Jon boat won’t get on plane anymore

Richardsjs

Recruit
Joined
Aug 12, 2018
Messages
3
Hello, I’m having issues with my boat getting on plane. I have a 1648 mod v Jon boat. I recently changed motors, before I had a 85 johnson short shaft 30 hp tiller steer. Worked great just wanted a console. Traded a guy and now have a 84 Mercury long shaft 35 hp with controls. With the old motor with out normal fishing gear and 2 guys I could do almost 30
mph. With the new motor I could only do 18 with fishing gear and by myself. I believe I’m on plane by myself but add one more person, it drops to 15 mph, third person down to around 10 mph. The motor sounds like it’s performing correctly, I raised the transom and the cavitation plate is flush with the bottom of the boat. Prop is in decent shape. I didn’t figure I’d gain much with this new motor in terms of power but damn sure didn’t think I’d lose power. Help!!!
 

Woodonglass

Supreme Mariner
Joined
Dec 29, 2009
Messages
25,930
You can get a Compression Checker on Ebay for around $10 bucks. Check every spark plug hole. If any are under 100psi you've got your answer.
 

GT1000000

Rear Admiral
Joined
Jul 13, 2011
Messages
4,916
Also check and make sure the prop hub is not stripped and the prop is securely attached to the drive shaft.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
51,150
you went from a short shaft to a long shaft on the same boat. if so, start by selling the long shaft and put a short shaft back on the boat.

the lower unit length must match the transom height. putting a long shaft motor on a short shaft boat will be a slug.
 

Richardsjs

Recruit
Joined
Aug 12, 2018
Messages
3
I don't believe the prop or hub are spun. It acts as it should, and does not sound like its blowing off or over revving or anything. Im gonna get a hold of a mechanic buddy and have them check compression. The more im thinking about that the more it may make sense. i hope its something easy and cheap but we will see. Ive got the motor mounted at the correct height, i built the transom up to do so.
 
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