Nissan/Tohatsu 4/6 hp 4 stroke cam

[jmoser]

Thanks to much help from this site I am considering buying a Tohatsu 4hp 4 stroke and rejetting the carb to the 6 hp specs.<br /><br />A trusted mechanic friend warned me to check the camshaft P/Ns first as he experienced differences on other makes [Honda I believe?]<br /><br />In any case if anyone can confirm factory camshaft or powerhead P/Ns for the Nissan/Tohatsu 4 strokes in 4 and 6 hp I would greatly appreciate it.<br /><br />All my web searches failed to turn up a Tohatsu or Merc parts fiche for these models.

 

[TOHATSU GURU]

Re: Nissan/Tohatsu 4/6 hp 4 stroke cam

They are the same. Once you replace the three carb parts you will also need to enlarge the fixed air jet too.

 

[WillyBWright]

Re: Nissan/Tohatsu 4/6 hp 4 stroke cam

Please explain to me how you expect rejetting to magically give you more horsepower. The mixture is rather specific. Straying from that mixture is going to give you Less power, not more.

 

[TOHATSU GURU]

Re: Nissan/Tohatsu 4/6 hp 4 stroke cam

By changing the two jets, one nozzle and enlarging the fixed air jet you end up with the same carb as on the six hp. Then you have a six hp...It's not that you are increasing hp so much as you are bringing the retarded 4 back up to the design spec for the 6.

 

[jmoser]

Re: Nissan/Tohatsu 4/6 hp 4 stroke cam

Elvin you are a godsend.<br /><br />What diameter should the fixed air jet be reamed out to?<br /><br />Many thanks -<br /><br />Jeff

 

[TOHATSU GURU]

Re: Nissan/Tohatsu 4/6 hp 4 stroke cam

I have no earthly idea. We don't have a spec for it.

 

[TOHATSU GURU]

Re: Nissan/Tohatsu 4/6 hp 4 stroke cam

Update***You may not have to do anything other then changing the two jets and nozzle. No one is certain sure that anything else is really different

 

[jmoser]

Re: Nissan/Tohatsu 4/6 hp 4 stroke cam

Thanks again Elvin - I'll look into this some more. I had to drill out the air hole on my motorcycle carb when I rejetted it, that kit came with the proper drill bit!<br /><br />To expand on WillyB's inquiry:<br /><br />A) If done properly you will not change the air / fuel ratio (mixture,) just allow more of it to flow into the engine. Checking spark plugs for proper color, etc. is a good 'poor man's' method of verifying proper mixture.<br /><br />B) Gas engines run 'best' at a precise air fuel ratio of around 14.5-14.7, known as stoichiometric. If you tune the A/F ratio Leaner (more air) to around 15.0 you will increase fuel economy but decrease power. Maximum output (torque) of any gas engine is reached at A/F ratio of around 13.2; this is quite rich and fuel economy suffers, emissions also increase. <br /><br />New outboards are tuned to meet EPA emissions regulations, meaning they are running right at stoich or possibly slightly lean. A little 'richening' will significantly improve power and be smoother running. Only if you lean it out will power suffer.<br /><br />(P.S. it is 'officially' illegal to rejet any engine certified to EPA regs: cars, trucks, boats, even lawn mowers. All this is just hypothetical!)<br /><br />Many thanks again!