]

I,ve got a Morgan Carb tune excellent bit of kit for setting the vacuum

I also use a method explained to me by a guy who used to repair lotus cars.

Set the float heights.

set the miture screws to the base setting in the manual (three out i think, but check )

Start engine and carefully pull the Ht leads off one at a time.

The Ht lead that has the greatest effect on the RPM is the cylinder working hardest so lean that one off a quater.

Check again, and adjust so the engine RPM drops by the same amount for each HT lead as you remove it.

Once there all the same, then pull off the HT leads, front two first and back then back two. Whichever pair stalls the engine. lean them both off a quater and check again until they both have the same effect.

BINGO!!!!!!!!!!!!!!!

[slewthy]

Yes, it would seem the front carb could be the problem.

Before I tuned anything, all the needles were 3 1/2 out, as per powertune book. The carb balancing screw was also backed off.

When I then started the engine, it actually sounded quite good but the vacuum on 1&2 was up off the scale and for 3 & 4 was low. When the balance screw was applied, the vacuums came close to equalising but the engine got rougher and misfires obvious. So, the sound of the engine seemed to counter what the carbtune was telling me.

It was after all the tuning, according to vacuum pressures was done that I found the HT leads on 1&2 had little impact on engine speed. Maybe I should try them before tuning from the baseline.

[fenton]

I,ve also used a timing light that has a tachometer on it as part of the process as i could never get it right with a colour tune. connect to the plug lead with the clamp on the timing light and adjust the mixture on that venturi until you find maximum rpm, back it off an 1/8 th move on to the next one. Then go back to the plug lead pulling off to see if they all have the same effect in rpm loss again and finally two leads at a time to see if one bank causes the enine to stall.

Takes time but you start to get a feel for it, part of the joy of esprit ownership.

[slewthy]

Thanks guys - I'm going to try that.

Will report back.

[slewthy]

OK,

Made notes of where I had got to and then undid all my previous tuning.

Then I set all carb screws to 3 1/2 out, as per powertune book and undid the balance screw completely.

Started up...

Sounded alittle rough, so gradually screwed in the balance screw until it sounded smoother. Note 'sounded', no tuning tools here. Surprisingly narrow smoothness window.

Then tried all the HT leads - all seemed to have an equal effect on rpm. Removing both HTs from front/rear cards had equal effect and didn't stall the engine, so that must mean both sets are working equally hard?

Holding my hand over the exhaust, it was obvious there was an intermittent misfire still but not a regular one.

My next step is to remove HT leads one by one until the misfire disappears, then I can nail it.

Once thats done, I might try an emmisions test again.