Need some Tohatsu advice

[rogerwa]

I just bought a 25 ft pontoon with a 92 tohatsu m40D outboard. It was outfitted with a 12 or 12.5P prop.

It starts easily bout when I give it juice, it will either bog and die or if I advance the throttle enough it will stumble and then take off. A smooth advancing of the throttle will cause it to stumble and not rev up to where it should be.

when I do get it to about 1900 and leave it it will lose RPM, like it is flooding or not getting enough I am not sure.. Alone in the boat at WOT, it will do about 4400-4500 RPM. Low I know but it will run consistent. It performs pretty well from 2000-WOT.

The old couple I bought it from never did much to it, so I know I need to at least replace plugs and impeller.

Tips on the problem above?? I am guessing a carb rebuild is in order, but would like opinions on whether it is running rich or lean in the low RPMs..

Any opinion would be great.. Thanks..

 

[TOHATSU GURU]

Re: Need some Tohatsu advice

The prop pitch is too high. Take the engine into a dealer to have the carbs cleaned and don't worry about whether it's too lean or too rich. If the carbs are dirty it's starving for fuel on a jet.

 

[rogerwa]

Re: Need some Tohatsu advice

What would you recommend for a pitch?? A 10 or a 9?? We often have quite a few people on board..

I will probably rebuild them myself, I have done that before.. I just have never calibrated three at a time. But I will probably order the service manual and parts from your site..

 

[TOHATSU GURU]

Re: Need some Tohatsu advice

I would say either a 7 or 9. Nine would be conservative.

 

[rogerwa]

Re: Need some Tohatsu advice

A couple of questions on prop selection.. I am thinking about picking up this prop.

http://www.iboats.com/mall/propfind...id=328&**********=963809864&*******=115616163

It is an 8p prop. The turning point site recommended either the 10.5p or the 8p depending on usage.

So with the real world numbers with 11 people on board, I was only getting about 3800 WOT with a 12P. According to the calc, assuming a 10% slip, it says I should be doing 21 MPH.. Obviously I was doing nowhere near 21 MPH. Without a GPS onboard, I am guessing that my slippage was somewhere in the 30-50% range. With a speed of 11-15 MPH.

If I went to an 8, would it be reasonable to asusme I would get less slippage, more RPM (800 potential by reducing 4" in pitch), and greater speed?

It certainly would be healthier for the engine, but want to know if the lower pitch would just give me higher RPMs and a still slower ride or if it would increase the performance by reducing slip.

 

[TOHATSU GURU]

Re: Need some Tohatsu advice

I can't really advise you on aftermarket propellers as we have no experience with them and only deal in OEM material. But, in general, an 8 pitch would seem to be in the right range as it should, in theory, reduce the slip and increase the RPM as you have surmised.