TOPIC:

Yesterday I performed a compression test on an 82 KZ750R. I pulled all the plugs, opened the throttle completely and checked each cylinder with a Motion Pro gauge. This bike still has the vacuum petcock and I did NOT disconnect the fuel tank. Will that affect the results?

No, max vacum is achieved on a closed throttle. With  throttle open it won't effect the results

Did you check valve clearances and warm the engine first?  Also realize, if the bike sat for a long time, the rings may be gummed up, resulting is false low readings.  

What were the results?

Just FYI start with a leak-down test, always my first go to. If an engine has been sitting for some time and as normal some valves have been sitting open you will get a false reading initially. If after starting, then take your compression test and then with a Leak Test. You'll figure out mechanical tolerances quickly. 

Before leak testing ensure the bike is in gear, the rear wheel is clear of the ground and the rear brake is jammed on (the motor can try and turn when air pressure is applied to the piston and if in gear it can kick it off the stand)

Highly unlikely the pressure will spin the wheel on a 4-cylinder?? Unless You Have it in gear and have your compressor set to 500psi, ha. Just take a straw or similar tool and "feel" the piston crown, both ways till you establish what feels like TDC. Like Wookie said the piston "can" slide down but that means, you didn't have it at true TDC yet. This can be a challenging test on the twin cylinder CB350-400's because of the lobe location but a walk in the park for the Z1/KZ models. 

asphalt900 wrote:

Highly unlikely the pressure will spin the wheel on a 4-cylinder?? Unless You Have it in gear and have your compressor set to 500psi, ha. Just take a straw or similar tool and "feel" the piston crown, both ways till you establish what feels like TDC. Like Wookie said the piston "can" slide down but that means, you didn't have it at true TDC yet. This can be a challenging test on the twin cylinder CB350-400's because of the lobe location but a walk in the park for the Z1/KZ models. 

Clay I'm not looking to start an argument and you obviously "know your stuff" but I have seen cars "load up" on the E-brake at 120 psi when leak testing. One thing age has taught me is that if something has the potential to "ruin your day" then it's best avoided if practically possible.

Friendly banter Wookie and Safety First by golly!

Yeah, possible "perfect storm" scenario - all plugs out, piston near TDC, bike in gear, apply 150 psi to cyl (about the same as normal compression, btw), and wheel could spin.   Better safe than sorry - murphy's law can strike when you least expect it !