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Message started by beacon on 03/13/14 at 13:15:10

Title: fuel line size
Post by beacon on 03/13/14 at 13:15:10

Can anyone tell me the size (ID) of the fuel line? I need to order some hose and I'm not near the bike.
Thanks

Title: Re: fuel line size
Post by Fishbulb on 03/13/14 at 13:59:16

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

Title: Re: fuel line size
Post by Greg on 03/14/14 at 09:37:06

I used 1/4" fuel injector line for mine. Worked well. And fuel injector line won't rot nearly as quickly.

Title: Re: fuel line size
Post by Oldfeller on 03/14/14 at 10:01:09

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.

Title: Re: fuel line size
Post by Dave on 03/14/14 at 10:28:12

Just in case you don't think that a change from the 5/16" to 1/4" is no "big" deal.

5/16" = 0.3125"x0.3125" x 3.14/4 = 0.0767 square inches.
1/4"" = 0.2500" x 0.2500" x 3.14/4 = 0.0491 square inches.

The area is reduced by 36% when you use a 1/4" instead of a 5/16".

This reduction becomes even worse if you use fittings for 1/4' fuels lines (fuel filters or Chinese petcocks), as the inside diameter of the nipples is far smaller than the original 5/16" (8mm) hose that was originally installed without a fuel filter inline.

Title: Re: fuel line size
Post by Steve H on 03/14/14 at 11:35:35

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.

Title: Re: fuel line size
Post by Dave on 03/14/14 at 11:45:17

342F76757473717E470 wrote:

The fuel/air thing comes into play whenever you run the fuel line dry, and it gets air trapped in it. This was not ever an issue that we thought about, until some folks put a clear fuel filter in the line.....and then could not get fuel to actually flow down the fuel line. The entrapped air acted as a block....and prevented the fuel from actually flowing down the line to the carb.

This was documented and discussed in an old thread.....and as I mentioned with 1/4" fuel lines and filters, and with the nipples designed for 1/4" fuel lines....the holes in the nipples might only be 3/16" or less....and the air would sit on one side of that hole trying to flow up while the fuel was on the top side of that hole trying to flow down....and neither would make room for the other to pass by. The small holes prevented the fuel from being able to flow hard or fast enough to flush the air down the fuel line to the carb.....and I suppose maybe some capillary force may have also come into play....whatever the cause the problem went away completely when the 1/4" filter was removed.

When the fuel line is purged of air, you are right....there is no air flowing up or down the fuel line.

Title: Re: fuel line size
Post by Oldfeller on 03/14/14 at 11:47:41

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.

Title: Re: fuel line size
Post by maxtowers on 03/21/14 at 15:00:21

I'm always amazed by the quantity and quality of info's that are available on this forum...
I have the very problem that Oldfeller mentioned; as i put the reserve on the raptor petcock the bike start to run oddly, barely idle, and i have to rev up the engine to prevent the engine to shut down.
Right now i'm running a 7mm line, but monday morning i will get myself a 8mm hose and see what happens (i trust Dave math ;)) i'm quite sure it will run better.