Blown Gasket

Well i dont know if this has ever happened to anyone here but me and my dad took the bike for a short trip and when we got home i noticed oil on the crank and part of the gasket between the crank and the pots blown out, can anyone tell me why this has happened and also on the ride the bike didnt want to get past 4500rpm could this bet a jet size problem and if so what size would i goto i am running standard 932 amals with standard jets, also if it changes anything it is a 1972 750 combat.

Has the motor been apart recently ? Have you changed any settings or anything else ?

Blown base gasket could be due to lack of check tightening flange fasteners. I might also surmise (but have no proof) that thrashing from cold might not help in this area - differential expansion and all that. How's your Dad's mechanical sympathy ?

Carb settings cannot just change (well, apart from main jet holders undoing themselves or needles dropping in or floats becoming "sinks" or something like that. If you haven't removed air filters or something, I would leave jet sizes alone. Does the motor just run out of steam and "hold back" or does it "8-stroke" and stumble ?

First things to do are check the timing (electronic or points ?) and strip, clean and balance the carbs, looking for any obvious mechanical problems - I've seen broken slides as well so check everything.

I have a feeling it could be due to not being tightened after the motor had been run in and warmed up as the bike was rebuilt about 2-3months ago.
With the power problem it just seems to get to 4500rpm and run out of leggs and not climb any higher and the motor hunts which has made us think it is to small a main jet, they are 220's and i was thinking of buying a pair of 230's and 240's also when you change the main jet do you have to change the needle or is it always the same.

Leave out the base gasket and use an aerobic sealant like Loctite Master Gasket 518. This will "glue" the cylinder and crankcase together but still allow them to be removed. This was a factory service note in the early seventies. If the base nuts are allowed to work loose, the cylinder can lift and fracture the crankcases.

Oops. That should read anaerobic sealant.

Thanks for that mate i was planning on using a gasket silicon and now you have made up my mind and also this way i wont need to full the head and everything off just to change the gasket.

Check-tightening is certainly worth doing. Not just engine components either.

220 was standard main jet for Combat and if you haven't changed anything else, it shouldn't need to be changed.

Carburettor settings are varied by throttle position rather than revs and whilst all sorts of carb problems can cause rough running, I don't think I've ever known anything give the symptoms that you describe.

What colour are the plugs coming out ?

Have you checked the timing? If you still have points then stuck bob-weights could give that sort of problem.

If you do decide to change jets, you don't need to change anything else but might find yourself wanting to alter needle position for a smooth changeover.

I assume valve timing hasn't been altered ?