2006 6hp 4 stroke stalls after 20 minutes and will not restart

I have a 2005 4hp 4 stroke Tohatsu that I converted last year to 6hp by swapping out the carb. Ran great after the conversion even putting a 10ft air floor inflatable up on a plane. Great upgrade.

This year, I put in fresh gas and the motor ran great: for about 20 minutes after which it would not restart. First time it stopped while idling at the dock; once it stopped while under load at about 1/2 throttle. It will not restart even with engine start spray. It won't restart even if it sits for 2 hours. If it sits overnight, it starts again with a single pull.

I drained out all the gas and put in fresh; replaced the fuel filter; throughly drained and cleaned the carb; put in a fresh correctly gapped plug and ran it through a full tank of fuel, much longer than 20 minutes. Stumbled a bit going to full throttle but basically ran well. It seemed to have somewhat less power than before. Problem (mostly) solved! Or so I thought...

Yesterday, it died again and would not restart.

Thanks for any and all suggestions.

Re: 2006 6hp 4 stroke stalls after 20 minutes and will not restart

Of course this calls for the Factory service manual (if at all possible) and basic 4-stroke troubleshooting. Here is the shade-tree procedure:

1- Verify ignition. If you have a coil or CD that is failing when hot, get the motor to fail, then remove the plug (inspect its condition) and hold it to the engine block ground. If you have ignition, you will see a spark for each compression stroke when the plug is held to ground and the recoil is pulled.

2- Verify fuel delivery. Does the plug look wet? If so, it is getting fuel, but not igniting it. Do you have the internal/external tank setup? If so, are you on the internal tank, or the external tank? The internal tank will likely deliver some fuel by gravity, but if that tank is low, or if you are on the external tank, the fuel pump must be working. You can disconnect the fuel line at the carb (use due care around gasoline), and cycle the recoil to see if fuel gets delivered to the carb inlet.

3- The 4 strokes (as defined by F1 racers) are "suck, squeeze, bang, blow". As long as the motor is mechanically sound, we are mostly concerned with the 1st and 3rd strokes. If you have verified good spark and good fuel delivery to the carb, yet have a dry plug, the problem is likely in the carb. You could have a sticky needle, shutting off fuel even though the bowl is low.