engine wont accellerate

[horizonRoyaldridge]

I have a 1980 Johnson 25 hp 2cycle tiller motor on a 16' flat bottom aluminum boat. It will start and idle perfectly and even accelerate while in yard, however on the water my I can not get over a few miles per hour. Could it be the linkage? I have changed the coil, added a new carburetor kit, and fuel pump, but no change.

 

[racerone]

Information needed.-----Post compression numbers.--Does throttle plate open all the way ?-------Have you checked flywheel key ?----Does spark jump a gap of 3/8" or more on oth leads, yes or no ?----Sounds like it is running on 1 cylinder.

 

[saltchuckmatt]

Just to check a couple of things....what size transom and what size outboard shaft length?

Your prop could be bad too....is the motor spinning up?

Post pictures of your motor on the boat.

 

[tphoyt]

This just reminded of a friend that put a long shaft motor on a Jon boat with a 15” transom.
I warned him it wouldn’t be safe but he did it anyway. He ended up skipping across the pond and getting ejected from the boat. Of course he wasn’t wearing the lanyard either “Jackass” and it ran wild for about 5 minutes before crashing into a dock and sinking. Thankfully no one was hurt.

 

[airshot]

Some serious testing needs to be done here before anyone can help. Without test results, we all are guessing !!

 

[oldboat1]

Take out the hex nut at the bottom front of the carb, and clean the orifice (jet). Use soft wire if there is a blockage, and follow with carb spray using the plastic nozzle. The low speed jet is on top of the carb and the hight speed jet is on the bottom. Additionally, the nozzle gasket has to be in place between the top and bottom of the carb. First check out the high speed jet for blockages.