Johnson 85hp no power when in water

[mallard1123]

I just bought a used 1976 johnson 85hp motor that has been sitting for a long time according to owner about 15years. Motor has good compression and spark in all cylinders. Motor runs but backfires when hooked to muffs in driveway. 3 spark plugs are black and top righr if staring at back of motor is clean looking. The motor will go in and out of gear in driveway and sound like it's full power and speed but when launched the boat will not go fast when in gear. The throttle will be all the way and the motor will not high rev and push the boat fast.

 

[flyingscott]

Do a compression and spark test.

 

[mallard1123]

100psi in all cylinders and spark.

 

[flyingscott]

Are the carbs opening all the way. That compression is low re-test with a different gauge.

 

[interalian]

And don't rev the wee out of it in neutral on the muffs or on the lake. Bad things can happen, and it proves nothing.

Some backfire/popping is normal when idling on muffs.

If you have a clean plug on the top hole (#1), it's possible you're not getting any fuel due to blocked jets on the carb throat for that cylinder.

 

[Chinewalker]

That motor has was amounts to four individual carbs, each throat feeding one cylinder. If any one throat is not getting fuel fed through it, that related cylinder will starve for fuel. Might be worth getting a squirt bottle of mixed fuel and spraying it in each carb throat and seeing if it picks up.

 

[mallard1123]

I have cleaned the carbs twice. I have another motor that's the same i can swap carbs and see it that works. I like the spraying gas in carb. So should I spray when throttle is all the way down on the lake and see if it goes full speed?

 

[oldboat1]

Do the spray test, but check spark with a tester. If running, use a timing gun and see if the spark stops at some point. If you confirm the ignition is producing spark, it's still possible the plug is not firing (bad plug or incorrect gapping, if appropriate). Blackened plugs can mean rich running or cold running. Should test them all with an adjustable spark tester.