Johnson 150 HP Compression

[FrankN]

I have an 84 Johnson 150HP V6 Outboard, The starter is missing 3 of the 8 teeth. It still starts, just sounds weird while the starter is cranking but when starts, seems to run fine. I have a new starter on order but I went to get a compression test and it tested low (70 psi on all cylinders). would the missing teeth on the starter have anything to do with lower compression?

 

[JB]

Re: Johnson 150 HP Compression

Your compression is okay. What makes you think it is low?? Those big V6s usually compress around 70psi

 

[jonesg]

Re: Johnson 150 HP Compression

I have an 84 Johnson 150HP V6 Outboard, The starter is missing 3 of the 8 teeth. It still starts, just sounds weird while the starter is cranking but when starts, seems to run fine. I have a new starter on order but I went to get a compression test and it tested low (70 psi on all cylinders). would the missing teeth on the starter have anything to do with lower compression?

No, the low reading has more to do with cheap compression guages and carbon on the rings, do a decarb with seafoam.

 

[emdsapmgr]

Re: Johnson 150 HP Compression

OMC came out with the low compression (bathtub) heads starting in 1983. You have those heads on the basic 150 powerhead. Their compression was significantly lower. Those heads ran compression around 90+. Your 70 reading is low, even for these low compression heads. Slow rotation will affect readings: I'd make sure the engine will rotate at least 200 rpm with the plugs in, then remove all 6 and restest. Try a second gauge to verify the readings. Follow Jones' advice and do a decarb, then retest. (the 150S XP/GT models ran different heads and ran somewhat higher compression.)

 

[FrankN]

Re: Johnson 150 HP Compression

Thanks for the replies, I was reading around and am thinking maybe I didn't do the compression test correctly. How important is it to crank while the throttle is wide open? would that make any difference?

Thanks

 

[JB]

Re: Johnson 150 HP Compression

That would make no difference, Frank. Look in the Top secret File ^^^ for one good procedure.

What do you figure the odds are that six cylinders would all go bad by exactly the same amount at exactly the same time?? The reciprocal of infinity? That's my guess.

You can change guages, technique, barometric pressure, warm engine up (you said she runs fine), freeze engine, decarbonize, shoot oil in the holes, etc, etc. The readings would probably change, but the thing that is important, I sometimes think it is the only thing that is important, is that the six holes all compress the same psi +/- 10%.

There is not a compression problem in your engine. Quit freaking about it and go fix something that is broke.

 

[FrankN]

Re: Johnson 150 HP Compression

Thanks again for the Reply JB, I'm not freaking out about it that much because it seems to run pretty good, but I'm trying to sell the boat because we purchased a newer boat, and when people come to look at it, The first thing they want to know is what the compression is, and when they hear numbers lower then 100, they immediately think the engines shot and tell me the compression should be no less then 120 etc... and look elsewhere for a boat, so I don't know how to convince them the engine's ok.....lol

 

[jonesg]

Re: Johnson 150 HP Compression

squirt some oil in the cyls and do the compression test when they come to look at it.
If it runs good nothing else matters.

 

[JB]

Re: Johnson 150 HP Compression

Good idea from Jonesg.