Misc: Everything Else Smoking Engine

[n650ss]

I'm hoping I might be able to get some help from you guys.

I have a 1950 Comet with a Bob Dunn big bore/stroked engine.

Due to illness, it has taken me a long time to build the bike. It runs well for 50 miles or so, but then starts to smoke a lot - still runs well & smoothly.

I presume that the valve area is under pressure. I've put on a vented valve cover - but before I try it, I'd like some opinions re cause and effects please.

Many thanks - Richard

 

[timetraveller]

I doubt that this will be the cause of your problems but if you have still left the original engine breather intact then the atmospheric breather on the valve cap and the timed breather are working against each other. Just block off the timed one somehow and then try again.

I am not sure what could cause the engine to smoke only after 50 miles but it would be worth checking the lower valve guides, just incase one takes some time to come loose and then tightens up when the engine cools down.

It might also be worth considering what oil you are using. If it is a thick oil then it could be that it takes that time to warm up and become thin enough to show up some other fault.

 

[Peter Holmes]

All good suggestions, but one would hope that if the engine was rebuilt by Bob Dunn, that it would not be suffering from loose valve guides, or any substandard mechanical work of any kind.

 

[Bill Thomas]

Don't use too thin an oil !.

 

[lee_812d]

How long ago and how much use has the bike had since being rebuilt?

My Comet was smoking and it turned out that the lower exhaust valve guide was loose. Other people have had the same with the guide breaking up. The easiest way to check for this is to take the exhaust off and have a look, see if the guide moves, or evidence of excess oil seeping through. I caught mine early because I stopped riding as soon as the smoke screen started, before that the smoke was hardly anything, just as much as you'd expect from an old bike.

 

[brian gains]

my Comet suffered from intermittent smoking bouts which I put down to a tired engine. having looked into the timing cover for another un associated reason i found some small segments of steel elliptical swarf at the bottom of the case. spent some time cogitating on where this could have come from due to distinct appearance, after some general firkling i found the E80 on the half time pinion had all but stripped due to over tightening?. replaced , tightened, punch fixed and no more issues.
regarding securing the E80 there are thems that punch, and others that swear by just loctite.

 

[chankly bore]

_______and yet others who fit a lock tab.