cranking compression

[HARTWOOD]

I have a 1988 Johnson 100 hp outboard motor what would be a good number for compression, engine runs good but starts a bit hard.

 

[Rudemen]

Re: cranking compression

Welcome to the Forum

120 to 125psi would be great and normal, it will run fine with compression as low as 90 to 95psi. What do you mean by it starting hard? Not spinning over very fast? Battery is weak or not fully charged. Ground wire cable at battery and motor needs to be cleaned well. Wont hurt to clean the Positive side while you're at it.

 

[JDusza]

Re: cranking compression

Concur. 120 psi would be a terrific number.
Also, not sure what you mean " starts a bit hard".

Clean all the battery, starter and relay terminals with a wire brush. High resistance through poor / corroded connections can send your diagnostics into the weeds. Be sure your battery has a good charge.

Pull the spark plugs and see how the engine spins without compression. It should spin freely. Do your compression test.
Then do an air gap spark test. Spark should jump 7/16" air gap.
After that, it's fuel delivery.
If hard starting means cranks and cranks without firing, then maybe fuel delivery is your issue.
Review all three, compression, spark and fuel.

 

[emdsapmgr]

Re: cranking compression

Those later crossflows had the lower compression heads from the factory. Compression probably in the 90-100 range.