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Hey All,

My 1974 R90S started smoking heavy after start up. Compression is 150psi both sides. Checked with camera, cylinders have cross hatch and look excellent. It will run perfect for a few minutes then both cylinders start to smoke excessively. Checked needle and seats in both carbs, no leakage. Seems like oil. Crank vent problems? Any advice would be appreciated. Thanks!

not enough details
is it running perfectly and smoking or running crappy?
could be:
too much oil (VERY common issue). Don't fill to full mark, should be between the upper and lower marks
with dipstick not screwed in.
or...or....or...
broken /bad rings and or valve guides
breather malfunction
wrong oil
choke malfunction
accelerator pump malfunction (if Dellorto carbs)
blocked/dirty air filter or intake

first thing i'd do is dump the oil and refill with fresh 20w50, then take it for a good boot and see what happens

Aron wrote: ↑Fri Nov 27, 2020 8:43 pm Hey All,

My 1974 R90S started smoking heavy after start up. Compression is 150psi both sides. Checked with camera, cylinders have cross hatch and look excellent. It will run perfect for a few minutes then both cylinders start to smoke excessively. Checked needle and seats in both carbs, no leakage. Seems like oil. Crank vent problems? Any advice would be appreciated. Thanks!

Do you leave it on the side stand after a run... If you do, don't.

Rob

No I haven’t, it will idle fine for a few minutes then both cylinders start smoking. Upright clamped on my work table.

Ok... does the smoke go away when the bike is ridden for a while? Is this a new issue? Has the bike been laid up? Has there recently been any work to the top ends?

As a starting point I would certainly be checking the breather valve to see if it is damaged or gummed up.

Rob

Is it oil smoke, or heavy condensation from cold weather?

Oil smoke never goes away while condensation will stop once the exhaust is fully warmed up.

Mike

It started on a ride. Happened abruptly but I limped it home. Definitely oil. Pipes are black w/oily film. It will run great for a bit then both pipes start to smoke. Steadily worsens. Thanks!

Hate to say this bit of both pipes are smoking equally on a '74 machine, it's unlikely to be the breather. On machines of that age, the breather only vents into the right hand carb...

One thought, On one occasion, I had oil smoke and leakage on a later machine. Turned out that the bolt that holds the airbox onto the transmission had worked loose and come out. The engine was pulling gearbox oil in and burning that. On the earlier bikes like yours, the bolt is still present but holds the steel strip which secures the clamshell halves of the air felter housing. Might be worth a look.

If, as you say, both sides are smoking equally, either both sides have broken at the same time (not very likely) or there is one fault that affects both sides... that pretty much means the airbox area so that's where I'd be looking first...

Rob

Thanks for the info, I’ll check it out when I get back to Wisconsin. I’m in Florida for the winter.