Topic: Burning oil, or unburnt fuel?

[The Lone Builder]

I have a problem with smoke coming out of exhausts 3 & 4. There is also a black deposit below the balance pipe which doesn't really feel oily; unburnt fuel?

The smoke I am told is whitish and appears "on acceleration and trailing throttle". The bike responds well to the throttle, but idle can be difficult to control - it tends to speed up when hot and be hard to set when cold; air screws are at +/- 1 turn out.

The plugs are as in the pic attached; 1 & 2 look OK, but 3 & 4 are definitely sooty with 4 being worse.

I've checked compression cold - 120, 120, 125, 115, and the fuel level in the carb bowls is good as per clear tube method.

The engine is stock K2/K4, with 110 main jet and needle set in 3rd slot.

What else can I check/do to try to resolve this?

[PeWe]

The black flowing stuff can be condense water mixed with soot coming out from cyl 4 pipe.
If run it warm, rev it a little with white soft tissues behind each pipe might show sooty spatter.

Clean off when warm and condense away.

[The Lone Builder]

But what will that tell me PeWe?
What do I do next?

[PeWe]

If bike is stored cold, or ridden in cold humide climate, condense is normal.

Plug colors can be something else.
If everything is stock, stock jetting must work fine.

[HondaMan]

Check the intake manifolds for slight vacuum leaks. (what follows next is a detailed explanation of WHY intake boot leaks act to cause fouled plugs - some call it controversial, I call it tested science from 50+ years of working on the CB750...)

The suspect here, if I was working on it, would be #1 intake boot, at the head side. If it has a significant vacuum leak it causes the engine to slow slightly ('stumble'), which causes #3 to draw fuel/air mix more slowly than the engine's speed, which means it will draw a slightly richer mix - because as the airspeed in these carbs slow down, the mixture gets richer. In this one, I would also suspect #3 is leaking because #4 is also rich. The #2 plug stays cleaner because by then in the revolutions the #2 intake is happening at the proper speed, and the #1 follows at that same speed.

The combination of having 2 cylinders running about the right mix & speed and 2 cylinders not will cause the other running symptoms you are seeing: differing idle performance when hot vs. cold. Just look at the correct cylinders to solve the problem.

BTW: if the cam in this engine is the K2, the #110 mainjet is correct. If the cam is from the K4 then the mainjet should be #105 instead.

[The Lone Builder]

Thanks Mark.
I did that, and to be honest I couldn't convince myself there was a change.
However I did discover a considerable - frightening actually - leak at the manifold on No 4 exhaust, so tomorrow I'll try to investigate that and then try again with the vacuum leak.

[The Lone Builder]

So I got my compression gauge hose replaced - it blew out last time and I did the test again, after adjusting teh cam chain and checking the valve clearances.

Cylinder      1          2          3          4
Dry           140      140       125      110
Wet          150                   155      140

This tells me the rings need attention (yes?) and so there is little to be gained by doing a leak down test - I don't have the kit to do this anyway.

Is my conclusion correct and should I just get on with the strip down, or ...?

[gearsoup]

So I got my compression gauge hose replaced - it blew out last time and I did the test again, after adjusting teh cam chain and checking the valve clearances.

Cylinder      1          2          3          4
Dry           140      140       125      110
Wet          150                   155      140

This tells me the rings need attention (yes?) and so there is little to be gained by doing a leak down test - I don't have the kit to do this anyway.

Is my conclusion correct and should I just get on with the strip down, or ...?

Interested to hear the response and workflow on this. My '76 (new project bike) is even lower than that and plugs look about the same

[HondaMan]

The rings? No, not likely unless it has over 40k miles on it, with not enough oil changes along the way.

If running it doesn't bring back the compression in about 300-ish miles (hiway riding), the valves will prove to not be sealing well. The rings are good for at least 50k miles even with poor oils, but the valve guides in the post-3/72 engines (excepting the 750F0 engines) are simple cast iron, and if the bike was ridden much during the years of MTBE gasolines, will be worn enough to prevent accurate valve alignment on the seats. The intake valve is the culprit then, although the exhaust guide will be worn even more (but usually still seats fine, outside of leaking oil out the exhaust port). This is the common thread in these engines when I see them.

For these reasons, I get LOTS of heads needing rebuilds. Sometimes I just get the head for rebuild, from others in these forums. Then they get new bronze valve guides, which will probably outlast the owner.

[Tracksnblades1]

So I got my compression gauge hose replaced - it blew out last time and I did the test again, after adjusting teh cam chain and checking the valve clearances.

Cylinder      1          2          3          4
Dry           140      140       125      110
Wet          150                   155      140

This tells me the rings need attention (yes?) and so there is little to be gained by doing a leak down test - I don't have the kit to do this anyway.

