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MG MGB Technical - Revs up & down at Idle

Hi all, I have a '77 Roadster with some idle problems. When waiting at lights, roundabouts etc where you pause/stop for a short while the idle revs between (low) 100/200 & then conks out if I don't give the accelerater a blip. At other times it will idle at (high) 1000 then in a short spac eof time creeps up to about 1200 revs. The up & down idle speeds can happen on the same (short or long) journey so there's not really any set patten to this as in the engine does this if it's hot or cold. I have no problem starting it in the morning & it's used daily, outside of this idle prob it runs smoothly. Any suggestions greatly appreciated.

"To sunny day's"

Barry
Barry S

It definatelt sounds like a combination of carb problems and a vacuum leak, which could be at the carb gaskets, intake manifold, or distributor. Have you ever had the carb professionally rebuilt? Is there any play in the throttle shaft? Spray carb cleaner around the throttle shaft and intake manifold (engine cold)to see if the idle increase. An unlit propane torch will also work to check for leaks.
Jeff Schlemmer

Thanks Jeff for your input. No the carbs haven't been serviced for at least 3 yrs, the time i've had the car & the state it was in when I got it, it totally lacked any TLC. Maybe an exchange for a serviced/rebuilt pair will do the trick but I will try your ideas first & wait & see what else might appear on this thread.

"To sunny day's"

Barry
Barry S

Hello barry
check out my thread su carbs my car also had similar problems the solution to your problem is in the reading
good luck my friend i know how frustrating this can be it has to be said the guys on this site really know their b,s
Richard H
Richard H

Barry

Sounds like carbs to me, best investment I made was to have mine rebuilt when I had similar problems and an inability to get the mixture right. Carbs were well worn. Rebuilt units bolted on and were so easy to tune.

Colin
C J Bryan

Barry, There could be many reasons. If you're not yet up to speed with these engines seek out a local classics garage and have him take a look. Phone around the garage trade to see if the same name comes up a couple of times or ask the MGOC for a local area contact you can ring for advice on somewhere.
Carbs are expensive. It takes a lot of wear before SUs need a complete overhaul yet only a minor problem may be the reason for your troubles. Is there oil in the dashpots? It could be a sticking float valve or erratic fuel pump. Then again, an induction or breather air leak or sticking distributor advance would do similar things. Rich.

Rich

This thread was discussed on 09/06/2005

MG MGB Technical index

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