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Discussion Starter · #1 ·
this might be long,but please bear with me. my bike has 6000 kms,left my bike outside this weekend,tried to start it up sunday morning(temp was -3C,27F) and it started very,very hard. thought it was valves and checked them and they were good. dicided i would check my compression,the engine was cold,and got 90 PSI,put a little oil in the cylinder and got maybe 93-95 PSI. the bike has been using a little oil lately and can see blue smoke when you are really in the throttle hard. would that low of compression make it start hard? does it sound like rings?
now another question,if it needs a top-end rebuild,would you recommend a stock rebuild or go with a HC piston,cam,exhaust,filter and jet kit? do the timing chains wear on these bikes? i need this bike to be reliable and start good and run good at any temperature,even down to -20C.
looking forward to your responses, thanks
 

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G-Man
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I personally would do a wiseco piston kit and gasket kit stock size and stock compresstion if you dont wanna spend a ton of money
 

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If the oil did not raise it much it sounds like valves not cylinder.
How much blowby is coming ut the dipstill hole when running?
(Edit. guess that is a tough call on the blowby because I check my father in laws tonight and it seemed like a lot or air coming out the dipstick hole)

Sevice manual list the compression at 188 lbs at sea level so yours is way down. 163 is Min compression in the yamaha manual.
skidsteer.ca
 

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X2 on that. If you added oil and the compression rose up substantially, that would indicate a worn top end. If it was the valves the compression would have stayed about the same, but a 3-5psi increase isnt much. I would look into the valves first, maybe head gasket. No white smoke right?
 

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Discussion Starter · #5 ·
if i remember correctly i think there is presure there when i remove the dipstick and it is running. there is a little pressure on the crankcase vent line that goes to the airbox. i have no white smoke at all and have not put a drop of antifreeze ever. i dont recall ever seeing bubbles in the rad with the cap off either. i am kind of at a loss with this one. could i have jumped a tooth on the timing chain? could it be carbon deposits on the valves preventing them from closing all the way? man this is getting frustrating!
 

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See my edit on the blowby.
Do a compression test first, 163 lbs is minimum.
If it fails, I'd loosen off the valves and have someone crank it by hand listen for air escaping into the carb or exhaust pipe.
Or you could make a old spark plug or compresion tester so you can hook it to the air compressor and see where the air like to leak out.
Skidsteer.ca
 

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Discussion Starter · #7 ·
the compresion tester i have can also do a leak down test. i tried it once but i think my air pressure was to high. do you know what pressure to do a leak down test at. the problem was i could not keep it a tdc with the shop air hooked up.
 

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You don't need full shop pressure, I have never used a leak tester, just kind of rigged up my own, but if you can hear any air leaking past the valves into the intake or exhaust then you have a problem. This of course assumes you can hold it a top dead center "tdc" or you have loosened the rocker arms off.
A leakdown tester will always put some air past the rings, so you will hear some leakage into the crankcase too.
Skidsteer.ca
 

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Discussion Starter · #9 ·
well,i did a leak-down test and had a fair amount of air coming out the exhaust,and a little in the crankcase. took it apart tonight and found the exhaust valves are not sealing very well at all, and the cylinder has light scratches and one spot that looks and feels like a rough texture. going to take cylinder and head to machine shop tommorow and find out the damages.
 

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Discussion Starter · #10 ·
well,have to bore out .5mm and the shop said he could grind the valves and the valve seats. i was thinking about changing the timing chain,the guides have no wear and looks like the tensioner could still come out another half inch. would the chain be stretched with 6000 kms?
 

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Discussion Starter · #11 ·
well, its all back together. where i bought the piston they sent me a hc piston by accident so that is what i put. started it up last night and it started on the first crank,idled nice. tommorow is the big day ! have to break it in on a good ride. cant wait to see if that hc piston makes any difference!
 

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Well glad to hear its back together, was the compression down as a result of valves and ring problem both?
Hopefully it is good for at least another 6000
Skidsteer.ca
 

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Discussion Starter · #13 ·
haha,hopefully! i believe it was just the valves. the exhaust valves were not sealing good at all,and the intake were sealing pretty good with just a little leakage. i did the piston and rings because it was using a little oil,once i got it apart i found play in the wrist pin too.
 

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Discussion Starter · #14 ·
took it out over the weekend and seems to work really good !
 
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