I just bought some locally, 99% sure it was 5/16.

Guys, 5/16 ID fuel line hose is better for fit and function.

1/4 is a little tight on the nipples and it has a problem flowing air and fuel at the same time (which occurs when you run the bowl down at a reserve changeover point).

Folks don't realize that an equal volume of air has to go back up the hoses at those "run it dry" changeover points and they wonder why their bike stutters and pops so much when they go to reserve.

Going larger on the hose helps during the transition periods.   This issue is aggravated in the Chinese Rip Off Raptors because the internal nipple size ID on those is smaller yet than the 1/4" ID hose we recommend you not use.

Best results come from full sized nipples ( the Stock Raptor Petcock) and 5/16" fuel hose.

I am in complete agreement that using 1/4" line will shortchange you in flowable area which will probably be needed at or near full throttle.

I debate the idea, as posted by OF that you need to flow fuel and air in the line at the same time.  There is a reason the bowl is vented.  It's so air and evaporation can escape without pressurizing the fuel system.

Steve H,

Good point about the air volume inside the bowl itself, you are correct about that largish bowl volume being able to be vented out through the air vent hoses.

I was talking about volume in the hoses and I really wasn't very clear that I wasn't talking about the bowl volume because in all our previous discussion threads we all knew the action area was above the needle valve as gasoline stacks up over the valve making an air-proof plug in the system.

Fuel filter volumes however and the volume of the empty hose itself above the needle valve must still be back vented up to the gas tank by dual flow of air and gasoline at the same time, gas down and air up.

The first half inch of gasoline that arrives does make a solid fill above the needle valve restriction which acts as an air plug.  All the air above it has to either percolate back up the hose or else it would have to move down into the bowl as interspersed chunks of air and fuel going through the needle valve.  

Both likely take place to some degree or another at the very very beginning of the fill up flow.  Logically, when the fuel begins to stack up longer and longer on top of the needle valve it gets harder to see that any trapped air will make it out that way -- so we can see the air having to go up the hose instead.

Clear hose people report that the air does go back up and there is  active interchange activity taking place inside the 1/4" hose volume and that the larger 5/16" hose ID helps to get the dual flow all over with much quicker.

But, if you were curious, Lowes sells clear nylon hose ....  it ain't fuel hose but then again you aren't going to leave it on there for very long either.   Buy a foot of 1/4 and a foot of 5/16 and you can satisfy your curiousness empirically.

And you can tell us what you find out and then we can call you "the fuel flow expert" from now on and refer to "the flow testing done by Steve H back in the day".    




Both the guys who did the all the original petcock filter and line flow stuff aren't on the list any more, so we are due to have a new expert on the subject.