Rev drop when clutch engages

[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,

[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.

[Ian F]

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

[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

[Dave Linkson]

Mine also does it on a recent engine, idle speed varies from 850 to 1000 RPM but I put that down to wear in carb spindles and may drop slightly when clutch pressed and of course when fan cuts in but yours seems excessive, must be annoying..