Topic: Oil change

[Bodain]

The first CB I worked was a 1975 K unit. When it was time to change the oil I droped the drain plug on the bottom of the engine. About 3/4 quart came out. At the time I thought very odd. Then I dropped the oil filter and I got the amount associated with such... At the time I was a bit confused because I had no experience with a dry sump. A few calls and I learned I also needed to drain it from the oil reservoir. Ok I got the experience I need for that situation... All was well...

Time passes and now I'm working a 78 F... This is different in that whenever I drain the oil. The majority of the oil comes from the engine instead of the oil tank as I would expect. I just thought this difference odd and worthy on mention.

The other odd thing is...Bike is running, runs great but smokes real bad and it's not the valve guides. So the bike is up to operating temp and I check the level in the oil tank. It's at a correct level. Again we have good oil pressure bike runs great, but poors a huge amount of the smoke out the exhaust pipe. If I remove the baffle it's just saturated in oil...

Now I let the bike sit 3 to 4 days... Today I'm pulling the engine again. I remove the oil. First thing I do is drain the oil tank. To my suprise less than half a quart in it. The majority of the oil is in the crankcase.

So 3 days ago the oil level in the oil tank was full. The bike hasn't been run since. Today the oil tank  level is much lower than I expect....

It almost appears the oil is moving from the tank to the engine as it sits??? Comments...

If the level in the crankcase was excessively high, this seems like a possible cause of all the smoke???

When you change your oil. Where does the highest volume of oil come from? Oil tank or crankcase?

I'm preparing total upperend disassembly chasing this smoking problem. I'm was planning to focas on pistons / rings since I've had valve guides replaced.

[Bob Wessner]

It almost appears the oil is moving from the tank to the engine as it sits?

It is. You have a leaking check valve in the oil pump assembly. When the engine is shut off, a spring loaded valve is supposed to close and prevent the oil from the tank leaking down. There's a rubber tip on the valve, probably old and not sealing well. Mine has leaked down for more years than I care to think about. Many have this same problem. As far as I can tell, it poses no problems other than checking your oil level needs to be done immediately after shutting down or you risk overfilling.

[Raul CB750K1]

It almost appears the oil is moving from the tank to the engine as it sits?

It is. You have a leaking check valve in the oil pump assembly. When the engine is shut off, a spring loaded valve is supposed to close and prevent the oil from the tank leaking down. There's a rubber tip on the valve, probably old and not sealing well. Mine has leaked down for more years than I care to think about. Many have this same problem. As far as I can tell, it poses no problems other than checking your oil level needs to be done immediately after shutting down or you risk overfilling.

I learned that leason the hard way......

[Jonesy]

There's a rubber tip on the valve, probably old and not sealing well.

...that is, if the PO ever had it apart and remembered to put it back!

When the oil level is too high, the spinning crankshaft will whip up the oil into like a foam. This can contribute to heavy oil consumption. How do the plugs look? How do you know it's not the valve seals? (asking out of curiousity)

[Bodain]

There's a rubber tip on the valve, probably old and not sealing well.

...that is, if the PO ever had it apart and remembered to put it back!

When the oil level is too high, the spinning crankshaft will whip up the oil into like a foam. This can contribute to heavy oil consumption. How do the plugs look? How do you know it's not the valve seals? (asking out of curiousity)

Cause I just spent $327.00 for a valve job... It smoked heavy prior to the valve job. The smoking hasn't changed...

[goon 1492]

sounds like its not the rings as much as the crankcase filling up with too much oil causing it to foam up and will actually starve the motor of oil if it foams up too much,try checking compression it will give you hints of bad rings with bad compression, you might pull the oil pan too and check the check valve before tearing open the top end. just things to keep in mind, the fundamentals behind mechanics is a process of elimination so use that and these guys and gals great info. peace

[Bodain]

Ok... Good idea's

Since I have the engine torn down and looking at things. Here are some facts.

This was a stock 78 F with major top end problems. I found another 78 F that had RC engineering 836 pistons in it. Imagine my delight!  I installed them.... About 150 PSI across all 4 cylinders. I fire the beast up.. Smokes like a #$%*... Time passes... I get a valve job and replace all 4 exhaust valve guides...  Install the head again... Compression came up a bit as a result of the valve job... It's now about 160 PSI all the way across.  I fire it up...   Still smokes just like before. Still runs great.....

Hmmm... Today I pull the engine and disassemble. I'm actually getting good at this. From a running engine. I can now have the engine out in less than an hour.

