Author Topic: Cb750 number 4 burning oil  (Read 878 times)

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Offline graeme7

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Cb750 number 4 burning oil
« on: September 01, 2023, 01:24:55 PM »
Bit of an odd one this.
I have owned my cb750 for about 15 years. Engine was rebuilt before I got it. I've had the engine apart before too to fix some leaks. Ive only put maybe 2000 miles on it in those years.
It has a tendency to wet sump if left for weeks without running, and then would smoke a little when started until it was all back in the tank. If started once a fortnight, no smoke.

Anyway, couple if days ago I had it out for a run, all good. Next morning I start it and it's smoking quite bad. Bit rough at idle, presumably as it was fouling the plugs. Went out on it assuming it would clear. Got home 2 hours later and there is oil leaking from no4 exhaust port. It was now smoking badly and no4 plug was black. The smoke wouldn't clear, figured head gasket maybe.
It needed primary chains anyway so I pulled the engine and just stripped it. I can find no reason for it burning oil though.
It obviously was, and pretty much just on no4. Head gasket looked fine. All other head seals were fine, no cracks in the head, rings are spot on with good gaps still. Bores are perfect and still with some honing marks. It had new stem seals when last apart but I've not had the valves out yet to check those.

Plan now then is just to do the chains, and put it back together with new gaskets and seals, and hope for the best.
I was hoping to find an obvious issue.

Any other thoughts on why it would have suddenly started burning oil on one pot?

Thanks.

Offline denward17

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Re: Cb750 number 4 burning oil
« Reply #1 on: September 01, 2023, 01:35:14 PM »
Maybe more experienced folks will chime in, but are you going to repair the wet sump issue?

Offline graeme7

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Re: Cb750 number 4 burning oil
« Reply #2 on: September 01, 2023, 01:39:08 PM »
Maybe more experienced folks will chime in, but are you going to repair the wet sump issue?

Yeah, just wants a new stopper in the check valve.

Online PeWe

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Re: Cb750 number 4 burning oil
« Reply #3 on: September 02, 2023, 02:05:15 AM »
Bad piston rings should smoke all the time.

Or if engine has been stored long time, piston rings stuck in need of a ride.
Wet sumping pump, no problem. My K6 was like that from the beginning. Replaced pump after ca 100.000km. Old better with new seals.

It can be a leaking valve guide seal or two.
Valve spring must must be taken off.
While there with springs off. Let valve open ca 10mm. Wiggle it sideways from chamber side.
Not a clonky play. A small "softness" is ok.
Ex guides usually go first.
If guides are bad on cyl. 4. Fix all guides, cut  seats with new valves + seals.
« Last Edit: September 02, 2023, 02:07:18 AM by PeWe »
CB750 K6-76  970cc (Earlier 1005cc JMR Billet block on the shelf waiting for a comeback)
CB750 K2-75 Parts assembled to a stock K2

Updates of the CB750 K6 -1976
http://forums.sohc4.net/index.php/topic,180468.msg2092136.html#msg2092136
The billet block build thread
http://forums.sohc4.net/index.php/topic,49438.msg1863571.html#msg1863571
CB750 K2 -1975  build thread
http://forums.sohc4.net/index.php/topic,168243.msg1948381.html#msg1948381
K2 engine build thread. For a complete CB750 -75
http://forums.sohc4.net/index.php/topic,180088.msg2088008.html#msg2088008
Carb jetting, a long story Mikuni TMR32
http://forums.sohc4.net/index.php/topic,179479.msg2104967.html#msg2104967

Offline graeme7

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Re: Cb750 number 4 burning oil
« Reply #4 on: September 02, 2023, 02:20:32 AM »
Bad piston rings should smoke all the time.

Or if engine has been stored long time, piston rings stuck in need of a ride.
Wet sumping pump, no problem. My K6 was like that from the beginning. Replaced pump after ca 100.000km. Old better with new seals.

It can be a leaking valve guide seal or two.
Valve spring must must be taken off.
While there with springs off. Let valve open ca 10mm. Wiggle it sideways from chamber side.
Not a clonky play. A small "softness" is ok.
Ex guides usually go first.
If guides are bad on cyl. 4. Fix all guides, cut  seats with new valves + seals.

I did wonder if it was a stuck, or broken ring, but they are perfect.

Head will be stripped and fully checked, but I don't think it could be guide wear (that's not to st they might not  e worn) causing the recent oil burning as it happened all of a sudden.
Unless they were worn enough to pass loads of oil, but the seals prevented it, then a seal failed. The seals look into t and in place, but impossible to tell really untill I take the valves out later today.

