1980 1850 auto rough idling

[Dunhookin]

Hi guys,

Hoping for some learned opinion on the rough idling problem with my 1980, 1850 auto. The car has contactless ignition and usually idles well. I’ve just had to change the rubber carburettor mounts for the umpteenth time.

At engine speeds from about 1,100 rpm, the engine revs smoothly and feels normal. However engine speed at 1,000 or less seems to cause it to misfire...and the idle becomes instantly rough, it feels like the engine is about to stop...?

Anyone seen this before, or have suggestions/ideas/advice? Any intel gratefully received.

With thanks and best regards

Andy

[cleverusername]

My money is on the mixture being wrong. If your rubber mounts were leaking and now that air leak has been cured, the mixture could now be out.

I had a similar problem with my Sprint engine, though in that case it was far too lean. Which could be caused if you new mounts are incorrectly fitted or faulty. Maybe something else hasn't been refitted properly, a vacuum hose for example?

If it isn't that the most likely cause is a bad ignition component. I had duff leads and a duff rotor arm, leading to misfires. Which I used a timing gun to find.

Anyway good luck finding the problem

[Dunhookin]

Thank you for your ideas/suggestions Clever. I had tried adjusting the mixture, made no appreciable difference. Tonight, I checked the ignition system, found all well.

Turning the engine idle speed up so it wouldn’t stall, I removed number 1 cylinder plug lead, the engine didn’t notice! I tried a spare spark plug and a spare HT lead - no change. Clearly there’s a compression problem on number 1 cylinder - so the next move is to compression test the engine.

That will be on Thursday evening - it could be tight valve clearances, but I strongly suspect a head lift beckons...gloom!

[cleverusername]

Well I hope you're wrong, I would still suspect an ignition fault and make absolutely certain you're getting a spark. Much easier to fixed than valve problems, worn rings or bore.