Bad piston rings? Smoking engine.

[surfish84651]

Hey guys, I just wanted to double check with the experts, make sure I did my tests right and came to the correct conclusions. I've got this 1980 cb750k that I purchased for a couple hundred dollars. It was not running when I got it so I figured it would be a fun project to do something with. To start out I replaced the starter motor with a new one, rebuilt the carbs (have not vacuum synced yet, only bench synced), took the engine out for a deep exterior cleaning, repainted the frame and for the heck of it repainted the gas tank.

Now I am getting to the actual engine. The engine starts quickly, warms up and will idle at around 1000 RPM (choke it pulled out a little less than half way). It sounds a little rough (maybe due to non synced carbs), and sounds like the valved are knocking. When I twist the throttle and let it go, it'll accelerate up the RPMs but comes back down very slowly, and sometimes I have to give the throttle a quick little twist to bring the engine RPM back down. Any idea how to fix this?

Also a whitish, and maybe slighlty blue smoke blows out of the exhaust pipes. This happens on start up and when the bike runs at around 3000 RPM for maybe 30 seconds. Then when the RPMs go back down, the smoke stops (at least visibly). I pulled the plugs which came out black. Then I did a compression test and the cylinders tested 95-90-90-90. I did a leak down test, putting a teaspoon of SAE 30 oil in the cylinder, redid the test and got 125-120-120-120. This I am being told by the compression tester instructions, means I have bad piston rings. Is this correct? If so, it sounds like I will be removing the engine again, and doing a top end rebuild, new rings, adjust the valves etc? Does this all sound like I'm on the right track, or am I not thinking of something? Thanks guys for any help and guidance.

 

[dirtdigger]

teaspoon of oil is actually quite a bit for a small bore motorcycle cylinder and that may be contributing to the large amount of increase in compression. Also assuming you are holding the throttle wide open when doing the test. When rings are bad you normally get oil at a constant rpm or under acceleration and high rpm so I would say you could have a ring issue. Intake valve seals/guides show up mostly on deceleration when vacuum is high. Exhaust guide/seals can show up at just about any rpm. Guides/seals like to have a puff of smoke on startup after sitting overnight.

 

[surfish84651]

dirtdigger, would the teaspoon of oil that I put in do any damage? I used a funnel, so it wasn't quite a teaspoon, but still.

 

[dirtdigger]

as long as it didn't act like turned over hard or acted like each cylinder wanted to stop on the compression stroke you will be ok. Too much fluid and you will hydrolock the cylinder in which case you can break pistons or bend connecting rods. all you are trying to do with the oil is coat the rings as it helps to seal them better which would indicate worn rings and cylinder.

 

[amc49]

Hope that guy never tried to ride that bike with the brake torque rod missing off the rear brake...................deadly.