Fuel tap options

[Digs]

Having some fuel starvation issues with my 7/11. Any thoughts on options? 

I’m unsure if I should rebuild either of my stock fuel taps. They both fit fine, but only one currently flows enough to keep the bike on idle in the prime setting. 

Bike runs beautifully off an auxiliary tank with just a gravity feed, but it bogs even on prime as it is and the ally casting seems fairly worn on both of my OEM taps. 

Vac line is fine and free of leaks. 

[Joseph]

Whats the engine/carb spec ?

[Digs]

1127 GSX-R 1990 L engine, BST 38 carbs, 155 main jet, Needle in middle grove, Pilot jet 3 turns out, K&N RU-2922, 750 L frame.  

1100 tank, un-modified. 

Edited July 10, 2023 by Digs

[Upshotknothole]

Open up the petcocks and check the diaphragms. Stock petcock should be fine with BST38s. May be able to rebuild one good petcock out of the two, I've never really heard anything good regarding the aftermarket rebuild kits. Other option is to stick a pingel on it, just gotta make sure to turn it off. I'd suggest double checking the vacuum line, but if they're barely flowing on prime, they're clogged up internally.

[george 1100]

Usually when they don't flow on prime it's because the cast alloy ramp of the centre part that turns, has worn out. They are in general good units but if you use the prime feature often, the cast alloy wears out real quick

Edited July 11, 2023 by george 1100

[Digs]

6 hours ago, Upshotknothole said:

Open up the petcocks and check the diaphragms. Stock petcock should be fine with BST38s. May be able to rebuild one good petcock out of the two, I've never really heard anything good regarding the aftermarket rebuild kits. Other option is to stick a pingel on it, just gotta make sure to turn it off. I'd suggest double checking the vacuum line, but if they're barely flowing on prime, they're clogged up internally.

Good to know about the 38s. 

I’ve had them both apart and they’re both clean and free of obvious damage, though one is looking a little more perished on the diaphragm. 

The vac line is brand new, aand I even tried the old one. I’ve also double checked the tank breather. Can’t see anything immediately wrong. 

[Digs]

6 hours ago, george 1100 said:

Usually when they don't flow on prime it's because the cast alloy ramp of the centre part that turns, has worn out. They are in general good units but if you use the prime feature often, the cast alloy wears out real quick

So I’ve got no idea on the history of either tap, one is definitely more worn as it flows better when you press on the tap. Think it’s a realistic that they’ve both worn out. 

[Joseph]

Well the issue is always coming up isn't it, they are just not durable. 

i've given up on any type of tank fitted tap, i use a brass spigot off the tank with an inline tap on the hose. Can't go wrong

 

[Digs]

Makes sense, I think I might make up and adapter plate and single inline tap for now as it runs fine from the auxiliary tank on a single tap. Or I could strip out the gubbins from the oem tap and add an inline tap on each line.

Pingle sounds like the long term solution though! Just stupid money for a tap! I can buy an rf900 engine for the price. 

[Upshotknothole]

6 hours ago, Digs said:

Makes sense, I think I might make up and adapter plate and single inline tap for now as it runs fine from the auxiliary tank on a single tap. Or I could strip out the gubbins from the oem tap and add an inline tap on each line.

Pingle sounds like the long term solution though! Just stupid money for a tap! I can buy an rf900 engine for the price. 

If you ever swap out the BST38s with big flat slides(RS38-40, FCR39-41), you'll be more inclined to spend the money on the pingel as they flow way more fuel than the stock tap. I'm currently running a cheap aftermarket petcock on my 750 with BST38s and it's fine for them. You can find them on Eblag,  but it's damn near impossible to switch that thing from On to Res while riding, so it lives on Res. One of these days I'll swap a stock one back in there.

[Digs]

7 minutes ago, Upshotknothole said:

If you ever swap out the BST38s with big flat slides(RS38-40, FCR39-41), you'll be more inclined to spend the money on the pingel as they flow way more fuel than the stock tap. I'm currently running a cheap aftermarket petcock on my 750 with BST38s and it's fine for them. You can find them on Eblag,  but it's damn near impossible to switch that thing from On to Res while riding, so it lives on Res. One of these days I'll swap a stock one back in there.

Eventual plan is fuel injection for ease of use, but the flatsides are tempting and I love the sound. Anyway, that’s the main reason for not wanting to go pingel, that and I can’t see the reason behind cost of the product in, its very simple. 

I know what you mean, I borrowed one of the aftermarket taps below and besides not having the space for it, there was no way I could change the position. 

 

[Digs]

Should say I also robbed the tap of my 750wp and had the same issue with fuel starvation. I also tried a bit of reinforced fuel injection line off my Daytona which improved the vacuum,  but still not spot on with any of the now four taps. 

[Upshotknothole]

Just now, Digs said:

Should say I also robbed the tap of my 750wp and had the same issue with fuel starvation. I also tried a bit of reinforced fuel injection line off my Daytona which improved the vacuum,  but still not spot on with any of the now four taps. 

It shouldn't take much in the way of vacuum to open up the tap at all. Bit of hose on it and you should be able to just suck on it and it will open up.

[Digs]

I did try that initially, it idles fine, just doesn’t rev out properly. I’ve only had 1-2l in the tank as I keep draining it to swap taps so potentially that’s an/the issue? Just seems odd when it runs so well on a gravity fed aux tank. 

[Upshotknothole]

1 minute ago, Digs said:

I did try that initially, it idles fine, just doesn’t rev out properly. I’ve only had 1-2l in the tank as I keep draining it to swap taps so potentially that’s an/the issue? Just seems odd when it runs so well on a gravity fed aux tank. 

Make sure the lines aren't getting pinched under the tank when it's back on? Don't need much fuel in the tank to rev it in the garage.

[Digs]

I’ll have a check tomorrow with fresh eyes.

[Upshotknothole]

I bought a hacked up GSXR 1100 off some kids years ago because they swapped the tank and petcock and used tiny diameter vent hose for the fuel line. Idled and rode at low rpm fine, but would starve as soon as you opened it up. How are you hooking up the external fuel tank to the carbs? 

[Digs]

8mm ID line on a 300ml auxiliary tank hung 1m or so above the bike. 

[Digs]

I’m using 8mm ID 14mm OD braided line for the fuel. 

[gsxwill]

Probably to simple but often gets overlooked but is your tank venting OK seems strange that all your taps are failing at the same time but runs OK on auxiliary tank

[Upshotknothole]

59 minutes ago, gsxwill said:

Probably to simple but often gets overlooked but is your tank venting OK seems strange that all your taps are failing at the same time but runs OK on auxiliary tank

What they said, try running it with the fuel cap open. Or at least checking the flow with the cap open.

[Digs]

Yeah I mentioned it briefly at the beginning of the thread, it's venting fine. I've run with the cap open and shut and blown a bit of compressed air through the tank breather no change unfortunately. 

Edited July 12, 2023 by Digs

[Gixer1460]

7 minutes ago, Digs said:

and blown a bit of compressed air through the tank breather no change unfortunately. 

? ? ? - tank vents through the cap assembly so how did you blow air through it?

[Digs]

1 hour ago, Gixer1460 said:

? ? ? - tank vents through the cap assembly so how did you blow air through it?

I blew 30 psi through this hole and the cap assembly is new so I didn’t do anything to it. 
 

 

 

[Gixer1460]

That's a drain for water that collects around the cap - it is connected internally to the bottom of the tank and then via a tube to below the engine. Many have assumed the same thing!