The bore across all four cylinders is about 2.55  65MM   My measuring tools arent incredibly accurate. I'm just using dial calipers. The top of the piston measures 2.53, the bottom measures 2.550... The bottom of the piston appears to be slightly larger than the top. The top of the piston appears to be perfectly round. The bottom is slightly out of round .

Clymer manual states that if the wear limit between piston and bore exceeds .012 replace pistons and rebore.
I am indeed out of that spec at .020 difference. Is this causing the smoke??  I don't know. I'm just a shade tree mechanic. If this is the problem the cure gets expensive.

All the rings are free and nothing is broken... 2nd compression ring has notch down on all four pistons. Rings and piston look good.

I think oil can enter the combustion chamber in 3 places..  Valve guides, rings, head gasket.  We can rule out valve guides. I'm considering the the head gasket and mating head cylinder relationship. I  don't see how oil could leak from an oil passage into the combustion chamber  and still maintain 160 PSI.

High oil level?  Assuming the rings are good.. The rings should prevent the passage of oil up from the bottom even with a slightly high level... In my mind anyway.

The head gaskets is a used head gasket made of aluminum...

So. I've spent the entire day looking at it...

Yes the piston bore clearance is a bit big. I don't really have enough hands on experience to say this IS the problem.

I'm ordering a new non aluminum head gasket....If nothing else I'll try that and reassemble. The alternative is just too expensive this time of year.

[Jerry Rxman Griffin aka MuthaF'er]

I'd ALMOST bet you have 1 piece oil rings??!

[Bodain]

You loose the bet... 3 piece oil ring.

[Jerry Rxman Griffin aka MuthaF'er]

Owe you a beer. I was hoping I was on to something.

[bryanj]

All pistons are bigger on the skirt than the top and all quality ones are turned oval, both have to do with expansion when warm.

What do the bores look like? if VERY shiny they may be "glazed" by running in too gently--this was a problem with Kawi 1000 cure is new rings and glaze bust or lightly hone the bores.
Cant help on piston clearance Mike Reike is best there.
Did whoever did the valve job fit seals to the valve guides at all? as early 750 didnt have exhaust seals and they may have got confused?

[Bodain]

Measuring the pistons reveals they are about right... Slightly wider at the bottom and too my suprise slightly oval on the skirt of the piston, yet round up top...  The bore actually looks quit good.The exhaust and instake guides all have brand new seals. I spent the good part of yesterday eyeballing it all. Then I had a phone conversation last night with a gentleman in California.. One of his suggestions was the same as yours.. Lightly hone the cyclinders... This was last night. Ok I have a plan.
I even have a hone because I bought one way back when I first started this 78 F  mess with the intent of honing and putting in new rings.

So today at about noon I have it all back together. Time to start it up.... My expectations were to see a reduction in white smoke bellowing out the exhaust pipe. Even if the reducation in smoke was temporary I would have been happy because I would have seen the change and pinpointed the source. No such luck...
The minute is starts. The smoker is on...No hesitation, no warm up. Instant white smoke... Giving it throttle just increases the volume of smoke...

I let the bike heat up then cool for an hour.... Start it up again... Starts effortlessly, by the way... The smoker is on again.... Time to step away from this and refect...

It occurs to me... Years ago when I worked my first CB ... Was a 75 CB 550...I eventually got it running, but poorly..The compression was about 85 to 90 PSI... I got pistons and got it bored, but even in that terrible state it didn't smoke like this...

I'm starting to think there is something out of the ordinary going on here. Other than rings and valve guides.

[wardmoto]

Where does the crank breather line go on a 750?  That is your 4th source of oil ingestion.  Any progress since your last post?

[754]

Everything seems to point at the check valve.

I think/hope you meant.002 piston clearance which is about right. Honda manual refers to cast, you likely have forged pistons.. should have .002.

other than over oiling the crank and tranny, it may be oil starving at times..

order the pump parts, you can have them in and buttoned up in a couple hours..

[Sam Green Racing]

Clymer manual states that if the wear limit between piston and bore exceeds .012 replace pistons and rebore.
I am indeed out of that spec at .020 difference. Is this causing the smoke??  I don't know. I'm just a shade tree mechanic. If this is the problem the cure gets expensive.

Bodain, this mesuring problem has come up before but I can't remember the thread.
I take it you have inserted a .020 feeler gauge between piston and bore.
I think that the maximum wear limit of .012 is all around the bore, meaning you have only got .010 in fact and are within the limit.
Perhaps someone can explain it better than me.

Sam.