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Unread 05-27-2019, 03:08 PM   #1
Not Yet Wild
 
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Default 1991 G8 - "0" Compression. Seals?

Hello all,
Bought this '91 - G8 blowing blue smoke a few years ago. I rebuilt the top end and went 0.020 over on the bore size. Sold it to a friend and it was running strong until just recently when it got sluggish, slowed down and died.
He asked me to look at it to figure out what went wrong. It turns over well (no bad knocking noises) is producing enough vacuum to dribble gas to the carb and has a strong spark. I then checked compression and surprisingly found it to be "0".
I don't think it's the rings or valves. I did notice when it was turning over it would occasionally cough and blow dust from the right side of the engine (side opp clutch). Would bad crank seals cause the a total loss of compression?

Any and all input is much appreciated.
SDR
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Unread 05-27-2019, 04:49 PM   #2
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Default Re: 1991 G8 - "0" Compression. Seals?

Check pushrods, one may have popped out of place.
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Unread 05-27-2019, 06:36 PM   #3
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Default Re: 1991 G8 - "0" Compression. Seals?

Quote:
Originally Posted by cgtech View Post
Check pushrods, one may have popped out of place.
Thanks. Will do when I get a chance. Is this a common happening? Would something be out of adjustment to cause this?
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Unread 05-28-2019, 12:02 AM   #4
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Default Re: 1991 G8 - "0" Compression. Seals?

Way out of adjustment, or gummed up intake valve sticking open are the likely causes. I wouldn't call it a common problem, but I have seen it more than one time (take into consideration that I been working on yamahas over 11 years, I seen a lot of things more than one time).
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Unread 05-28-2019, 10:26 AM   #5
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Default Re: 1991 G8 - "0" Compression. Seals?

Thanks for the info. What is the correct setting for the valves?
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Unread 05-28-2019, 11:02 AM   #6
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Default Re: 1991 G8 - "0" Compression. Seals?

Can someone confirm that both should be set at 0.004 from TDC? Thanks.
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Unread 05-28-2019, 11:08 AM   #7
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Default Re: 1991 G8 - "0" Compression. Seals?

CG Tech,
If they are gummed up what is the easiest way to clean them? Any issues if I pour some Marvel Mystery oil directly on them? That stuff is amazing. I had a pair of needle nose that was frozen from rust. Penetrating oil didn't help but MMO had them free in just seconds.
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Unread 05-28-2019, 11:42 AM   #8
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Default Re: 1991 G8 - "0" Compression. Seals?

Yes, both valves should be .004.

Best way to clean them is to pull the head, remove the valves, and clean them on a wire wheel. Since it's a pushrod motor, it's easy to pull the head off. It will take you longer to remove all the shrouds than it will to pull the head and clean the valves.

You may find that you have a blown head gasket as well
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Unread 06-09-2019, 07:46 AM   #9
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Default Re: 1991 G8 - "0" Compression. Seals?

CG Tech,
Well I finally got around to checking the values and you were right. The push rod on the exhaust had popped out. Is this common? What might cause this? Should I just put in back in and readjust the valves or pull the head? Also they looked pretty dry. ??
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Unread 06-09-2019, 09:54 PM   #10
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Default Re: 1991 G8 - "0" Compression. Seals?

Reinstall, set clearance correctly (0.010mm/0.004in), put a little oil on the parts and try again. Might want to remove and clean the breather valve on the side, if it's not working, oiling will be poor. If it pops out again, pull the head. Usually the gummed up one is the intake.
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