Yamaha 2b Strange fault

[KAM]

My trusty Yamaha 2b has developed a strange fault. Apart from cleaning out the carburetor now and again and changing the water pump its been amazingly reliable. Now if I run at full power for a few minutes it stops very suddenly, just as if I have run into debris, it starts again straight away and will run again for a few minutes. It will run all day at one third to half power. There is nothing wrong with the fuel supply as far as I can see and the carburetor is clean. If it was fuel starvation I would expect a slow down and stop and then difficulty restarting. It did this before and after cleaning the carburetor the floats and jets all look clear and in good condition. Any help would be much appreciated.

 

[lw395]

Things to check:
fuel not flowing into carb fast enough, could be rubbish in the tap or filter, or float valve.
tank vent stuck. I've seen the rubber washer weld itself in place so unscrewing does nothing, or nothing much....

I've had the first problem on a bike, I had to work out say 50mpg at 80mph, how long should it take to fill a jam jar.....

 

[KAM]

There seems to be a good flow of fuel when I pull the fuel pipe off the carb but yes it sounds like its running for a float chambers worth then cutting out. Its about the same distance from the boat as when I forget the tank vent but the difference is it restarts instantly. It does not sound like the tank vent as when I forget this the engine slows before stopping. This is a very sudden stop almost with a backfire.

 

[twohats]

Similar sounding problem with the same motor - turned out to be tracking across the plug cap.

 

[Fantasie 19]

Similar sounding problem with the same motor - turned out to be tracking across the plug cap.

Yeah - change the plug and see if you have the same issue...

 

[KAM]

Ahh yes I'll check out the HT. Still using the plug which came with the engine. Must be about 15 years old but usually looks fine when I do a visual on it.