Motorcycle Repair: CB175 oil burning question, valve stem seals, honda cb175

Question
Hi, Bill. Have a puzzling oil burning problem on a 73 Honda cb175. The left cylinder smokes a lot, particularly if the engine is left running while the bike is on the kickstand--it really starts to pour out then. Seems simple enough...except that when the cam chain went out at 12k miles, I had a full top end rebuild done by the Honda shop at the same time--rings, valves, springs, guides, and so on. Yet after all that I have this smoking problem, which the bike also had before the rebuild. I adjusted the left side carb float so low the bike can barely run, so THAT'S not the problem. Somehow I'm getting oil leakage on that side, but can't figure out why. Is there something about the 73 that this is an endemic problem? I noticed a low-mile '73 on ebay was noted by the owner as also having some oil burning problems. Any ideas? I'm stuck.

Answer
Vince... ALWAYS hone cylinders when installing new rings. The walls get glazed and the new rings have a tough time catching hold and making friends with the cylinder walls. Make sure that rings were installed with the markings UP, too.

Bill







Vince, perhaps the valve stem seals were not changed before and during the engine repairs. You mention "guides" though, so that may not be the case, unless they reused the seals.

http://www.cmsnl.com/honda-cb175-super-sport-175-k7-usa_model423/partslist/E04.h  #9
on the exhaust valve side, but that doesn't usually account for huge volumes of smoke.  

The valve guide could be loose in the head, as well, but it sounds like either the cylinder bore is not round (you didn't mention reboring the cylinders), is tapered or the rings are either broken or perhaps upside down. Were the cylinder bores honed properly at a machine shop?

Do you have a lot of fumes coming from the breather tube, as well?  I am assuming that you are talking about gobs of blue smoke, not black smoke, which is fuel related.

Lowering the float level down will only make it liable to piston seizure, which will of course create oil smoke. Make sure your carbs are clean and jetted to specifications.

I hope you took your time in allowing the new rings to seat in properly, which takes about 500 miles of easy riding.

You can have the shop do a leak-down test to see where compression is being lost, if you have different readings from left to right side or the values are too low. If compression readings are both even and about 175 psi, then it is more likely about cylinder bore condition. When the bores are not round, the rings will rotate around until the gaps all line up, which will aggravate the condition. Initial ring gaps should be around .006 when everything is new and to specifications.

These small bikes work hard to keep up with today's riding needs, plus the fuel quality is poor in comparison to when the bikes were built. I am finding that there is a need to re-jet vintage Hondas up about one step, to compensate for alcohol in the fuels now. I am not aware of any tendencies for 1973 175s to be oil burners.

Pull the motor and have the shop recheck their work. You probably need to have the cylinders bored and new over-sized pistons/rings installed.

Ensure that both head gasket surfaces are clean and flat and that the cylinder liners are secure in the cylinder block.

Bill Silver