TOPIC:

Fitted a fuel tap recently from a GPZ (ZX) 900R.
This is the vacuum feed type tap.

The bike ran very well at low revs but once I got to above 60 mph it began to missfire & bottom out (fuel starvation).
Pull into the side of the road battling to keep the engine running & turn the tap to PRIME & it eventually gets back to normal..........but even in PRIME position, once above 60mph I have the same starvation issue.
Tried the 2 vacuum pipe feed method, & it made no difference what-so-ever.
I know its fuel starvation as just before it begins to misfire, the temp reading goes UP, due to the LEAN mixture it is running (HOT).
It would appear the tap cannot supply enough fuel for the 3 large float bowls quickly enough........which is a bit of a mystery, as i know other owners have said they have fitted these taps with no issues (ANY EXPERIENCES VERY WELCOME TO COMMENT!)

Took the tap off & fitted all new seals & diaphragm............no better, just the same..........awful.

Replaced the whole tap with the old (leaky) type standard one.............problem solved.

I now intend to fit new seals in the standard (leaky) tap & fit a second inline fuel tap under the tank on the opposite side, so even when the standard tap allows fuel past in the OFF position the inline tap will stop it flooding the carbs.

What a carry on !

Pete F
UK

Hi Pete

mine works, do you use a filter in your system or can some trapped air block the fuel going thru ?

Holger

I know the way the fule-hose are worked has some impact, however I do trust you to have that done well.

Pete:

In my tool box, I have a hand actuated vacuum pump with gage and allows a person to check vacuum diaphragms.

Whenever I run into these situations, I drag that tool out and verify the operation of the membrane.

If you pump it down you can check a couple of items with the tank.

1: Flow rate into a graduated or measured cylinder. (i.E. ratio rite)

2: Does the membrane hold a vacuum after it is pumped down? it should hold a vacuum without leaking.

Regards.

Todd.

Thanks for replies Holger & Todd, as ever I appreciate them.

The way I tested the tap is to connect it to a vacuum pipe from the inlet with a dummy fuel tank feeding the carbs so I can have the tap in my hand to inspect.

Its quite interesting as with the tap lever removed from the tap body, you can actually see the diaphragm valve opening & closing in relativity to the revs of the engine (pulse). It obviously "Sucks" the diagram open & is closed by the small spring when idle.

This all works exactly as it should & a little better with a T Piece & 2 vacuum pipes fitted but the only explanation I can arrive at is it does not allow enough fuel past.

All pipes are brand new too, so no blockages.

Bit of a bugger really, as I thought it was the answer I needed, but no luck !

Pete F

What condition is the petcock in? Does it have a screen on the top? If no screen exists, it's relatively easy for the petcock to get clogged with junk coming from the bottom of the tank. If you haven't already, I would take the front of the petcock apart just to make sure there's no particulates in it. Also some petcocks come with an internal screen and those can get clogged up as well and require cleaning. If you're using a screen on the petcock and also using a fuel filter there may be too many obstructions for the fuel flow.

Could also try using a larger diameter fuel line that way more fuel rests in the line ready for the carb to suck up.

Let us know what you come up with .

Thanks Mike,

Petcock was in very good condition & from a low mileage bike that had been used until it was parted.

I completely stripped the tap before use & cleaned it & replaced every seal etc, it was all good & clean inside.

Tank is resin lined so no crap or rust what so ever coming from there, just clean fuel.

I like your thoughts on the screen though, as mine does have one fitted, but tried the "sink" test with that in a bowl of fuel before fitting it & it absorbed the fuel instantly so by rights should be clear for use ?

I have 7.5mm bore fuel line fitted (as standard) but with fluid feed engineering, the restriction is always the narrowest point no matter how wide the bore of the pipe is, so I dont think its that & I used the same pipe with the standard tap no problem ?

I am coming to the conclusion it just cannot let enough fuel past to feed the 3 carbs at higher revs.

Pete F