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MG MGB Technical - Intermittent rough idle

1972 MGB starts and runs fine; however after fully warmed up, the idle will occasionally drop about 200 rpms and idle rough. The rough idle continues to shut down. If started while warm, idle will be low and rough. Cold start will again be fine.

I have checked and adjusted valves; checked timing, checked spark plugs (all grey/tan); verified power to spark plugs; adjusted carbs. I checked compression (1-165, 2-173, 3-155, 4-170). At idle, I disconnected one plug lead at a time: no change for 1 or 2; idle dropped for 3 and 4.

While it appears that I have two dead cylinders, there does not seem to be a loss of power while driving.

Thoughts and suggestions will be gratefully accepted.
Gerry L

when you say adjusted the carbs-it sounds like the carbuettor balance needs checking, but first things first--floatlevels and clean the dashpots and pistons -top up the oil in the dampers etc and check for sticking linkages ,like choke sticking on and not returning properly , same with throttle linkages
William Revit

Carb dampers won't affect the idle, but nothing wrong in checking them.

It's 'normal' (for a car like the MGB) to have varying drops in idle as each spark is disabled in turn, one may make very little difference, but I'd say two making no difference at all does indicate a fault. As those two are on one carb it does look as if that carb is not 'working' at idle, so a full set up for airflow and mixture balance needs to be done. Also check that both pistons act pretty much the same as the throttle is opened smartly i.e. damped rise and rapid drop, and with the engine off that both drop smartly with a click when fully raised manually.
paulh4

of course the dampers effect idle, if there's no oil in there the piston rattles up and down at idle and the mixture runs lean
William Revit

Having said that, I'm not suggesting low damper oil is causing Gerry's issue, it would usually show up as hard starting cold and a constant idle issue
It was more of a reminder to top up the dampers if he had the dashpots/pistons off for checking/cleaning
Cheers
willy
William Revit

Thanks for the comments. I will do a full carb set up.
Gerry L

ETA: I didn't see Gerry's reply so perhaps too late.

Just quick thoughts -
. grit between dashpot cover (suction chamber) and piston edge on front carb
. misfire to 1 and 2.
Nigel Atkins

I'd have said no damping in a carb would be more noticeable from stumbling on acceleration, and increasingly so as oil was lost, but there we are.
paulh4

Spent time checking carb setup; all seemed good. Examination of intake manifold identified a possible vacuum leak - bolt not fully tight. Bolt tightened; idle speed adjusted. Engine running fine now.

Cheers, G
Gerry L

I originally thought about putting a post including vacuum leak as that's been a favourite of mine recently but I decided to let those with more knowledge come in first.

I think as Willy didn't mention it I wandered off the idea.

The lean running giving the symptoms of your original post and the pairing of 1 & 2 suggest your loose bolt was at the front.

You'd have found it if it got worse, certainly if the manifold gasket goes it can make quite a (metal) racket.

Sorry I should have had more courage.

Glad you got it sorted.

Nigel Atkins

Be brave Nigel--you can do it--got myself mixed up there a bit, like you I've been following a similar thread in the MGA section and wrongly thought Gerry had already checked his manifold for leaks-my bad-

Nice find Gerry-good stuff

Paul--just some useless background to the damper theory--and, yes I agree, flat-soggy acceleration is definitely the usual sign-
A bloke up the road from me here has a 6cyl Healey -2 carb model and has been complaining of it being hard to start cold since the weather cooled off here a month or so ago---then the other day he rang me and said his car had a rattle and was running rough----Went to have a look and got him to start it up, wind wind wind on the poor old starter and then it fired up running really fidgety and seemed like on 3 cylinders and this rattle that sounded like half a dozen loose rockers coming from the rear carburettor, checked the damper oil and he was empty, topped them both up and away it went--this was fresh in my mind when i mentioned the damper oil with Gerry's issue
Cheers
willy
William Revit

Another part of my hesitation was all the emissions stuff that can be on the NA models and from Ice Road Truckers I know Manitoba gets really bl**by freezing. plus believe it or not I am conscious of perhaps over contributing. 🤔

Gerry by himself done the right thing and rechecked what he'd already checked to find, like all of us have, what he'd previously missed, unlike some who take the attitude that "it won't be that, I've already checked".

My 'adventures' last year including farting with the carbs also reminded me not to dismiss something that you thought you'd sorted, in case you haven't, as was the case for me. 😖
Nigel Atkins

This thread was discussed between 24/06/2021 and 26/06/2021

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