is your bike losing power at top speed?

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2door

2door

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Sep 15, 2008
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Freewheeler,
I don't know what you're hearing but I promise you...it ain't "sucking". You have to know the principle of the carburetor and how the engine gets fuel from it to understand why you could not hear a sucking sound through the fuel tank. What I think you hear is a differene in resonance through the tank when the cap is off or loose.
Tom
 
corgi1

corgi1

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Aug 13, 2009
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is it the ocean sound like in a sea shell,and my real question is why do you have your ear down on an open gas tank while traveling down the road at WOT?????




lol
 
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FreeWheeler

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Jun 22, 2010
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Goat Herder said:
That sounds like some trick phony riding:D
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Nothing phony or tricky about it, WFO pull the cap. If youre hearing is reasonably intact, you'll hear something that doesnt require you to risk going over the handle bars.

2door got it right, there's sucking going on just not fuel.
 
Goat Herder

Goat Herder

Gutter Rider
Apr 28, 2008
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On my Morini ride I had to leave the moped tank cap loose on it I know what you mean.
I meant to say pony sorry accidental pun there.
 
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Black_Moons

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Oct 25, 2010
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LoL, Look for a tiny dent in the side of your gascap, It needs to be drilled out. Thats supposed to be the vent.

Since I drilled mine out, Iv drained the fuel using the tanks fuel line, Makes no diff at all if the gas cap is on or off.

PS: With stock petcock that only takes about 10 mins to empty the entire tank. Clearly not a flow problem.
 
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Black_Moons

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Err ok I just opened up the stock petcock... It was.. Scary. No wonder it leaked, the sealing surfaces are rough inside, they just expect it to seal against a gasket.. a poorly made gasket. I have never seen such a poorly made valve. Just buy a 1/8" NPT petcock anywhere and screw it in with lots of teflon tape. problem solved. (Any loose tape will get caught by the downstream fuel filter anyway. You DO have a fuel filter.. Right?)
 
happycheapskate

happycheapskate

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Nov 26, 2009
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Yeah, and it looks good too. It was kind of hard to thread into the HT gas tank, maybe too much chrome or different thread pitch. But once its in, its in for good and shut off is tight! I can park in the house! (still use cardboard though, good idea)

What's a NPT valve? Is that the little brass ones from the plumbing aisle?

Yes, I use an inline fuel filter from the auto dept made for cars or lawn equipment.
 
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bairdco

bairdco

a guy who makes cool bikes
Aug 18, 2009
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NPT = National Pipe Thread. it's tapered threads instead of straight cut, so the more you tighten it, the tighter it gets. it makes a leak-proof seal.

ideally, you want to tap the threads on the tank with a 1/8 NPT tap, 'cause if you just jam it in there, it might never come out. but if it works for you, don't sweat it.
 
biknut

biknut

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Sep 28, 2010
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Teflon paste works great to seal the threads on a petcock. I like it better than the tape.
 
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bitsnpieces

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Dec 9, 2010
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Is anyone able to describe the amount of flow between the stock to the new value everyone is using?

I mean, how little is too little? I think I get a consistent flow on mine; just have to double check.

I'm going to have a look at increase my fuel flow nonetheless, but at this current point in time, I found one problem I'm getting is any vibrations will reduce the flow; it somewhat pushes the fuel upwards, therefore starving the engine for fuel.
When my bike vibrated less, I noticed I could WOT for a longer duration of time before it would starve.

Whilst still trying to remove all vibrations, or at least most of them, I'm guessing increase flow may add the weight needed to counteract against the minor vibrations and keep a good flow?
 
happycheapskate

happycheapskate

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The new one (sick bike parts brass valve with blue or black handle) flows noticeably better than the stock China petcock, from a visual test, with same level of fuel in the tank. The SBP doesn't leak, and its ON or OFF at its stops, does not spin around or get bumped easily. Its easy to operate while pedaling.
 
happycheapskate

happycheapskate

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No. You can remove the stock little red plastic filter and use an inline lawn mower filter. That helps some. You could go to the plumbing section and use a non-petcock barb-end hose coupler to replace the petcock, but you will need an in-line shut-off or the carb may vaporize fuel or leak, esp if the float/pin is not adjusted right.

Here is a brass shutoff valve from a marine engine shop.
http://store.hamiltonmarine.com/prodimg/MOE-33306-10.JPG

http://store.hamiltonmarine.com/bro...e-3-8-hose-1-4-npt-brass-731238-/4,39851.html

Ball valve demonstrated 0:45-end http://www.youtube.com/watch?v=txDANNh4_ME
 
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Black_Moons

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Oct 25, 2010
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Stock valve is poor flow when its on, and leaky when its off. Toss it into the trash. Its so fail, its not even funny. I pay like $13 for a new high quality 1/8" NPT petcock, Totaly worth it (though I guess a ball valve would be good enough, you know a petcock is not gonna be affected by gas, because its metal on metal, Ball valves usally have a plastic seal, Gotta check if they are gas rated)
 
Goat Herder

Goat Herder

Gutter Rider
Apr 28, 2008
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By 2 Door : Think of it this way: Fill your fuel tank then open the petcock and drain the tank back into a can. Time how long it takes to empty the tank. Now fill the tank again and go for a ride using the advertised gas milage of most 2 stroke kits, 100+ mpg (probably more like 80). Run your tank dry and time your ride from start to stop. I will guarantee that the ride will last a lot longer than it took to drain the tank. Even with 3/16" line. I can't imagine that even a modified pocketbike motor would require more than the amount of fuel that could flow through a 3/16" line. That's a lot of fuel. Just for comparison, you can successfully feed a 300 hp small block Chevy with a 3/8" line and never starve for fuel at highway cruising speeds; and it has seven more cylinders :)
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bitsnpieces said:
Is anyone able to describe the amount of flow between the stock to the new value everyone is using?
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