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high compression build problem
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Greg Riddle
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PostPosted: Sat Oct 19, 2013 10:48 am    Post subject: Reply with quote

just had another thought what if there isnt enough spring pressure to push the valve shut. maybe i just need to shim the springs
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clay
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1972 Chevrolet Nova

PostPosted: Sat Oct 19, 2013 9:16 pm    Post subject: Reply with quote

I've never replaced a valve guide in a motorcycle engine but mine weren't as large of a displacement per cylinder as yours and had smaller valves. I'm still betting you don't need to replace the guide. I agree with 10sec.et - a quick lap will show you where the problem is and I would guess valve as I haven't seen a seat problem either. Only bent valves I have seen the problem was obvious - usually owner never checked cam chain and it got too loose and jumped. On yours maybe you bumped it when you were putting it together and getting everything timed. Either way you are narrowing it down. As far as weak springs it usually shows up at higher rpm's. I have seen really weak springs cause problems on a car with hydraulic lifters but I think you have mechanical so I don't think that's it. Let us know what you find. Clay
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jeep_406
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PostPosted: Tue Oct 22, 2013 4:25 pm    Post subject: Reply with quote

I may be off base, like that's never happened before Shocked
Did you check how much piston to valve clearance you have. Don't know anything about bike engines. Didn't know if both cylinders share the same cam timing? Obviously if that was an issue, you'd see it on the piston.
I would think if you had low spring pressure it would rear it's ugly head at RPM.
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Greg Riddle
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PostPosted: Tue Oct 22, 2013 8:43 pm    Post subject: Reply with quote

jeep_406 wrote:
I may be off base, like that's never happened before Shocked
Did you check how much piston to valve clearance you have. Don't know anything about bike engines. Didn't know if both cylinders share the same cam timing? Obviously if that was an issue, you'd see it on the piston.
I would think if you had low spring pressure it would rear it's ugly head at RPM.


yes i did check piston valve clearance there was more than enough clearance. there was room for even more lift. each cylinder has its own cam and dont share any timing. what would it do at high rpm. and what would you consider high rpm. this engine has a 3 1/2 piston and a 4 1/8 stroke. it only runs to 5850 rpm
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clay
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PostPosted: Wed Oct 23, 2013 1:07 am    Post subject: Reply with quote

Two things happen with springs that are too weak. One it can basically toss the lifter or in your case rocker arm off the cam at the top of the lobe. From what I gather this isn't the biggest problem. The big problem lies when the valve closes and bounces on the seat. Once this happens you lose power and usually a lot of it. The once or twice I have "floated" the valves in something I didn't want to do it again. Not only does it lose a bunch of power it makes a sound that just isn't good for lack of a better term and makes you not want to do that again. There are other problems that can occur dealing with spring harmonics that cause dips in power but I don't think any of that is your problem. Clay
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Greg Riddle
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PostPosted: Sun Oct 27, 2013 8:42 pm    Post subject: Reply with quote

i was bs-ing with a neighbor that had built an engine or 2 for woods buggies. he said the whistling sound cumming out of the exhaust is a sign the valve is burnt. im now wandering if the jetting being so far off when i first ran the bike on the build. i started the jetting with a 225 main. the correct main jet ,since i tuned the carb, is a 245 main
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clay
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PostPosted: Sun Oct 27, 2013 11:20 pm    Post subject: Reply with quote

I wouldn't think your main being lean would cause and exhaust failure in your case. Here's my reasoning - usually a street bike spends very little time on the main - especially something that makes some decent power because you get going so fast - well - so fast. Now if you had a 250 that spends a lot of it's time with it's tongue hanging out then that's different. That was just my experience when I had street bikes - yours may be different because I really don't know how hard you run yours. Clay
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Greg Riddle
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PostPosted: Mon Oct 28, 2013 12:50 pm    Post subject: Reply with quote

clay wrote:
I wouldn't think your main being lean would cause and exhaust failure in your case. Here's my reasoning - usually a street bike spends very little time on the main - especially something that makes some decent power because you get going so fast - well - so fast. Now if you had a 250 that spends a lot of it's time with it's tongue hanging out then that's different. That was just my experience when I had street bikes - yours may be different because I really don't know how hard you run yours. Clay

Your right I don't spend a lot of time on the main jet but with the main being that lean it will also lean the mixture in the mid range where the needle controls flow.

What ever happened, ill be taking it part soon its getting colder here
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clay
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PostPosted: Mon Oct 28, 2013 7:20 pm    Post subject: Reply with quote

Agreed - main will have effect on the needle. I just don't think that is your cause as you primarily run on the pilot, slide cutout and just start on the needle during normal cruising. However I've been wrong before though and pretty much keep an open mind anymore so please let us know what you find. Clay
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Greg Riddle
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PostPosted: Thu Oct 31, 2013 8:22 pm    Post subject: Reply with quote

ordering parts tom. its november and it will not be long. will see when it snows
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Greg Riddle
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PostPosted: Sat Nov 09, 2013 7:31 pm    Post subject: Reply with quote

got all the parts in. supposed to snow tuesday and wednesday. if they salt the roads the riding season will be over and ill start teardown
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Greg Riddle
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PostPosted: Mon Nov 25, 2013 2:31 pm    Post subject: Reply with quote

its apart nothing obvious as to why the valve is leakig
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Greg Riddle
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PostPosted: Mon Nov 25, 2013 3:49 pm    Post subject: Reply with quote

found the problem. there is a bad dent in the valve seat. my initial inspection revealed a spot but wasnt sure about it. i started to lap the new valave and inspect the seat and it was more obvious. here it is



Last edited by Greg Riddle on Mon Nov 25, 2013 6:47 pm; edited 1 time in total
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Paul P
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PostPosted: Mon Nov 25, 2013 5:45 pm    Post subject: Reply with quote

That will do it. That shiny spot there right?
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Greg Riddle
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PostPosted: Mon Nov 25, 2013 6:45 pm    Post subject: Reply with quote

Paul P wrote:
That will do it. That shiny spot there right?


yes it spans across the entire seat face. i lapped it out and its ready to go back together. i replaced the valve seal retainer and clips.

im going to fab a test jig to see how it holds pressure

its nice to know i put it bag together correctly but it sucks that something else happened
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