Mid 70's Johnson 70 HP doesn't run full power

[Korn24]

I own an older Larson, with a mid 70's Johnson 70 HP engine on it. I fixed it up, rebuilt the carbs, changed the coils, and the spark plugs. The engine runs great with the muffs on in the drive way, and shifts into gear no problem, but when we get it out into the water, it won't go faster then about 5-10 mph. I don't understand what the problem, and have already thrown alot of money into fixing this boat, any addvice???

 

[iwombat]

Re: Mid 70's Johnson 70 HP doesn't run full power

I'm guessing you've got a cylinder that's not firing. No backpressure on the muffs, so you're not noticing it until it hits the water. In the driveway put on a thick glove and try removing one spark plug wire at a time. If the rpms don't change when you remove a plug wire, you've found your culprit.

 

[aguitarestv]

Re: Mid 70's Johnson 70 HP doesn't run full power

is the motor running poorly at the 5-10mph? and is the timing and carbs,all synced and linked, also make sure that all 4 cylenders are firing i ran into this problem the other day when i realizes the the high tention leads where chaffed and arcking out on the flyweel. if thats not the case than i would go back to making sure your advance arm is going all the way to the stop (under the flyweel) and that the carbs are opening up all the way when the throttle is pushed down.