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Old 12-08-07
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One cylinder won't fire...

My 1986 S3 Spider has a problem I can't fathom. All ideas welcome.

No.1 cylinder (front of the block) is not firing. There is a very good spark. There is good compression. But, it won't run. Have cleaned the dizzy cap, changed the lead and plug - nothing. The butterflies on the carbs are moving freely to the eye (I haven't had them apart).

The car drives but is obviously a bit rough.

Carbs somehow? Valves (but there is compression)? Dizzy? Please help.....

I also posted this on AROC, in case you thought there were two of us...
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Old 13-08-07
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Idea Re: One cylinder won't fire...

Mmm , there may be a spark at the lead but is the plug doing its stuff, swap it with another cylinder and see if the fault moves with the plug. Also, if number 1 cylinder is not firing, is the plug getting wet with fuel when cranked from cold, if not then you may have some kind of fuel delivery problem
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Old 13-08-07
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Re: One cylinder won't fire...

You may have the wrong mixture settings for that cylinder, either the carb is out of adjustment or you may have an air leak. I had one cylinder that I couldn't get adjusted and it turned out to be an air leak where the PO hadn't used any gaskets between the manifold and the rubber carb mounts - themselves another possible source of leaks. Being cylinder 1, also check the small vacuum pipe that runs from the front of the manifold.
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Old 14-08-07
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Re: One cylinder won't fire...

There's a tiny air leak when the chokes are open. I ruled it out because it would affect all 4, wouldn't it? Why would it affect only one pot?

And I've tried a new plug and lead - no change.

Thanks guys...
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Old 14-08-07
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Re: One cylinder won't fire...

Strange that! Pop the plugs on an AC Doctor or similar to check them out? Not short of a compression are you?:
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Old 14-08-07
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Re: One cylinder won't fire...

Originally Posted by Malfa View Post
There's a tiny air leak when the chokes are open.
An air leak will only upset the mixture if it is up stream of the butterflies - worn spindles, gasket leak, cracked rubber mounts etc. If the leak is in the airbox somewhere it won't have an effect on a single cylinder.

I would recommend that you tune your carbs and get the mixture set right. It can be a bit of a fiddle unless you have a carb synchroniser and/or Colortune to help you out. I shelled out 40 odd quid for a 4 way carb synchroniser and it immediately pinpointed two air leaks as well as making the carbs very simple to set up.
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Old 14-08-07
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Re: One cylinder won't fire...

If it isn't spark then it is fuel as you have checked compression whch means valves OK and the engine runs on at least 3 cylinders indication thaT timing is OK.....these engines are simple compared to modern engines. Blocked carby inlet jet for example.


I would have thought that if there was an air leak on No1 cylinder inlet that car would actually run on that cylinder but just run lean.

So is No1 cylinder actually not firing or just running lean???? You can test by starting the car with no1 cylinder spark plug lead off and see if the engine runs any worse. You can remove whilst running but should take precautions against electric shock.





Last edited by Stori; 14-08-07 at 08:27.
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Old 14-08-07
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Re: One cylinder won't fire...

First thing I'd do is swap plugs around - 5mins immediately tells you if the plug is sound and also the rest of the ignition system. If the replacement plug in No1 gets wet it sounds like you have an ignition problem. If the plug that was in No1 (and which you have in the meantime cleaned and heated on a gas ring or using Bunsen) works fine in another cylinder, then you had one problem and you've isolated it.
If No1 gets wet again and the thing runs (or tries to) on two cylinders you know you have two problems, and what they are.
Someone might have already mentioned a bad distributor cap. Try going out to it in the dark, opening the bonnet and firing it up, suprising how visible tracking can be.


Doesn't say that in the manual
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Old 15-08-07
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Re: One cylinder won't fire...

I've tried the plugs - no difference. Plug from no1 works fine in no2, that from no2 makes no difference to no1.

No1 doesn't run lean - it just doesn't run! Nothing, nada.

The carb air leak is, I think, through the spindles; would that affect only no1?

I'll probably sort the leak and see what happens.
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Old 15-08-07
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Re: One cylinder won't fire...

