'78 70HP Johnson no power low cold start comp.

[Ditch Angler]

I purchased a '78 70HP johnson outboard for my 16' glastron runabout. Boat has zero power. Feels like it goes in reverse faster than foward. Motor purrs like a kitten when idling and seems to rev fine however has no power or speed and unable to plane. I did check the compression however I checked it a good 45 minutes to a hour after I had it running. I removed all the spark plugs and checked the compression in each cylinder. The dial read 65 PSI in each cylinder. I sprayed some oil in the center cylinder and compression improved to around 75PSI in that cylinder. I did not spray oil in the other cylinders. Motor starts with little difficulty. Any thoughts? Should I put much faith in the value of the compression check, or is it more important that they are all equal? Is this motor a lost cause?


David

 

[rudedude]

Re: '78 70HP Johnson no power low cold start comp.

Its not a lost cause. Your compression is very low, the book says 100 isnt very good but its more important that each cylinder is with in 15 psi of each other. 100 is time to do rings, if you have 120 in two and 75 in one its ring time. Putting the oil in and rechecking the compression is what your supposed to do. That tells you the rings need replaceing and time to check the cylinder walls. Your looking at a rebuild for the off season, minimum ring replacement and cylinder hone.

Its worth rebuilding in my book.

 

[_brad_]

Re: '78 70HP Johnson no power low cold start comp.

Before you get all worked up here, perform a decarb.

Its been known to up compression numbers.

 

[tashasdaddy]

Re: '78 70HP Johnson no power low cold start comp.

you have a very tired motor, time for a rebuild or replacement. don't believe a decarb, will bring the compression up on all them. i some times works if one is low and has a stuck ring. if you price replacing it, you may decide to rebuild.

 

[Ditch Angler]

Re: '78 70HP Johnson no power low cold start comp.

Thanks for your quick responses. In place of rebuilding the power head (as what I believe you are indicating) should I consider replacing it entirely? I have found a '78 70HP power head with compression stated as 131/131/132 or something to that effect for <$350. Or in your opinions would it be less expensive to overhaul the existing power head? Which would be easier/ cheaper?

David

 

[rudedude]

Re: '78 70HP Johnson no power low cold start comp.

Get the $350. power head, put it on then you can rebuild the old one in spare time and a little at a time .

 

[water n oil]

Re: '78 70HP Johnson no power low cold start comp.

Get the $350. power head, put it on then you can rebuild the old one in spare time and a little at a time .

Good idea.

 

[Ditch Angler]

Re: '78 70HP Johnson no power low cold start comp.

Thanks for the input. I did take the boat (and motor) to a marine repair shop today and was given some good news and discovered some bad news. The good news was the compression in the motor was good (140 across all cylinders). Must have had a bad guage or not installed properly. Perhaps this is a stupid question but was I suppose to remove only one spark plug and check that cylinder's compression? I removed all the spark plugs and checked the compression in one cylinder. The fact the marine repair shop said the motor didn't seem to have any problems. So to the bad news, apparently I should have done some reading on how to drive a boat with power trim and tilt (this is my first boat). It is more likely that my problem was I did not drop the trim. I left it raised and mearly brought the motor down to the already extended trim cylinder rods. So, this at least was a learning experience for me. I appreciate your input.

David

 

[bgreg]

Re: '78 70HP Johnson no power low cold start comp.

Hi David, proper procedure for a compression check is to remove all the spark plugs. Crank engine for at least 20 seconds for each reading. After this test is completed you can do a wet test by put some light wieght motor oil in each cyl., crank engine to coat oil in the cyl. and take a second set of readings. If you have big differances, I would be concerned 5 to 10 lbs. differance between cyl. would not be a concern.

B Greg

 

[HybridMX6]

Re: '78 70HP Johnson no power low cold start comp.

You just revived an almost 2 year old thread.