Topic: Oil consumption/leaking, Crankcase pressure issue? Aftermarket Breather Filter

[Mandic]

Well I have had my 77 CB750F on the road for about a month now.  It has been running pretty well for me and leaving me with a smile.  However it has a bit of an issue.  The thing smokes once it warms up.  Not constantly but randomly it just smokes like crazy out the exhaust.  Bluish grey tinted smoke too.  The other issue I have had is the tach drive leaking oil.  I tried RTVing it(before I did the proper research and realized the seal is likely gone) and it still leaks a decent bit of oil out of there.  In 500 miles the bike went through 1qt of oil running 10w-30. 

The engine supposedly only has 12,000 miles on it, so for the rings to be that bad strikes me as odd.  However it did sit for quite a while before I got my hands on it and got it running again. 

Anyway I put 250 highway miles on it yesterday(70mph taching about 6,000 most of the way) and it went through a quart of oil in that time.  The only leak is the tach drive, everything else external on the engine was just resealed when I redid the bike. 

Compression was good last time I checked, though it has been a while so I can't say what the numbers were.

The only thing I can think is, the breather isn't breathing well enough.  I installed some cheapy PepBoys special filter right onto the breather box.  My factory breather setup was rotten and the plastic box was broken, so this seemed a better streamlined option.  This wouldn't be the first time I've seen an engine smoke due to excess crankcase pressure.  If I had the stock airbox I'd be tempted to install a port into it so there was a slight bit of vacuum on the crankcase.  As it is I am tempted to install a PCV system into the exhaust header and go that route. I've run this style setup with success on some circle track cars and it is fairly common in auto drag racing. 

Anyone think I am on the right track here?  My next step is an oil change with 20w-50 or straight 40 weight, however I don't really WANT to do either of those.  Also I need to get a seal for the tach drive and get that installed. But I cannot see a full quart leaking out of their in 250 miles. 

[MCRider]

In my opinion...

The tach drive seal is a no brainer. Just follow the tips here, and its a 5 minute fix. I call these a 5 and 5. That is: 5 hours, days, weeks, months, of procrastination for 5 minutes of work. Maybe a 10 and 10.

We got to remember that on our old bikes age is as imprtant often as mileage. Even with low mileage, its been sitting and you might have some stuck rings and/or dried out valve stem seals. These are the 2 most likely sources of oil blowing out the tail pipes. Each of these if leaking will pressurize the crankcase and cause smoking out the breathers.  dealing with the breathers is in fact off track, again in my opinion. You have to deal with the source.

So, short of dissassembly, I'd go to the autoparts and in the words of Click and Clack the tappet brothers "buy anything with "miracle" in the title." Truthfully, many of us have had great luck with SeaFoam, which should have miracle in the title, or Marvel Mystery Oil. It will free up stuck rings and freshen up dried out valve stem seals.

Ultimately, your looking at a simple overhaul. Remove the engine, remove the head and cylinders, re ring the pistons, dingleberry hone the cylinders, decarb the head, lap in the valves, replace the stem seals and reassemble. You'll have a spirited ride for as long as you want. (Of course you'll check dimensions etc for anything way out of whack, but I'd bet money you'll be OK).

Oops I see you "redid" the bike. What did you do to the engine already?

[Mandic]

Nothing internal to the engine, just external reseal on it. 

It had a valve or two stuck when I got it so I did put some SeaFoam in the oil already, however it wasn't a lot in there.  Didn't want to overdue it and risk wiping out bearings(first time I've used SeaFoam).  Cylinder 4 was misfiring and the pipe was cool.  I poured SeaFoam through the tappet covers and sure enough it came to life just fine. 

I pretty much figured I will have to end up overhauling the engine so I guess I will stick with that thought.  Winter project, since for now it runs just fine.  It is simply a bit embarrassing to see it smoke screening people, ha.  Keeps them off my tail though.

And I haven't noted smoke out of the breather, I was just thinking maybe the breather isn't letting the engine breath enough.  Which I guess would be simple to figure out, just pop the filter off next time it starts smoking.  It kind of strikes me like a valve seal leak.  In that it clears up when you wind it out.  So it seems like it is loading up on oil and after you rail on it the oil clears out.

[MCRider]

Nothing internal to the engine, just external reseal on it. 

It had a valve or two stuck when I got it so I did put some SeaFoam in the oil already, however it wasn't a lot in there.  Didn't want to overdue it and risk wiping out bearings(first time I've used SeaFoam).  Cylinder 4 was misfiring and the pipe was cool.  I poured SeaFoam through the tappet covers and sure enough it came to life just fine. 

I pretty much figured I will have to end up overhauling the engine so I guess I will stick with that thought.  Winter project, since for now it runs just fine.  It is simply a bit embarrassing to see it smoke screening people, ha.  Keeps them off my tail though.

And I haven't noted smoke out of the breather, I was just thinking maybe the breather isn't letting the engine breath enough.  Which I guess would be simple to figure out, just pop the filter off next time it starts smoking.  It kind of strikes me like a valve seal leak.  In that it clears up when you wind it out.  So it seems like it is loading up on oil and after you rail on it the oil clears out.

OK then, you've got your answer I think. I wouldn't hesitate to put SeaFoam in the gas and oil per its directions. For ever. Many like to put it in the oil soon before changing the oil as they don't like to drive around with it in the oil. There is no warning on the can to this effect and I don't do it that way. But whatever makes it for you.

You've already had good luck with it, so I'd carry on.

As to rebuild you can go all crazy and spend a lot of money, but the simple overhaul I outlined will give you a crisp motor for a long time.