Okay, here's one I haven't seen before: The car is a 1958 MG ZB Magentte
(think MGA drivetrain with the clutch slave on top of the bellhousing).
Recently rebuilt engine (about 1500 miles ago). New clutch and carbon block
release bearing as part of the rebuild. Fresh oil change with Castrol Syntec
(first time synthetic oil use for me). Carbs with new needles and jets (one
needle and jet had worn because of a jet bearing misalignment). Carbs
balanced nicely. The engine idles very smoothly at around 870 RPM, and runs
well.
Since the oil change, we've noticed a significant drop in idle RPM when the
clutch pedal is depressed. The clutch travel and function seem unaffected.
I am totally guessing here.
Is it possible that the Syntec is so slippery that at 870 RPM the drag of the
release bearing is now a significant factor? It seems unlikely to me, but I
have little experience with synthetic oils. It seems more likely that the
release bearing has screwed up somehow, but I'm having a hard time figuring
what can go wrong with a carbon block release bearing that wouldn't also make
one hell of a noise when the clitch is disengaged. Besides, it's a little
wierd that it would happen at the same time as an oil change.
Any ideas/thoughts?
Chris K.
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