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MG MGB Technical - Idle Problems

Have a '74.5 GT which does not maintain consistent idle. Weber Carbs rebuilt, timed at 12 BTC degrees at 1500 rpms. Problem is the idle will stay set but within a few minutes/secs it will gradually fall to the point of engine shut down. I hit the accelerator, goes back up then eventually will fall again. Adjusting the idle screw doesn't seem to fix the problem. Any ideas? Thanks, guys.
JOHN
JW Colson

John,

is your dizzy compartible to the vac-pickup at your carb? Are there no air leaks in the whole installation? Is the fuel delivery O.K.?

Ralph
Ralph

hi,
i have the same problem with my b,it has a weber 45 dcoe, on a 1950 stage 2 engine, so i tryed twin su's on it and car car was to smooth and not as sharp,plus i lost the nice sound it made with the weber on it, so i put the weber back on and put up with the bad tick over,
i did put up a post on the angle of the dcoe, is your carb on a steep angle ,i had my manifold face changed to lower the carb angle,
da wright

Took for a speed run and did better after re-timed but after the car got speed (smooth) the engine cut out as if there was a govener on it, sputtered for a bit, down shifted then it got back to running better! I am guessing the float, carb pump or other fuel issue present. Any ideas? John
JW Colson

Your first post is a relatively 'normal' problem for HS SUs, fuel pools in the carb throat and gradually chokes the engine, which is why during setting-up of carbs if you don't complete it within a given (quite short) time you have to rev the engine for several seconds to clear the throats before carrying on.

But make sure the carb is resting on the idle screw and there is a little bit of slack in the accelerator cable. What idle speed are you trying to achieve? Maybe it is just too low, try setting it to 800/900 or so.

Dizzie, vacuum advance and vacuum source specs are nothing to do with this. You should be able to get a consistent idle with any combination, although the wrong distributor/vacuum capsule will affect overall performance when driving. You can even tune out the effects of an air leak at idle with SUs, although it will result in rich mixture off-idle. Webers being fixed jet are not so easy without swapping jets and stuff.

Your post timed at 5:12 is more like something else, could be fuel delivery or ignition. If the tach was flicking around at the time it will be ignition LT. If not then HT or fuel. You can check fuel *delivery* by disconnecting the supply pipe from the carb and directing it into a container. Switch on the ignition and it should deliver *at least* one Imperial pint per minute with negligible bubbles, and in practice closer to two. If you have an SU pump it should deliver in a consistent stream of pulses. Other pumps can be different, and if it is an after-market pump then the idle problem could be excessive pressure overwhelming the float valve and flooding the carb. That is best checked for by removing the overflow pipe from the carb float chamber and switching on the ignition but not starting the engine (disconnect the coil to protect it). An SU pump shouldn't click more than once every 30 secs, and after-market pump may click constantly. But leave it like that for several minutes to see if any fuel comes out of the overflow.
Paul Hunt

I don't think I can match the technical responses so far other than to say I have a 1972 BGT with a DCOE 45 and stage 2 tuned and unleaded head. I overcame the problem you describe to a large degree by fitting a set of triple electrode spark plugs. The car, once it has warmed up properly, has a smooth and generally constant tick over and the engine rarely dies on me.
Good luck.
Doug
Doug E

This thread was discussed between 18/09/2008 and 24/09/2008

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