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Tohatsu 15HP strange missing

J

Jimrem

New member
Model: MFS15B2
SN: 019278XE

This is a low hours motor that has been kept inside and is like new. I recently had the WP, all of the fluids, filters, etc. replaced and had the carb and fuel system serviced to get it ready for use this season.

The carb was removed, soaked, cleaned, and all of the work was done by a very experienced tech at our local marine service center. We use only non-ethanol gas so it was very clean.

Here's the trouble:

On the ears/hose: Starts right up, idles perfect, revs to full throttle in neutral, revs to full throttle when in gear. Purrs like a kitten.

In the water: Starts right up, idles perfect, revs to full throttle in neutral, put it in gear and start to get on plane and it starts sputtering like someone is flicking a light switch on and off. Idle down and it runs rough. Take it back to the dock. Let it sit for a few minutes and it acts fine like in the On ears/hose description above. Back to the water, start up on a plane and the sputtering starts again.

The red warning light never illuminates when it is acting up.

I took it back and they went through it and could find no problems. Seems to only happen when on the boat and actual torque is demanded.

Any idea what is going on here?
 
Don't ever go to WOT without a load on the motor. You could really scatter things. The conditions under load are significantly different than idling or unloaded. The fuel metering must be more accurate. Verify that you don't have any flaws in the fuel delivery system. Start at the tank. Make sure the venting is clear. Confirm that any tank strainer is immaculately clean. Double check the primer hose connectors for lazy or worn o-rings. Run a vacuum test on the fuel pump. If you draw a significant amount of fuel for WOT under load, any minor vacuum leak in the gas line can admit air into the system. Quite common on some water-separator filters. If your plugs are not coming up dark, that is a possible culprit. Also confirm that the valve lash is correct, and that the ignition system is top notch.
 
Thaks Paul. I know that the valve lash has never been adjusted. I would like to check. What is the lash supposed to be? Check at TDC for each cylinder correct?

I'll also go through the fuel system, vacuum test, and look for any vacuum leaks. Good ideas.
 
Full information is in the Fcatory service manual, available from any dealer, including me. The cold valve lash specs are:
Intake - 0.008 - 0.040 mm
Exhaust - 0.025 0 0.057 mm

Start by diagnosing the fuel system, then verify the electrical system, and finally the mechanical system. Odds are you have a fuel delivery malfunction, or a vacuum leak.
 
Going through the checks, I found that the thermostat is frozen shut. I want to replace it.

What is the part number for the thermostat and where can I get one?

Model: MFS15B2
SN: 019278XE
 
3R3010300M THERMOSTAT (60 DEGREES CELCIUS) $36.49
346010320M GASKET, THERMOSTAT $1.30

Available from any dealer, including me. Did you test the stat in a pot of water? Is the motor overheating?
 
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