I JUST RAN A COMPRESSION ON MY DS650. I JUST SCREWED IN MAC MAC TESTER. AFTER CRANKING I ONLY GET 90LBS. IS THIS RIGHT OR DOES IT SEEM A LITTLE LOW.

 

To accurately measure compression, you have to disable the decompressor lever on the exhaust cam. This device allows easier turning by the starter motor, and disengages via centrifugal force once the motor starts.
Seems to me though, 120 is what you should get without disengaging if stock cams are installed, if memory serves me correctly. With you getting only 90, I would suspect something wrong, and remove the head cover to disable the decompressor for a more accurate test. When you remove the head cover, you can quickly and easily check valve clearances while you're at it - it could be a valve that no longer has clearance is being held open slightly, causing the compression loss. Clearances decrease over time.

 

i belive there could be a problem. by taking the cover off is it possible to shim the valves without pulling the head?

 

yes, the head must be installed to measure and shim. I advise getting a shop manual, allthough somebody has a link to a website with detailed instructions, somewhere.
When you pull the head cover, you will see the cams. measurements must be taken @ tdc of compression stroke. if you need to adjust, get the detailed instructions with torque specs and stuff. If they never have been adjusted, and you have considerable hours on the motor, I'd just about bet an intake valve has no gap left.

 

Valve Adjustments

 

thanks for the link!!!!!

 

Saint650 - besure to do the test with the throttle wide open after you disengage the decomp mech.

 

Yes or take the carb off you wont get it right. Also may be a little tight on the exhuast size

 

By disableing the De-commpresion? Do you mean take out the lever or what do you do exactly? Also about how much compression should you have with a 12:5:1 piston at 1000 ft.?

 

Tony,
I usually snip 1/4" piece of the clear breather tubbing that vents the gas tank and stuff that behind the decompressor lever.
I usually take it in and out with a pair of needle nose and it works just fine. Just remember to take it out when you're done.