Tohatsu 8hp difficult starting and dies after running

[Tnora]

Hello, I have a 2004 MFS8A 8hp and after cleaning the carb completely, I still have difficulty starting "cold" motor. After 20+ pulls, the motor sputters to a start and then runs perfect. But after running at full throttle, crossing the lake, the motor dies after 10mins, for no reason. Then the motor starts right up but then dies again after about 10 mins full throttle. Runs great and never dies at low trolling speed. I've also checked plugs and they are clean and correctly gapped. I also noticed that my 3gallon gas tank builds up a lot of pressure sitting in the sun and at 6000 feet sierra mountain elavation. I've opened the cap to release pressure but not sure if that has helped the hard starting part of my question. Im not sure if these starting and full throttle problems are related but any help would be very appreciated.

 

[pvanv]

Sounds like a fuel delivery problem. What method did you use to clean the carb?

If starving for fuel over time at high speeds, you could have a vacuum developing in the EPA tank (on that model year, your motor does not require a sealed EPA tank). If the tank vent is not open, that could also case that problem. Likewise, if there is restriction, such as a partially-clogged filter, that could limit running at high speeds. Another possibility is an air leak in the fuel system, allowing air to get in instead of fuel. Remember that everything between the fuel tank and the fuel pump is under suction (not pressure), and a failing o-ring in a primer hose connector could cause high-speeds to "run out of gas".

As for cold starting, your motor should start after about 2 or maybe 3 pulls at most (1 pull is ideal). If not, something is wrong -- either technique, an adjustment at the carb, or something else. It's always good to throttle up a little when cold starting; If you are at dead slow idle setting when starting cold, that could be the issue -- or if the adjustment for the throttle advancer arm isn't correct when choking, you won't be getting the extra throttle that is needed for cold starting.

 

[Tnora]

I used gumout carb spray cleaner. I dismantled the carb completely so I could spray out the high speed jets and low speed jets, and the float and needle valve. I got all the ethanol jelly cleaned out. I'm using this 3 gallon Atwood fuel tank. It says the single cap is vented. There is no relief vent screw on this tank. You said the fuel system is under suction so would the expanded pressure in the gas tank from the sun exposure affect the suction properties or not? You also mentioned a partially clogged fuel filter. I can replace that with a new one. This last Sunday I used the motor at high speed and low speed and it started with 2 pulls and it didn't stall at either speed. I had reinstalled the motor on my boat from it laying flat it the back of my suv with the carb drained. I had done this same situation with laying my motor in my suv, carb drained, at another high altitude lake a week earlier, and that's when I had to pull 20+ times to get the motor started. I have to pack the motor this way because I have a wooden transom and some of the back roads to the lakes are very very rough which could damage the motor or transom from all the bouncing. Anyway, the motor has been very temperamental but worked great this last Sunday. BTW, is the throttle advancer arm the linkage from the throttle knob to the carb butterfly plate or is it something else?
Thanks.

 

[pvanv]

You did not clean the carb. You made it shiny. Do a 4-hour soak of all disassembled parts (including jets and emulsion tube nozzle) in a 1-gallon can of real carb dip. Then blow out everything with a whole can of generic carb spray.

If you can get a real original type Factory fuel tank with vent, life will be easier. No, your tank does not vent in the conventional sense. It requires about 5 psi to either let fumes out, or let air in. Due to the EPA, they are NLA new.

You must disconnect the gas line at the motor and idle it out, or drain the carb with the drain screw, every day. Otherwise, varnish will accumulate. Regardless, you must disconnect the primer hose every day, or the increased pressure in the tank will overwhelm the carb and fuel pump (it can blow out the diaphragm in the pump), and will both dilute the oil and flood the motor.

You can use a transom saver if you want to keep the motor on the boat when trailering.

The advancer arm is on the side of the carb. You may want a mechanic to help you with that if you continue to have problems.