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I quess you all your answers to new members are so rude. That was not helpfull at all.Only an idiot would put jets for a 20 hp in a 15 carb. That would make the motor way way rich. If you want 20 hp, you need a 20 hp carb -- with its bigger throat.
No, Rob, you are mistaken. I was in fact originally helpful. In May... Half a year ago. Please review the beginnings of the post. I was not guessing, I was stating fact. I continue here:
The carb throat sizes are different. Period. Yes, the castings are similar, and the main/low jet locations do get shuffled between the models, but the correct air/fuel ratio must be maintained.
Yes, the 15C and 20C share blocks. That happened when Tohatsu dropped the MFS9.9/15B2 in favor of the MFS8/9.8 and simultaneously increased the overall frame size of the 15/20C. And, just as with all (yes ALL) Tohatsu small four-stroke families, the carb throat size is how the smaller-output motors are de-tuned for lower-output applications.
Examples:
MFS2.5/3.5... different carb throats.
MFS4/5/6A2/B/C... different carb throats.
MFS8/9.8A/A3... different carb throats.
and the winner, in your case:
MFS15/20C... different carb throats... once again.
Also, in most cases, the more modern carbs are even leaner, as they comply with more stringent EPA rules.
Carbs 101: The fact remains that if you add a ton of fuel by changing jets alone, you will end up with a too-rich mixture, as I noted originally in post #2, back in May, and as you have experienced... Unless you have some sort of magic manner to stuff more air into the mix as well. You need to maintain the correct air/fuel ratio... not just add gas. Motors do not run by gas alone.
This is the same situation that often comes up when someone wants to upgrade an MFS4 or 5 to a 6... or an 8A3 to a 9.8. They too must swap carbs.
BTW, tinkering with the low-speed pilot jet can mask the problem, as was noted... might even run sort of OK. Not only is that a violation of Federal law, but after disturbing it, getting it back to the correct adjustment without very good instrumentation and an exceptional ear is difficult at best.
Yes. Swap the carb.
Yes, remove the complete assembly.
Yes, use new gaskets.
Just dry gaskets.
No. Rev limiter will not engage if everything else is correct.