Topic: cb750K1 - foaming oil + dripping out the tank breather

[Salman_z8]

Assuming i an pulling the motor, what are we looking for ?
Any bets ?

I'm going out on a limb to say it's a problem with one of the piston rings?

You reckon it's compression pressure leaking excessively via the rings into the case ?

[grcamna2]

Assuming i an pulling the motor, what are we looking for ?
Any bets ?

I'm going out on a limb to say it's a problem with one of the piston rings?

You reckon it's compression pressure leaking excessively via the rings into the case ?

It's possible.

[Stev-o]

Assuming i an pulling the motor, what are we looking for ?
Any bets ?

I'm going out on a limb to say it's a problem with one of the piston rings?

You reckon it's compression pressure leaking excessively via the rings into the case ?

Are you using an automotive type compression tester?  They are known to give false readings on these motors...

[HondaMan]

While this description is a little bit "in the weeds", it is what I have seen happen before with these engines, many, many times.
The 750K0-K2 engines made before 3/1972 had 1-piece oil rings in them, and the 3-piece versions showed up after that, from Honda (although, I have seen them in virgin engines from the K3 era, which probably were reworked engines that came back into their production line). The 1-piece ring is much more of a square cross-section than the 3-piece version. This causes the piston's oil ring land (groove) to wear fully into the piston from the stiff 1-piece ring: in fact, with the 1-piece oil ring there is usually NO step in the upper edge of the oil ring grooves after many thousands of miles. The opposite is true when the 3-piece oil rings are used. Both methods will seal the piston in the bore, but only once.

The 3-piece oil ring has a thin top and bottom ringlet that borders and supports a wavy steel breather, but without actually touching the inside of the oil ring's groove: these are about 80% as wide as the 1-piece oil rings. When new pistons are installed, their oil ring groove is slightly undersized on purpose (about 0.02mm or 0.0008") to allow the new oil ring to seat itself into the upper (more important) and lower edge of the groove.

When these engines get "re-ringed" with the original pistons still in place it is therefore vital to use the same TYPE of oil ring (and for that matter, the 2nd compression ring, but I'll leave those details out here...) in the oil grooves on the pistons. Since this is an early engine, the chances are 100% that it started life with 1-piece oil rings, so the new ones must also be that type if the same pistons are re-used. This will not yield long life, but maybe 10,000 miles before they lose full function and the engine starts slowly using some oil. Only a step-bore of at least 0.25mm/0.010" with new rings (then using rings mated to the piston type) will make it last longer, as the bores are already not round and the pistons are matched to those bores: the new rings are not. This then forces the new rings to have to wear into the non-round holes, which they won't do effectively. When attempts at very heavy honing have been done to make new rings on old pistons into an "overhaul" by forcing the new rings to wear into the new, heavily honed surface, the end result is too much piston clearance (usually more than 0.0020"), which causes blow-by and oil leakage past the new rings on a permanent basis.

It is this blow-by that I suspect is causing the oil frothing in this engine. It is worst at low engine speeds and makes the exhaust smoke at higher speeds, burning oil in the process.

[scottly]

Oil rings are supposedly rik 3 piece oil ring, on original pistons.
I could possibly try to swap the oil pump with one which has not been rebuilt, and test ??

About half a quart of oil in the crankcase, however there was some wet sumping before the engine was started and ran a few mins.

I just put the automotive 20w50, not the 4T.

But frothing is excessive, you can see bubbles as the oil is returning to the tank, and the amount of aeration is directly proportional to the revs.

It is perfectly normal for the oil from the scavenge pump to be aerated, as it's pumping a lot of air along with the oil as it sucks from the sump. This has NOTHING to do with piston rings, valve guides, compression, or the phases of the moon.
If the tank has been overfilled from wet-sumping, or gas has drained into the sump from leaking floats, then it will overflow out the tank breather as the oil/gas is pumped from the sump into the tank. (I think we all have over-filled a 750 tank at least once. ) There is an oil/air separator inside the oil tank, which drains through the spigot on the back of the crankcase; if this is not properly connected, it will occasionally puke oil on the ground, proportional to the revs.     

[Salman_z8]

Thank you for the detailed replies.
I will be looking into it more, and more than likely will pull the topend off.
Only then i will know more.

From the limited testing i have done, it seems to be worse at high revs and not much a problem at lower revs.
I will have a chat with the buddy, if they are ok with pulling with apart, then apart it will come.

I am a sucker for a good puzzle.

[scottly]

Thank you for the detailed replies.
I will be looking into it more, and more than likely will pull the topend off.

WTF!!?? NONE of your symptoms have anything to do with the top end of the motor!! What is the level of the oil in the tank on the dipstick?

[scottly]

This is a pic of a K1 drain from the oil/air separator in the tank to the back of the crankcase. The K1 tank also has a vent hose, which is normally routed down near the right side of the swing arm pivot. The valve cover breather hose is routed over the top of the carbs, then down on the right side along with the tank vent hose. Any pressure in the crankcase from blow-by will be vented through the valve cover hose, and will not force oil out of the tank, provided everything is plumbed properly. Do some more research before you touch a bolt on the top end.

