Topic: Cb750 k2 burning oil from cylinder 4

[RacingSaints]

Hello, I'm a new cb750 owner and recently ran into a problem. Also I'm new to this forum so I apologize if this is something that's already been discussed. So a couple months ago I bought a cb750 k2 that had a full engine rebuild less then 500mi prior to me getting it. Rode it around town for a little and it seemed to run good. Just a couple weeks back I went out for a group ride and one of my buddies behind me asked if it was normal for my bike to smoke a little from the exhaust and I told him it probably was as the bike was running fine and the smoke wasn't significant, I could tell when I stopped as I'd have some smoke come up to me. When I got home later that day I checked my engine oil level and it was close to the end of the dipstick. Went out and bought some Lucas 10w40 motorcycle oil and filled it right between the low and high mark. I didn't get to ride it right away and had to wait a day or two because of the weather, but when I took it for a short ride it blew clouds of smoke. Came back home and saw that the spark plug on cylinder 4 was fouled with oil. Got a new plug in and just after a short ride it was all oily again. I noted a month or so back ago when I was looking at plugs that cylinder 4 seemed a little wet with oil, but now it was a lot more. Did a compression test and got about 155 psi which is fairly consistent with the other cylinders too. Added a bit of oil in cylinder 4 and did a compression test again but got the same number so I'm pretty certain that it isn't bad rings, which I wouldn't expect anyways as the engine has been rebuilt fairly recently. Talked to the PO and he suggested I check valve clearances which were spot on. I'm sort of lost now and dont know what to do. It's smoking a lot and doesn't seem to clear up when I punch it a little. Any help or direction would be very greatly appreciated. Lmk if theres more info that I can provide.
Thanks!

Harnaad Singh

[newday777]

Welcome aboard the forum Harnaad
Ask the PO if he checked the valves and guides for wear/slop.
Also what did he use for the head gasket  and the sealing rings in the head.

[RacingSaints]

On it, thanks!

[RacingSaints]

Ok so the PO says that the valves had been machined and gaskets should be good as he uses them in his other engines too. He also said it could possibly be a valve guide that had gone bad.

Got any other ideas on what it could be?
Thanks

[bryanj]

If it was rebored by a car shop the piston to bore clearance is wrong, could be bad guide BUT any half reasonable mecanic who fitted valves should have noticed that so i think you need to remove the engine and tear it down to inspect properly

[RacingSaints]

Yeah, the other 3 cylinders are running fine and well with no oil deposits on plugs or anything so that makes me think the job was done decently.

Seems like there isn't much I can do without having the engine out..

[scottly]

Just a couple weeks back I went out for a group ride and one of my buddies behind me asked if it was normal for my bike to smoke a little from the exhaust and I told him it probably was as the bike was running fine and the smoke wasn't significant.....
..... I didn't get to ride it right away and had to wait a day or two because of the weather, but when I took it for a short ride it blew clouds of smoke. Came back home and saw that the spark plug on cylinder 4 was fouled with oil. Got a new plug in and just after a short ride it was all oily again. I noted a month or so back ago when I was looking at plugs that cylinder 4 seemed a little wet with oil, but now it was a lot more.

So the smoke has increased significantly in a short amount of time? You can have good compression if the top piston rings are good, but high oil consumption and smoke if the oil ring is bad; one piece oil rings can break, and 3 piece rings can be miss-assembled rather easily. The early bikes had no seals on the exhaust guides, but any oil usually gets blown out the exhaust where it burns in the pipe, so won't foul the plug. If the seal has failed on a intake, the oil gets sucked into the combustion chamber, where it can foul the plug, but the guide would also have to be loose or damaged to admit a large amount of oil in a short time. One test for an intake guide is to compression-brake down a hill with the throttle closed for a bit, which increases the suction on the guide, and then open the throttle. If the smoke drastically increases, compared to cruising at a steady speed, it points to a problem with the guide or it's seal.   

[Medyo Bastos]

Are you sure its oil and not fuel? A fouled plug can look like oil


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[scottly]

Are you sure its oil and not fuel? A fouled plug can look like oil


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Oil smoke is blue/white, while fuel smoke is black.

[Medyo Bastos]

Hmmm, I didn’t read what color smoke was seen


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[HondaMan]

What is the serial number on that engine, and the build date on the label attached to the steering tree?
Reason I ask: builds made before 4/1972 had engines with Stellite valve guides, which last over 100,000 miles. The ones made after that have cast-iron valve quides which can wear quickly if the bike was parked (and not prepped for storage) for a long time. Those later guides get rust in them while parked, and if not carefully restarted (as in, valve removed, guides cleaned and stem polished again) they can end up with too much clearance, and then leak oil. The exhaust guides made before 4/1972 are distinctive: they have no valve seals, nor a groove to hold one.

Ask your friend expressly: what piston-to-bore clearance was used in the "rebuild".

[scottly]

Reason I ask: builds made before 4/1972 had engines with Stellite valve guides,

Is there any documentation about that Mark? I've never heard of stellite guides before?

[scottly]

Hmmm, I didn’t read what color smoke was seen


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Neither did I, but since you raised the question, I thought it might help if Harnaad knew how to tell the difference.

[HondaMan]

Reason I ask: builds made before 4/1972 had engines with Stellite valve guides,

Is there any documentation about that Mark? I've never heard of stellite guides before?

