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

[Salman_z8]

In my efforts to try to help a buddy out, my curiosity got me intrigued and now i am scratchin my head.

Its a 1971 cb750, re ringed about 1500 kms ago, new seats were cut for the valves.
Runs good, decent bit of power. A touch rich, but i think the needles need to be dropped a notch.
I believe it has a cycleX rebuilt oil pump.

Compression is 115 psi all across.
Plugs are sooty, i am chalking it up to carbs being a bit rich.

But it foams up the oil as soon as it goes past 3k revs.
Even on my lift, if i hold the revs up, i can see oil all frothing up.

Eventually the breather system gets overwhelmed, and out the tank breather it comes.

Valve cover breather is clear, back of the case inlet is clear, oil tank seems all clear.

How the heck is the crankcase being pressurized to the point that oil aerates and increases in volume.

Its the same castrol 20w50 that i have used for years in my cb750's, so it aint the oil.

Anything i should look at ? if its worn guides, or too much crank case pressure, why does it not breathe out the top ??
All the blow by or vapour seems to be in the oil tanka nd coming out of there.

IS the oil pump over pressurizing ? worth it to replace pump ? or the three o rings ?
Doesnt seem like its the rings since the compression is decent.
No funny noises, no ticks .. oil smells terrible, like exhaust or gas in the oil .. which is why i thought of guides.

Perhaps i should just put a catch can ? This is not a bike that routinely sees huge mileage.

Any thoughts ? i am open to anything

[newday777]

"Oil smells terrible, like exhaust or gas in the oil .. which is why i thought of guides"

It could be worn guides, did you measure the deflection?
Or did you put in stock rings after honing? Did you check the ring end gap in 3 different depths of the cylinders? Too big end gap will allow blow by and pressure in the crankcase.

[grcamna2]

subscribed

[Salman_z8]

I didnt re ring it, thats how it came to me.
No clue about guide to valve clearance either.

Does any thing else come to mind or i can find answers once the top end is torn down, which i am trying to avoid.

Any other solutions ?

I am more than sure, it was probably honed, ( i can see the honing marks in the barrels ) and re ring'd.
From what i was told, guides were never replaced, machine shop had ok'd em, which usually works on diesel engines.

[Salman_z8]

Plugs show presence of oil for sure, so do the exhaust tips.
I dont see much for visible smoke. The exhaust guides do not have seals, that i know for a fact.

[newday777]

The only other possibility from the plugs is too rich mixture.

[Salman_z8]

I checked the float heights, they were all ok, number 3 was a touch too high, but not more than 2mm

I dropped the needle to #3, with 120 mains and 40 pilots

[Salman_z8]

Blown head gasket ?

[seanbarney41]

sump is full of gasoline?

[scottly]

sump is full of gasoline?

1+

[HondaMan]

If the oil rings are not seating (due to several possibilities) then the oil will froth.
If the cylinders were honed and new rings installed, something to look at in the K1 is: are they 1-piece oil rings, like RIK or MC brand? They need to be if the pistons are the OEM pistons. Installing 3-piece oil rings into the genuine Honda early pistons will not seal. The 3-piece rings are slightly thinner height than the 1-piece oil rings, making the ring-side-clearance value too big.

And, while rare (and recently new in the marketplace, it seems): there are some 3-piece ring sets being sold (eBay) for standard size pistons that do not match the K0/1/2 pistons that used 1-piece oil rings. I recently got some of these from someone who didn't want them (he used 1-piece rings in the overhaul instead) and 'twas a good thing he didn't try. They will probably work OK on the K4-later engines' pistons, though.

I've not seen the oil pump itself "froth" the oil unless the oil is a detergent type. You mentioned the Castrol 20w50 oil, it is also the "4T" motorcycle oil type? If it is the Castrol car oil, regardless of weight, it has detergent in it, which will foam. And, a fresh oil pump with new seals will make detergent oils foam a LOT. When these oil pumps age, the paper gasket between the top and bottom portions of it weep a bit, and if the oil is frothing with one of the 'leaking' pumps, less froth makes it to the tank. This seems counterintuitive, but is real.

