Topic: Thar she blows (gray smoke)

[BlackMax]

This has been beaten to death I'm sure, but I'll make it quick.

What is the likely culprit when:

1) blows a big puff of smoke (gray or blueish) immediately on startup

2) then, at idle, constantly blows less.....but it's still there and probably more than
     you would consider "normal"

3)  Under hard throttle, it blows LESS or NO smoke, but when you back off the throttle, it goes back to puffing.

[WHALEMAN]

Valve guide seals? Dan

[Borkunit]

Valve guides/seals would be my guess too.

[MCRider]

As stated, guides and/or seals explains your symtoms. Oil runs down the valve stems into the combustion chambers when sitting. On startup , that oil blows out. It will soon get better. On accel or Deccel, it all depends as to whether more oil gets sucked in or held out by pressure/vacuum.

What bike?  Early CB750s don't have exhaust seals. Some high performance rebuilds leave them off, at least the exhaust side. They will smoke, though not bad if all parts in like new condition.

[BlackMax]

As stated, guides and/or seals explains your symtoms. Oil runs down the valve stems into the combustion chambers when sitting. On startup , that oil blows out. It will soon get better. On accel or Deccel, it all depends as to whether more oil gets sucked in or held out by pressure/vacuum.

What bike?  Early CB750s don't have exhaust seals. Some high performance rebuilds leave them off, at least the exhaust side. They will smoke, though not bad if all parts in like new condition.

It's a '78, and I think the previous owner had it set up as a "hotrod" bike.  I think the cam is hotter than stock.  Maybe it just doesn't have seals. 

My engine builder guy quoted me $750 labor to totally go through the head.  Is that reasonable or.....?

[MCRider]

Depends. On or off the bike. On or off the engine?

Does "go through" mean replace the guides and seals, or just the seals.

Does it include a high level valve job? Or just lap the old ones in?

Usually when you replace guides you have to recut the valve seats to get them to line up with the guides.

He may be just fine, $750 is probably fine for what he has in mind, I tend to think people want to be fair, but there are a lot of unanswered questions. If the head is off the engine, you could ship it to any of several specialists on the forum here. They charge based on the level of service. Sky's the limit.

[BlackMax]

Depends. On or off the bike. On or off the engine?

Does "go through" mean replace the guides and seals, or just the seals.

Does it include a high level valve job? Or just lap the old ones in?

Usually when you replace guides you have to recut the valve seats to get them to line up with the guides.

He may be just fine, $750 is probably fine for what he has in mind, I tend to think people want to be fair, but there are a lot of unanswered questions. If the head is off the engine, you could ship it to any of several specialists on the forum here. They charge based on the level of service. Sky's the limit.

He said "it sounds like seals...to do that, the top end of the motor comes off and all valves must be lapped and all seals replaced"   So, it sounds like there will be no valve guide replacement and re-cutting of valves.      He also mentions that the pistons get polished and all gaskets get replaced (I supply parts).   

While he's there, should I "upgrade" to valve guides and a re-cut of the valves (or new valves)?  Is it a good idea to upgrade the timing chain and tensioner at this time, or is that getting into a whole new can of worms?

[MCRider]

So he's pulling the engine too? As that's worth a few hundred probably.

Top end means pulling the cylinders, which opens the question as to putting new rings in.

He may just mean pulling the head, which is not the full top end.

He'll check the guides for wear and discuss replacing as necessary. If they are old style guides they don't have the groove for seals and they'll have to be replaced anyway.

Never heard of "polishing pistons" but maybe de-carbon (or de-coke) as the Brits say.

Cam chain requires bottom end teardown. Whole new can of worms.

[LoopsAndLogic]

Well, by the sounds of it, it looks like your valves are just fine.

If you have a puff of "blue" smoke only on start up, then yeah, your valve seals are showing there age.

But if your getting "white" smoke Only during idle and deceleration, then you have a ton of moisture in your
exhaust system.

Now when you have a water cooled bike, or a bike that has a separate transmission with Trans oil, then I would suspect your
burning one of those fluids, but since you don't have neither of those on you SOHC (it is a SOHC??) I wouldn't worry.

Take her out for a nice long ride (35mins +) and report back after that.

Cheers

LL

P.S. I had this problem on a couple of bikes including a VF500F Interceptor. Thought it was a water leak, but it went away after 2 days of riding. And it has never returned