Air in fuel line

I had a problem this race weekend that is baffling me. My normal procedure at the end of race day is to empty the gas tank and fuel line. Race day I fill the tank and purge the line till it is free or air. First practice it ran fine for 8 laps or so then I started getting a brief lag like I was running out of fuel. Returning to the pits I noticed about 1/3 of the line had air in it (nearest the tank).

I cleared the line again and returned to the track and the same situation occurred. About 8 laps in I would get a brief lag. Back in the pits I noticed the same issue with air in the fuel line. I replaced the fuel line thinking maybe a small hole. First qualifier I had the same issue with stuttering lag about 8 laps in. This time I clamped the ends of the in tank fuel line with safety wire.

Next qualifier the same issue. For the final it was suggested to try a fuel filter at the low point in the line to act like a reservoir and while that did take care of the stutter / lag I still had air in the line as before. The tank is new (3rd race) and the only thing that was changed between last race and this race was I replaced the diagram only on the carb (KA100).

I’ve never had this issue before, but in talking to other racers some have this issue but never to the extent that it causes performance issues. Any ideas?

Start with the obvious and replace the diaphragm again, double checking that the diaphragm and the pump gasket are installed in the proper order. Verify the weighted end of the fuel pickup is still attached and the tank vent is open.
Has the carb been off of the motor?

The week before the race I did an on the kart change of the diaphragm, did not remove the carb and reused gaskets. Yes the weighted pickup is attached and secured to the fuel line.

I don’t see popoff mentioned in your post. Has pop off been checked on your carb?

A picture of your fuel tank, along with the nature of the pickup tube in the tank, or whatever way the tank uses for its pick up device, would be a big help. The fuel, sloshing in the tank, can momentarily uncover the pickup tube in the tank. At that very brief moment, air can get pushed into the fuel line. Someone else will have to tell you why that air stays at the lowest point in the fuel line.

James - Last time I did a complete carb rebuild (about 4 hours of run time ago) it was set at 11psi.

Al - This is a new 5qt Freeline tank with the supplied weighted pick up. The pickup hits the bottom of the tank and bends in one direction. I run the tank about half full. The air is from the high point and then down towards the engine for almost 1/3 of the line that is about 30 inches long. Prior to this weekend at most I would get maybe 2" of air at the high point.

Was it checked again (Both pop and hold) after the diaphragms were replaced?

Basically we want to rule out your carb as the issue.

I’m not paying too much attention to the air pockets because sometimes that happens. But the performance dropping right after the carb rebuild makes me suspicious.

Sidenote: I like to run the filter/weight in the tank slightly above the floor so it is free to move with the fuel at all times.

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Sometthing to try. Fill the tank to the top and see what happens. 4 hours run time, how does that compare to sitting time. I’ve seen tank pick up tubing harden over time. Something to look at.

I had this issue just recently, daughters kart was dying through a long flat out corner. When she came in there was a huge air bubble in the line. The pickup was like you described, against the bottom of the tank but bent a long way to one side, enough to be touching the side of the tank. I replaced the line inside and shortened it, so the pickup barely scrapes the end. Worked well this past weekend, first time i have seen a solid feed with zero bubbles.

When I first started building floor tanks, about 1974 I think, I pretty much stumbled on to the way I designed the sump. I’ve never heard of anybody having any trouble with air bubbles since. Tab Bell, my welder back then, is still building tanks if you’re interested.

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3 posts were split to a new topic: Why do most karts not use a gravity feed fuel system?

Could be sucking air somewhere in the chaser hose assembly inside of the tank.