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MG Midget and Sprite Technical - High Idle Speed

I am running twin 1 1/2" SU's with 35mm stub stacks and K&N filters. However the problem I have was the same when running the standard filtration.

When I start from cold the engine requires choke. After a few moments I can push the choke in (actually I remove the clothes peg and it goes in by itself) The engine idle speed is about 8-900 and will sit there all day. As soon as I raise the revs and drive off the idle speed raises to 15 to 1600 rpm, sometimes a little higher. This can be temporarily settled with a blip of the accelerator only to raise again when I next bring up the engine revs. There are no power loss issues and from looking at the plugs the mixture isn't far off being right. Can anyone throw some light on this one for me please. I'm thinking vacuum leak somewhere but I don't want to pull everything apart without an idea where to start
Nick Sayle

Sounds like the throttles are not closing correctly. Next time the idle is at 1500rpm try getting under the bonnet and push the throttles more closed. If this slows the idle then the throttle discs and springs require sorting out so that they return to the same point each time
Robert (Bob) Midget Turbo

Kind of the same issue as Bob describes: but I would suspect that one of the jets isn't centred properly, preventing one of the pistons from dropping down fully.
Guy Weller

nick, i had the same sort of thing on mine and fixed it by degunking the carb with a can of carb cleaner and replacing all the springs on the carb which cost about £5. wish all fixs were that cheap and easy.
roy j

thanks for the help, it was a loose butterfly! poked my finger it while it was over revving the there was movement on the spindle to. I am going to put a DCOE on today but will replace the SU's as soon as possible. Will give me a chance to get them fully sorted without taking the car off the road
Nick Sayle

I have my carbs in bits on the kitchen table. The butterfly's have those annoying valves fitted (they are troublesome and they interfere with air flow) I have 2 options as far as I can see. One is to renew the butterfly's with new replacement parts and the other is to remove the valves and silver solder the holes up. However the other option I am wondering about is the possibility of swapping them with butterfly's from the earlier HS4. Is this possible?
Nick Sayle

nick your a brave man. im more scared of touching my carbs than i am of flicking a rottweilers nutts. i sprayed carb cleaner on my carbs and changed the springs and felt brave doing that. dunno what it is with carbs but i think of them as a bit like playing with the dark arts. im thinking of getting my carbs tuned on a rolling road. do you reckon its worth it?
roy j

Nick... now's the time to 'Vizard' the spindle... done this to 3 carbs in past to good effect.

A
Anthony Cutler

Anthony, I have had the dremel out believe me. I have a flexi extension on it and have managed to remove all the steps where the various parts fit together. I dare not take any more off the spindles!! I have knife edged the butterfly's and polished the insides. After lunch I am going to fill the stupid holes where the valves were. If that fails I am going to try fitting non valve butterfly's
Nick Sayle

job done, works perfectly.


Nick Sayle

This thread was discussed between 23/04/2010 and 24/04/2010

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.