Full open throttle on compression test??Why

[PULLNIT]

I am about to do a compression test on my Johnson.I have read alot of post that tell me to do the test at full open throttle.Why?Can I just move the shifter to warm up throttle all the way?? Thanks John

 

[seahorse5]

Re: Full open throttle on compression test??Why

On a 4-cycle motor you need to open the throttle to minimize the vacuum restriction when intake valve opens and the piston "sucks" in the air to compress. On a 2-stroke motor, the exhaust port is open during the bottom travel of the piston and the slow cranking speed allows the cylinder to fill until the exh port is covered, and some air comes thru the intake ports. You can check compression with the throttle open or closed and still get the same reading.

 

[skier]

Re: Full open throttle on compression test??Why

Pullnit:<br /><br />I'm not sure if you realize or not, the warm up throttle only advances the timing, nothing else. Just do you know.<br /><br />Craig

 

[seahorse5]

Re: Full open throttle on compression test??Why

The warm up lever on OMC's both advance the timing and start to open the throttle.

 

[SeaDawg]

Re: Full open throttle on compression test??Why

The main reason I check compression at wide open throttle is if your carbs are not exactly syncronized (meaning you have the same vacuum readings on each one), it will give you a slightly different compression reading for each cylinder when checked with the throttle at idle.<br /><br />You eliminate this when you check it at WOT. And, since all manuals I know say check it that way, you can then compare your results to what they publish.