What to Fix on Vacuum Prob?

Hi, I've read the forum on fuel delivery problems and run the tests e.g ran bike with fuel cap open and on prime.

The bike a 1980 Z500 runs well through the gears until you want to cruise on the motorway then it starts to hold back like fuel starvation. I've run the test as per forum and the bike runs perfect with fuel cap open and tap on prime, but I can't find on the forum what parts to replace for the fix!

E.g do I need to rebuild petcock or fuel cap or both. Or is it just to replace diaphragm in petcock.

Can anyone please guide me

Cheers
Colin

Try running with the tap on "on" and cap closed. While cruising along with your problem happening open cap and observe. If problem goes away it's the cap, if not then it's the petcock. Take cap apart and make sure "vent" is open and make sure you have a good vaccum source for the petcock (which does not need vaccum on "prime") If either one fails to solve problem then check fuel flow from tank to carbs. Check for volume (have container handy) May have a restriction in petcock (dirt, crap, etc) Let us know...

I'd take apart the cap and clean the vent. Then take apart the petcock. I'm not sure about the vacuum ones but the manual ones have a screen and a bowl. Just be careful of the diaphragm. I'd also confirm the vacuum line is ok and not cracked or plugged and coming from a vacuum source. Good time to add an inline fuel filter if you do not already have one. GLuck!

The problem is still there with tap on "on" and cap opened, I could here the cap hissing when problem started after I got back from work! It might be a mixture of both cap & tap. Would it pay to get a new diaphragm for the petcock and a inline filter + check petcock for any restrictions?
Cheers
Colin

When I had fuel flow problems I ended up buying a different bike's petcock, brand new (and from a Suzuki, Shhh!). It was much more modern and had a great big screen on it that allowed me to run without an in-line filter, to improve the fuel flow of course.
Took me about a fortnight to finally suss it out. Good hunting!!

I've read/heard about how the alcohol/gas mix is destroyng some rubber carb parts. I'd replace the fuel lines as they are constantly exposed to this mix, potentially swelling and restricting the fuel flow.

gravityoutlaw wrote:

The problem is still there with tap on "on" and cap opened, I could here the cap hissing when problem started after I got back from work! It might be a mixture of both cap & tap. Would it pay to get a new diaphragm for the petcock and a inline filter + check petcock for any restrictions?
Cheers
Colin

If your diaphram was bad it would be spraying fuel as one side has fuel and the other side has vacuum. Where do you have vacuum hooked up to?

Sometimes a careful and thorough cleaning of the petcock will resolve a problem with fuel flow through the petcock.

This entails removal of the petcock from the fuel tank, and complete disassembly of the petcock.

Correct reassembly is of course also required.

Fuel flow through vacuum operated petcock may be tested with engine not running by detaching the fuel line from the petcock, turn vacuum operated petcock to ON or RES (not PRI), detach vacuum hose from carb, use a syringe to supply vacuum to hose, and watch for fuel flow from petcock nipple.

On a vacuum operated petcock in ON or RES position (not PRI), the fuel flow is blocked by operation of a diaphram/o-ring/spring until vacuum is applied (which vacuum usually comes from a running engine).

Do you have for reference a parts diagram of the petcock?

Good Luck!

If needed, here's a diagram from kawasaki.com of fuel tank and petcock on 1980 KZ550-A1.

[Click on image to enhance view]



Good Luck!

Thanks guys will try a strip & clean up of the petcock.

Cheers for all your inputs
Colin

Make sure fuel line from petcock to carbs have no kinks in it either! Let us know...