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  #1  
Old 04-15-2009, 08:08 PM
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Big Bear Compression, My compression is fine.

Hi Track Share mighty and wise guru's. I'm just wondering about if anyone knows what the compression should be on a 2000 Yamaha Big Bear 350. (And now the story why....)

My dads machine when you hit the throttle backfires, and blubbers out. But, release the throttle and idles just fine. Roll on the throttle slowly and all is OK. He has done a lot including , cleaning then replacing the carb, replacing spark plug, replacing the stator, replacing cdi box (computer), replacing the coil and in the back of the machine the relay's etc, basically anything electrical. Can anything top end be the issue? Cam, valves, timing chain? He did tighten the timing chain. Are we chasing a ghost here, cause we don't have a clue. is this a rings, piston and valve issues? 5000 hard belair km's are on it (very dusty and sandy). Compression is 110psi on the cylinder.

Help? Any help would be very much appriciated and thank you in advance!

Thanks,

Mark
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  #2  
Old 04-15-2009, 08:43 PM
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110psi should be alright compression-wise,almost sounds like a timing issue,chain could have skipped a tooth?.
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Old 04-15-2009, 09:36 PM
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Outlaw brings up a good point. Check the timing. Is it possible the diaghram is not put in place correctly? What about a carb vent line? is it plugged?
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Old 04-15-2009, 10:03 PM
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Thanks so far guys, we shall check the timing chain, as for the carb vent line I will pass it on to pops!
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Old 07-30-2017, 02:26 PM
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I've got a 98 Big Bear 350 and the compression on the piston is only 45 pounds Does this mean the motor's SHOT?

**Edit: I used a better compression tester and found out the compression is at 90 psi which is better but still bad. So I'm trying to adjust the valves and having a hard time finding out how to set the cam at Top Dead Center on the compression stroke. Any Ideas???
Thanks so much

Last edited by RI Deer Hunter; 08-01-2017 at 09:28 AM.
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Old 07-30-2017, 05:11 PM
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Old 07-31-2017, 12:54 AM
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Spray brake clean all around the intake boot from carb to head while idling.Those intake boots are known to tear and cause air leaks.The main knozzle slide tube in the carb is probably worn out from the cv diaphragm needle causing the needle to suck to one side interfering in fuel atomization it will look egg shaped if worn.Replace both pieces if so.Remove the the valve caps and inspect valve clearance and cam lobe wear at time of checking timing.

Last edited by pipes; 07-31-2017 at 12:57 AM.
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Old 07-31-2017, 07:14 AM
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Quote:
Originally Posted by Shoemaker View Post
Hi Track Share mighty and wise guru's. I'm just wondering about if anyone knows what the compression should be on a 2000 Yamaha Big Bear 350. (And now the story why....)

My dads machine when you hit the throttle backfires, and blubbers out. But, release the throttle and idles just fine. Roll on the throttle slowly and all is OK. He has done a lot including , cleaning then replacing the carb, replacing spark plug, replacing the stator, replacing cdi box (computer), replacing the coil and in the back of the machine the relay's etc, basically anything electrical. Can anything top end be the issue? Cam, valves, timing chain? He did tighten the timing chain. Are we chasing a ghost here, cause we don't have a clue. is this a rings, piston and valve issues? 5000 hard belair km's are on it (very dusty and sandy). Compression is 110psi on the cylinder.

Help? Any help would be very much appriciated and thank you in advance!

Thanks,

Mark
Hey shoe buy yourself a 900 rzr and give the 570 to pops .problem solved
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  #9  
Old 07-31-2017, 08:06 AM
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lol. 8 years ago....I'm hoping he knows what it is by now. lol
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  #10  
Old 10-08-2017, 08:28 AM
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This thread is 8 years old but am piggy backing as I'm having the same issue. Machine not wanting to run and backfires.....

I bought a very clean Yamaha Big Bear from original family. The machine belonged to dad and uncle of seller, he tried to get it running and gave up. I jumped on the deal because the quad didn't appear to have been abused.

They replaced all filters along with a rebuilt carb and still wouldn't run. Its in my hands now, checked compression its good, valves adjusted.

Should I be looking at timing chain and read somewhere the "keyway".

The machine is a 94 Yamaha Big Bear 350 4x4.....

Thanks,
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