[Salman_z8]

This is a pic of a K1 drain from the oil/air separator in the tank to the back of the crankcase. The K1 tank also has a vent hose, which is normally routed down near the right side of the swing arm pivot. The valve cover breather hose is routed over the top of the carbs, then down on the right side along with the tank vent hose. Any pressure in the crankcase from blow-by will be vented through the valve cover hose, and will not force oil out of the tank, provided everything is plumbed properly. Do some more research before you touch a bolt on the top end.

Yes, everything is plumbed as it should be.
The crazy thing is, all the vapour ends up in the oil, and froths up in the tank.

Oil level was correct. The motor is not wet sumping.
I do not think fuel is making its way in the oil, rather combustion leaking in the case.

I am thinking excessive blow by.

I am building a motor of my own, once that is complete, then i will tackle this one.

This situation, in this case, is certainly not caused by over filling or incorrect plumbing.

Why does this motor not let any pressure out the top cover, beats me.

I am open to more ideas, since i wont be doing anything drastic to it for a bit anyway

[Salman_z8]

This is a pic of a K1 drain from the oil/air separator in the tank to the back of the crankcase. The K1 tank also has a vent hose, which is normally routed down near the right side of the swing arm pivot. The valve cover breather hose is routed over the top of the carbs, then down on the right side along with the tank vent hose. Any pressure in the crankcase from blow-by will be vented through the valve cover hose, and will not force oil out of the tank, provided everything is plumbed properly. Do some more research before you touch a bolt on the top end.

I do not get anything out of the valve cover vent, but it is clear, i checked again.
The oil pump has shimmed relief valve to help with pressure, from cycleX

I havent put an oil pressure gauge on it, so cant tell you, if its excessive or not.

But i am stumped, not gona lie

[Don R]

 The top vent not working is a big clue, I wonder if someone made a full gasket for the cover? Have you tried blowing through the vent?
  It might be a perfect storm of gas in the oil, case not venting, detergent oil and rings not seated.

[grcamna2]

The top vent not working is a big clue, I wonder if someone made a full gasket for the cover? Have you tried blowing through the vent?
  It might be a perfect storm of gas in the oil, case not venting, detergent oil and rings not seated.

I think that's a good point Don;a gasket may be blocking it.

[Salman_z8]

The top valve cover vent is not blocked.
However it doesnt spit any oil from the top, nor i can feel any excess pressure coming out.
Everything ends up in oil, regardless of which oil is it.

[Salman_z8]

Rings not seating, bores being less than perfect, no clue about the mileage when re ringed, possible too  much taper or too much out of round, i find very plausible.
And i am not convinced its coming from the valves.

May be the increased pressure from the oil pump, coupled with the excessive blow by, is the recipe for the perfect storm in the oil tank.

[Don R]

 I thought of the suction line from the tank to the pump sucking air but can't imagine where or how it would let air in and not leak when parked. I was handling an oil pump once when the casting fell apart in my hands, I only took it out because of curiosity, I'm glad I did. I think it was the thread on the relief valve part.
 The oil pump in our dragster lost a soft plug from the pump body once, it came out on the drain plug magnet. We dropped the pan and pump, put it back in and staked it with a punch. My engine builder said just add a quart and run it, if it breaks, I'll build you a new one! 

[Geoff Hastings]

If the rings are good ,which the compression test shows they are ok and, this also rules out a head gasket issue. Worn valve guides tend to cause excess smoke after the over run as the vacuum in the inlet sucks the oil down the valve stems. A good oil will have anti foaming agents added so even if it has a high detergent content it should not cause frothing. The sooty plugs are a red herring, in the garage you’d never hold the engine under load long enough to change the plug deposits. At this stage, if the crank case breather is functioning ok then I’d be looking at where the air is being added to the oil, I’d go for the pump seals, a long shot could be the sump pick up that feeds oil to the pump, the tube could be cracked.

[Geoff Hastings]

I keep seeing excess blow-by being cited as the cause of the oil frothing, the compression test seems to me to rule that out but, if it did have excessive blow-by you’d have smoke and pressure felt at the oil filler or the vent pipe from the back of the cases.

[Salman_z8]

Hi All, took me a while to get to this.

Did another compression test before disassembly, 92-95psi cold across the board - ( engine had sat for a couple months )
Took the engine out and started disaasembly.

Alot of soot in the combustion chamber considering its only been run 1000 miles.
Intake ports are clean, and stems clean.
Exhaust ports are sooty, #1 and #3 have oily guides. Shiny sticky spot through the soot.

Piston tops have carbon, and oil burning deposits, this engine was surely burning oil. All 4 cylinders, have exactly the same buildup.

Pistons do not seem sloppy in the bores. Pistons are not showing signs of blow by, atleast not on the skirts.
Bores are glazed, but i can see the cross hatch very visible, under the glaze.

#1 Ring end gap is measuring .016 goes but .020 doesnt.

i also had a spare set of new std rings, and i measured the ring gap, in all cylinders, same reading as above.

i have not yet taken the valves apart.

i only took off the top ring off of #1 piston, they are the correct side up. The ring edge where it rides against the cylinder, doesnt seem to be touch at full face, only 50% of the edge seems to have made contact.

