Tohatsu 3.5 4 strokes low RPM at full throttle - SailNet Community
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post #1 of 12 Old 08-14-2015 Thread Starter
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Tohatsu 3.5 4 strokes low RPM at full throttle

Hi there, I'm new around...I bought last autumn a brand new 2014 Tohatsu MFS3.5b (3.5 hp) which I had many problems with.

I brought it back to the reseller many times, they fixed most of the problems. But the last one it seems we did'nt find it yet.

Let me explain :

(The carb is brand new, changed 3 weeks ago)

When I use it on the water, with a 14" fiber fishing boat, there's no difference in RPM between full or 3/4 throttle. So it work fine at 3/4 throttle. The tachometer I bought give me around 4500-4800RPM. But in the user manual it said should run between 5000-6000 rpm at full throttle.

If I turn a bit more then 3/4 throttle it seems to want to go higher but stall at this rev around 4800 rpm...

I tried a mercury 3.5, a few year older than mine, which is the exact same engine, and difference in full throttle was very noticeable. Almost got my fishing boat on a plane with it.

So if somebody know those engines well, if there's adjustment to do, please let me knpw.

Also, it get it's 6200rpm on neutral without any problem. I have the same 7" pitch propeller which comes with the outboard.

Thank you

Last edited by Antares555; 09-01-2015 at 08:54 AM.
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post #2 of 12 Old 08-14-2015
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Re: Tohatsu 3.5 4 strokes low RPM at full throttle

You are exhibiting the classic symptoms of 'over-propping', ie, the prop's pitch must be reduced -and fairly dramatically, I might add.

The prop that came with the motor presumes a smaller boat/lower loading which will allow the boat to 'come on plane'. Not being able to run on plane, the boat is over-propped....the engine will not reach anywhere close to rated RPMs

Find a prop with a pitch of somewhere around 4.75" instead of 6" and try again....
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post #3 of 12 Old 08-15-2015 Thread Starter
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I bought a 4.5" pitch this week...I will try it this week end.
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post #4 of 12 Old 08-15-2015
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Re: Tohatsu 3.5 4 strokes low RPM at full throttle

Excellent choice. Please let me know how it goes....
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post #5 of 12 Old 09-01-2015 Thread Starter
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Ok...here's what I got with my Tohatsu 3.5hp MFS 3.5 2014...I made proper more accurate testing with my tachometer.

14' fiber fishing boat

7" of pitch 4500-4800rpm
6" of pitch 5100-5200rpm
5.7" of pitch 5300-5500rpm
4.5" of pitch 6200rpm...it reach the cut off at full throttle.

According to Tohatsu, this is the only propellers that fits my engine.

The thing is, when I tried the Mercury 3.5hp which is the exact same engine (a few year older then mine), It was reaching the 6000 rpm even with a 6" of pitch propeller on the exact same boat...like if the engine was more powerful.

So, again almost no difference between full and 3/4 throttle with a pitch of 5.7" and more. Is there something about the break in ? If I never reached the full throttle rpm (6000rpm) during the break in procedure, should I continue using it with a 4.5" pitch to make sure it reach it ?

Last edited by Antares555; 09-01-2015 at 08:49 AM.
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post #6 of 12 Old 09-02-2015
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Re: Tohatsu 3.5 4 strokes low RPM at full throttle

Did u test the skiff 'empty' or loaded as u would usually run it, ie, equating its real usage condition?

If u tested it in its 'fully' or 'as usually loaded' condition and it hit 6200 rpm thus, u might want to revert to the 5.7" prop. If the skiff was almost completely empty when u hit 6200, leave the 4.5" prop u have now because engines do not get stronger as they get older....
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post #7 of 12 Old 09-02-2015 Thread Starter
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I was with somebody on board and fully loaded with fishing gear, cooler, battery, electric outboard and stuff when it reach the 6200 rpm easily with the 4.5" of pitch.

My testing was with the same conditions everytime.

Since then, all the propellers have 7 3/8"of diameter. But I have one that I did'nt try yet, this one have 7 1/4" x 6" pitch. I will try it this week end...

Will the diameter difference can affect the RPM too ?

Last edited by Antares555; 09-02-2015 at 06:02 AM.
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post #8 of 12 Old 09-02-2015
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Re: Tohatsu 3.5 4 strokes low RPM at full throttle

Technically yes, diameter will also affect RPMS but the effect of that small a change in diameter will be negligible.

I think u will be happy with the 6" pitch....

Last edited by Galo; 09-02-2015 at 11:57 PM.
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post #9 of 12 Old 09-02-2015 Thread Starter
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I'll let you know...thank you very much.

Last edited by Antares555; 09-02-2015 at 11:50 AM.
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post #10 of 12 Old 09-02-2015
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Re: Tohatsu 3.5 4 strokes low RPM at full throttle

Thks
And sorry I initially recommended the ~4.5" pitch prop, but....I did that because your first post said your pitch was 6", not 7". Yeah....from 7" to 4.5" would have been too much a drop in pitch -as u found out the hard way...

The 5.7" pitch should be perfect, coming down from the 7" prop, but I suspect the 6" with the smaller diameter will be just as good....should get u into the 5200-5300 RPM range when loaded and up to 5600-5800 when lightly loaded


Last edited by Galo; 09-03-2015 at 12:27 AM.
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