My buddys 660 Grizzly sputterd today and had gas flowing out the upper carb vent. He went threw a small bog and it started sputtering. The gas tank had alot of air pressure built up in it. We got out of the bog and went down a hard pack trail and it would start sputtering again, he would on do his gas cap and it would run fine. I looked down into the tank and there was bubbles coming up threw the tank making it look like the gas was boiling. Do you thik he has a pinched vent some were or is this a sparkplug cap/water/steam problem or what? Any help would be helpful.

 

The pressure in the tank can only be caused by one thing the vent aint venting. Combo vapor lock, gas cap isn't venting right, probably with a need for dilectric grease too. Easiest fix, try grease on the plug boot. Second fix, make sure that his tank cap is venting, it seems there's a check ball in the grizz cap, that may be locking too much pressure in the tank. Third fix add additional heat shielding around the exhuast, although my grizz has extensive heat shielding already installed, I'm not sure if it's the same as a bit older grizz 660.

 

Do you think the heat from the head is building the pressure in the tank? I know the gas is not boiling, if it was that hot the tank it self would melt. I'll try the grease thing, and take the gas cap apart and see if its all working correct.

 

There is a good chance that your gas WAS boiling. Gasoline boils at a much lower temp than water does (you can put your hand in it), and what you described sounds exactly like what happened to my machine. It seems that there was (is) a problem with some bikes having too much heat build up in the engine compartment, and its actually vaporizing gas in the carb and/or causing the gas to boil. Yamaha has a heat shield kit (stickers I think) that is supposed to rectify the problem. If your machine does it again, try removing the engine covers. If this stops it, there is definitely a heat problem. For the record....I think Yamaha should be doing something about this problem. If it was a car doing this, you can bet the sh%t would hit the fan, why should we be treated any different?

 

I actually had this happen once. Never believed it until then, but it was a crock. I backed the bottom of the gas tank in reflective heat tape, and wrapped the pipe from the header to back past the carb with Highlifter heat wrap. Have not seen that problem since. knock on wood