'88 Johnson 30HP bogs down on full throttle

AM12

Recruit
Joined
Nov 19, 2025
Messages
2
I got a new to me 30HP Johnson on a 15' jon boat. Rookie mistake that I didn't have him show me the boat on the water, but he did show me video's of it running. In the parking lot, he dry cranked it for about 1 second to show it cranked and cut it off. With the video's it was running full throttle no issue, it was dated a month or so before I bought it.

When I finally got it on the water, it was running a little rough on idle but some minor adjustments, got it running fine. Pissing great, idled, forward, reverse no issue on throttle to about 3/4's the way. Once full throttle, it bogs down and still runs but sputters, shakes and make a lot of racket.

I changed and gapped new spark plugs and compression tested, both at ~95 PSI. I also got it tuned from a local mechanic who got it idling perfectly and looked to diagnose the problem. He seems to think it was the throttle trigger or something like that where it won't hold time on full throttle. I've also spoken with someone who thinks it's the link sync?

This is my first boat/engine and struggling with what to do next. Planning on cleaning carb and running some seafoam through it this weekend. I'm trying to get it ready for duck season to get to the blind, and while it will get me there, it's just slow and won't get on plane.

If anyone has any resources/videos/knowledge on how to trouble shoot this, I would be greatly appreciated!
 

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
39,164
Well---95 PSI is NOT good enuff.-----Did you install a new water pump impeller ?-----Mixed gas /oil at 50:1 in the tank?-----Does spark jump a gap of 3/8" on both leads ?-----Is the flywheel loose?----Nut properly torqued?----Checked the gear oil ?----Inspection is up to you.----I can not see , feel your motor.
 

AM12

Recruit
Joined
Nov 19, 2025
Messages
2
Well---95 PSI is NOT good enuff.-----Did you install a new water pump impeller ?-----Mixed gas /oil at 50:1 in the tank?-----Does spark jump a gap of 3/8" on both leads ?-----Is the flywheel loose?----Nut properly torqued?----Checked the gear oil ?----Inspection is up to you.----I can not see , feel your motor.
I'll check these - As mentioned this is my first boat/engine, so I'm still learning.
 

saltchuckmatt

Captain
Joined
Jul 19, 2019
Messages
3,016
I'll check these - As mentioned this is my first boat/engine, so I'm still learning.
NOT good enough....compression.

First line of racerone's comments.
Time to give up on this motor. Did the mechanic who "tuned it up" know about the compression numbers?

Might want to seek out someone else if he did.
 
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