40hp johnson top end knock

[sambrigman]

I just purchased 1988 40hp johnson with a top end knock. I pulled the pistons and everything looks great. Rods, bearings, and crank look excellent. I put everything back and turn the flywheel, It turns smooth until the top cylinder gets to the top and has a hard spot. I think the knock happens here. I can turn the flywheel backwards it turns smoothly always.What could the top end knock be,not pistons or bearings. This is way over my head. Any help or suggestions would be greatly appreciated. Thanks guys

 

[gm280]

Do you have the head off the engine? And if so is there still a hard spot with the top cylinder? If no, then the piston has to be hitting something with the head installed. JMHO

 

[racerone]

Did you carefully line up connecting rod caps ?-------Or did you just assemble and torque them up ?

 

[humleridder]

Does it make a difference whether it is in neutral or not? Just to rule out anything pump or gear related.
It may not be advisable to turn the flywheel in the wrong direction due to the functioning of the impeller in the water pump.

 

[flyingscott]

Pull the lower unit off and see if the powerhead turns freely then.

 

[Silvertip]

Incorrect spark plugs perhaps (too long a reach). Wrong head gasket thickness. Rebuilt at some point with the a wrong replacement connecting rod and/or piston. Marks on the crown of the piston would be a clue. Carbon build up or incorrect cylinder head. New rings and pistons installed at some point without removal of the ridge at the top of the cylinder. Lots of possibilities. Could even be an incorrect crankshaft if the engine was every rebuilt a novice.

 

[sambrigman]

yes, I have removed the head again, and taken the pistons out, and removed the lower unit. There is a noise when I turn the crank. I have not removed the flywheel yet, It has got to be something under the flywheel or not???

 

[racerone]

???----You took the pistons out without removing the flywheel??-------Explain how you did that .

 

[oldboat1]

^^can't quite tell where you currently are with a teardown or reassembly, but suppose the noise could be something under the flywheel -- think in terms of what you worked on under there. Maybe the charge coil, stator or sensor is binding. Maybe the plate assembly is loose or incorrectly installed.

 

[sambrigman]

racer one, removing the head and intake.. pistons are easily removed

 

[racerone]

Yes , but then how do you line up the connecting rod caps on re-assembly ?---Failing to do that step will possibly result in a " dynamic halt " of the motor at full throttle.