Compression on my 78 75hp johnson?

[OpA28]

Around what psi should the compression b on my 75 hp stinger?

 

[iwombat]

Re: Compression on my 78 75hp johnson?

That's somewhat of a loaded question with no real definitive answer. You're looking for even numbers across all the cylinders. Pressure gauges vary so much that paying attention to any one particular number isn't the best of advice.

Why are we checking compression?

 

[OpA28]

Re: Compression on my 78 75hp johnson?

When my boat is sitting at idle n i throttle up it gets power 4 about ten seconds n then slows rite down. I wasnt sure if i was gettin drag from the motor bein 2 low, or i was gettin cavitation, or losin compression.

 

[iwombat]

Re: Compression on my 78 75hp johnson?

During acceleration is when you're testing the torque of your motor the most. So, if it's got good hole shot, but just can't maintain the speed, I think you're looking at something besides a compression/performance issue.

Best bet is to take a reading and post the results. 100psi or better and you're certainly looking good. 60 or less and you've got some problems. The middle ground may or may not indicate issues depending on gauges, particular motors, and measurement procedures.

Any variation of 10% between cylinders indicates something as well.

 

[OpA28]

Re: Compression on my 78 75hp johnson?

Yea the only time i get speed is when i go from a dead stop 2 full throttle. If i was 2 slowly throttle up i might get 10 mph at most. The motor never races as if my prop was slippn. the motor itself stays at low rpms as if it was gettin drag. Not really sure where 2 start.

 

[iwombat]

Re: Compression on my 78 75hp johnson?

Compression check and spark test are certainly the places to start. That whole 10% variance thing is what you're primarily concerned about most of the time with compression checks. Spark test will indicate if you've got a weak spark on one or more cylinders.

 

[OpA28]

Re: Compression on my 78 75hp johnson?

Thanx 4 the help ill c what the comp. says n go from there.

 

[iwombat]

Re: Compression on my 78 75hp johnson?

Next question:

Is this a new condition, or has the boat always had this problem. If it's always been there (or new boat to you), Where is the ventilation plate in relation to the keel?

 

[OpA28]

Re: Compression on my 78 75hp johnson?

This is a new boat 2 me n the problem came with it. The plate is mayb 3" below the keel. the motor is as low as itll go on the bracket. My 1st thought was the motor was 2 low n creating drag but wasnt sure it would make that big of a difference.

 

[iwombat]

Re: Compression on my 78 75hp johnson?

It'll make a huge difference. Raise the motor until the plate is even or slightly above the keel (1" above is a good starting point).