compression reading question

[bob johnson]

went to look at a 1990 Johnson 15 hp saturday . i took my compression guage. the unit isnt that great and I remember it holding the readings, but it wasnt doing that this day...as the guy pulled the rope the guage went to 60 lbs and fell off...each time doing the same...it was hard to really know the reading..

it was a super clean motor and the guy was a small engine mechanic who I had bought a chainsaw from last sunday. I had asked if he had a welder for sale that i needed and he said no..i said I am also looking for a 15 hp outboard...got one of those...he says yes I do!!!

since they canceled the shad fishing in the hudson....he figured hed sell it

it hadnt run in a year...

but I really felt i could trust the guy....just had that feeling.. so i bought the motor.

it ran fine in the 55 gallon barrel...but didnt push the water out like would have thought for a 15.

it was a good price it seemed and i bought it.

well i bought a new guage and checked the compression today...

first pull guage goes right to 60..every time..second pull guage moves up to 90 every time..third pull guage runs up to 107-ish

both cylinders the exact same.

my question..

is the compression really 60 lbs...or 107??

seems on a electric start it cranks many times over for the readings..

and should a 15 hp boil the water out of a 55 gallon barrel when in gear and reved up????

thanks

bob

 

[bktheking]

Re: compression reading question

107 seems right, I get 95psi on both on my 76 15hp and that was with a bad lower crank seal, 4 pulls on the rope. I had my 9.9 extra long shaft in the barrel last year, no way I could rev it up past half without covering myself with water, I suspect a short shaft 15 would be even worse for clearing water out of the barrel.

 

[fattypackard]

Re: compression reading question

i am assuming it has 2 cylinders?
if so, take both plugs out. and crank it a time or two.
then put the gauge in and pull it. see what it says.

if it is reading near 107 at all.....thats more than likely the correct reading than 60.

just my opinion...

 

[FIRE34]

Re: compression reading question

The compression seems great. With a pull start motor its pretty much just pull until it stops increasing compression from what ive read and been told. But i have a 72 20 h.p. and had the same problem. Except i was in my old swimming pool and it was half full. I though it should move more water around, so i just twisted the throttle as hard as i could. and well the stand broke and i was holding on for my life as the motor ripped off of the stand. turned out i just had a stuck throttle. May be the same for you. Or your throttle adjustment may be off. Is it tiller or rigged with cables?

 

[bob johnson]

Re: compression reading question

this is a tiller model long shaft.....

the water boils pretty good, I was just thinking I remember running an outboard in a barrel before the same size and the water was coming out like crazy when in gear and the rpms were up.....

maybe needs a carb clean.... i tested the compression with both plugs out.

one other thing, it SOUNDS like the motor will stall when the throttle is turned down as far as it will go....you needed to give it a little throttle to keep it running..not too much...but you could let go and it was stay running fine...but twist it all the way and she sounded like she would konk out!

thanks for the replies.

bob