Motorcycle Repair: rear cylinder smoke on pipe tube, valve stem seals, air filter system

Question
QUESTION: hello-I hope you can help me i just repear the 2 cylinder heat complete -honey cylinder-the piston clean piston ring new on 2 piston carburetor clean the 2- mark time ok good start at first good very good compresion test 175psifront 175psirear is a honda shadow vt750c 1983-the guestion is Im finally the brake-inrun the bike on more rpm and start to smoke on the rear cylinder pipe tube-Im do acompresion test and is 175psi rear and front 175psi spark plus are sooty oily color dark brown tob black the bike rear cylinder smoke the 3 05 minute an stop-ron and not smoke more engine hot or lukewarn not smoke next day at first start yes smoke the 3 to 5 minute disappear the smoke the end of pipe tube oily but is not much the pipe tube how i to boil water excuse me english i hop you can help me thanks-gamalier

ANSWER: Gamalier, It sounds like you have done most of the repairs correctly.

Did you clean the piston ring grooves, so new piston rings can seat into the piston grooves fully?

Did you replace the valve stem seals on the cylinder head. After 28 years, the valve stem seals are hardened and not able to keep oil from running down the valve guides/stems and being drawn into the cylinders.

Oily spark plugs are getting oil past rings, if not broken in all the way or from valve stem seals. Also be sure that crankcase venting system is clean and clear, so you are not drawing old oil back into the air filter system. Oil smoke is blue. Be sure air filter is clean and installed correctly.

Black smoke and sooty spark plugs are due to excess fuel mixture (or lack of sufficient air). All carburetor air and fuel passages must be clean and clear. Carburetor jet sizes must be correct. Some V-twins have leaner jets on the rear cylinders than on the front one.

If you have any overheating problems, replace the thermostat and be sure to bleed all the air from the system at the bleed valve fitting, usually next to the thermostat housing. I hope that answers your question. I wasn't too sure about the "boil water" comment. Otherwise, I think I understand your questions. Break-in period for new piston rings is generally about 500 miles. Do not use synthetic oils until engine break-in is complete.

Bill Silver

---------- FOLLOW-UP ----------

QUESTION: hi bill Im look the oil on crankcase and the oil is black smell a burning oil Im think the crankcase is blocked off and the drain passages too to use a sea foam to cleare up  if not the engine out fron bike=Im back to more valve stem seas will leakin small amountsand its not that noticeable while engine is running after I shut the engine  down all the oil that is in top en has drain back down as hits doin this and part of it seeps in to cylinder in small amounts so that on startup theengine will smoke I THINK-SO I think that the engine has good conpression it has 175psi on both cylinder the piston and ring cylinder are good please  tell me if im on the right track abat what im talking  about I take out the engine again the valve steam seal ar new I THINK THE gasket set is no good is a cometic gasket set and the stem seal for this gaket set no good thanks you gamalier

Answer
Gamalier,

When you had the engine out before, did you notice large amounts of sludge and varnish build-up inside? Did you change the oil before you started it up again? If new oil is dark and smelling burnt, then there is some kind of overheating of parts, possibly from underside of pistons where the oil will turn black and get charred from heating.

Cometic gaskets seem to be okay, but I don't have a long history with them. They are making gaskets for me now for a 1959 Honda. A lot of racing teams are using them, as well.

While you stated that you changed the valve stem seals, did you check the valve stem clearances in the guides? If the stem-to-guide clearances are excessive, then even new stem seals will have trouble keeping the oil away. Oil should not be pooling around the valve guide/seals after the engine is shut off.

If you noticed that the ring gaps were all lined up on the pistons..... at least the top two, then the cylinder is somewhat out of round. When the gaps line up, there is more blow-by past the rings than normal. I will assume that you installed the rings in the right order and placed them right side up. Top ring is == 2nd ring __-- in the cross section.
Top ring is shiny compression ring, second one is the scraper ring. Marks on end of rings go UP when installed. Ring lands on the piston grooves must be clean.

If you notice a build up on the back sides of the valves with some soft brown deposits, that is usually a sign of valve stem leakage due to excess clearances or stem seals.

The bike will smoke for awhile during the break-in, especially if you haven't honed the cylinder on a wet-hone machine and checked for taper and out of round conditions of the cylinders.

Bill Silver