crankcase oil blow-by

I have a 75 model that has recently developed an oil blow by problem. It comes out of the crankcase vent tube after only 5 miles of riding, it sprays out about a few ounces of oil, enough to cover the tire huger. A compression check yielded 135 on one, and 140 on the other 3 cylinders.I switched from synthetic oil to conventional, no change.
The pistons, rings and bore has 35k, bottom end has 60k miles. Oil level is correct. There is no exhaust smoke, or other problems. Runs fine, plugs are clean.

Bill, This is just an offering, and hopefully completly wrong.. but, if you have an escaped wristpin circlip/worn/gone teflon button, I've seen instance (1) where wrist pin has worn cyl. causing blow-by once piston was mid-way down stroke. This would explain good comp. clean plugs, yet blow-by. Exhaust all other alternatives 1st, hope I wrong G

Have you measured your oil pressure?

If your oil level is correct? and your oil pressure is correct? I would bet on a piston or rings, being defective.

I my personal opinion! 35,000 miles on some top ends, are alot. And 60,000 miles on the bottom end, thats alot. If you are the second, third or tenth owner, who knows? Thats why they can't talk.

If you love the bike and plan on keeping it? Let it rest in peace and give it a complete rebuild.

gane wrote:

Bill, This is just an offering, and hopefully completly wrong.. but, if you have an escaped wristpin circlip/worn/gone teflon button, I've seen instance (1) where wrist pin has worn cyl. causing blow-by once piston was mid-way down stroke. This would explain good comp. clean plugs, yet blow-by. Exhaust all other alternatives 1st, hope I wrong G

if he had an escaped circlip, he d know it!!....if he used buttons, they are not known for longevity...could very well be the problem.......35k.. might just be time for a top end freshening up ..good luck ....

My oil pressure checked out fine despite the high miles on the bottom end. At 3k RPMs it is 10psi. I'm sure this is not related but, the blow by problem started all of sudden. It did it for the first time after I replaced several electrical components after a faulty voltage regulator spiked the system.

"At 3k RPMs it is 10psi."...
@ 3k you should measure 2.8 lbs, not 10 lbs...???

I recently just replaced my oil pump where I measured exactly 2.8 lbs at 3k rpm... In reviving up to 4K I then went to 10 lbs...

I would think at 3k if you're already at 10 lbs then you must be developing some serious case pressures at higher rpm's henceforth blow by...

Or am I missing something here...

somebody might of shimmed up the pump/milled case to get higher pressure.what does it do at higher rpms for pressure?

Doesn't this engine have the typical "pop off valve" to prevent excess pressure made up of a steel ball bearing and spring? My 750 has that. I see a bypass valve in the picture but I think that just goes around the oil filter?

Are you saying above normal oil pressure is created by ring blow-by ? Is there anything in the oiling system that can create a sudden increase in oil pressure ? In addition to losing oil from the vent, I recently developed 2 oil leaks.
Thanks for your input.

"Is there anything in the oiling system that can create a sudden increase in oil pressure "...

One thing I can think of is a blocked cylinder passage up to the head/valves/camshafts... Meaning your oil is trapped down in the lower case, no flow...

Since you mentioned new oil leaks....

What about your oil filter itself, when was the last time you replaced... Just something to think about, no need to answer.... Possibly old gaskets and new pressures due to as Plummen menitoned, shiming the oil pump for increased flow....

OMR

I found my oil blow by problem. The number one cylinder sleeve liner came loose, and could be slid in/out 3-4 inches easily by hand. Rings, pistons, cylinder surface all were ok.