It is possible that the spindle would only leak on one cylinder but it should only be a problem at idle. When you open up the throttle the effect of a small leak will disappear.

It would be worth while taking out the carb jets and cleaning them - use a squirt of carb cleaner and then blow through some air (computer air dusters work well). Do *NOT* use anything solid to clean out the jets. You could also swap the jets for cylinders 1 & 2.
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Old 15-08-07
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Re: One cylinder won't fire...

Now you are talking. Try removing the gubbins on the end of the spindle, and make a felt washer up which you saturate in thick oil, where it joins the carburettor body/bushing, see how this improves it? Could well be the spindles/bushings, bleeding excess air?:
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Old 15-08-07
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Re: One cylinder won't fire...

I't seems I'm unclear about these symptoms. I took it that it wasn't running at all on No1? Is it then the case that it pulls moderately well on all four under load and misfires at idle/light throttle?
I don't suppose yuo have a vacuum tapping into the manifold or throttle body for No1 do you? If so perhaps you have a leak into that vacuum circuit? Cracked brake servo hose of duff distributor vacuum advance capsule??? Also leading to poor running and very common. I'm sure there are many cars run regularly without the drivers being aware of duff vac. advance caps.
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Old 18-08-07
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Re: One cylinder won't fire...

Sorry to hijack thread But ! Cosmo What 4 way carb synchroniser did you buy and can you reccomend it..easy to use etc ?.. I didn't realise it can pinpoint leaks !? Cheers N.B.

I would recommend that you tune your carbs and get the mixture set right. It can be a bit of a fiddle unless you have a carb synchroniser and/or Colortune to help you out. I shelled out 40 odd quid for a 4 way carb synchroniser and it immediately pinpointed two air leaks as well as making the carbs very simple to set up.[/QUOTE]
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Old 18-08-07
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Carb synchroniser

My carb synchroniser is actually designed for motorbikes but works just the same for any 4 venturi set up such as the Webers or Dellortos commonly found on our Alfa's. From the picture you can see that it attaches to a take off point on each venturi on the carb - I have them on my DCOE40/32's but some early Webers do not have a place to attach the vacuum hose to.

Tuning by ear isn't actually so difficult but relies on everything working. I couldn't get mine set up correctly, one cylinder was way out of adjustment compared to the rest and fiddling with the mixture, even using a Colortune, didn't help. When I used the carb balancer it was very obvious that number 1 cylinder and number 4 cylinder had lower vacuum than the rest. The rear leak proved to be the brake booster hose, the front leak was *no gaskets* between manifold and the rubber carb mountings.

After fixing this, getting the two carbs balanced was a cinch. Then I used a Colortune to get the mixture correct. The engine would idle down to 500rpm if I wanted to but is set at about 750rpm. I had to get an electric rev counter as the cable one is useless at such low revs and it also means you can watch it from the engine bay.
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Old 18-08-07
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Re: Carb synchroniser

Is it possible the spark is at the wrong time, due to distributor cap tracking inside. Usually this would mean 2 cylinders not firing. If a brown cap you should see the black tracks and it is possible to scrape them off to get it going. (Done it by roadside). More likely air leak, we used to find these by wiping grease around suspect places. Not much help here, a lot easier with hands on.
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Old 21-08-07
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Re: One cylinder won't fire...

'Tracking won't affect timing, just spark strength. If you've a deposit causing tracking you're lucky, it's usually a hairline crack or some kind of porosity and means a new cap for sure.
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Old 21-08-07
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Re: One cylinder won't fire...

It can be useful to watch the engine running in complete darkness. It is quite surprising how ignition problems show up like a set of Christmas tree lights.
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Old 21-08-07
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Re: One cylinder won't fire...

Made the odd leather washer up, last longer than felt ones, soaked in grease/ hypoid oil, helps with spindle bush wear for a time.:
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Old 21-08-07
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Re: One cylinder won't fire...

I seem to have a vague memory, or dreamt it maybe, that certain compound carburettors had nylon bushings, which when worn you could pop out, and stick a replacement in Could of been a dream I suppose?
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