New Top End, and Zero Compression?!?!???

xscessivecompulsive

xscessivecompulsive

Live 2 Ride, Ride 2 Live
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Hey Guys, So me and buddy just rebuilt my top end! We used Mikes XS replacement cylinders with new pistons, rings, and the whole meal deal! Had the head sent to a machine shop and received a clean bill of health! So today we checked it to see if she will turn over and ran a compression check. We had both spark plugs removed and got zero readings on both sides! We set the valves when we had the motor out, so the only thing I can think of is that the timing might be off when we installed the cam?? What do you guys think, it sucks that I might have to pull the motor after just going over the whole top end?? Any suggestions or help would be greatly appreciated!

XSC
 
Yes, I thought we had the marks lined up correctly! But we were using two different books! The book we went off was the factory manual, it was the yellow and black colored shop manual! I also have a clymer but we used the yamaha manual! My bike is a 81, you wouldnt think there was a difference would you??

XsC
 
the dot on the cam sprocket if your looking at the right side of the motor as in your right if your sitting on the bike, should be facing the back of the motor on a horizontal plane with rocker box mating surface. and the mark on the rotor should be a the tdc mark. There is a hole on the right outside of the cam (used for the advanceer), on the advance side that should also face 12:00...this is all from memory..so someone else chime in
 
that should be correct, i remember wheeling the motor around many times in front of me to make sure the marks were lined up....the cam mark is opposite side of the rotor/stator side of the motor.
 
Thanks for the help CDNTX650, That link you posted just might be the ticket as I can check cam timing with the motor still in and I will do that tomorrow morning when I get back at it! And no we didnt reset the valves, we set them when we put the head on and that was it! I was also gonna check the valves again to! Im hoping I can fix this without having to tear into the engine again after it was just done and Im so close to riding my bike down the road, I think??

XsC
 
Ya fellas, It feels like it has compression! And Im pretty sure that my guage is solid! When we turned the engine over with the push button you could feel a good amount of air coming out of the plug holes! I'll try and put my finger over one of them tomorrow! Im almost wondering if I went by the old book when putting the engine together is there enough changes in the older engines compared to the 81??? To maybe throw things off a bit??

XsC
 
No, all the year time the cam the same way. To add a bit more info to what was said.
On the cam you have threads on one end. This is where the advancer goes on the points ignition. This end should go on the right side of the engine.
With the timing marks on the stator and rotor lined up at TDC. As you install the cam there is a notch on the left side of the sprocket boss, this notch points straight up.
On the right side of the sprocket is a dot out by the teeth and as CDNTX650 said it lines up to the rear and level with the sealing surface. This is with the cam chain tight on the front and the slack to the rear. If you have the slack to the front and the rear of the chain tight the cam marks won't line up right.
The cam can be put in with the threads to the left and you can still get it right. and run fine but you have to line up the marks reversed.
It's better to do it right.
Did you use the stock size or the 750 size cylinder kit? I used the 750 kit, very nice. A bit more power, a lot less vibes. More fun to ride.
 
Oh I was going to say, try a different compression gauge. Some parts stores have tools like that to loan. On a fresh engine I would adjust the valves a bit on the loose side, leave the carbs off to test compression. The loose valves ensure they are seating. Carbs off ensure they aren't adversly effecting the reading. No carbs will read a bit higher. After the compression check set the valves to spec. .003 intake, .006 exhaust. It can take 5 or 6 revolutions to get the compression up to the max reading.
 
Thanks XSLeo, I really appreciate it!! It is 3:30 in the morning here and Im going into work waaaay early to get started on figuring this out! I will report back this afternoon when Im off! And Highspeedhamish, everything is brand new I used Mikes stock 650 replacement kit! So everything fit as it should there!! Wish me luck fellas!!

XsC
 
i would check your compression guage. on my craftsman guages theres a tire stem core type valve at the end of the line, sometimes the spring gets weak and it won't allow the compression to build in the guage. my gauges came with plenty of replacements so i gotta figure its common for them to go bad.
 
Last edited:
Hello Again fellas, Well I got in to work at about 3:30am as I had a hunch that the cam timing might of been off, and sure enough it was!! I took the engine out and ripped the top off and followed CDNTX650's link and reassembled, then set the valves and I damn near had to put the majority of the bike back together! After I had it all assembled I put gas in the tank, hit the choke and bam she came alive!! Ive still got a few loose ends to tie up around the electrical box and shes ready for the road! Keep in mind that this is my first build and Im a very novice mechanic but Ive learned a great deal, from reading this forum and couldnt thank all the good dudes around here enough!! When I first got the bike I never did a compression check and it turns out the bike had a burnt exhaust valve and would only run on one cylinder and I would take short runs with it! So needless to say I still havent felt the true power from the XS engine! But now she is all back together and I feel she will rock and roll!! Thanks again guys!! Im going in super early again tomm and will post some pics!!

XsC
 

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