Is my conclusion correct and should I just get on with the strip down, or ...?

Interested to hear the response and workflow on this. My '76 (new project bike) is even lower than that and plugs look about the same

How much oil did you put in each cylinder to get your “wet” compression readings..?

[The Lone Builder]

Not very much; a squirt or two from a plastic container of light oil. probably half - one teaspoonful.

The engine is out, so Ill be opening it up once I've given it a wash.

[Tracksnblades1]

Not very much; a squirt or two from a plastic container of light oil. probably half - one teaspoonful.

The engine is out, so Ill be opening it up once I've given it a wash.

You’ll know for sure soon enough… 👍

[scottly]

There is also a black deposit below the balance pipe which doesn't really feel oily; unburnt fuel?
Yes, it looks like un-burnt fuel.



The plugs are as in the pic attached; 1 & 2 look OK, but 3 & 4 are definitely sooty with 4 being worse.
Both the 3&4 spark plugs are fouled by too much fuel, or too little air: you have a carburetor problem.

[scottly]

So I got my compression gauge hose replaced - it blew out last time and I did the test again, after adjusting teh cam chain and checking the valve clearances.

Cylinder      1          2          3          4
Dry           140      140       125      110
Wet          150                   155      140

This tells me the rings need attention (yes?)

The excess fuel that was getting pumped into the exhaust by the non-firing 3&4 cylinders also washed the oil off the piston rings, reducing the seal until you added oil. Put the motor back in and fix your carburetors.  

[M 750K6]

Can lower compression cause a rich fuel:air mix?

[The Lone Builder]

So I got my compression gauge hose replaced - it blew out last time and I did the test again, after adjusting teh cam chain and checking the valve clearances.

Cylinder      1          2          3          4
Dry           140      140       125      110
Wet          150                   155      140

This tells me the rings need attention (yes?)

The excess fuel that was getting pumped into the exhaust by the non-firing 3&4 cylinders also washed the oil off the piston rings, reducing the seal until you added oil. Put the motor back in and fix your carburetors.

Thanks Scotty, when you say "fix my carburettors", fix what?

Looking when I removed them, the balance seemed OK, and before removing them I checked the float level with a clear tube. All the air screws were 1 turn out.

I don't think I said earlier, that the exhaust gas coming from No 3 & 4 pipes was colder that that coming from pipes 1 & 2. No 4 was cooler than No 3. I thought maybe it was a plug issue, but switching the plugs between 1 & 4 cylinders made no difference.

[The Lone Builder]

The rings? No, not likely unless it has over 40k miles on it, with not enough oil changes along the way.

If running it doesn't bring back the compression in about 300-ish miles (hiway riding), the valves will prove to not be sealing well. The rings are good for at least 50k miles even with poor oils, but the valve guides in the post-3/72 engines (excepting the 750F0 engines) are simple cast iron, and if the bike was ridden much during the years of MTBE gasolines, will be worn enough to prevent accurate valve alignment on the seats. The intake valve is the culprit then, although the exhaust guide will be worn even more (but usually still seats fine, outside of leaking oil out the exhaust port). This is the common thread in these engines when I see them.

For these reasons, I get LOTS of heads needing rebuilds. Sometimes I just get the head for rebuild, from others in these forums. Then they get new bronze valve guides, which will probably outlast the owner.

Sorry Mark, somehow I missed this post.
The rings were replaced during the rebuild in 2018, some 20-25,000 miles ago; valves and guides were NOT replaced. Oil changes have been regular even if the oil used has varied considerably depending on availability, although it was always good quality.

I think that since I have the motor out already, that I'll open it up and check the valves rather than just attending to the carbs, which I'll do anyway.

[scottly]

I don't think I said earlier, that the exhaust gas coming from No 3 & 4 pipes was colder that that coming from pipes 1 & 2.

That's because those cylinders were not firing.

[The Lone Builder]

The head is off, and I think I may have discovered the problem; several, in fact most, of the cylinder head nuts/bolts were much looser than I expected. Some felt barely tight at all!

My notes from the last time I was in here say that I tightened things to 210-220 inch/lbs, waited 24 hours, had to torque them again, "almost forgot to check them again after another 24 hours." So either I did it wrong, my torque wrench is off, or they worked loose.

Anyway, it is what it is.

I've added some pics of the valves, pistons and cylinders; any thoughts would be appreciated.

In spite of all the oil around, piston No 4 especially, there was no hint of an external leak. The cylinder bores looked OK, with no worrying scores.

[Tracksnblades1]

I don't think I said earlier, that the exhaust gas coming from No 3 & 4 pipes was colder that that coming from pipes 1 & 2.

That's because those cylinders were not firing.

Proof is in the pudding….

http://forums.sohc4.net/index.php?action=dlattach;topic=193946.0;fs=139446;attach=488018;image

Was the oil from the tear down or misfire pump..?

