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Message started by seve321 on 04/06/18 at 12:41:31

Title: gravity fed petcock
Post by seve321 on 04/06/18 at 12:41:31

I changed out my stock gas tank to a benelli mojave tank. the petcock is a gravity fed one. I know to cap off the vacuum. my problem is that it was working fine for a little while then I started to notice that it wasn't sending gas fast enough. the next day, no gas is getting through.

I've checked all the lines, filter (inline fuel filter), petcock for blockage and it's all good. It will flow gas if the hose isn't connected to the carb. but once I put it on it stop flowing immediately. Is there something I'm not doing right?

Title: Re: gravity fed petcock
Post by Gary_in_NJ on 04/06/18 at 12:49:40

Remove the inline fuel filter.

Title: Re: gravity fed petcock
Post by verslagen1 on 04/06/18 at 13:00:41

What size fuel line?

1/4" lines don't clear air bubbles as well as 5/16"
Also, a little water in a paper filter and it's done.

Check the screen above the float valve.

And I agree, remove the in line filter.

Title: Re: gravity fed petcock
Post by seve321 on 04/06/18 at 14:16:40

Its a larger sized paper filter with 1/4in fuel lines. but I replaced it with a new filter and same thing. Also, the petcock outlet is smaller than a 1/4in so it's hard to go any larger than what I have. I'll try with no filter and see what happens.

Title: Re: gravity fed petcock
Post by verslagen1 on 04/06/18 at 14:31:03

57617261373635040 wrote:

That's your problem right there.
1/4" will be fine for cruising on level ground.
But will die on a hill or any spirited riding.

Title: Re: gravity fed petcock
Post by Dave on 04/07/18 at 04:38:35

4E786B782E2F2C1D0 wrote:

The problem with the 1/4 inline filters is the hole in the nipples are too small. The plastic housing is thick, so the actual hole in the end of the filter is very small.....and air bubbles get stuck in them and cause an air embolism, and it won't allow the fuel to flow down to the carb.

And......when you reduce the diameter, the change in flow area decreases dramatically as show below:

Stock fuel line is 8mm, and the area is 0.0778 square inches.

Most hardware and auto supply stores don't carry 8mm - so we buy 5/16" hose which has an area of 0.0766", which is a flow area of 98% of the stock fuel line.

When you buy the Chinese copy or other petcocks with 1/4" fuel line, the flow area is now 0.0491 square inches - which provides 63% of the stock fuel line area.

I measured the inlet hole in a 1/4" plastic fuel filter, and I found the hole size to be 5/32", which has an area of 0.0191 square inches - and provides only 24.6% of the original flow area.

So....how well would you perform if I cut your food supply down to 24.6%? ;)

Title: Re: gravity fed petcock
Post by Gary_in_NJ on 04/07/18 at 05:21:22

Dave, your mathematical explanation was spot on, but you lost me on the example; most men would perform much better if their food intake was reduced by 24.6%.

Title: Re: gravity fed petcock
Post by Dave on 04/07/18 at 08:47:25

OK....I screwed up - the stock fuel size is 6.5mm on the parts list - not 8mm. No wonder we can't locate the stock size anywhere.

When I go to a cycle shop and buy new tubing.....I use 8mm as it is something the local bike shop stocks in bulk and they just cut off 2-3 feet for me when I stock up again. The 8mm fits snugly on the petcock and carb, and I do secure it with a couple spring clamps on each end.

The 6.5mm fuel line has an area of 0.0514074 square inches.

The 1/4" fuel line is similarly small at 0.04906 square inches, which is 95% of the stock size. If you buy reinforced fuel line from an auto store that is used on carbs with fuel pumps - it will be difficult to push this small tubing onto the diaphragm and carb (the 5/16" reinforced fuel hose is much easier to get on the fittings).

The 1/4" plastic fuel filter has 37% of the flow area of the 6.5mm stock fuel tubing. (Which is a 63% reduction in flow).

5E54505A51585C550B0D09390 wrote:

Dave, your mathematical explanation was spot on, but you lost me on the example; most men would perform much better if their food intake was reduced by 24.6%.

Gary - They are only getting 24.6% of the food, which is a 75.4% reduction in food! Enough of a reduction to make anyone anemic.

Title: Re: gravity fed petcock
Post by Matchless G11 on 04/07/18 at 18:13:47

Dumb question,

Is your tank gas cap properly vented?

Title: Re: gravity fed petcock
Post by seve321 on 04/09/18 at 11:09:30

Removing the filter helped. I am only able to use a 1/4in hose because of the outlet on the petcock. It is enough to get everything flowing again though. Thanks for all the input.

Title: Re: gravity fed petcock
Post by batman on 04/09/18 at 12:13:05

The Mojave tanks I've seen on line were supplied with two petcocks ,due to the tunnel down the center nearly dividing them in half, and the reason two were used was to prevent complete stoppage of the flow if the bike was leaned away from a single petcock . If your tank is set up that way you could run two petcocks (1/4 inch) into a 1/4 x1/4 x 5/16 tee and then feed the carb.

Title: Re: gravity fed petcock
Post by batman on 04/10/18 at 00:02:57

Having looked at the Mojave tanks again it seems the two stock petcocks are only 1/8 inch , which is confirmed by the fact that your using 1/4 inch fuel line. I think your in trouble as even 2 petcocks of this size should only feed about 28% of the fuel normally supplied to our 40mm carb. You may need to customize the tank to except one(5/16) or two(1/4) inch petcocks.