Is it worth rebuilding?

[RoughFishAssassins]

I recently aquired a 1974 85hp johnson. It turns over nicely but 1, it has no spark, 2, it has low compression, 3, ??
The cylinder walls look ok, thinking that is might just be bad rings, but not sure.
Any one have any ideas? I am looking for more power for my boat and this would do nicely but needs to be rebuilt, new plugs, coil packs, power pac etc...
So the question is, is it worth rebuilding? What else would cause low compression (BTW the head gaskets looked good).

 

[oldrudedude]

Re: Is it worth rebuilding?

What are your compression numbers? Try spraying some fuel oil mixture into the spark plug holes to give it a chance.

 

[BonairII]

Re: Is it worth rebuilding?

We need more concrete numbers. What exactly were the compression numbers? Are u confident you performed the comp test correctly?
By "low" compression do you mean in 50s-60s low? More important that all cylinders are within 10% from lowest to highest.

 

[RoughFishAssassins]

Re: Is it worth rebuilding?

Ok, I will have to get the numbers again, maybe Ill burrow a better gauge this time. But when I did it I think the numbers were ranging from 50 to 100, I think one was even at 70. Not really sure I will try to get better numbers.

 

[BonairII]

Re: Is it worth rebuilding?

If you have accurate numbers ranging from 50-100 and the head gasket is ok....prob not a good sign.

 

[1946Zephyr]

Re: Is it worth rebuilding?

You have to be sure. This isn't a game of darts. LOL Make sure you have a good guage and crank it 5 times with the throttle wide open for each hole. If the cylinder walls look good, then that is one less thing you have to worry about. Who knows, maybe the rings are carboned up and sticky. Has this motor been sitting for some time?

If you can get spark back into it, I would get it running and do a de-carb on it and try your compression test again.

 

[RoughFishAssassins]

Re: Is it worth rebuilding?

Well finally got to putting a battery on this motor to check the compression. Well the guage that I got isn't the best but it works. I came up with in order (1,2,3,4) 50, 50, 20, 30
Now what do you guys think, should I attempt it?

 

[freddyray21]

Re: Is it worth rebuilding?

those numbers are very low. It's a choice. If you rebuild it you will have a new motor at a fraction of the cost of a new one.

 

[JB]

Re: Is it worth rebuilding?

I don't trust those numbers.

Go to the Top Secret File and do the compression test exactly as described there. Don't worry about the throttle, it makes no difference. Get a known good guage.

When all four are below acceptable numbers I think there is more wrong with the test than the engine.

I don't think you want to waste hundreds of bux fixing what ain't broke.

 

[emdsapmgr]

Re: Is it worth rebuilding?

Are you sure your starter is cranking the engine at a minimum of 200 rpm? If the battery is low and results in low cranking rpm's that could be the problem. The engine needs to crank at least 200+ rpm's in order for the ignition to fire. If you still get the same low compression numbers, pull both heads and check the head seal rings to be sure they are intact. If the engine was severely overheated by the previous owner, that could have damaged the head gasket seal rings-resulting in low compression readings.

 

[RoughFishAssassins]

Re: Is it worth rebuilding?

Well the starter isn't really spinning the motorfast at all, I'm not sure if the starter is tired, it is very sticky, had to lube the starter to get it to fully ngage into the flywheel, or the battery is junk, but I did have the charger on it when turning over. It was like it had a stutter when cranking it, never went fast like my other johnson.

 

[emdsapmgr]

Re: Is it worth rebuilding?

Possible the contacts in the starter solenoid are burned and not transferring enough current to spin the starter rapidly. Try using jumper cables from a good battery, direct to the starter itself. If it spins at a fast, normal speed-replace the solenoid. The recheck your compression and see if you have spark.