I think the old wet sumping, smoke on start up issue is unrelated to the recent constant smoking from No4. I just cant work out how so much oil was getting into that chamber. Enough for it to be dripping out the exhaust port!
It all looked dry under the pucks too, but even if it hadn't been, it was smoking alot and No4 plug and combustion chamber are black and oily.
I can only assume that although the head gasket looked fine, it must have been leaking.
« Last Edit: September 02, 2023, 02:26:13 AM by graeme7 »

Online bryanj

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Re: Cb750 number 4 burning oil
« Reply #5 on: September 02, 2023, 03:35:44 AM »
You dont say what model 750 it is, the F2 was notorous for badly wearing exhaust guides that give exactly your symptoms, 350 miles per pint oil till stem seal gives out then 150 mpp and liquid oil at exhaust
Semi Geriatric ex-Honda mechanic and MOT tester (UK version of annual inspection). Garage full of "projects" mostly 500/4 from pre 73 (no road tax in UK).

Remember "Its always in the last place you look" COURSE IT IS YOU STOP LOOKIN THEN!

Offline graeme7

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Re: Cb750 number 4 burning oil
« Reply #6 on: September 02, 2023, 03:44:32 AM »
You dont say what model 750 it is, the F2 was notorous for badly wearing exhaust guides that give exactly your symptoms, 350 miles per pint oil till stem seal gives out then 150 mpp and liquid oil at exhaust

Sorry, I forgot.

It's a 1976 cb750f sohc.
It was smoking so bad, just from the one cylinder, it was like having a bonfire.

This is what I don't understand, one cylinder, with no warnings suddenly starts burning LOTS of oil, and yet on taredown I can find no evidence for why. Plenty of evidence that it was, but can't find the source.
Stem seal exists
Head gasket showed no signs of a leak
Rings are perfect
Bore is perfect
Piston is fine, no holes or cracks
Head isn't cracked

Camt think of any other reason it could suddenly start burning so much!

I'm beginning to think I must just be wrong about the Head gasket.
Not much I can do other than check every component and reassemble with all new gaskets and hope for the best.

This kind of stuff is far from new to me, I've stripped, repaired, reassembled, built from scratch, many engines. As I say, it's not even the first time I've had this one apart.

This post was to try and find out if I'm overlooking something simple as I'm too close to the issue, or in case there was something cb specific I'm not remembering. Probably 8 years since I had this engine apart.

Online bryanj

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Re: Cb750 number 4 burning oil
« Reply #7 on: September 02, 2023, 03:51:55 AM »
How much play between valve stem and guide, anything more than just feelable is too much, also possible for guide to crack
Semi Geriatric ex-Honda mechanic and MOT tester (UK version of annual inspection). Garage full of "projects" mostly 500/4 from pre 73 (no road tax in UK).

Remember "Its always in the last place you look" COURSE IT IS YOU STOP LOOKIN THEN!

Offline graeme7

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Re: Cb750 number 4 burning oil
« Reply #8 on: September 02, 2023, 05:17:12 AM »
How much play between valve stem and guide, anything more than just feelable is too much, also possible for guide to crack

I'll know that in a few hours when I strip the head.

Offline newday777

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Re: Cb750 number 4 burning oil
« Reply #9 on: September 02, 2023, 05:47:07 AM »
also possible for guide to crack

Yes indeed.
And 8 years lots can happen. A chunk of a guide could have broken off at the seal to allow it to leak.
Stu
Honda Parts manager in the mid 1970s Nashua Honda
My current rides
1975 K5 Planet Blue my summer ride, it was a friend's bike I worked with at the Honda shop in 76, lots of fun to be on it again
1976 K6 Anteres Red rebuilding project, was originally my brother's that I set up from the crate, it'll breath again soon!
Project 750s, 2 K4, 2 K6, 1 K8
2008 GL1800 my daily ride and cross country runner

Prior bikes....
1972 Suzuki GT380 I had charge of it for a year in 1973 while my friend was deployed and learned to love street riding....
New CB450 K7 after my friend returned...
New CB750 K5 Planet Blue, demise by ex cousin in law at 9,000 miles...
New CB750 K6 Anteres Red, to replace the totaled K5, I sold this K6 at 45k in 1983, I had heavily modified it, many great memories on it and have missed it greatly.....
1983 GL1100A, 1999 GL1500 SE, 1999 GL1500A

Offline graeme7

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Re: Cb750 number 4 burning oil
« Reply #10 on: September 02, 2023, 05:50:12 AM »
Only got the valves out of no4 so far, but the guides have 0 wear and the seals are perfect.
I'll take some photos

Offline graeme7

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Re: Cb750 number 4 burning oil
« Reply #11 on: September 02, 2023, 06:02:11 AM »
Pics

Offline graeme7

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Re: Cb750 number 4 burning oil
« Reply #12 on: September 02, 2023, 06:05:28 AM »
Struggling to post pics.

Managed to post one of the surface of the barrels, but can't upload the others.

