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Yamaha Tech Originally the Yamaha XT600 Tech Forum, due to demand it now includes all Yamaha's technical / mechanical / repair / preparation questions.
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  #1  
Old 25 Sep 2015
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Low Compression
Hi all, I picked up a 84 tt600 for 100 bucks. Shop was going out of buss told me it needed new valves.
I tore into it and sure enough all valves were bent but the weird thing was they were brand new and bent in fact everything in engine looked new/cleaned.
I ordered new valves/head and upper gaskets. Put it back together and forgot to adjust valves before starting and they were very tight enough to keep intake open and blow air out carbs. I didn't hear any noise but I adjusted valves to spec and tried starting to no avail. I did compression check and I'm only getting 90psi. I'm going in again to check if i damaged valves. I have timing set up right "crank notch to arrow and cam lobes down cam gear marks level with the case.
Is there anything else I should look for while im in there?
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  #2  
Old 25 Sep 2015
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Did you do the comp test on warm eninge and throttle fully open?
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  #3  
Old 25 Sep 2015
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He tried starting, no avail. 90 lbs is extremely low. Maybe too low to run. Normal compression is 150 to 170lbs. My 86 has never been apart, has about 15000 miles on it and still has 170 lbs on a new gauge.

A good test of valve sealing is to remove head, turn face up, pour gasoline in the combustion chambers and see if any leaks past the valves, if so, you have found part of the problem at least. You may have bad rings/cylinder etc as well.

I just remembered, did you disable the compression release by removing the cable?
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'84 XT600 is now bored to 2nd oversize and new OEM pistons and rings installed. No more smoking.
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  #4  
Old 25 Sep 2015
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Holyeeee CRAP! well I know the decompression valve works. That was freakin hard to kick over w/out it.
Stop home to try it w/out decomp lever during lunch and got the same 90psi.
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  #5  
Old 27 Sep 2015
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Put a little oil down the cylinder and kick it over a bunch of times. I have had some motors that when freshly rebuilt and just kicked over/spun over a few times the compression was way down. My hi-comp motor when first tested only had 75psi and didn't want to start(it was more a carb issue). I oiled it up and ran the starter for 30 seconds and then got it to pop off for a minute. Compression was then 190 , after another couple minutes running it was 208 and stayed there. I now usually lube things up good with assembly paste and spin them over a bit before even trying to start. Take the sparkplug out for easier and faster spinning. You can use the flywheel nut to turn over a kicker motor

My point , if the rings are new it may just not be seated yet so comp will be low. @90psi though it should start.
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  #6  
Old 27 Sep 2015
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Thanks jjrider, Just got done taking off the head. Valves are still in good shape. Piston comes up to tdc and is flush with top of cyl jug. It's not going to far up so it looks like right piston/rod.
I will reassemble with some oil in cylinder and try it again.
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Old 25 Sep 2015
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hi is the piston a standard piston ?,I was thinking if a high compression piston had been put in or a different con rod (longer) that could account for all the valves being bent while the engine still looks as new? steve
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  #8  
Old 25 Sep 2015
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Hi thanks for the fast replies. I haven't measured piston or rings yet just looked at them.
When I got it I did take head off and did the gas squirt around valves and all leaked. After i changed them I did it again and it was sealed no leaks.
I did not disconnect the deomp lever duh!! i'll try that and test again.
I have carbs off right now while testing. i just squirted some gas into intake to try and start it and it wouldn't fire.
The spark is good but white not blue. not sure if that's an issue.
I'll post my results this evening.
Thanks again all.
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  #9  
Old 25 Sep 2015
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Yes SSBON, I was thinking along those lines to. My first ?? was how did the valves get bent in the first place. My gut tells me there bent again.
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