Thick Blue-ish smoke 1978 CB750F - head gasket? Rings?

[conradcb750philly]

Hello forum,

I need some advice from the experienced folks on a blue smoke problem.

I purchased a 1978 CB750F Super Sport last week. It has 29k on the clock.

The previous owner had it for four years, and it seemed to run well when I gave it a close look before purchase, but I think I have discovered a (somewhat?) (serious?) problem.

Here are the symptoms:

- blue-ish smoke from the tailpipe that is clearly visible in bright daylight
- smoke is subtle when engine is cold, and gets worse as the engine warms up
- smoke gets thick under load/acceleration
- the front of the valve cover has some seeping oil, at the edge under the round caps where you adjust the valves. It is not clear if this is related, or if this is just a coincidence. The oil does not seem to be from the round caps themselves.

Based on my internet research, I believe I may have either of these problems:

- a failed valve cover gasket
- some bad piston rings/oil control rings

A few specific questions:

1) How serious is a blue smoke symptom on an air cooled engine? Is it a terrible idea to ride the bike a few miles in this condition? Am I doing damage to the engine? Some internet forums suggest that this is risky for the engine, but that seems to apply to water-cooled engines, not air-cooled engines.

2) Is oil seeping around the front of valve cover (near the spark plug holes) likely related to the blue smoke, or just a coincidence?

3) Am I correct that for a bike that is over 35 years old, with old 30k miles, an old gasket is much more suspect than bad rings on the pistons? Or am I making some wild leaps of logic here?

4) Given the age and relatively low mileage, should I first try to replace the valve cover gasket? Is seems that would be much easier than dealing with pistons.

5) Is a valve cover gasket replacement a pretty reasonable DIY project for somewhat who is comfortable, say, adjusting valves?

In case anyone can speak to their experience, I would greatly appreciate it! I'm somewhat mechanical, but this feels like I'm getting into more complicated work, and I wanted the wisdom of those who have gone before me.

Thanks kindly,

Conrad
West Philadelphia PA
1978 CB750K Super Sport with 30k

 

[amc49]

Could be bad valve seals too. Mileage that low I'd think rings were OK unless the mileage is fake or abuse of bike. Bear in mind if rings you are eating oil all the time, it will be lowering in the engine, next you'll be out of oil and stuck on side of road. If rings you should possibly have some abnormal blowby at the crankcase vent hose off top of motor. Compression test to see if the rings. Your plug on the errant oil burning cylinder will tend to foul pretty fast. Check plugs to see how many have the problem.

Valve cover leak is a separate problem likely.

 

[dirtdigger]

It has worn exhaust valve guides, just about guaranteed. The 77,78 had issues with valve guides and valve spring retainers that wear and pull through causing the valve to drop. They have been know to start burning oil at 10000 miles so your mileage is not unusual to have oil burning. My suggestion would be to send the head to Cycle X and have them do the upgrades...iron valve guides and new design valve retainers.

Either way you have to pull the engine even to replace the valve cover gasket.

 

[conradcb750philly]

amc49, thanks. I'll do a compression test, check the plugs and look at the vent hose.

Just curious, if it's bad valve seals, is that something that a moderately competent DIY'er could handle? I'm looking online, and it appears there are valve "seals", as well as "guides" and they are not the same.

thanks.

 

[conradcb750philly]

Thanks dirtdigger. Am I correct that if a compression and leakdown test both look good, that means the rings are NOT suspect, and I should be looking toward valves? I'm just scared about the $$ involved in sending to Cycle X!

Is there anything that I would be able to visually see when I remove the round "port covers" over the valves and look at the area where valve adjustments are done?

Thanks.

 

[amc49]

Nope. Nothing helpful with your issue anyway.

I'm more of a DOHC guy-the guides themselves don't wear but the seals cook pretty quick on those. Yours being SOHC may well have a guide issue like he says, they are rocker type and the rockers sideload the guides a bunch to wear unlike the DOHC. And those years were known for issues with them.

10K miles and already worn guides...whew! But then the DOHC could burn valves in less than 5k If you got one with specs off a bit.

 

[dirtdigger]

No you cant check anything by looking through the valve adjust holes. You got two options ride it the way it is or tear it down and fix it. It is what it is. $600 would be close. APE charges $700. I would go with Cycle X