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Engine bumping ! Rpm increase then decrease

LYF

New member
1996 Johnson 50
motor almost feels like it slips out of gear momentarily , Rpms will increase from 5800 to 6000 for a split second and the engine will noticeably bump. It is now doing the same thing but dropping to 5600 after the “bump”
Initially we thought it was the clutch dog being worn or the shift being out of adjustment , that’s not the case.
the motor has a change in tone when it drops rpms , kind of feels like you hit something submerged , all the tell tale signs of it physically slipping out of gear , but with a drop in rpms when it catches
I recently replaced the coil with a “good” used one from another motor , there was some wiring issues that have been rectified.
any suggestions other than a brand new motor with a warranty and a new boat and trailer to go with it ?
could a failing power pack , coil , or stator cause this bump and momentary rpm increase , if it is indeed an increase , may be a false reading from the tach ?
 
Any metal bits on the lower unit drain plug magnet ?------Inspected flywheel key and flywheel magnets ?-----Run with a timing light to see if you can find an ignition issue.-----Post compression test values too.
 
The feeling of hitting a submerged object is created by one of two things.... you have either hit a submerged object or the lower unit is jumping out of and back into gear.
 
No metal bits in lower unit , does not clatter when shifting
timing is correct and flywheel key is okay , I checked the compression and it was good (within 10% of each other) can’t remember the numbers though
could it be a power pack or coil failing in a weird way ?
 
Was the lower unit off for water pump work ?----Shift rod turned ?-----Test run with another tank and hose.----Possible water in ther fuel.------Run with cowling off and timing light to see if there is an ignition fault.-----Up to you to do some testing before spending money on parts !
 
shift rod properly adjusted to spec
no water in fuel , checked the water separator 3 times with now water
timing was done a couple of weeks ago but haven’t run it without the hood yet
 
NO,no------You use the timing light at full throttle !----Observe the light when motor falters.----Steady flash means all OK.----Flickering / black spots indicates spark / ignition trouble.----Trouble shooting is so easy.---Best to post you actual compression numbers today.-----Running with VRO in service , yes or no ?
 
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No metal bits in lower unit , does not clatter when shifting
timing is correct and flywheel key is okay , I checked the compression and it was good (within 10% of each other) can’t remember the numbers though
could it be a power pack or coil failing in a weird way ?

What is your (or perhaps someone else's) method of shifting?... Slamming into gear as fast as possible... OR... shifting into gear as easily as possible for the purpose of taking it easy on the shifting components?
 
Vro is deleted , timing light is akin to a fast paced strobe light at full throttle , almost on all the time
 
One cylinder is 130 and one is either 125 or 135 , I can’t remember but I know one is 130 and the other is 5psi difference
 
Could prop slip cause these symptoms?

No..... A slipping propeller hub would cause a loss of forward motion and the rpms to increase at a high rpm and stay there until the throttle is lowered, at which time... if the hub is not completely destroyed... the propeller would grab again if the rpm was low enough, BUT the jerking, sudden grab sensation and feeling of hitting a submerged object would not exist.
 
The lil 2 cylinder gearcases on these engines are unforgiving..you have a gear problem. Fix it and the tach/RPM problem will go away....
 
Agreeed those gearcases are NOT robust.-----But I think LYF is till waiting for the solution / answer he wants to hear.
 
Not to hijack but...i have a 94 120 v4 with a similar problem. Only the motor has no noticibal rpm change. Does it about once a minute or two. Thanks for a clue that i can pursue. (Lower). But whats the repair? Shift mechanism? Gears?
 
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