Author Topic: No compression....No reason?  (Read 1971 times)

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Wykydtron

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No compression....No reason?
« on: December 19, 2006, 04:41:17 PM »
Ok this one is just for the WTF factor.  Some may have noticed my other post "anyone need xs500 parts" where I describe my parting out of a yamaha xs500 that apparently had no compression.  I got this bike for free, the title situation is bad because the PO that gave it to PO that gave me the bike :P signed and dated it 1996.  In other words the guy I got it from never transfered the title to his name.  Whatever.  The bike leaked oil like hell from the alternator cover and between the oil filter and its housing when turned over, plus it had no compression so I figure hell, part it out on ebay, make a couple bucks.  Now I never did the formal compression test by hooking up a guage..didnt seem to be a reason to.  I could turn this thing over by pumping the kick starter with my hand using minimal force.  I also tested it by putting the bike in gear and pushing it with the clutch out and there was only slight resistance.  The plugs where in it and tight.  So in my mind theres only three things that would cause this:  bad rings, hole in the piston, or stuck valves/timing way the hell off.  I assumed that since It came to me with such a terrible oil leak(I mean it was pouring out when you turned it over with the starter) that the PO ran it out of oil and killed the rings that way.  So I'm tearing down this motor, valve cover comes off, valves look great from up top and the cam chain has good tension.  Break the cam chain and head comes off, flip it over, yep valves are fully seated, look great.  Now I get a look at the cylinder walls.  Push each piston down so I can see and feel inside, smooth as can be!  Pull the clyinder block and the pistons look great and the rings look ok too!???  Can someone tell me what the hell I missed?  Its no big deal cuz I got it for free, its completely torn down now and partially sold on ebay.  But damnit, this was a cool bike, almost all there and in great shape with 18k on it.  Did I make a big mistake by making assumptions?

Offline Sam Green Racing

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Re: No compression....No reason?
« Reply #1 on: December 19, 2006, 06:33:32 PM »
You did mention a possible cause but you didn't mention if you had checked it.

Perhaps the PO had been messing about, not knowing what he was doing and got the valve timing out a bit.

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Offline bryanj

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Re: No compression....No reason?
« Reply #2 on: December 22, 2006, 04:33:58 AM »
If this was the Yamaha 8 valve 500 twin the only better thing you could have done with it would have been to use the optional spade from the toolkit to dig a LARGE hole and bury it! They were never reliable, leaked like a sieve and not quick!
Semi Geriatric ex-Honda mechanic and MOT tester (UK version of annual inspection). Garage full of "projects" mostly 500/4 from pre 73 (no road tax in UK).

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Offline 750goes

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Re: No compression....No reason?
« Reply #3 on: December 22, 2006, 07:05:46 PM »
Lousy valve timing will have a fair impact on compression readings, also if a motor has been sitting for a while rings can get stuck to a degree which also does not help...
were the carbs off or jammed open when turning the motor over??, choke open or closed??


Offline kghost

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Re: No compression....No reason?
« Reply #4 on: December 28, 2006, 02:01:34 PM »
I would bet that when you removed the cam chain / cam to remove the head you masked the problem.

if you really are curious pull the valves and look for sign's on the seat/face of the valve of blow by.

With the tension off the cam and or the cam out ...of course the valves will be closed.

Valve lash could also have been misset. I dunno if it has screw adjustment or shims or what...but I can't count the times I've seen people set the valves wrong.

If that aint it did the head gasket look like it was sealing?

Given the fact it was a thumper did it have a compression release? If it did that could have been stuck/failed open.

Stranger in a strange land