Topic: Oil fouled plugs on a '78 CB550

[jwalters]

Hello, now that I have my CB 750 running welll, I decided to take on a new fall/winter project.  A friend has a '78 CB550 which has oil fouled plugs.  She was told by our local honda mechanic that the bike needs a new head gasket, and from the looks of the oil leakage onto the fins, I would agree.  I did a compression test and all cylinders are in the 120-130 range (which seems ok).  I did not add a teaspoon of oil as some users recommend to identify bad rings. (I wish I would have)

Anyway, could a bad head gasket cause oil fouled plugs?  Or should I be looking for an additional culprit, such as valve gaskets, piston rings etc.

jesse

[TwoTired]

Your compression test proves that the head gasket is fine and that the local Honda mechanic cannot be trusted to properly diagnose the bike.  The oil weapage at each end of the cylinder bank is likely from the orings that are supposed to seal the oil galleries from cylinder block to head.  The heat makes these brittle with age and they lose their seal. Some weepage can be tolerated, but it will, over time, become annoying enough to address.   The o rings are not part of the head gasket.  But, the head does have to come off to replace the o rings.  However, this is an external leak, and does not effect the cylinder operation or put oil into the cylinder to oil foul spark plugs.

Who diagnosed the oil fouled plugs?  If it was the same Honda mechanic, you'd better check for yourself.  They could be only fuel fouled (soot).  Possibly from using the wrong heat range spark plugs.  The Honda dealer has tried repeatedly to sell me plugs for a CB750; D8EA instead of the proper D7EA.  The colder heat range plugs (D8EA) don't self clean in the Cb550 due to colder combustion temps, and that allows a buildup of deposits on the plug.  Other factors could be changes in exhaust or intake restriction that can effect mixtures and deposit buildup on the spark plugs.

Truely oil fouled plugs receive oil past the piston rings, or via a worn valve guide(s).  If, indeed, you are getting oil fouled plugs, then temporarily sealing the rings with oil during a compression test should show an increase in measured pressure.  If not, then perhaps the valve guides need work.

Your compression test numbers looks good to me.
Cheers,

[jwalters]

Thanks, I was just reading about the oil gallery o-rings.  I diagnosed the oil fouled plugs, and I feel confident that it is not the flat black color that is caused by richness.   I think I need to do the teaspoon of oil compression test as mentioned, to figure out if its a piston or valve issue.  Thanks!

 

[oldbiker]

A compression test does not tell us anything about the condition of the valve guides. If these are worn or if the valve guide seals are toast then oil can be drawn into the combustion chambers and be burnt onto the plugs.

[jwalters]

Correct!  But by adding some oil to the chamber before doing a compression test, I can see if the rings are shot.  Granted, both rings and valve guides/rings could be shot (in which case this test doesn't accomplish much), but I might be able to eliminate piston rings as a culprit.
-jesse

[jwalters]

By the way, do any of you have a good valve guide/seal replacement howto anywhere?  My service manual doesn't cover it at all.    To be honest with you, I am not completely sure I know what a valve guide is?   From what I've picked up from google  its the port which the valve sits.  Sounds like the constant movement of the valve wears away the wall of the cylinder the valve sits in, similar to the way piston's cylinders have to be rebored after they gain too much clearance.  I'd image this problem might take either some machining or possibly over sized valves.  Hope, its just the seals.... is this a common problem?  I know someones going to ask me milage, but I am not sure!

[TwoTired]

Assuming you know what a valve looks like, it has a head and a stem.

The valve guide is the part between the head casting and the stem which guides the valve in it's up and down excursion.  When guide seals are fitted they are located under the valve spring and seal the stem to the guide or head casting.  The seals are made of some rubber or polymer and gets hard and or brittle with heat and age.  Rubbing on the valve stem wears them, too. Leakage is then likely.  If the valve guide is not worn too badly, seal replacement is an effective repair.

Unless you have machining capabilities at home, replacing and reaming guides at home is not prcactical.  Also, finding new guides may need knowledge of one who is in the business.  Either make friends or pay someone to do it, I think.

The valve head rests on the valve seat when closed.  For aluminum heads this is a steel ring pressed in the a pocket in the combustion chamber and ground to an angle matching the valve face.

To my knowledge there are no oversize valves available for the Cb550 head, as there is no room for larger ones to fit.  They are already close to one another.  Better breathing can be accomplished by a cam of higher lift and duration, though.  Check out Megacycle for that.

[rsmith550]

I have a 78 550 and I also discovered a problem like yours, I tore the engine down to clean and replace all gaskets, o -rings and seals and found many valve stems had no valve stem seals!! That def. explained the "unexplained smoking and fouled plugs". Good thing I did not listen to the DIP of a mechanic at the local shop, That ass said the old engines are prone to smoke and without a complete rebuild it was pointless to try. In mechanics " real " words - " If you are not going to pay me HUGE in greenbacks I am not going to even pull the throttle!!!!!!!!!!!!!!!!!!!!!!!". - Ryan