QuestionHello, I have a few questions about rebuilding an older Honda engine. I have a 1972 Honda CB350K3. Bike and engine have approx. 17,000k miles on it(That is what the odometer reads). I bought the bike a few years ago with the intent to get it running again. When I got it home and went thru everything, checked points ,condensers, new oil, plugs, carbs rebuilt, valves adjusted and I was able to get it running. The right cylinder seemed to be firing great but the left not so good. It was burning lots of oil, oil would spray out the exhaust. I checked compression on both cylinders and they both were in spec. I figured that there must be a valve guide or seal leaking. (I have later found out there are no seals on the valve stems). So 3 years later I finally have the engine out and the top half disassembled. I do not see any thing immediately wrong. Valve stem diameters are within spec and the guides seem to be in good shape. There was a lot of carbon built up on the valves, piston heads, and combustion chambers. I guess my first question is where could the oil have been coming from? And my second question, since I have the top half apart what else should I replace. I did take the head and jugs to a local motorcycle shop that has a machine shop and they are going to check the head, valves and guides, and measure and hone the bores for me. They suggested at least a set of rings. Cylinder walls don’t look bad but I haven’t measured them yet. Thanks for your help.
AnswerKevin, Was your "carbon buildup" black fuel based or brown-ish oil based deposits?
How long did you run it after you rebuilt and replaced all the parts you have listed in your question? Sometimes, bike engines, which have been sitting for years can suffer from piston rings gumming up, which can cause some oil control issues. The bottom oil ring is the widest and most likely to gum up and stay in the ring land, unable to expand/contract and move up and down, in relationship to the piston's motions. If you have dismantled the pistons and rings, check each ring in the bottom of the cylinder bore and measure the ring gap. If the rings were sticky and hard to remove then the oil comsumption questions are at least partially solved. Excessive ring gaps indicate worn rings, due to wear.
As you have surmised, excessive oil burning can come from worn guides/valves, loose piston clearances, ring gaps with excessive specifications or some kind of crankcase pressurization issues (pinched off breather tubes, for instance).
If the bike was in a poor state of tune or poorly maintained before you got it, then there can be residues left in the exhaust port/mufflers that take awhile to burn out.
If there are any issues with carburetion metering or ignition system woes, then the misfiring on one side will wash oil off the cylinder walls and the intermittent firing will cause an rich fuel/oil mixture to be blown out the exhaust system.
Other misfiring conditions can come from air leaks at the intake manifold connections, condensers which are not grounded properly or are out of range. The ignition point faces must be absolutely clean and gapped to about .014" on Honda twins. The points should never be closed at the same time, during firing sequences, as the excessive current draw of two coils will cause misfiring issues. Check the primary coil wiring resistance values. They should be around 4 ohms. The spark plug caps should be about 5k ohms.
Finally, your question about what to replace with the top end off should include the camchain tensioners, guides and rollers. Check the timing chain for looseness on the sprocket. If the camchain is stretched, you can find a chain with a masterlink and replace the original endless one with that type, to avoid having to split the cases and remove the crankshaft in order to string a new chain around the crankshaft sprocket.
Make sure that your cylinder head and top of the cylinder block are FLAT before reassembly. This will help the head gasket seal properly.
Bill Silver
