1982 70hp Johnson engine troubles

[Brendan1996]

Hey guys, my father and I bought a boat . It has a 70hp Johnson 1982 model. We bought it with a dud motor. Got a second hand block barely used, still had the red paint in the water gallery's. Bored it out 30 thou over sized. And we're only getting 89-87-87 compression top to bottom. On the rebuild. When it's out on the water. The boat struggles to get up on the plane. Going from idle to full stick to get it up on the plane. It will bog right down and lift the nose of the boat up. But if you hit the choke. It will pick itself straight up on to the plane. We have had a few different marine mechanics look at it. One said water was getting into the cylinders. We had the head back off and there was no water markings in the bore. Spark plugs aren't fouled or burnt white. We can't really work out why it struggles to get on the plane. It's a 4.7m fibreglass boat. The motor could be a little small for the boat. But the motor does feel really under powered. We have had the electrics tested and the trigger came back at 1.0 ohms instead of the 1.5 ohms it's ment to be. We're really stumped. Any help or advice on where to start would be really appreciated.

 

[faztbullet]

Get a different gauge and retest compression as that's way low. Sounds like several issues going on.....compression,fuel delivery and sync and link

 

[racerone]

If you use the choke to get it to run properly it points at the carburetors.-----Clean them and make sure the high speed jet in the carburetor bowl is clean.

 

[Brendan1996]

Thanks guys, did do another compression test today with a different gauge got 120-120-100 on the motor. But I feel old mate wasn't holding the gauge properly on the lower cylinder. And the carbs are clean and all floats are working properly. High speed jet is clear. What do you mean by sync and link ?

 

[kevinj]

What do you mean by sync and link ?

Ensure that in neutral throttle position all 3 carb butterflies are completely closed. Also make sure the choke butterflies are completely open and that they close uniformly when the choke solenoid is engaged.

When the cam on the throttle engages the roller you need to make sure that all 3 butterflies are opening at the same rate across the entire throttle position range. If any of the butterflies are different from the others it will result in an imbalance between cylinders.

I'm with Racerone. A poor performing engine that magically kicks up when you hit the choke is a typical sign of an engine with one or more carbs starving for fuel. When the choke engages is makes the mixture richer and compensates for the lean condition.

KJ