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Acceptable / normal oil consumption

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K2-K6:
Just watched the whole video posted early in thread, quite a few concerns he has there  :o obviously experienced at assessment but didn't come to a conclusion in that one. Is there another ?

Clearly there's oil getting to combustion chamber/ports somewhere, statement of the bleedin-obvious there. Can only really be head or barrels/rings/pistons etc.
There's nothing to judge the tolerances used for rebore, just an assumption that he's familiar with just how concise they have to be. No indication of rings (may have been prudent when he took head off to quantify these for failures etc) but certainly a question mark over them. Very often machine shops don't, or unwilling to, get the tolerances where they need to be. Have you compression checked it warm with throttle wide open ?
Head, not that confident with that guide fit if they can be pulled out like that, what was outcome of refit on these?

While head was open, that bearing for camshaft next to #3 looked shagged  !   Another thread on here had a crack into exhaust port with the the same oil burning as you're experiencing on one of these engine, It may be worthwhile trying for another head to work on as a way forward.

Odd thing to try. Glass of engine oil, place breather from cases into it and run for decent period of time to see if it sucks up any oil (mark the glass) ordinarily you'd get outflow very gently, but if it's slurping the oil fast enough may show as vacuum here.

Certainly it's an elusive problem, you know where it is, but exactly what's causing it yet to be found.

K2-K6:

--- Quote from: Cappodimonte on September 25, 2022, 09:14:49 AM ---K2-K6, I don’t intend to get into ‘Oil Wars’ but simply put old engines use thicker oil, if you wish to use synthetics designed for modern engines in old classics, it’s your call.

--- End quote ---

I think you've missed what I've written there. There's no recommendation from me for others to use any specific type of oil, old or new. Understanding how viscosity interacts with any mechanical device is valid in best lubricating them with some of the currently available oils helping to understand why certain choices are made. It is a technical forum which entails discussion at various levels which if clearly written the people present here can then form an opinion of and contribute/question the ongoing scenario. That's what a forum is about surely......

If anyone is curious about the viscosity question, the thicker oil is the OW30 product. But probably for another thread in reality that doesn't get it mixed in here with the very real problem the OP is experiencing.

Oddjob:
Was following a Triumph 900 the other week coming back from an oil meet. It started to smoke, I could smell oil being burnt. I signaled him to pull over and his left leg up to the knee was saturated in oil, it was pouring out of the engine and forming a pool on the floor. He'd only had the bike a month so he was understandably a little upset. Turns out one of the half moon shaped rubbers sealing the camcover to the head, where the cams would be, had popped out. We managed to force it back in enough that he could get home.

The next week he turns up and explains the breather pipe had been fitted wrong by the PO and instead of being fed to the bottom of the airbox they'd fitted it to a blank on the airbox. This compressed the cases so much the seal popped out.

This got me wondering, could the engine be forcing oil back into the airfilter via the recycling system fitted to the 400, this causes the smoke from the exhaust. Which leads you to think it must be head related when in fact it could be something else, like a failed ring allowing compression into the crankcase.

Have you checked the airfilter to see if it looks really wet with oil?

400 Cafe Racer:

--- Quote from: Oddjob on September 26, 2022, 12:49:23 PM ---Was following a Triumph 900 the other week coming back from an oil meet. It started to smoke, I could smell oil being burnt. I signaled him to pull over and his left leg up to the knee was saturated in oil, it was pouring out of the engine and forming a pool on the floor. He'd only had the bike a month so he was understandably a little upset. Turns out one of the half moon shaped rubbers sealing the camcover to the head, where the cams would be, had popped out. We managed to force it back in enough that he could get home.

The next week he turns up and explains the breather pipe had been fitted wrong by the PO and instead of being fed to the bottom of the airbox they'd fitted it to a blank on the airbox. This compressed the cases so much the seal popped out.

This got me wondering, could the engine be forcing oil back into the airfilter via the recycling system fitted to the 400, this causes the smoke from the exhaust. Which leads you to think it must be head related when in fact it could be something else, like a failed ring allowing compression into the crankcase.

Have you checked the airfilter to see if it looks really wet with oil?

--- End quote ---

Also check the accuracy of the breather pipe inner diameter to ensure fully unrestricted breathing over the full length. Some of these breather pipes can Kink or partly deform which could lead to choked breathing, crankcase pressure build up and force thin hot oil past the valve stems and piston rings.

Laverda Dave:
Just to add my bit for what it's worth.....
This sounds like the same issue I had with my rebuilt CB250RSA just before it seized on the M40!
The oil consumption was very high yet I couldn't see any smoke whilst I was riding it. I got the wife to ride it whilst I rode behind on my VFR. It was only then I could see it smoking, 30 miles later it went bang. It was only when I stripped the motor I noticed the excessive piston to bore clearance and this was on a freshly rebored barrel with new piston. When I asked the engineer who bored the barrel if he had used the Honda spec clearance he said no, he thought it was too tight so he bored it to a bigger clearance that HE thought was correct! Result was it drank oil. The individual has now gone out of business (I wonder why?).
About 30 years ago I had my 400/4 rebored by a m/cycle engineering company and it also drank oil. Turned out they hadn't rebored the barrel square!
Reboring motorcycle engines and especially modern close clearance motors is a very specialised field and you have to trust who is doing the work. If it were my engine I think I'd be stripping it down by now for further investigation to check the bores for clearance and straightness and especially if someone else has rebuilt the engine before you got it. If Trigger is still working I would take it to him.

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