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Yamaha Big Bear 350 carb problem


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ok.  I got the vin, but left the paper in my shop lol.  I put some WD-40 in the cylinder and retested compression.  now it al 80-90psi.  So it is rings, but also valves prolly.  I'm thinking it ran rich (carb flooding) for long enough that it built up crud in the cylinder and the valves arnt sealing properly and it blew the rings.  I just thought it would be smoking if the rings were shot....

but yea for those out there wanting to know if their low compression is due to rings, squirt/poor 1 ounce of oil into the spark plug hole and hook up the compression tester, then turnover once or twice by hand to make sure it's not going to hydro-lock, then test compression (with electric start).  if the reading is better, your rings are shot.

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owner gave me permission to dig in.  Head is off.  Inside looks exactly like the bayou.  I'm thinking rings, valves, and a gasket set?  Should I replace the cylinder and piston also or just rings?  Trying to do it as cheaply as possible for this owner.  But if its not a good idea just do rings and valves, I'll get the whole sha-bang

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ok.  how do you clean the piston, and also the head?  Its got black build-up and I'm worrying about the new valves sealing correctly.  I tried carb cleaner on the bayou with no effect... maybe oven cleaner?  I herd that stuff will cut through some baked on crap..  Maybe I could try it on another piston first (I still have the old bayou piston with crud on it)

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btw, thanks to quad crazy for making this forum easy to use!  my bike's derailleur hanger broke and I was trying to ask for help because I couldnt find the part online....  first the forum wouldnt let me post a pic till I had 10 posts (whats the purpose of that????)  then it wouldnt let me post more than 5 posts (and I was being as helpful as I could) in 24 hours...  it's like they are trying to make it as hard as possible to use their forum:aargh:...  THANK YOU to quad crazy for an easy-to-use forum!!

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update:  Rebuilt the bike with:

New piston

New rings

New top end gasket kit

New valves

Honed cylinder.

Compression rose from 60psi to 120psi

Runs great and a lot stronger.  Owner pleased.  I guess this is my first successful rebuild lol- I think I was losing compression again with the last one because I didnt replace the valves or hone the cylinder.

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