I pulled my fuel tank check valve and it is bad. I have been having problems with the fuel system holding prime. The supply line goes dry after a few hours and the pump can’t pull up fuel to start the engine. The pump is a brand new mikuni part. Could the vent check valve be to blame?
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Can a bad fuel vent check valve cause fuel to drain back/not prime? (94 gtx)
- Thread starter Alexh886
- Start date
Yes it can, until you get your new valve just open the gas tank for a second
It lets air go both ways. Does this still apply?
It shouldn’t let air both ways. There should be one under the rib rail that lets air out only and one up higher sometimes under the front of the seat that lets air in
From what your describing it sounds like it’s the valve up high that is bad
Yes it’s the one up high that’s supposed to let air in. It’s letting air both in and out. My thinking is that it’s letting pressure out of the tank/creating a vacuum so the fuel supply line is running dry. When I pull the full supply line off it dribbles a bit and then is dry.
I guess in theory maybe while your driving the air passing over the end could create a very low pressure vacuum but I would think it’s negligible and would correct to atmospheric pressure the second you slowed down if in fact its letting air in and out. Also very odd that you’re only experiencing problems after a couple of hours of driving. Either way you’re going to want to replace it. The fuel tank should have a pound or 2 of positive pressure
The fuel system loses prime after sitting overnight. I’ll go to start it up and it just cranks, there’s no fuel in the supply line. If I reprime it by blowing in the return line it starts and runs fine.
Gotcha, I was thinking you meant after driving for a few hours it wasn’t able to pull its fuel, I didn’t realize you meant after sitting it would lose its prime. Now I’m only guessing but I’m guessing there’s a check valve in the carb that would stop this from happening but I’m not sure on it. Perhaps the 2 Mylar discs in the fuel pump block? I would have to read through the mikuni carb manual again. I suppose it definitely wouldn’t hurt to replace they check valve either way, as I mentioned there is supposed to be a couple pounds of pressure in the fuel tank and there’s not.
Yeah it’s not pulling fuel. It’s really strange. The fuel pump body is a brand new mikuni part. I got it from summit racing
The pulse feels decent as well. The line is a new piece of 1/4 fuel line and the nipple is clear
Can you explain which component of the fuel pump it is that’s causing his problem where the fuel drains out of the supply line while the machine is sitting? Just curious, is it one or both of those Mylar discs ?
I wish I knew man. I have a pressure tester coming tomorrow that I’m going to do some testing with. It must be something else on the carbs because I pulled a pump off of another set and the same thing is happening. This as well as the fact that the pump body and all rebuild parts I’m using are brand new oem.
Sorry I can’t help more, did you replace the 3 Mylar discs in the pump? There’s also a Mylar piece in the metering block, did you change it? For the ones in the pump, did you put the blue stripe up? You didn’t punch a hole through the grommets by accident did you? What type of hose do you have on the oulse line? Black fuel line is best. Once you get your tester in, see if the fuel pump will hold 4 pounds
This is the pump body I got. Everything was pre-installed. I used black fuel line for the pulse line. I’ve checked pulse a number of times while cranking and it seems to be fine. The ski will start and run but it is a pain. After sitting there’s no hope of it starting on its own. I have to dump a little fuel in the carbs and reprime the system by blowing air through the return to the tank.
what about the oring in the fuel filter has it been changed? If not, you can pressure test it as well. That could be a possible cause
