jared_bortel Posted August 16, 2008 Share Posted August 16, 2008 This is more a question of basic engine theory I suppose. Single cylinder 500cc four stroke engine burning oil after a rebuild. Original failure had wrist pin wallor out hole in piston cuased major piston to jug contact. Pulled top end off and cylinder off, 0 play in connecting rod to wrist pin. Ordered an over bore kit with a domed piston, machine work done by reputable shop that I've had experience with before. Get everything back and reassemble it, thing runs like a scalded dog. Go out riding runs great through 5 hr of serious mudholes, obviously wasn't broken in properly but that's not a cause of my concern. Changed the oil during the next weekend and went riding again, running well until about 3 hr into it, then it turns into a rolling mosquito fogger. Burns a half a quart of oil in 45min. I go straight to valve guide seals, ending up replacing whole head assembly with one off a friends bike. Reassemble and test drove new plug once again covered in oil and it's still smoking. We're trying to repair this damn thing but don't know where to go. Basic symptoms include loss of oil, no external oil leaks, no loss of power, crankcase breather has a foam filter over it that is completely dry. Bike that replacement head was sourced from was stolen and recovered so history is somewhat unknown but there is no way it could have over 80 hr total on motor. Piston rings and cylinder wall are known good, reinspected before placing new head and crosshatch is still present and not a scratch on it anywhere. Possible failure scenarios I've thought of but have disproven: Poor seal of ring to wall, should cause oil venting both into and out of crankcase therefore should have oil on breather and furthermore a loss of compression there by creating a noticable loss of power on a single cylinder engine. Piston cocking on up and down travel in cylinder, piston ring spun and stacked up the gaps in the two compression rings, or cracked ring. Shouldn't all of these, however unlikely, create the same symptoms listed above as well as show markings of some sort on the cylinder wall? Leaving the final answer of new head's valve seals are crap as well cuasing the same concern as previous head. Does this make sense to anyone else? I want to double check that I'm not missing something stupid. I will be running a wet and dry compression test after work as well as a cylinder leakage test, maybe that will come up with something I can use. Compression testing is just for comparison of wet to dry because I have no spec for this motor now that it's been overbored and had a domed piston popped in it. Quote Link to comment Share on other sites More sharing options...
Bruce Amacker Posted August 17, 2008 Share Posted August 17, 2008 I had a similar problem on a motor I did a few years ago in one of my cars. I pulled the (perfect running) engine out to detail the engine compartment and thought I'd take a look inside for S&G. Big bore ridge, so off it goes to the machine shop for a major- +.030", all new guides, cam, -.010"/.010" crank, the full meal deal. I assembled it with no problems and it smoked like a frigging freight train. The story gets longer but the problem turned out to be screwed up valve seals. The machinist couldn't figger it out, but it was actually very simple- I tore the motor down while it was warm and the valves were wet with oil to the touch between the seat and guide (under the head of the valve). This is an older engine that did not have room for umbrella style seals, so we cut the tops of the guides for positive style seals. Now it doesn't burn a drop between changes. Run it till it's hot, and figure out a way to look at the valves. I'm betting it's going through the heads somehow. Could the be a missing oil drain/baffle/tube/ or some weird widget that we're not familiar with in a car application? Mebbe check the parts guide really close for details. Good Luck! /forums/images/%%GRAEMLIN_URL%%/grin.gif Quote Link to comment Share on other sites More sharing options...
LARRY BRUDZYNSKI Posted August 17, 2008 Share Posted August 17, 2008 Jared have you pulled the oil fill plug while running to see if its building pressure in the crankcase? Maybe a plugged crankcase vent??? Quote Link to comment Share on other sites More sharing options...
Jim Warman Posted August 17, 2008 Share Posted August 17, 2008 I'm with Larry on this one.... The Japs seem to like having really complex things.... Could it be as simple as a misrouted vent hose of some sort... FWIW.... when it comes to things mechanical, I am usually first in line at the service manual counter.... My some wasn't exactly overjoyed when one of his Xmas presents turned out to be the WSM for his quad - but the pages are dog-eared and dirty.. Every scooter I've had... my gen-set.... you name it.... the number of times that a manual is going to have that one little line that saves your sorry ass.... How many times, at work, do we utter "how can I work on that without a manual?"? Quote Link to comment Share on other sites More sharing options...
Aaron Posted August 18, 2008 Share Posted August 18, 2008 How many times, at work, do we utter "how can I work on that without a manual?"? PARTS 'TILL IT STARTS! /forums/images/%%GRAEMLIN_URL%%/rofl.gif Quote Link to comment Share on other sites More sharing options...
jared_bortel Posted August 18, 2008 Author Share Posted August 18, 2008 I agree with what's been said, original problem was definitely a valve seal as backside of intake valve was loaded with oil. I have the manual for the bike and reassembled it as directed, however it's fairly simple so I didn't necessarily consult it for every step, but I read the procedure twice beforehand and once afterward. Compression and leakage tests were inconclusive as I only had a 5 psi jump with wet compression and less than 5% leakage. I'll pull the fill plug tonight and let you all know but I'd be very surprised to see that as I have no coresponding symptoms. I'm thinking that the new head may have the same problem. Not like there's ever been consistent failures with vehicles before. Quote Link to comment Share on other sites More sharing options...
DwayneGorniak Posted August 18, 2008 Share Posted August 18, 2008 Jared, what kind of oil are you running in it. A good buddy of mine is a Bike tech and said that if you are not running four stroke oil in it and are using automotive oil, it will foam and not cause the rings to seat. He just overhauled my other buddy's Foreman beacause he pooched the engine with automotive oil. Just an idea. Quote Link to comment Share on other sites More sharing options...
jared_bortel Posted August 18, 2008 Author Share Posted August 18, 2008 Off the top of my head it's SAE 40 weight, but I know that it's right from Honda's parts counter at the powersports store. Just a thought but with a few hours already on the bike and it running fine is it possible that the rings "unseated"? I've seen cylinders washout before but I've got good compression, which should eliminate that as a cause. Quote Link to comment Share on other sites More sharing options...
jared_bortel Posted August 20, 2008 Author Share Posted August 20, 2008 I think we've found the problem. Last night we had decided to remove the valve cover for fun. After popping the springs off the valves and using zip ties to hold them up, found no tension what-so-ever on the intake valve. It wasn't real tight on the exhaust valve but you could feel the seal holding it somewhat. Having 2 seals overnighted so we can ride this weekend. Thanks for the input. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.