Rpm drop on Johnson 25HP and 30HP

Stevo777

Recruit
Joined
Feb 3, 2020
Messages
1
I all I am very new to this but have a weird issue going on.

i has a 20HP Johnson 2 stroke motor on my Tinnie, for 12 months it ran great then began developing a issue where it would run any speed for maybe 10 mins then it would drop revs and backfire/misfire.

eventually the motor began idling rough and if I turned the tiller throttle to say halfway the revs would eventually drop and it would stall (progressively getting worse).

i took the motor off and put a good, known working motor 30HP Johnson on, then the same problem, the higher the load (faster) the quicker it would misfire and drop to no acceleration and when I try to accelerate it would spit and fart at me.

i have removed carny and cleaned it, blew down all of the lines, checked gauze on carby and all appeared fine. On the original motor I did find water in the fuel. Maybe a tablespoon worth.

on the new motor I have changed the fuel lines and primer bulb including the tank itself just to eliminate everything. When it drops its power it’s like I just put boat in neutral.

then sometimes I can stick motor into neautral or reverse and then back into forward and it will go for another 20 mins then same thing again.

i am going to empty the fuel and try a different servo and new oil but any ideas what could be causing this? Would it be fuel or something else?

i hVe almost changed everything from sparks to hoses etc and for 2 motors to do it seems quite weird.
 

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
38,892
Is this a 1953 model 25 HP or something else.-----Any pictures / model numbers ?----What is a " servo " on this motor ?-----Have you run with a timing light hooked up to see if it is an ignition problem ?
 
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