( Rings lands being worn ? )

The head gasket was covered in oil, but that no signs of blown gasket, or exhaust residue or discolouration, across the head deck or the cylinders.

I am more confused now than i was before i took it apart.
I was thinking, that i will find obvious blow by marks past the rings, or carbon by the valve train, ( Leaky exhaust guides )

I did find some loose carbon like particulate in the valve cover breather cover, and in the casting pits by the cam ends.

any ideas ? i will be calling the machinist folks tomorrow, to see if they can help measure things up.

[HondaMan]

I put in the automotive oil, 20w50, since that was sitting in shelf, and i just didnt want to do any diagnostic work with that dirty oil in it. Sorry i f i have confused you

That's likely. the issue. Castrol's automotive oil is high in detergent and has ZERO zinc. Both will make it froth.

[Salman_z8]

The bike was burning oil before i changed the oil, and frothing up the oil even prior to castrol 20w50.
The combustion chamber is sooty with oil deposits. Oil was certainly making its way into the combustion chamber.

Lets assume its not the oil, what else should i be looking at or for ?

Cylinder deck not being true ?


Rings not seating fully ?
Worn valve guides ?

I feel like i am more confused now than i was prior to disassembly lol

[Salman_z8]

I put in the automotive oil, 20w50, since that was sitting in shelf, and i just didnt want to do any diagnostic work with that dirty oil in it. Sorry i f i have confused you

That's likely. the issue. Castrol's automotive oil is high in detergent and has ZERO zinc. Both will make it froth.

Appreciate the input HM, but all the problems exhibited were present with and without castrol 20w50

[PeWe]

Thoughts....
K1 heads Ex guides have no oil seal. It will take some oil.

More if oil level is higher inside valve cover so oil can overflow the higher non sealed K1 guides.
Must be possible on cyl 1 when parked on sidestand leaning to that side, more oil goes there.

Thicker cyl studs and headgasket with tight oil return holes might make oil return slower and oil level higher.

If no oil restrictors mounted (forgotten by a PO) oil must flood extra.

[Salman_z8]

With the amount of oil that was on the headgasket upon disassembly, this very well could have been just a very slightly internally leaking headgasket.
Cylinder deck has two low low spots, .002, and .004, between 1and 2, and 3and 4 respectively.

The smoking might have been from the exhaust guides, the pressure could be the gasket.

I probably should have tried torquing the headgasket first, but impatience got the better of me.

Does this seem plausible ?

I will probably change the exhaust guides to seal type regardless

[PeWe]

I have an extra K1 head that will get new guides and valves.  I have all parts, just the job left.

Best to do while my local shop's mechanic is till working despite soon 73 yo.
I do not know another shop nearby that know these heads.

A fresh head make a difference.

[HondaMan]

Both the cylinder deck and the head's surface must be milled flat following disassembly in most of these engines. The #1 and #4 cylinders warped as the engine broke in (first time around) - this is a 100% 'statistic' - and when taking the head off and replacing the gasket without milling them both flat again, they seldom seal well, if at all. In addition, the modern head gaskets are too thick at 1.00mm thickness (some are even thicker!) while the OEM head gasket made from asbestos was 0.8mm thick. The use of newer head gaskets requires than not only should the cylinder deck be milled flat, it must also be milled down 0.010" for a 1.0mm thick head gasket, or correspondingly more if some of the thicker MLS head gaskets are used (some of those are up to 1.6mm thick).

I also have some thicker O-rings for the 2 oil ports thru the cylinders to the head: if you don't already have some of these, PM me your address and I'll send you a pair. They are 2.6mm thick instead of the 2.4mm in the gasket kits, which were only correct if the head gasket was 0.8mm thick, which it isn't. This makes up the extra thickness so as to apply full sealing pressure to the oil ports. This site is the cause of the many "oil-leaks-out-the-front-of-the-engine" issues you'll see in build threads for these bikes. But - this won't make the oil foam.

Oil foaming comes from either detergent oils being used, or blow-by down the sides of the pistons, thru the rings (or both!). In these engines (all SOHC4 and most DOHC4, even the wonderful CBX Six) the outer 2 cylinders warped into an egg-shape by about 8k miles or so, stopping the increase by around 10k miles. This means the outer 2 cylinders are ALWAYS egg-shaped: I have never seen this fail with one exception, which is: 750 Hondas sporting the [rare] Vetter Phantom fairing (full-length, full-width predecessor to the Windjammer) of which Craig Vetter made only a few hundred, in which case the outer cylinders' airflow was blocked 25% around the outer sides. This was by design, which would take a long post to explain. Those are the ONLY multi-cylinder transverse bike engines I have ever seen that didn't warp the outer 2 cylinders.

The upshot of the warping is: when the cylinders are then just honed and new rings installed (or just new rings installed sans honing) they never seal (at least in the many times I have seen this done, but quit doing it myself in 1972...). If, instead, the cylinders are now bored (less than 1.5mm bore increase) the holes will stay perfectly round forever. Then they don't experience blowby of the rings.