I've only seen a Manufacturing Notice from 1973 that was something the Honda Rep (who was a good friend of mine in those days) had, but you can't miss the guides: the exhaust guides in those heads have no valve seals, just pointed tops (I might have some pix of them) and no place to install a seal - they are so tall that the valve spring "misses" the top of the guides by just 1mm on full downstroke with standard cams. This was an issue with some of the hotrod cams from guys like RC Engineering, when used with the specially-modified rocker arms that had wider wiping faces and lots of lift (like 5mm). You had to carefully grind off the tips of the valve guides (exhaust side) about 2-3mm to use those cams.

If you've ever torn down a K0 or K1 engine (or an F0 engine) and cleaned off their valve guides, you'll find them smooth as glass: those are Stellite (if original). The only reason I know them as "Stellite" is because that was what Honda called them: it is an alloy of cast iron that is very, very hard, slick to the touch, and the sharp tips of which often chipped. I had 2 chipped ones in my K2 in 2006 when I took it down from not riding it 5+ years after cancer: the valves rusted and one of the exhausts wouldn't close. They finally wore to the wear limit at 131k miles (on 3 of the exhaust guides), that's how tough they are, and the reason why so many F1 bikes last so long. Honda's later valve guides are simple cast iron and last around 20k-30k miles if excellent oil is used, or 10k miles if not.

[scottly]

My K1 engine has bronze guides with no seals on the exhaust.

[RacingSaints]

Just a couple weeks back I went out for a group ride and one of my buddies behind me asked if it was normal for my bike to smoke a little from the exhaust and I told him it probably was as the bike was running fine and the smoke wasn't significant.....
..... I didn't get to ride it right away and had to wait a day or two because of the weather, but when I took it for a short ride it blew clouds of smoke. Came back home and saw that the spark plug on cylinder 4 was fouled with oil. Got a new plug in and just after a short ride it was all oily again. I noted a month or so back ago when I was looking at plugs that cylinder 4 seemed a little wet with oil, but now it was a lot more.

So the smoke has increased significantly in a short amount of time? You can have good compression if the top piston rings are good, but high oil consumption and smoke if the oil ring is bad; one piece oil rings can break, and 3 piece rings can be miss-assembled rather easily. The early bikes had no seals on the exhaust guides, but any oil usually gets blown out the exhaust where it burns in the pipe, so won't foul the plug. If the seal has failed on a intake, the oil gets sucked into the combustion chamber, where it can foul the plug, but the guide would also have to be loose or damaged to admit a large amount of oil in a short time. One test for an intake guide is to compression-brake down a hill with the throttle closed for a bit, which increases the suction on the guide, and then open the throttle. If the smoke drastically increases, compared to cruising at a steady speed, it points to a problem with the guide or it's seal.

Ahh okay, yeah the smoke increased pretty drastically. I can try the intake guide test tomorrow and see if that helps narrow down things a little.

Thank you!

[RacingSaints]

Are you sure its oil and not fuel? A fouled plug can look like oil

Yep definitely oil. I have a oily residue at the tip of my exhaust too, also it smells like burning oil haha
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[RacingSaints]

Hmmm, I didn’t read what color smoke was seen

Haha my bad I should have mentioned that. The smoke coming out is a white ish blue colour
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[RacingSaints]

What is the serial number on that engine, and the build date on the label attached to the steering tree?
Reason I ask: builds made before 4/1972 had engines with Stellite valve guides, which last over 100,000 miles. The ones made after that have cast-iron valve quides which can wear quickly if the bike was parked (and not prepped for storage) for a long time. Those later guides get rust in them while parked, and if not carefully restarted (as in, valve removed, guides cleaned and stem polished again) they can end up with too much clearance, and then leak oil. The exhaust guides made before 4/1972 are distinctive: they have no valve seals, nor a groove to hold one.

Ask your friend expressly: what piston-to-bore clearance was used in the "rebuild".

Hmm okok, I'll try to get that info. I do have a picture of the little label.

[RacingSaints]

Hmmm, I didn’t read what color smoke was seen


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Neither did I, but since you raised the question, I thought it might help if Harnaad knew how to tell the difference.

Lucky for me the very strong smell of burning oil plus the steadily decreasing oil level leads me to belive im not imagining the smoke screen i leave behind for the poor drivers in my rear view hahaha

[bryanj]

That pic is the frame/vin number, the engine number is on a rectangulsr pad on the top crankcase and will be different, the engine number is what Mark needs

[RacingSaints]

Also, I forgot to mention but when I open the oil reservoir there is some smoke in there too. Not a whole lot but I don't know if thats normal or not

[RacingSaints]

That pic is the frame/vin number, the engine number is on a rectangulsr pad on the top crankcase and will be different, the engine number is what Mark needs

Oh okay got it, my bad

[MRieck]

If it smokes a lot on cold start up the exhaust guides are NG. If it smokes all the time all the guides are NG. These "rebuilds" often don't include real valve train work......just lap the valves for 3 minutes and get a .100 contact area on the seat.
Given the bikes age I'd say guides and valves need replacement and recut the valve seats on a Serdi etc. Also....have the new guides honed.......not reamed.

[RacingSaints]

If it smokes a lot on cold start up the exhaust guides are NG. If it smokes all the time all the guides are NG. These "rebuilds" often don't include real valve train work......just lap the valves for 3 minutes and get a .100 contact area on the seat.
Given the bikes age I'd say guides and valves need replacement and recut the valve seats on a Serdi etc. Also....have the new guides honed.......not reamed.

Yeah it smokes a lot all the time, but its only cylinder 4 that burns oil. Would that still mean all the valve guides r bad or its just the one that went south and needs to be taken care of.
...anyhow it sounds like a fair bit a cash to get her healthy again. How damaging would it be to still take the bike for short rides?