Rev drop when clutch engages

Technical MGB discussion
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Trouble Monkey
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Joined: Thu Aug 13, 2020 11:25 pm
Forename: Andrew
Surname: Bennett

Rev drop when clutch engages

Post by Trouble Monkey »

Ello,

More out of curiosity than anything as the engine runs fine otherwise - and was rebuilt fairly recently, but if I set the idle to less than 1k and put my foot on the clutch the revs drop to about 500rpm. If I happen to do it while the radiator fan and lights are on it pretty much stalls. Is this normal behaviour?!

It’s a 78 uk BGT with a few mods, but it’s done it since I bought it and before i made any changes.

It has a standard carbon throw out bearing,

Thanks,
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Bumpa
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Forename: Mike
Surname: Howlett

Re: Rev drop when clutch engages

Post by Bumpa »

The first thing that springs to my mind is the crankshaft thrust washers. If they are worn then putting pressure on the flywheel by operating the clutch can cause the crankshaft to move forward by a couple of thousands of an inch which loads up the crankshaft bearings and con-rods. With the engine off, get a stout lever behind the front pulley and see if there is any amount of fore and aft movement. Or with the engine running get an assistant to operate the clutch while you watch very carefully the front pulley. Ideally there should be no front to back movement.

Personally I never try to get carburetted engines to idle below 1000 rpm. They seem happiest at that sort of level. Oh they might have been set to 800 or less when everything was new, but that was decades ago and, with worn carbs and distributor, perfection is hard to achieve.
Mike
1969 MGB GTV8 3.9 Conversion, 1977 Triumph Dolomite 1850HL, 1971 MGB roadster undergoing restoration.
Ian Fozzard
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Forename: Ian
Surname: Fozzard

Re: Rev drop when clutch engages

Post by Ian Fozzard »

My Midget has always done this, even after a freshly rebuilt engine (948 cc mildly tuned, otherwise standard). My BGT may be the same, but with more bhp it perhaps isn't noticeable. I just put it down to the friction from the carbon thrust bearing.

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear
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Trouble Monkey
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Joined: Thu Aug 13, 2020 11:25 pm
Forename: Andrew
Surname: Bennett

Re: Rev drop when clutch engages

Post by Trouble Monkey »

Thanks. I’ll check the end float on the crank. Good to know I’m not alone though.

Perhaps I just need more bhp- some kind of supercharger perhaps :D
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