compresion reading for 75 20 hp merc

[ooldschooll]

I am thinking of buying a 20 hp merc motor and he says it reads 100 psi on both. It aparently starts and runs but rough.

I am going to look at it tomorrow. Any tips? Has a new fuel pump kit, and I would think plugs and such

 

[oldman570]

Re: compresion reading for 75 20 hp merc

If the compression is 100# and even or within 5# of one another should be OK. Check the shifting of the gears and water in the LU for bad seals. All it might need is a good tuneup and maybe motor mounts if bad. Best to see it running before hand, if seller will not run it. I would head for the door.
Oldman570

 

[jbjennings]

Re: compresion reading for 75 20 hp merc

Hey Randy,
My dad and I have owned quite a few 70's mercs. The '75 model came with a mild steel driveshaft and if original, it's likely pitted and grooved, making it difficult to reseal the lower unit without sleeving the shaft.
The lower units were darned good when new, but are difficult to find GOOD used parts for due to the rust problem. There's probably a hundred thousand of those across the country with good powerheads but bad lower units. 100psi is low in my opinion if accurate, but probably fine since they're even and you know how compression gauges are. They also have some expensive electrical parts on some of those---triggers, switch boxes, etc.
I love to run one in good shape---they're really powerful for a 20hp, and have an awesome sound with a 2 blade prop in particular. Just some things to think about.
the '76 came with a stainless shaft, and I think it will fit your '75 model if you can find one. I might even have a 2 blade prop for one that you can have.
JBJ