New Developments on my Tohatsu 15 4 stroke

[boatster]

So I took the motor out today for a test run again. It is behaving very odd.

Starting isn't too difficult anymore. My new problem seems to be the motor surges in fits at full throttle. The low pressure oil lights up intermittently. Eventually the motor will just die. I can immediately restart the motor again but running at full throttle results in the same surging with the eventual die. I can reasonably run the motor at just above minimal planing and not have the surging but it seems to be a delicate balance.

I checked the oil level and it is full. I have not replaced the oil filter though but I doubt that could be the cause. The motor oil on the dipstick looks brand new clean.

Is this a fuel delivery problem? Inline fuel filter? (not likely I think). Fuel pump?

What about the timing belt? The timing belt feels a little loose. Shouldn't it be extremely tight?

The spark plugs look clean and brand new. They are gapped correctly. I poke a bright flashlight into the piston walls and they are not scored.

???

 

[TOHATSU GURU]

Re: New Developments on my Tohatsu 15 4 stroke

The first thing you need to do is stop and begin troubleshooting the entire engine fuel system, electrical system and mechanical components. Start with either the fuel or electrical system and begin running every check in your service manual.

 

[Snoopywoopy]

Re: New Developments on my Tohatsu 15 4 stroke

Looking back at your posts with this motor, I notice that you just bought it this year.

Were you able to run at WOT (verified on a tachometer) before the carburetor got all cleaned up?

If not, can your WOT surging problem possibly be from ventilation or cavitation?

My point is, if you've never run this engine at WOT on this hull, you might have an issue with the hull, lower unit, prop, or installation thats causing the prop to ventilate or cavitate. You didn't mention if the surging is "WOT then loses RPM" or "WOT then gains RPM". This can make a big difference in trouble shooting the problem.

Just a thought.

 

[boatster]

Re: New Developments on my Tohatsu 15 4 stroke

Hi SnoopyWoopy,

It is actually dropping rpm from WOT.I didn't verify if its actually running the full rpms at WOT with a tachometer though. But I know the carb is opening up to max. Do you know if this motor has a temperature sensor of some sort? The model is NSF15b. I wonder if its simply the t-stat not opening properly and it overheats and bogs. This happened with an older Tohatsu 8hp I had. I replaced that t-stat and it was happy again.

So its running WOT then drops rpm then surges, then drops rpm etc etc. Eventually the motor will just die. I will come off plane then simply restart the motor again at idle. Could the motor be overheating? I'm going to replace the t-stat and give it a go again. Does that make sense?

Either way I'll have to get a service manual like Elvin stated.

 

[TOHATSU GURU]

Re: New Developments on my Tohatsu 15 4 stroke

It has a T-stat, but that's not going to be the problem.