Running on issues

[grimboh]

We've owned our 4a for 6+ years now and during that time it has always run on, to varying degrees at times, but it's always been there.

Brief history of what I've tried and what's changed:

Constantly fiddled with idle to no great satisfaction. Receipt for 'standard' Moss camshaft from PO so assuming that's what's in it. New plugs, then new titanium plugs. Carb air leaks checked and new diaphragms. Refurbished cyl head, close attention to intake installation def no air leaks, runs great but no impact for running on. 123 Tune+ distributor, big improvement, reliable advance curve, and resolved black sooty plug and chamber on no.3 cyl, but no impact on running on - even though it cuts the spark. Ball jointed throttle linkage with fully free movement and return. New choke cable.

Fiddled with idle many times (and choke idle adjustment), always unsatisfactory, lumpy, varying idle at diff stages of 1st 30-40 minutes of running, always still running on. Been using the clutch for along time now to stop the engine. Fiddled yet again today and decided to adjust the throttle linkage between the 2 carbs. No gauge, but by ear each carb sounded different, so adjusted in stages until sounding the same. Each stage seemed to improve the idle smoothness, allowing a low idle speed (which had been set high at around 1000rpm).

So now idles happily at 800rpm reading on the taco - which I know overreads from the 123 Tune+ app, so really 750rpm. So that's great - much smoother idle and at an rpm that it should be. But what I did not expect is that it now does not run on. Which got me to thinking Why?

The best theory I have is that one of the carbs doesn't shut off the butterfly fully, whilst the other one does. Backing off the 2 throttle stops does not solve this as the idle is then too poor for any reasonable town driving, and likely to stall. So no amount fiddling with the idle can solve it. And a higher idle speed for smoother idle and to stop stalling only makes the potential for running on much higher. The carb that's not fully shut off will, of course, still let air and fuel into the 2 cyls it serves, and they continue to self ignite easily with that fuel - usually causing the engine to run backwards, which risks the timing chain jumping as the tensioner only works in the correct rotation.

So if you have an unsatisfactory idle and running on problems then re-balance the carburettors. It's likely to make a big difference. I didn't even use a gauge, just by listening with ear close to the intake filters. It's actually quite easy to hear when both carbs are the same.

Another long essay but I hope this helps others in due course...