Offline graeme7

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Re: Cb750 number 4 burning oil
« Reply #13 on: September 02, 2023, 06:22:26 AM »
Try again

Offline graeme7

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Re: Cb750 number 4 burning oil
« Reply #14 on: September 02, 2023, 06:23:01 AM »
Again

Offline graeme7

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Re: Cb750 number 4 burning oil
« Reply #15 on: September 02, 2023, 06:23:36 AM »
Again

Offline graeme7

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Re: Cb750 number 4 burning oil
« Reply #16 on: September 02, 2023, 06:24:11 AM »
Again

Offline graeme7

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Re: Cb750 number 4 burning oil
« Reply #17 on: September 02, 2023, 06:24:46 AM »
Last one

Offline denward17

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Re: Cb750 number 4 burning oil
« Reply #18 on: September 02, 2023, 09:52:14 AM »
Now that the cylinders are off, you can take measurements to see if things are in spec.

An internal bore gauge will help with cylinder roundness and cylinder to piston clearance as well as ring end gap measurements.

If no tools to perform said measurements, maybe a local machine shop could help, to make sure where you are while apart.

Online PeWe

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Re: Cb750 number 4 burning oil
« Reply #19 on: September 02, 2023, 10:41:59 AM »
Not only cyl 4 has oily chamber.
CB750 K6-76  970cc (Earlier 1005cc JMR Billet block on the shelf waiting for a comeback)
CB750 K2-75 Parts assembled to a stock K2

Updates of the CB750 K6 -1976
http://forums.sohc4.net/index.php/topic,180468.msg2092136.html#msg2092136
The billet block build thread
http://forums.sohc4.net/index.php/topic,49438.msg1863571.html#msg1863571
CB750 K2 -1975  build thread
http://forums.sohc4.net/index.php/topic,168243.msg1948381.html#msg1948381
K2 engine build thread. For a complete CB750 -75
http://forums.sohc4.net/index.php/topic,180088.msg2088008.html#msg2088008
Carb jetting, a long story Mikuni TMR32
http://forums.sohc4.net/index.php/topic,179479.msg2104967.html#msg2104967

Online Don R

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Re: Cb750 number 4 burning oil
« Reply #20 on: September 02, 2023, 10:45:14 AM »
   If it's never been bored, I'd do a clean-up bore anyway, even a relatively new Honda engine can benefit from getting the bores rounded out.  I've never understood why you can't re-assemble used rings, but it's always been standard procedure to hone and re-ring any time it's been apart.
  I see Cycle X has an 812 kit available. It leaves a lot of material for strength and future rebuilds. Also, a nice little power increase.
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Offline graeme7

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Re: Cb750 number 4 burning oil
« Reply #21 on: September 02, 2023, 11:22:48 AM »
I have a full write up of bore sizes and ovality in the paperwork somewhere from last time it was apart. It's done a couple of thousand miles TOPS since then. I'll be measuring them as a matter of course, but the bores really aren't the problem.
I also have absolutely zero issue with rebuilding it with the same rings. As long as the ring gaps still spec up, which I will be checking, then it will go back together as is. Absolutely no reason to hone and change rings just as a matter of course! Just because the rings have come out of the barrel, what is suspected of changing?
Obviously while it's apart, if there is any issue, you would correct it. But hone and 're ring just because? That's crazy. If its somehow managed to go oval from sitting under a sheet for 5 years, then ill bore it. But it wont have. The rings have done no work and the bores look great. It's not that far past run in from the last rebuild!

Yes, not just no4 that's been burning oil. As I said, it used to wet sump and burn oil on start up. An issue with the fact its only ever done a couple of hundred miles a year.
But you can see from the exhaust valves that 1/2/3 stopped burning oil. When cleaning up the head the blackness on 2/3/4 Was just dry soot. The blackness on 4 is sticky and oily.

I've been rebuilding engines, vehicles, machines, for 30 years. I constantly have at least one vehicle in parts, being restored, rebuilt. This is familiar territory for me, I just can't find any solid reason for how so much oil was getting in. I half expected to find a hole in the piston it was that bad, or a completely open passageway past the head gasket.

Offline Tracksnblades1

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Re: Cb750 number 4 burning oil
« Reply #22 on: September 02, 2023, 11:32:13 AM »
Does the slide in #4 carburetor go up and down with the other 3….?
Age Quod Agis

Offline graeme7

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Re: Cb750 number 4 burning oil
« Reply #23 on: September 02, 2023, 11:39:59 AM »
Does the slide in #4 carburetor go up and down with the other 3….?


Oooooo, I like your thinking.
You think it's sucking in oil as it can't draw air?
I will check.
I had it at 70mph on the way home while it was in this state. It felt ok, ish. It was raining, I just wanted to get home, it seemed down on power and didn't want to idle, but it didn't sound like it was running on 3.
Maybe at certain rpm it could get enough air, and not at others.

Online bryanj

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Re: Cb750 number 4 burning oil
« Reply #24 on: September 02, 2023, 12:38:29 PM »
If it has been apart before dont forget honing can make piston to bore clearence excessive as standard its only 1 thou, thats NOT 1 thou per inch but 1 thou total
Semi Geriatric ex-Honda mechanic and MOT tester (UK version of annual inspection). Garage full of "projects" mostly 500/4 from pre 73 (no road tax in UK).

Remember "Its always in the last place you look" COURSE IT IS YOU STOP LOOKIN THEN!