1978 Honda CB 750 F running hot with mystery oil loss. No leaks and bike only smokes a bit when cold. help!?

Question

mileage is unknown but high I am sure. bike just tuned and runs very strong. Live in NYC with lots of traffic and I know it is air cooled and heat is a issue but oil loss is about 1 qt per 150 miles. This sounds excessive with no leaks or smoke to speak of. I am lost on this one.

Answer 1

Hmmm,,no kiddin'

Now,,the experts know all about Valve Seals and Piston Rings.

I guess cuz that's all there is to know about "burning oil".

So I cant argue with that.

Lets see 1qt @ 150 miles

At about 70mph,,,thats a half quart an HOUR?

Or,,at 40 miles per gallon,,,
That's a 16:1 Oil/Fuel Ratio,,
aka 8 oz per gallon

I do not know of Any Engine or any Oil that is smokeless at 16:1

And that's 2 Stroke engines,,,with a Burn on Every Stroke.

NOT a 4 Stroke,,,which combusts on every OTHER Stroke.
And just pumps Raw Oil into Exhaust on the alternate stroke.

4 Cycle Oil smokes 10X as bad as 2 Cycle oil.
I've run PLENTY of 4stroke oil in 2 strokes,,and had to deal with a Lot more than that.
The Smoke is HORRIBLE Dense

Therefore,,,I would expect Both OIL & Smoke to be pouring out the exhaust
at 16:1 at LEAST as bad as any 2 stroke does at that fuel/oil ratio.

Your Honda beats hell outa spark plugs any chance it gets,,that's a traditional norm for 750-4's

I seriously doubt it would be running very strong at all with about 8Oz's of oil running thru it with every gallon of gas it burns.

I rather doubt it would succeed at burning a gallon of fuel at all ,,before oil fouling the plugs.

I cannot recall if those things have Condensation DRAINS on the exhaust?
Honda may not have put them on the 4-n-2 Supersports exh 'cuz of the crossover tube.

Moot point I 'spose,,
that much oil in the pipes,,sweat drain or not,, it would be smokin' like a mosquito fogger machine.

Not to Even mention the SMELL,,,Gawd!!

The Light smoke when cold thing,,,,,
a Cold oil-burner smokes Worst at start up.
Once engine comes to Op Temp,,,it's better able to combust the oil.
But WARM Oil is a Nasty Mess in a Cold combustion chamber.
..........................................................................
Did you check your CHAIN?
Or inside the chain gard for wet oil?

They can get a countershaft sprocket leak and puke oil pretty good.
It goes onto sprocket,,,slings out to Chain,,,and slings Off of chain.
Much of what is caught inside the chain gard just drips off the back edge of the gard from road wind.
The Rest drips right back onto the chain>>some gets slung off>>some flys Back up into Gard,,where it drips off,,rest goes BACK onto chain.
The cycle repeats .
A Front Sprocket leak just gets Looped thru that cycle,,,disposing MOST of the oil from back edge of chain gard.

Since the Oil is being put on the Front Sprocket,,,as the gear Turns the oil is almost Immediately being carried DOWN on the chain,,then back.
Majority of what leaks gets slung off Bottom of chain into thin air.

Some engines' construction is such that oil puddles real bad from countershaft leaks,clutch rod seal leaks ,etc.
Recess & nooks and crannies in the casting in that area.

750-4 are rather "clean" in that spot.
Plus the sprocket is very far Back on the engine,,
and Beneath it its almost totally open down to the ground

.......................................
No Way it can be burning That much Oil AND Running Hot.
Engines just dont work that way.
The oil drops combustion temperature a Bunch.
Which is part of the reasons why Excess Oil Burning engines usually have very low power.
It's a Non Contributor to combustion,,,a Suppresant actually.

................................
Real Easy to see any smoke.
*Run the hell outa it down the block.
*Then,,,Make a U Turn.
You'll ride right back thru the Smoke Cloud,,,to whatever extent it exists.

Simple enough,,,and beats the heck out of turning Back to LOOK at 50~60~70 while You're going one way and "Smoke Cloud" is going another.

Valve Stem Leak?
That's easy.
A Compression test or Leak Down test cannot tell you Anything about Valve stem Seals leaking,or Worn valve Guides.

*Park it
Rev it up to about 6~7000 RPM in Neutral and Hold it there 15~20 Seconds or so to load up Top End with oil.
Then Open Throttle for an Instant ,,,and let go of it --just let it slam shut.

That fills the Valve stem area with Lots of Oil,,,
and it Spikes the Vacuum to Force sucking oil thru the stems/stem seals.

Then soon as it has fallen back near Idle,,
Pop Throttle Open and rev to 4~5~6,000 or so.
That gives the combustion chamber a Good Burn with plenty of AIR to burn "all that oil" ya just inhaled under HI Vaccum Deccel mode with Closed throttle an no air to burn it

It'll PUKE Smoke when ya hit the throttle,,if it did indeed suck a bunch past rings or valve stems.

....................
Run it at a High Idle for a minute or 2,,,,1500~2000 rpms.
Kill it and look at a couple of it's spark plugs.
8 Oz/Gal?? 16:1 Oil:Fuel Ratio?
The Plugs will Definitely be Oil Wet,,if not Dripping Wet.

Normal is Totally DRY.

..................
Go Ride it some more.
Wind it out in Low Gear,,,Shut the Throttle and let it Drag Down in-gear to Idle( or almost idle).
Thats a HUGE Pump of Oil to Upper End,,,followed by A Huge Vaccum over and extended time period.
Open Throttle Fully to see how much smoke it makes.
It should blow Opaque reams of smoke.
IF it did indeed suck oil in.

...........................
If you cannot see distinct indications it's actually BURNING oil....
Massive,Dense,Purple~Blue Smoke,,,

....Then Ya need to check Close for a Leak

Put it on a Hard Surface,,,On the center stand.
Let it run several minutes.
Put a Fan on it to keep it cool.
Gravity sez,,,if it Leaks,,,it's eventually gonna end up on the ground.

If it DONT leak,,, Put it in gear and run it on center stand.
Why??
'Cuz if it IS a countershaft seal leak,,,it May only leak when the shaft is Turning in the seal.

It will approximate actual Riding conditon of Shaft Turning.
And the Chain will sling an oily stripe on the ground

............................
IT MAY BE that it Is actually Burning that much oil.
I remain Skeptical.
But it IS Possible


But about the Only place that I know of on a 750 Honda
where Oil can "sneak away without a trace"
is Countershaft seal cuz of it's exit path,,,how & where it leaks.

Just my opinion,,,,which is meaningless.
But that's the first thing I'd check.

Leak or Not...Multi Vis oil aint what it's cracked up to be.

You might consider switching to straight 30 or 40 oil

I seriously doubt engine is actually Consuming that much oil,,just doesn't stand to reason.

POSSIBLE,,,but not very likely according to your description.

Good Luck with it,,hope it's nothing terrible.
Those are really cool 'ol bikes.

Answer 2

Piston rings are gone with a oil loss of that amount.Keep the plugs clean and ride it till you can find another bike.Same problem with my 1979 750F got over 100.009 before i had to get another bike.

Answer 3

just a guess. your rings are worn, or your valves seats are worn, and you are 'burning' the oil as it escapes past rings or valves into the combustion chamber.

You only see the oil burn when you start it, when you are moving, you are dissipating too quickly.


I would do, or have a mechanic do a compression check, and a "leak down" test.

I bet you will find the valve guides are worn.

Answer 4

Valve seals. not a bad fix just have to know a couple of tricks.

Answer 5

I would have to agree with Bob D

Answer 6

dump lucas in it, pure petro