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Old 07-11-2014, 07:20 AM   #1
73Mustang OP
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Vacuum fuel valve disasble-able?

Hi. Anyone ever try to disassemble one of these valve? I don't see any screw holding this thing together. Not sure if it is permanently sealed.

It is vacuum operated; meaning fuel will flow out the valve only when there's a vacuum on the bottom port. And I'm trying to disable the vacuum part, so that fuel is always allow to flow thru. The reason is my Symba is getting fuel starvation at high speed, becasue there's not enough vacuum at wide-open-throttle. Alot of Symba people are having problems with this valve; so I think the problem can be addressed permanently by disabling the vacuum bits and installing a manual petcock at the carburetor.

The Symba is a Honda cub copy. Honda uses a manual petcock; that's why Honda is reliable. The copy cats who made the Symba strayed from the origin Honda design by changing to this vacuum valve which is automatic. The problem is now vacuum valves are not reliable.



73Mustang screwed with this post 07-11-2014 at 07:34 AM
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Old 07-11-2014, 10:04 AM   #2
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Need a real petcock

EDIT: Disregard- I didn't see that you were actually planning on installing the manual petcock at the carb. My Bad!
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Dabears screwed with this post 07-11-2014 at 10:10 AM
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Old 07-11-2014, 01:14 PM   #3
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I would look for a manual fuel tap from a scooter shop.

A lot of them have the same threads to install.

Have you considered bumping your main jet size?
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Old 07-11-2014, 01:17 PM   #4
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Originally Posted by brianwheelies View Post
I would look for a manual fuel tap from a scooter shop.

A lot of them have the same threads to install.

Have you considered bumping your main jet size?
Yeah, I'm seeing manual ones on Ebay. Buy don't know if the thread size is the same.

Most local scooter stores around here have gone out of business. The one left is like a used-car dealer. The owner wanted $75 for a clutch cable that they have in stock, a part that should cost about 10 to 20 bucks retail.
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Old 07-11-2014, 01:48 PM   #5
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Originally Posted by 73Mustang View Post
Yeah, I'm seeing manual ones on Ebay. Buy don't know if the thread size is the same.

Most local scooter stores around here have gone out of business. The one left is like a used-car dealer. The owner wanted $75 for a clutch cable that they have in stock, a part that should cost about 10 to 20 bucks retail.
Scrappydog scooters website. They have good prices too.

See if you can get confirmation on the fuel tap threads.
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Old 07-11-2014, 01:55 PM   #6
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I saw vacuum petcocks for Symba online for under $20. Are you sure your vacuum is weak or could the vacuum petcock just be toast?
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Old 07-11-2014, 02:00 PM   #7
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If you want to simply disable the vacuum petcock you could use an air compressor and nozzle to blast the rubber diaphragm. Should be able to rip it that way. Drop a little battery acid in there and let it eat away the rubber.

Definitely want to run some cleaner through it once you disable it. Cap off vacuum nipples at carb and petcock. You may even be able to see down into the thread side of petcock to poke some holes in the diaphragm.
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Old 07-11-2014, 02:09 PM   #8
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Originally Posted by brianwheelies View Post
If you want to simply disable the vacuum petcock you could use an air compressor and nozzle to blast the rubber diaphragm. Should be able to rip it that way. Drop a little battery acid in there and let it eat away the rubber.

Definitely want to run some cleaner through it once you disable it. Cap off vacuum nipples at carb and petcock. You may even be able to see down into the thread side of petcock to poke some holes in the diaphragm.
Won't that disable it in the closed state?
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Old 07-11-2014, 02:24 PM   #9
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Won't that disable it in the closed state?
When they fail they do so in the open state but they also leak out of the vacuum port.
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Old 07-11-2014, 04:11 PM   #10
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Just get a new one. Keep it stock.
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Old 07-11-2014, 05:11 PM   #11
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Vacuum fuel valves come in all different designs. I have rebuilt the one on my Vulcan 750 three times. It looked like a regular petcock. It costs $130+ to replace it. Each time I found the rubber diaphragm and seals melted. Just about had to be ethanol gas. The one on my Vino 125 also failed. It looked a lot like yours, but did not connect directly to the tank. It was mounted to the frame, and had 3 hoses on it, one from the tank, one to the tank, and one vacuum line. I just ran a new fuel hose directly from the tank to the carb and plugged the vacuum hose. Never got around to installing a manual petcock, but the float needle valve in the carb held until after I sold it. Yes I told the buyer. You can probably disable that one just by sticking a nail or something like that through it. Once you puncture the diaphragm, you should get fuel flow. Also, depending on the thread size on the tank, you might be able to get a non motorcycle fitting that would screw onto it, that you could attach a fuel line to.
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Old 07-11-2014, 07:14 PM   #12
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Originally Posted by brianwheelies View Post
I would look for a manual fuel tap from a scooter shop.

A lot of them have the same threads to install.

Have you considered bumping your main jet size?
I just found out somehow the float got messed up. I guess when I put the carb back together, I bend the float tabs and that caused the float height to be very low. So, the vacuum petcock seems to be working fine, and there seems to be enough vacuum. I verified it by riding with a manual hand vacuum pump attached to the fuel valve.

But I'll want to do this mod anyway for reliability. Don't want to be in the middle of TAT and have the vacuum port flood gasoline into the intake manifold.

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Old 07-12-2014, 02:29 PM   #13
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I was about to mention that the first suspect in a case of high-speed fuel starvation is the float level. Glad that you got it figured out.
A manual petcock is just as likely to leak fuel as a vacuum operated one is, and if your float needles are sealing you won't get any flooding even if the petcock is leaking.
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Old 07-12-2014, 04:01 PM   #14
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Right.

Quote:
Originally Posted by k-moe View Post
I was about to mention that the first suspect in a case of high-speed fuel starvation is the float level. Glad that you got it figured out.
A manual petcock is just as likely to leak fuel as a vacuum operated one is, and if your float needles are sealing you won't get any flooding even if the petcock is leaking.
Probably the part on my scooter I change most often is the manual petcock.
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Old 07-12-2014, 04:21 PM   #15
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My old Kawasakis had vacuum petcocks, and I never had an issue with them.

Since yours turned out to be fine, my thought would be to pick up another as a spare and carry it on your trip if you're worried about it.

Having a spare will virtually guarantee that this part will never fail.
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