[The Lone Builder]

I'm not sure what you mean.

That's just how it looked when the head came off.

[HondaMan]

Loose head nuts will cause the rings to leak. The mechanism is: uneven [mechanical] compression from one side of the crank to the other causes a flutter, a slight back-and-forth vibration, in the rotation of the crank, The crank weights will often carry it thru anyway (unless it is a lightened crank: these cranks DON'T fit in that category!) and this sudden acceleration change causes the rings to shift upward when it decelerates, then downward again as the cylinder fires - hence the term 'flutter'. Each time the ring(s) flutter, they pump oil upward. It doesn't take long for it to reach past 3 rings.

I went thru this exactly with a K4 engine back about 2008 or so. I was working at a place with a shop and they let me do the "bike thing" at night when there was enough floor space between machine projects (this was one of only 2 'perks' that job had, the other being free ammo...). They had a brand-new set of 'click' torque wrenches, so I thought I'd try them out. Big mistake: after I got the bike back together and went out for the shakedown ride, it vibrated a LOT for a CB750, and after 40 miles of trying to figure out what was wrong, it started smoking on pipes 1 & 3 - with new pistons and rings! After re-pulling the engine, despite having performed the [usual for me] torque-twice routine after 24 hours, there were several nuts on the left side of the head that were barely tight at all. I immediately suspected the 'new' torque wrenches, but re-pulled the top end and re-checked everything, even piston shape and cylinder bore roundness: nothing wrong. I brought in my trusty (1969 vintage) Craftsman beam-type torque wrench from home for the reassembly, and all was fine after that.

Later I tested those 'click' wrenches against the "old" beam wrench, measuring torques to 75% of full scale. The Chinko Harber Frate wrenches were 'off' more than 20% (smaller 18 ft-lb one) and almost 40% for the [larger, 40 ft-lb) wrenches I'd used, indicating MUCH higher torques than were actual at the 'click'. Lesson learned. Despite telling management about it, they continued to use them: I went along behind them and corrected their assemblies with my old one until it pissed them off, then I re-demonstrated it for them, a year later. They finally quit using them HF units then, and bought a Craftsman set. One night when we moved to a new building, I threw out the HF units. No one ever said a word about it, either.

[Tracksnblades1]

I'm not sure what you mean.

That's just how it looked when the head came off.

So when the head came off, was the oil from the valve cover area draining down or running down the cylinder stud holes during the removal (tear down) of the cylinder head…?  Or was the oil already present on the piston from the non firing cylinder (piston pumping)…

The Pumping I posted is merely a simple pressure differential.
Remember the cylinder is in a vacuum state during the intake stroke. The carburetor slide’s position and rpm will dictate how high the vacuum is. At the same time the crankcase pressure is “ambient+” (same pressure as we breathe 14.7#s sea level) + any positive blow-by pressure. This pressure differential of the  20+” mercury vacuum trying to suck oil up through the pistons rings while at the same time the slightly over 14.7# in the crankcase is trying to push (blow) the oil up through the piston rings from below.  The only help your ring package has is the compression/combustion cycle where your 120+psi of compression can “start” forcing the oil trapped in the oil rings rails, under the second ring, and in the combined ring lands, back down towards the crankcase. When the compression stroke becomes the power/stroke near TDC, the 120+psi of compression pressure will spike to somewhere near 300psi at an idle and 1000-1500+psi at WO throttle, on a firing cylinder. This increase in pressure can help return the oil or at least purge the oil that may remain in the ring package back to the crankcase side. Even if the rings weren’t designed for it..

Sorry, your linked pictures shows what Scottly posted : “That’s because those cylinders aren’t firing”….. Which was his response to your post indicating cylinder’s 3 & 4 exhaust temperature wasn’t as hot as 1&2…

The picture I linked of yours show these two cylinders with wet combustion chambers, shiny wet spark plug electrodes, and black exhaust valves. Notice the color of the exhaust valves that don’t have carbureation issues and that are firing consistently…

[The Lone Builder]

Tracks: I think since there was oil on both No 1 and No 4 pistons, that the oil leaked down.

Putting the previous two responses together:

The normal tendency for oil to come up past the rings is aided by the fluttering caused by the uneven torquing. This flutter also reduced compression.
More oil coming up fouls the plugs which then don't fire leading to lower pressure during the power stroke, reduced forced return of errant oil, and cooler exhaust gas.

So the solution is to:
          clean everything up,
          check valve guide clearances, 
          check rings (just in case),
          check/renew my torque wrenches and
          put everything back together, having cleaned the carbs.

What do you think?

[M 750K6]

New gasket? As you're checking the guide clearances, I'd also lap in the valves, to be sure.