8HP Yamaha f8SMHB (4-Stroke) Compression Test

[lenwilliams]

Gauge reads 75psi on both cylinders. Has spark, fuel is good, won't start. Do I need to tear down?

 

[racerone]

What if this motor has automatic compression release??-----Hard to believe that a motor could run and run to allow compression to get that low.----Do some orderly trouble shooting I say !

 

[boscoe]

Does the motor start, idle and run well?

 

[lenwilliams]

What if this motor has automatic compression release??-----Hard to believe that a motor could run and run to allow compression to get that low.----Do some orderly trouble shooting I say !

Thanks. I was not aware of "automatic decompression" (Yamaha's term), but it certainly makes sense to me --especially when both cylinders read 75psi. I suppose that's a good thing. Testing plugs/ignition with an inline device gave a positive result, but the motor still won't start even after spraying starting-fluid down it's throat. Thanks again for the heads up!

 

[TerryRauh]

I thought I would throw this in may help may not. If you put like a Philip screw driver in your spark plug wire and crank it how far will the spark jump? I had a outboard once that had fuel and spark but the spark wouldn’t jump more than the spark plug gap. It should jump at least 3/8 inch or more. Good luck.