1978 Mercury Outboard

[cferro]

Greetings...

Looking at a 78 Merc Outboard, on a 78 Glastron GT-150.
Tested compression and all readings 95psi...seller said motor had not been started in about a year and had a tough time getting it to idle initially.
We started it up on the muffs and got it to idle ok ..ran it up in it got smoother with the higher rpm.

Just wondering if she just needs to be run...and possibly just may need some Sea Foam treatment?
How do those compression readings fare for a 78 Merc?
And wondering if anyone has the 78 Merc service manual?

Thanks for your time,
CF

 

[444]

Re: 1978 Mercury Outboard

What size of mercury outboard? I had a 77 Glastron GT-150 with an 84 90hp inline 6. My compression readings were all around 135 and that engine idled smoothly and ran like a sewing machine.

Any compression under 100 is what I would consider to be too low for me. Even 110 would be borderline for me. But they say it's more about consistency between the cylinders than the actual compression #'s.

 

[Gomer50]

Re: 1978 Mercury Outboard

Ditto with 444.Since it hasnt been run in a year or so a good decarb (seafoam) to get those rings back up to par and then do another compression check.If all are about the same within 10% of each other its not bad,but if you would lets say have 2 that were way lower than the rest then I would be worried.Good Luck,,

 

[cferro]

Re: 1978 Mercury Outboard

I know I though 95psi readings were slightly low, but I thought it was more of a consistency between cylinders issue than anything. They were all 95 psi..

The engine started running better as we continued to run it but still skipped slightly abou 100 rpm drop at idle.

I was thinking the Sea Foam would work..

How much of it do you mix into the gas/oil mixture..Im not 100% familiar with the procedure

Thanks Again

 

[Gomer50]

Re: 1978 Mercury Outboard

First I mix 50:1 in a gallon jug and put roughly 1/2 can (seafoam)into the gallon jug and run it thru till its gone.I do this in the driveway on muffs.Be aware this will smoke up the whole neighborhood.After that I put about a double shot in every 6 gallons of premix and go cruising.Works for me.Nice clean motor. I cant promise your compression will go up but it wiill definitly clean all the gunk out.

 

[cferro]

Re: 1978 Mercury Outboard

ok great..thanks so much..

 

[CVX20SPRINT]

Re: 1978 Mercury Outboard

Could be just the guage.Try another one if you can get your hands on one after the decarb.