MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Tickover

Am having bother getting a consistent tickover on 1975 BGT. Have adjusted it several times to 850 revs after a run, but find after a further run it either rises or falls. Any ideas?
R E Merrall

Rod, I had exactly the same problem with my 1976 roadster. It was cured by changing the carb spindles, butterflies and spindle seals. It was the seals that had gone and were letting air in.

After checking all the obvious stuff, you certainly need to have a good look for air and vacuum leaks.

Andy
Andy Robinson

When high does it drop when you press the quadrants onto the idle screws? This can happen if there is stiffness in the linkages, or the cable does not have the required free play, i.e. the butterflies are hanging on the cable rather than resting on the idle screws.
Paul Hunt

You also need to check the poppets on the butterflies, I had one that wasn’t seating properly causing similar issues
R.A Davis

Another cause can be the temperature compensating bi-metalic valves that allow the carburetors to change idle based on the air temperature going through the throat of the carburetors. They can lose their tension, over the years, and cause erratic idling problems. RAY
rjm RAY

The only bimetallic temperature compensation I'm aware of is the HIF jet adjustment to vary the mixture, and the V8 air filter cans to vary whether it pulled air from over the exhaust manifold or not.

Are you thinking of the poppet valves on the butterflies? They work on air pressure to open on the overrun, they don't alter the standing idle as such - at least not when they are working as they should.
Paul Hunt

On US spec cars, the bi-metalic assembly bleeds in air to increase the idle speed and, presumably, reduce emissions. RAY
rjm RAY

Shifting timing will also cause variable ticker, Check that the distributor base plate is free to move, in reaction to the vacuum unit, by the time honoured method of sucking on the vacuum pipe. This will also confirm vacuum module condition. Also check that the Dis shaft is free to react to centrifugal advance.
Allan Reeling

"On US spec cars, the bi-metalic assembly bleeds in air to increase the idle speed and, presumably, reduce emissions. RAY"

What carb?
Paul Hunt

Tried spraying carb cleaner onto throttle spindles. No effect on spindles between the carbs or on the spindle nearest the bulkhead on the rear carb; but on the spindle at the front of the front carb nearest the radiator the engine almost died when sprayed. There is no discernible movement in the spindle but does this point to wear?

Rod
R E Merrall

"There is no discernible movement in the spindle but does this point to wear?"

Yes, or some other vacuum leak in the vicinity depending on how accurate your spraying was. Try dribbling thick oil onto the end of the spindle there and see if that makes a difference or not.
Paul Hunt

Thanks for the advice. I taped up the carb mounting points and sprayed; same result, engine spluttered. Smeared grease on the spindle end and sprayed, engine fine. Guess I have a leak in the spindle. The carbs were replaced with new SU HS4's in 2007 by Oselli and the car has done very few miles since, so it is strange one now leaks.
R E Merrall

By the way,if I have to replace the throttle spindle does the carb need rebushed and, if so, is it a diy job?
R E Merrall

Rod, you will find this site useful for not only parts, but also for technical info and guides on stripping, rebuilding and tuning SU's.

http://sucarb.co.uk/technical/

If your car is a 75, then I guess it would have had HIF carbs originally?

Andy
Andy Robinson

Andy

Thanks very useful. Yes it would have had HIF's but PO changed them. Easier to work on I think.

Rod
R E Merrall

Indeed. HIFs are better from an environmental point of view, but after that there are a number of draw-backs over HS.
Paul Hunt

IF indeed the carbs replaced by Oselli were new, then there is no way they will have worn in so short a time. Seems like "NEW" means "different"!!
You may be lucky and find a majority of the wear is on the butterfly spindle and replacing that will affect a marked improvement.
You can re-bush them. I think Burlen do supply bushes but you will need to enlarge the worn holes with a long parallel reamer after carefully drilling. If Burlen can't supply bushes you need access to a lathe.
Allan Reeling

You can also get oversize spindles. I just found a pair of .020" oversize ones that I've had lying around for over twenty years, still in the original packaging. RAY
rjm RAY

This thread was discussed between 27/10/2015 and 03/11/2015

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.