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Motorcycle Repair/69 honda cl175



ANSWER: Pat, you didn't give me a lot of information to work with here. How much "rebuilt" work was done...?  Hone and re-ring the old pistons? Rebore with new parts? Cylinder head rebuilt??? How long has it been running since overhaul?  

If the bike has been sitting up for years, it is normal for the exhaust system to smoke for about 10-15 minutes, while it burns out old carbon, varnish, fuel deposits that collected inside.

Check the breather tube for excessive crankcase pressure and look at the spark plugs after running it for a few minutes. If the plugs come out dry and there is no excess breather pressure, the smoke is just burn-off in the exhaust system.

If the plugs come out wet with oil and there is crankcase pressure and smoke after 20 minutes of running, then there may be engine issues. If the cylinders were rebored and clearanced to about .0015" on the piston skirts, someone could have installed the rings upside down. The 2nd ring is a scraper ring, which helps pull oil off the cylinder walls, before it gets up to the top ring. A ring may have been broken during assembly, if the mechanic was not careful enough.

Loose guides or excessive guide-to-valve stem clearance might contribute to oil burning out the exhaust system.

If definite oil drops are leaving the exhaust system then something is mechanically wrong. If it is just wispy smoke and lessens as the engine warms up and gets some miles on the piston rings, then it may be nothing to worry about.

Do a compression check, too. Should be around 175psi on both sides with the throttle held wide open.

Bill Silver

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QUESTION: Bill, I had a new cylinder,pistons ,rings installed when the mechanic did the rebuild. Also the head was sent out to be checked by a shop that specialized in rebuilding heads and they said everything looked good. They also installed new valve stem seals at that time. I checked the rings to make sure they were installed right, compression was good on both cylinders. I installed the engine again and started it up,but after running for ten minutes it had oil drops coming out of the drain hole in the muffler. Everything looks good and all parts are new honda parts. The mechanic I used doesn't have any idea what the problem is.

Pat, I'm not clear if your bike is a "sloper" or an upright motor model, but it shouldn't make too much difference. Basically, my reply is the same as last time, considering that you didn't mention/address any of the items that I answered in the last question....  

This listing: #18 has the tiny little stem seals that were used on that series bike.
or  #17

I'll assume that these were the seals that were installed on the valves when the head was redone.

Backtrack...asking again.

Is the compression at 175psi or close/even on both sides? D8HA spark plugs coming out clean and dry? Any excess breather fumes/smoke coming from the breather tube out of the cylinder head? Does it smoke after sitting at an idle for 60 seconds? Does it continue to smoke once the throttle is turned on, under power?

You may just have leftover oil burning off the insides of the mufflers from the previous owner, before the engine was even rebuilt. If the bike's engine was worn/smoking before you got it, the oil accumulates inside the mufflers and it takes a good 20-30 minutes sometimes to get all of the old residues to burn out of the mufflers and header pipes.

Pretend the bike is just fine.... run it for 10 minutes, shut it off, check it over and run it for 15 minutes. Check again, run for 20 minutes. The engine needs to be broken in gently so the rings can wear into the cylinder bores. That is what the 500-600 mile first service is for these bikes, when they were new.

If oil is coming out of the muffler/exhaust, remove it and see if it is wet in the exhaust ports and header transition. If it is burning oil, you will see it on the valve stems/heads and the exhaust ports. If everything is clean and dry (as much as it can be with a fresh rebuild), then assume that the oil is just residue inside the muffler.

Pull the plugs out after it has been running for awhile to see if they are wet/oily or wet/black from fuel or just clean and dry. You can also look into the spark plug hole and watch the valves open/close, looking for signs of oily deposits on the back side of the intake valve and on top of the piston crown. If it is wet and oily on top of the pistons, then the rings are not seated yet or they have been damaged on assembly.

Bill Silver  

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Bill Silver


Need help with vintage Hondas from the 1960s? I am an expert with 250-305cc bikes in particular and most all of the other pre-91 models, in general. I do NOT claim to have a great deal of experience on Gold Wings, Cruisers, ATC/ATVs and dirt bikes.


I have owned/ridden/maintained Honda motorcycles for 35 years. I have written five books on Honda repairs and collecting. I was a service manager for two Honda shops back in the 1980s.

VJMC (Vintage Japanese Motorcycle Club) of North America

VJMC newsletter, as editor for two years and as contributing editor currently.

3 years auto shop in high school, teacher's aide in Automotive Technology in Jr. College, Diesel mechanic course in college, self-taught mechanic and automotive writer.

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