Classic/Antique Car Repair: idle adjustment 71 MGB, new shaft, clutch release


Question
 I have a stock/original 1971 MGB RDSTR.with 66000 miles.I am having difficulty adjusting idle speed.I use a carb sync and seperate dwell/tach and get right-on between 900/1000,drive it for about ten minutes or so,and idle increases to 1250/1350. If I push-in clutch pedal,or turn on head lites,it drops to 900/1000.All emission gear on and working fine,no back fireing,pinging or orher running problems.NEW PLUGS,POINTS,GOOD COMPRESSION,GOOD VACUMN READING.Any help/ideas,would be most appreciated.This just started recently,and is has me PERLEXED,to say the least....THANKS...DAC

Answer
Dave:

Before you do anything with the carbs it really is necessary to make sure the valves are adjusted to spec. This controls the vaccum and affects carb operation. Also, disconnect the vaccum advance on the distributor before setting timing. One of the most common problems with getting a steady idle speed with an MG with higher miles can come from the carbs throttle shafts being worn enough to allow some air leakage past the shaft. With 66,000 miles this could be at least part of your problem. I'd check the shafts for "wiggle" or looseness. If the shafts are loose and excessively worn (usually from lack of periodic lubrication) then you will never get a steady idle. If this is the case, the only lasting fix is to take the carbs off and have a new shaft installed. [ They also make oversized shafts, and you may need to have the carb bodies "rebushed" where the shaft goes through the carb body- the body needs to be drilled or reamed to fit the new bushings].

The engine dropping in idle speed with pushing in the clutch / use of headlights is normal - in the first case due to the loading of the clutch release bearing on the pressure plate, and in the second case, due to the additional turning resistance of the alternator when it is under electrical load.

Brian