Smoking Norton Commando

[sprinta]

A friend has a Norton Commando and following and engine re-build it started smoking on one side. Lent him a pressure gauge and he found one side was down and squirting some oil into the bore did not make much difference.

So he stripped the head and barrel off and found a valve not seating. He also found one of three part oil ring not seating well, no damage to bore or piston.

Has re-built engine and now finds that when cold it is fine but as it warms it now smokes like a two stroke on both sides. Compression pressure is only around 90 psi, which is low. Valve seating OK and it has oil seals on the inlet guides. Gets about a cup full of oil from sump and oil returning find.

Have suggested he checks crankcase/engine breather is OK but beyond that he seems to have checked everything. Could be that rings have not sealed well but it is a mystery. He has suggested fitting the old single cast oil rings again but was wondering if anybody has some advise/suggestions before he takes it apart again?

Thanks

Andy

[Johnwebley]

Try a hone on the bores.Maybe the rings have not bedded in.3or
rings normally seal

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[MarkCR750]

Valve stem seals?, shine a torch down the inlet and exhaust ports to see if there is any oil on the valve stems, that's if the norton engine architecture allows you to see them, I know it's rebuilt but it's always possible that the new parts are faulty or the wrong ones.

[Bryanj]

This is a long shot but there was a batch of badly hardened rocker pivot shafts that let too much oil lie on top of the valve guide. Had one under warantee years ago and took a lot of time to find it

[philward]

Just a suggestion - could he not just swop it for a Honda! (Sorry to be so flippant but couldn't resist 😀)

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[royhall]

How many miles has the engine done since the rebuild. Similar symptoms to the one I had years ago, it took nearly 1000 miles before it stopped smoking. It was my only transport at the time or I would have taken it off the road before then for another rebuild. Glad I stuck with it as after the 1000 miles it was perfect.

[sprinta]

Thanks for all the replies and advice, even the the tongue in cheek ones.

The bores were honed prior to the re-build and the bike had only done around 200 miles with the smoking on the one side more or less immediate and as there were some issues found with a valve seat and an oil ring some logic to the possible cause. However,  this problem with it smoking on both side has started again straight from re-build but as indicated only once it gets warm.

Inlet valve guides have oil seals fitted so don't believe that oil is finding its way down there, though not impossible so worth checking if design allows?

The breather has been check and seems to be OK. A longer bed in time might be the answer but the amount of smoke just don't seem right? 

It sadly looks like another barrel and head off job for him, don't we just love our hobby!

[Tomb]

To check for valve guide wear or valve guide seal failure letting oil past I take exhaust and carbs off and poke a finger up the port feeling for where the valve stem and guide meet, if oil is coming through it shows on your finger, they should be clean of wet oil.