Norton single oil burning problem - help!

[youngjohn]

New member - first post. Hello all.
I've got a bike with a Norton Model 50 engine, great compression, runs like a watch, but sometimes the engine smokes really badly, and then sometimes it doesn't smoke at all. I took the crankcase drain plug out immediately after a run and about a quarter of a pint or more of oil came out. I know the non return valve needs looking at, as the oil drains from the tank when the bike is stood, but I would have thought that the scavenge side of the pump would keep the cases empty when the bike is running. Oil returning to the tank seems to be fine and has a good flow.

I'm hoping that this oil in the crankcase could be the cause of the smoke. Any remedies, advice or opinions would be gratefully accepted.

[L.A.B.]

Hello youngjohn,

I took the crankcase drain plug out immediately after a run and about a quarter of a pint or more of oil came out.

That's maybe a little more than I'd expect, but it's not an excessive amount, not that I know much about the Norton singles?

I know the non return valve needs looking at, as the oil drains from the tank when the bike is stood,

If you mean an anti-drain valve? Has it got one? As I don't think they had one as standard?

but I would have thought that the scavenge side of the pump would keep the cases empty when the bike is running. Oil returning to the tank seems to be fine and has a good flow.

The scavenge should be capable of clearing any excess drained-down oil from the sump after a few minutes running? So if it smokes on start-up then it should stop after a short while.

What engine oil are you using? As multigrade oil will drain through the pump quicker than monograde oil. Also as the pump wears, the clearances between the pump gears and pump body increase so the oil drains through the pump even more quickly! The remedy is to dismantle the pump, remove the gears, and 'lap' the body and end plates by carefully rubbing the faces down on fine abrasive paper placed on a flat surface.
Lap the feed side first, then the scavenge side, regularly washing and reassembling the pump until all traces of end play has been removed from between the gears and end plates until the pump just feels slightly stiff to turn when assembled. Lapping the pump also improves oil pressure.  

        

[youngjohn]

I never thought that it wouldn't have a valve... oops. The bike is a bitsa and has an inter oil tank, and the return to the tank may not be quite as good as I thought. Maybe there is an in-line screen of some kind on the return which is clogged. I just don't understand why it was smoking badly then stopped then started again..

[L.A.B.]

That's certainly an interesting bike you have there!

The bike is a bitsa and has an inter oil tank, and the return to the tank may not be quite as good as I thought. Maybe there is an in-line screen of some kind on the return which is clogged.

Normally I think there has to be a slight restriction (placed between the rocker feed connection and the tank), otherwise all the scavenged oil would return to the tank instead of some getting pumped up to the valve gear?  

I just don't understand why it was smoking badly then stopped then started again.

The oil could actually be draining down between the inlet valve and its guide? And it isn't unknown for oil to drain between a valve guide and cylinder head if the guide isn't a particularly tight fit in the head?

I can only suggest that you talk to somebody who really knows these engines, like Mike at: http://www.pushrod-performance.co.uk/