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2010 Tohatsu mfs15c carb upgrade

B

bmb779

New member
On a 2010 15 hp C motor to upgrade to 20 HP does the whole carb have to be changed or can you just change jets etc? Many thanks.
 
Left the 20hp jets and emulsion tube in place,ferreted around and found blanking plug just above throttle spindle removed plug and adjusted mixture in tank and fine tuned on the water,engine now pulling 6000rpm and purring like a kitten. A lot cheaper than a new carb.
 
Hi,
I just replaced jets for 20hp model, but engine is not running with throttle possition over 1/2.

what should be tuned/adjusted after rejetting ?
thanks
Rob
 
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.
 
pvanv said:
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.
Click to expand...
I quess you all your answers to new members are so rude. That was not helpfull at all.
You can always check that there are the same parts for 15-20HP 4str. like intake mainfold, and the same elements for carburator - just different main jet, main nozzle, main jet plug and slow jet (the rest is identical) so I think your bigger throat in 20HP carb doesn`t make sense.
Rob
 
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.
 
pvanv said:
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.
Click to expand...




On the off chance anyone is still following this thread... I too am looking to replace the carb on my mfs15c with a 20c. All the above info was very helpful. However, I'm struggling to find the right carb on various sites. Any help, links or direction would be most appreciated.

Many thanks,
Mike
 
3BJ031000MCARBURETOR, F20C (MF/EF)$266.37

3BJ031330MCARBURETOR, F20C (EP/EPT)$266.37


So there are 2 different carbs, depending on whether you have tiller or remote.
 
Yes. F is for "fishing" which is tiller. P is for "pleasure" which is remote (electric choke).
 
Hi Folks - just picked up a used Mfs15c tiller and was super delighted to find this thread and learn that carb swap could get me to 20hp.

Got the service manual and saw a video on YouTube of a tech taking out and replacing carb on Merc 15hp 4 stroke which looked identical to Tohatsu.

Before I start had a few questions:
- confirming all that is needed it straight swap of carb for 20hp carb?
- when removing carb looks like u take off manifold, carb and silencer as one piece
- do I need to replace the gasket that seals manifold to engine and gasket that seals carb to manifold (service manual says not to reuse)?
- any sort of sealant needed when reassembling or just the gaskets
- 20hp is listed as rev’ing higher - will rev limiter on 15hp kick on prematurely?
 
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.
 
pvanv said:
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.
Click to expand...

Excellent - a straight part for part swap without adjustments/tweaking i can handle:) Feel like i lucked out as initially i was going to try and sell the 15hp to get a 20hp but no need now!

For others who want to attempt i found this great video. It is of a Merc but believe this is Tohatsu made.

https://youtu.be/vlXNXKWUxC8
 
Last edited:
Hi Confirming which gaskets/seals need to be ordered in addition to the carb assembly.

Based on diagram - assuming parts 14, 7(x2), 8, and 2 would be needed?

TohatsuCarb.jpg
 
Just 2 base gaskets (7), and the intake gasket (2). You can re-use the plastic spacer and the o-ring.
 
Thanks so much Paul - really appreciate the guidance and patience! Do u think going from 15hp to 20hp will require a new prop? Was going to buy inductive tach to see what is maxes out at in current 15hp config.
 
Also to keep Carb running clean - any fuel additive recommended? Dealer told me to watch use of Seaform since it could harm plastics and fuel hose. On erect I used to run Etec cleaner twice a year through tank and never had issues.,
 
Just run the carb dry if sitting for more than a couple of days. Seafoam is just mineral spirits, so it does virtually nothing. If you forget to run it out, drain it with the screw.
 
According to service diagram there are two drain screws I-33 at very bottom and I-17 higher up. Assuming it is the lower one at very bottom of carb?
457B7B3D-6AAD-46AE-A659-47D0A0585377.png
 
The tohatsu 9.8/merc 9.9 is different from the 15; While possible, it would be very very difficult to remove the carb without taking off the carb and intake as a unit.
 
Good point Paul - i had a closer look last night now that motor in off boat in my garage. I couldn't fit a socket in there - the air intake bulges out and makes it virtually impossible. So i totally see why it has to come off as one unit now. No biggie - i got the seals for manifold on order.
 
Thx Paul for help! Got the swap done and seems good. At first the new carb was idling at very low RPM - about 850 vs 1200 on the old 15 carb. I adjust stop screw and have it around 1100 and not vibrating as bad. Only able to test in water tank so will need to wait until next season for WOT lake test. Assuming it was ok for me to adjust idle up? Was very rough below 1000.
 
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