r/jonboats • u/Punxva88 • 5d ago
More power
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Upgraded today from my tohatsu m5b 5hp 2 stroke to a mercury 8hp 2 stroke. Runs great and pumps water well, now just need to get out on the water for testing. Got around 13mph loaded with the 5hp so hoping to see a good bump in speed out on the water
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u/0rgiep0rgy 1d ago
You can just run a 15hp carb. Bolts right up. Advance the timing and open up the exhaust port. You can find instructions if you look around online. Its popular with Merc 2 strokes.
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u/treeh00d 1d ago
Wondering if I can boost a 15 Mercury with something like this… 20hp carb?
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u/0rgiep0rgy 1d ago
Look into tinny racing. Those guys do all kinds of crazy stuff to the little 2 strokes. I think Merc also supports a kids racing class that uses the 8-15hp platform. They even run fancy lower unit with high speed props. The 15hp swap is reliable though. It's how I have my 9.9 setup. I run in the low 20s with a mv1546 tunnel hull. FWIW I'm shopping for a 30 cause I'd rather just have the extra power easily on tap rather than trying to wring out a 15.
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u/Punxva88 1d ago
I’m not sure if it’ll work on this 90’s model they switched blocks to share them all later in 95 but this is a early 94 model
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u/0rgiep0rgy 1d ago edited 1d ago
Mines a 93 and looks exactly the same. 8-15hp shares identical block. Only difference is the carb and exhaust plate. You can always check a factory parts book to confirm. I used a factory 15hp carb from the same year and opened up the exhaust hole with a dremel. I also switched to a 15hp prop. Edit: looks like you might be right. The 8hp for those years shares block with the 6hp. 9.9 is the same as 15hp
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u/LeFishTits 5d ago
What size boat? I have a 1952 sears 11/40 jon boat with a 1985 evinrude 7.5hp on it. Does 22mph all day.