Tohatsu 4hp upgrade

I bought a Tohatsu 4hp M4C motor which was seized. Off I go to the marine wreckers to find a 5hp motor (M5B). The first difference I found was theyre the same motor except the 4HP has one reed valve and the 5HP has two. That is in the 4HP one is blanked off. OK. Now Im seeing much conversation about the jets being different. Yet, having tuned motorbikes and replacing carbies, I'm struggling firstly why the slow jets in the 4,5,6 need to be different since Im understanding the throat size is all the same. The three engines should idle the same RPM as at slow speeds they are identical. The main jets when made bigger are allowing a higher above 3/4 throttle WOT speed. ie a jet too big even for 6hp rating will make the engine bog before max 6hp rpm's and a jet too small for even the 4hp rating will make the engine lean off and die, overheat, even when trying to achieve 4hp. Clear me up because I think alone the fitting of the second reed valve has made my motor more powerful. ie the throat was restricted by only one reed valve. Anyone know the Tohatsu jet sizes in each size motor. I'm believing the slow speed jets should all be the same. Penny for your thoughts.

 

Can you just run it at full throttle and then check the plug?

 

I have the Merc version of that motor.

My understanding, and this goes back a few too many years, but the difference is the carb itself.

The carb throat size is different, but externally the same, so they're interchangeable. This requires different jetting.

 

Totally agree. I also pulled both the 4hp and the 5hp to make up one, being 5hp and the carbs were different model numbers. Main jet is actually the same 78. Slow speed jet on 5hp is actually smaller than 4hp being 52. 4hp is 55. Yet the 4hp has one reed valve and one dummied closed. The 5hp has two functional reed valves. I found an American website parts listing which shows a table comparing the 4c to 5b tohatsus. I will do a wot plug check, albeit i find with modern fuels on fine tuned engines one can go through several jet sizes before seeing a plug colour change. Since my last post i see many people correctly say the carbs are different which i have only just researched and found myself but I emphasize, oops, emphasise, the reed valves need modification too. Good to hear from you guys out there

 

Porting may be different as well, so too timing settings. There are usually a few different things in play.

 

Supposedly on these the carb is the only difference, well the Reed cage too. Porting and everything else is the same.

People changed the carb and reed cage to increase it to next HP range. All other part numbers were the same.

I haven't used mine in maybe 7 or 8 years, and not much then, although I may get it running this summer.

I need to locate a new fuel tank for mine, it became brittle and the threads broke off so you can't tighten the fuel cap.

 

You may be able to just put an external fuel line and tank on since the carby has an integral fuel pump. I chose the external option as i will be trolling most of the day with lures behind. Does sound like theyre a great little motor. Rumours are they are quivalent to near 6hp on other two strokes. See how I go next week.