Topic: Same S*** different day

[1979CB650_Chris]

Hey, ok heres whats up:

So once I first got my CB650 running and finally firing on all four cylinders, I noticed a pretty decent amount of blue smoke at take off and anything over 5500 RPMs.  After driving it around for a while like that the smoke stopped! 

Today I checked the oil and spark plugs and noticed that the oil was a bit low and one of the plugs was pretty fowled.  So I changed the plug & added some oil and guess what, the smoke came back just like before.  Smokes at take off and over 5500 RPMs.

Im thinking piston ring, can I get a second?

[Lumbee]

...could be a number of things...worn rings...leaky vavle seals.  Either way it points to a top end rebuild.  Check u'r compression and see what it looks like.  If compression is good, then I'd look at vavle seals..

[BobbyR]

You may have stuck rings. If the bike has been sitting for a log period of time, a lot of stuff may be stuck. Use produts like Marvel and run it for awhile before tearing it down. It may need time to re break in.

[TwoTired]

Check your crankcase breather system for blockage/restriction.

Do a compression test, dry and wet, to check cylinder integrity.  Report both sets of numbers.  This should indicate if rings are required before taking it apart.

Cheers,

[Jay B]

Marvel is Marvel Mystery Oil. Good stuff, but I'd try Ring Free from Yamaha Marine. It's made to decarbon 2 stroke outboards, but it's safe in anything. I'd run a couple tanks through with 1/2 bottle of Ring Free in each before I yore it down. Just my .02

[BobbyR]

What I meant was to pour some Marvel Mystery Oil directly into the cylinders and let it sit. It has detergents in it. I would also use the Yamaha stuff. When you tear things down. Something always goes wrong to make you nuts. Try the easy stuff first, then if you have no choice tear it down.

[MRieck]

Do a 2 minute standing burnout....it will free anything that is stuck.

[scunny]

IMHO if it smokes more the longer it sits means you have got oil seeping down through your valve guide(s). Do the compression test.

[HondaMan]

The CB650 was a 2nd generation engine, done by the young engineers that replaced the Honda veterans who designed the 90, 305, and 750. They did 2 things wrong: 1. shortened the exhaust valve guides too far, which overheats those valves until they burn and 2. used wilder valve angles in an attempt to make the engine breathe better in the "flat spot" that the 500/550 suffered.

Both of these items make the valve guides wear and burn oil, starting at about 25,000 miles or so. The exhaust valves on the center cylinders frequently burn. This will make it smoke at higher RPM.

Don't wish to be the bearer of bad news, so don't shoot the messenger. My brother struggled big time with this in his 650 after 100,000+ on his 500. After 2 rebuilds, he gave up and sold the bike at 40,000 miles.

[TwoTired]

Why didn't you report that history at the outset?  With the low miles, it seems unlikely to be valves or guides.  It's probably rings stuck into the piston lands, or the cylinders were rusted and pitted from those valves standing open, which now prevents ring seal.
The compression test dry and wet should confirm or refute the bad news.

You might be able to free stuck rings with a good soak of MMO and another hundred miles of flogging the motor.  But, if the cyl. walls are pitted, over bore and new pistons/rings.

Best of luck!

[cb650]

Chang all the seals and use new gaskets.   Bought a bike "with new seals installed" and found out they only replaced 2 and just gooped the hell out of the old gaskets.  Bought it cheap so didnt bother me to much.  Even with low miles they can go bad with age.



                Terry

[Zeke]

Anyway, if i change the valve guides should I change them all?  And out of curiosity have any other 650 owners had this prob?

Chris

Hi Chris:

I was rebuilding my 80 650 this time last year.  When I bought my bike, it allegedly had 12000 miles on it.  I say that because the odometer said 4000 and the PO said the OD that broke had 8000 on it...

Anyway, I was commuting at freeway speeds about 60 miles/day, and my #1 exhaust guide bit the dust.  Everything was good before that.  I came out of work with the onset of the flu, hit the starter, buncha smoke came out of #1 -- had no choice but to ride home (didn't know what was wrong at the time).  When I got home, there was oil film all over the back of the bike, so I knew something was wrong.  And I was sick as hell, so I didn't care.

When I took it apart it was obvious it had already been apart once before, but who knows why.  The machine shop only replaced the one guide -- they had my shop manual and measured everything though to make sure it was still in spec.  Finding a valve and guide wasn't easy, but it was possible.  And, they told me that if they couldn't find one they had suppliers that could make one.

Because I ran it with the bad guide I had to have the valve seats all recut, because that one was beat to #$%*.  I checked compression before taking it apart and it was much much lower than the others.

So, if I was you I would check compression and if it's low stop running it now -- you might save yourself some additional damage and machining bills.

It cost me about $400 for the machine work, which included rebuilding the head, valve recut and lap, cyl hone, and bead blast of everything.  This didn't include parts.

All the parts I bought from my local honda dealer.  I was warned against aftermarket parts and gasket kits, and didn't want to be haunted by that.  I blew at least $600 on parts.  Valve, valve guide, rings, gasket kit, new carb boots(had to saw off the old ones), 4 carb rebuild kits......

The carb kits were a rip off -- 40/each.

But, I now have a kickass running old bike.  I'm not going to do a lot of freeway riding with it now(I moved) the engine seemed a bit strained at a sustained 70 - 80 -- I'm probably just a worry-wart but there's a notch where the motor gets alot louder.  If you need to ride fast maybe gearing would do the trick.

Cool, I'm going to go out for a ride.

hasta,

Zeke

[cd811]

my money is on valve seals, not guides

what I want to know is how bad is the smoke?...can ya live with it?

[cd811]

I understand the 

I guess I was getting at... ride it tell ya can do a head job...a extra head would be cool to build while ya ride, then teardown time would be short

[Ibsen]

If you find that the compression is good, my vote goes to the valve stem seals as well.

The smoking at idle is a good sign of that. Also if it is smoking after you brake the bike on the engine with the trottle closed, and then open the trottle suddenly, leaking valve stem seals are usually the problem.

[Geeto67]

everybody is saying compression test but I still haven't heard "leak down test" to test the rings. A compression test, test just that therer is compression in the cylinder, however it is still possible to have small amount of blow by and still be within spec compression. A leakdown test tests the ring sealing as well as the seals in the engine. Basically you  plug the intake and exhaust (either by rotating the engine till they are both closed or just putting a plug in the exhaust port and intake, then fill the cylinder with 5psi of pressure. It should hold 5 psi for about 30 seconds and then slowly drop down. If it doesn't hold pressure at all you have blow by. If you plug your intake and exhaust you can test the piston at different heights to see if the bores are ovaled as well.

[Jay B]

If you are doing a leak down test, do it with the valves closed, and listen. Listen to the exhaust port, intake port, and listen at the oil fill hole to tell where it's leaking You'll hear air coming out if it very bad.

[Ibsen]

I agree about the leak down test. It's a good way, even the best, to test for valve and/or piston ring leaks.
But if you take a compression test, a low reading indicates a compression leak, and then pour a teaspoon with engine oil into the plug hole, and then take a new test, then the two readings will tell you at least  something about how well the rings or valves are sealing. If here is a significant increase in the compression with oil in the bores, you most likely have got leaking piston rings. If the reading is low and there is no difference, or only a very small difference, between the readings, you have got leaking valves. And if both readings are good, leaking valve stem seals will be one of the causes for the oil smoke that I would consider to be very likely.