 How much oil is sitting in the crankcase after the engine is stopped? The best way to measure this is to run it for at least 5 minutes (at speeds of 2000 RPM or more) and then stop and drain the crankcase to measure how much oil is sitting down there. If it is more than 3/4 quart then the scavenge pump side of the pump is having some trouble. This can happen from air being trapped in this half of the pump, which can be removed from the pump body by setting the pump in 2" of oil and rotating it until the bubbles finally stop exiting to the discharge port (that goes back to the tank). Be sure to rotate the pump gear in the same direction as the crankshaft.

[scottly]

 oil smells terrible, like exhaust or gas in the oil

[Salman_z8]

If the oil rings are not seating (due to several possibilities) then the oil will froth.
If the cylinders were honed and new rings installed, something to look at in the K1 is: are they 1-piece oil rings, like RIK or MC brand? They need to be if the pistons are the OEM pistons. Installing 3-piece oil rings into the genuine Honda early pistons will not seal. The 3-piece rings are slightly thinner height than the 1-piece oil rings, making the ring-side-clearance value too big.

And, while rare (and recently new in the marketplace, it seems): there are some 3-piece ring sets being sold (eBay) for standard size pistons that do not match the K0/1/2 pistons that used 1-piece oil rings. I recently got some of these from someone who didn't want them (he used 1-piece rings in the overhaul instead) and 'twas a good thing he didn't try. They will probably work OK on the K4-later engines' pistons, though.

I've not seen the oil pump itself "froth" the oil unless the oil is a detergent type. You mentioned the Castrol 20w50 oil, it is also the "4T" motorcycle oil type? If it is the Castrol car oil, regardless of weight, it has detergent in it, which will foam. And, a fresh oil pump with new seals will make detergent oils foam a LOT. When these oil pumps age, the paper gasket between the top and bottom portions of it weep a bit, and if the oil is frothing with one of the 'leaking' pumps, less froth makes it to the tank. This seems counterintuitive, but is real.

 How much oil is sitting in the crankcase after the engine is stopped? The best way to measure this is to run it for at least 5 minutes (at speeds of 2000 RPM or more) and then stop and drain the crankcase to measure how much oil is sitting down there. If it is more than 3/4 quart then the scavenge pump side of the pump is having some trouble. This can happen from air being trapped in this half of the pump, which can be removed from the pump body by setting the pump in 2" of oil and rotating it until the bubbles finally stop exiting to the discharge port (that goes back to the tank). Be sure to rotate the pump gear in the same direction as the crankshaft.

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.

I do not hear any piston slap when cold.
Compression was checked hot

[Salman_z8]

sump is full of gasoline?

Parked with fuel off.
I changed the oil as first thing, but i am leaning towards excessive blow by, or exhaust leaking via guides in the crankcase.

But that 115 compression, is confusing me

[Salman_z8]

And unfortunately, i cannot do any road testing, as we have full blown winter here in alberta.

I had an idea, to run a T, in between the tank and crankcase line, and add an extra vent with the house routed higher, to see if thats enough to get rid of the excess pressure.

If that works, i could possibly add a catch can with a return T’d into the rear case fitting.

I have had really worn engines in the past, more than 80’000 kms, showing 90psi tops, and never do this. Has this been encountered by anyone else ?

[Salman_z8]

Thank you everyone for your inputs, much appreciated

[newday777]

You have a problem.  It's winter.  Open it up and find out what the person did to it causing the foaming.

[Salman_z8]

 I got a couple projects going of my own 
But i guess there isnt much of a choice eh.

Such a pain to pull the motor from a nice shiny frame  :'(

Any other suggestions folks ?

[Salman_z8]

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

[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?

[PeWe]

Too much oil?
Some pumps leak oil down to the oil pan so level will be low until engine is fully warm, ca 20 min ride.
Fill up oil when cold will end up in too much oil.
Oil level in oil tank must be verified after min 20 ride.

[MauiK3]

Didn't I read that it's not motorcycle oil?
That won't do the clutch any good and it will likely affect foaming

[newday777]

Didn't I read that it's not motorcycle oil?
That won't do the clutch any good and it will likely affect foaming

Yes you did. Reply #12

[Salman_z8]

Didn't I read that it's not motorcycle oil?
That won't do the clutch any good and it will likely affect foaming

Yes you did.
However, the oil that has been giving this trouble for the last 1000 miles was castrol 4T motorcycle oil.

[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