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My 1975 R90/6 developed a lot of blowby and started fowling plugs. I took the top end apart and realized it was due to glazed cylinders. Too many slow rides around the yard with the grandkids (it has a sidecar). So, I rebuilt the top end, valve job, new rings, new crankcase breather valve. So far it's running better but I'm surprised by the amount of blowby that I still get out of the dipstick hole when the engine is running. I assume some is normal but I expected less than what I'm seeing. What is normal? (and how do you even answer that question...)

Did you also hone the cylinders when doing the top end? This would remove the glazing. Recently I had the same issue with a marine 3 cylinder Yanmar diesel engine. As we opened the throttle it blew the dipstick 3m into the air while spraying the side of a barn with oil. Once the cylinders were honed no blow by and the dipstick stayed in place. Problem solved.

K

Yes, absolutely. I aggressively honed the cylinders in order to break up the glazing and get a nice cross-hatch hone in the bore.

Define blowby? If you remove the dipstick, the air that is compressed when the pistons travel to bottom dead center has to go somewhere. There is a crankcase vent on top of the engine, but if you remove the dipstick that air is going to find the easy way out.

If you think you have blowby from the ignition inside the combustion chamber, you need to do a leakdown test to quantify how much and where it's doing.

Kurt

I agree with above, the volume of the crankcase is changing massively every time the pistons reach BDC both together. This is not blowby. Mine makes a god awful noise if I inadvertently start it up with the dip stick unscrewed. At least you can't forget and ride off it that state.

The purpose of the engine breather is to pump down the crankcase to a vacuum such that it's not losing power by pumping air in and out at every stroke. In theory this would mean no further air is expelled through the breather once the engine was running. That in practice they do expel air is down to blow by.

This is called crankcase pumping and is normal as long as the cylinders are sealed as in piston rings bedded in, with new rings you should take it for a good ride and get some load on vary the engine speed to get the rings run in, don't rev the crap out of it but don't labour it just normal riding.

As for the fumes from the oil filler you have 2 pistons moving outward away from the crankcase and then back in acting like a pump, the breather at the top of the engine is there for that so you may need to check it's not blocked.

On the bright side, you've got a fresh top end.
A plastic bag on the end of the crank case breather hose will show/disprove blow bye, if you don't have a leak down tester.