Topic: CB175 smoking bad

[JScottB]

I've got a '71 CB175 that I re-ringed earlier this year and while taking it for a long ride today, my son who was following me said the bike was smoking really bad. As the bike was moving and under power it was hardly noticeable but once I got back into the throttle specifically after engine braking, the left cyl would leave a plume of smoking as if I was fogging for mosquitoes. After the bike cooled a bit I did a compression check and both cyls came in at 130 lbs however the spark plug from the left cyl was clearly oil fouled.  Being my comp is decent and it only smokes immediately following an engine brake, is this a clear sign of a bad valve stem seal? Keep in mind that even though the cyl head was disassembled, cleaned and valves lapped, new stem seals were not used because I didn't have new ones and the old ones still seemed OK however it is possible I buggered one of them up reassembling the head. What do you think?

[svenD]

 yes,when you dissasembled the valves for the valvejob you very likley reemed the stem seal,sorry!

[JScottB]

My nephew says it could be a valve guide seal, either way it's gonna suck because I'll still have to take the head off to fix it but I'm grateful it's not a broken piston ring gouging the sides of my cyl.

[bailboy]

Sorry guys but I had to use my kids username to log on because I can't remember my password for mine and my email has changed but I do have some news. I know it's been a while but I've FINALLY gotten around to taking the 175 apart to see what the problem was and it turned out to be the exhaust valve stem seals. I wasn't necessarily that the seals were bad but more so because I had installed them incorrectly. I had the seal on top of the seal cap rather than inside of it. The rubber O-ring was under the cap like it should but without the stem seal in place, the cap was never able to compress enough onto the top of the valve guide to seal the oil. I found it odd that the intake valve has no seal at all while the exhaust does (friom my experience it's usually the other way around) but every parts fiche I looked at for a '71 cb175, not a single one of them showed a valve stem seal for the intake however at the same time I was examining the parts fiches, that's when I realized I had the exhaust valve stem seals installed wrong and seeing how wet the exhaust valve were with oil pretty much explained it all. I'm still going to replace the exhaust valve stem seals with new ones because I really don't feel like taking this motor apart again just for a puny seal.

[grcamna2]

How many miles do you have on the new rings ?

[bailboy]

The rings are lucky to have 50 miles on them. Once I noticed the smoking after engine braking and realized I had to take it all back apart to fix it, I just parked it for another day because I had so many other projects in the works and the garage was a mess. I can't say the garage isn't still a mess but at least it's organized. My kid is home from college for the holidays and he seemed bored so I thought checking the 175 out again would keep him occupied.

[grcamna2]

While you've got it open it would be a good idea to bring the cyls. to a friend who has an accurate inside micrometer(or machine shop who respects you)and just double check that your cylinder bores are completely round...the old twins(according to HondaMan) sometimes have that problem and need a .5mm bore job to get things completely "round" again...just my 2 cents.

[bailboy]

While you've got it open it would be a good idea to bring the cyls. to a friend who has an accurate inside micrometer(or machine shop who respects you)and just double check that your cylinder bores are completely round...the old twins(according to HondaMan) sometimes have that problem and need a .5mm bore job to get things completely "round" again...just my 2 cents.

I appreciate the input and if this engine was a high mileage engine I would take your suggestion under serious consideration however the engine only has 3,300 original miles on it and is in excellent condition in regards to wear. The only reason I re-ringed it was because I was having issues tuning the carbs and with 1 cyl weaker than the other on compression test and I wanted to make sure both cyls were sealing as best they could as evenly as they could and re-ringing it did the trick because afterwards both cyls comp test jumped up to where they should be and both were even. Upon initial teardown it was evident from very faint stains on the cyl walls that some rust had developed due to it setting for so long before I got it but even before I first tore it down running with its old rings and 1 cyl weaker than the other, it didn't smoke at all engine braking or not. The only diff between then and now is that it has new rings that it's comp test is showing are sealing better than the old rings and I had installed the exhaust valve stem seals wrong and the way I had installed them made them unable to seal at all. As it stands now the cyl head is assembled and ready to be put back on the engine however my son just picked up a 98 cbr600 earlier this week and right now that's all he can think about so putting the 175 back together will have to wait just a bit.

[grcamna2]

I hope he's enjoying the CBR...is it his first one ?