pressurized gas tank?

craziest thing happened while riding the other day have a yz426 supermoto and shut it off to figure out next spot to ride then it wouldnt start back up after kicking 4-5 times noticed no fuel in line going to carb so checked petcock and then removed gas cap when removing cap a ton of pressure released and some fuel even shot out every time i would kick the bike it was building pressure in the gas tank so we tried push starting it couple of times almost fired up but didnt about to give up and call someone to pick me up i kicked it one last time fired up and ran like nothing ever happened all the way home shut it down and it fired right up again first kick im so confused on what happened and what was causing the fuel tank to build pressure if anyone has any input that would be greatly appreciated  

Clogged vent line.  Vapor lock.

Cut the ends of all four of the vent lines.

yea thats what i did i pulled all the vent lines off and its possible it vapor locked then i guess when it cooled off it started breathing again we were riding in some heavy traffic was prob a little hotter then normal when i shut it down 

and a cloged vent line on the carb would cause it to actually send pressure up the fuel line and into the tank ??

and a cloged vent line on the carb would cause it to actually send pressure up the fuel line and into the tank ??


No.  Furthermore, there is no possibility of that happening.  There is no path by which either cylinder pressure or crankcase pressure can be introduced into either the fuel tank or carburetor float bowl.  


If pressure was building in the tank, there was another reason for it, but understand that the carb is fed only by gravity, and with air pressure in the tank, fuel would be forced into the carb, not out.  But you mentioned heat.  A lot of the fuel available these days is a bit sketchy as to vapor pressure and such, what with ethanol and all, so you could have been heating the fuel in the tank, but again, that should have led to a flood condition, not a dry one.


More probably, you have mistaken the "pressure" in the tank for vacuum, which would be the simple result of a clogged tank vent.  Blocked carb vents can also cause delivery problems.  Check the fuel flow by disconnecting the line from the carb, turning on the gas, and letting it flow into a container.  The rate should be steady.  If it tapers off, open the gas cap and see if that helps.  


No problem there?  Reconnect the fuel line.  Drain the float bowl, then close the drain and turn the gas on.  It should take no more than 3-4 seconds to fill it, so turn it on, count 4, shut it off and drain again, checking how much comes out compared to how much comes out when you just let it sit for several seconds with the gas on. 

I second what Greyracer said, It was likely a vaccume in the tank. I have seen this happen on a FI street bike, the vaccume was great enough the calapse the tank, (the owner had recently purchased an aftermarket gas cap, and the cap vent had not been tapped properly).

It was not vacuum when I opened the cap gas actually shot out of the fill hole it had so much pressure it was the craziest thing I've ever seen and I'm very mechanically inclined built bikes rebuilt car motors never seen this before

And I did remove fuel line gas flowed out fine I have a clear fuel line hooked it back up gas went to carb kicked it and fuel tank built up pressure squirted out vent hose and clear fuel line had no fuel in it was the strangest thing I removed all vent lines from carb pulled gas line off again fuel flowed freely so we push started it couple times nothing about to give up and kicked it as hard as I could jumped up then down and it fired and ran fine it was the weirdest thing and why I asked here because it dosnt meke sense mechanically wanted to see if anyone has had anything like this happen before or if it was vapor lock don't understand exactly how vapor lock works

A hot engine makes the fuel boil/evaporate quite a bit. That will cause pressure if the tank vent doesn't work. Had a Ktm that would push fuel out to the carb and the bike would stop running it was so rich. Problem was a plugged vent in the fuel cap

I have also seen bikes have a plugged vent and stop running due to no fuel. In that case it was a vacuum and the bike would run for a few miles then quit.

Also have had vapor lock issues in two different road race bikes. Took a long time to solve both of them. One bike always ran great in practice but would run like hell after a lap or two in a race. Turned out practice was always cool in the morning and races were run in the hot afternoon. Added a heat shield for the carbs and the problem never came back. Other bike would run great in practice but run progressively worse as the day went on. Turned out we were adding race gas at the track and the dealer who sponsored the bike used pump gas. So the more race gas we added the worse it ran. Fix was to run all pump gas.

Edited by slotracer577

Then the vent line was clogged and the heat of the motor built up positive pressure. Also common on 4 strokes.

Since it's the same pressure chamber as the float bowl, when the needle is open, maybe its closing the float bowl too early or with too low of a fuel level? Starving the jets of fuel?

It could of been just so hot it was actually vapor locking from the fuel line and bottom of tank and just a coincidencee that when I would kick it I would put the cap on so it would build up pressure from being hot idk but it was unbelievably strange the more I think about what happened the more it is seeming like vapor lock

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