Carb issues

[JeffH]

Also does anyone know what the measurement of the piston on the petcock diaphrams are suppose to be? I'm going to rip into mine again and check the measurements of the ones I am using.

I put K&L Petcock Kits into my 1978 XS1100E Project. The part number on the package(s) are 18-2697 which are labeled MADE IN JAPAN so I think that is the actual K&L p/n. Says FCK-12 Petcock Rep Kit: YAM on the package label. It's the same blue packaged K&L kit sold by many parts houses.

When I got the bike I had over two quarts of gas down in the crankcase with the oil. I was skeptical about the rebuild vs replacement but these K&L Kits certainly STOPPED the gas leak. In fact, petcocks work like new. As designed; OFF when I'm stopped, Fuel Flowing when I start the bike.

MEASUREMENTS:
I'm looking at the old diaphragm that came out. It measures 35mm x 35mm
The seal "O" ring is 8mm OD. On the opposite side, the disk is 25mm diameter across.
Piston sticks 10mm OUT from the rubber seal and has a diameter at the base of 10mm across.
Base of piston is 5mm OUT X 10mm across then the piston narrows to 8mm diameter for the remaining 5mm.

Hope that helps. BTW: my vernier calipers must have gone for an evening stroll since I can't find them but mm I provided should be correct.
Jeff

[DiverRay]

A quick sanity check is to run the bike on "prime". That way, the fuel flows, and does NOT stop. IF it runs well then, you know it's the petcock. Also, when you sync the carbs with a gauge, make sure you have a fan running to cool the engine. You CAN overheat the engine very fast while trying to sync...

[crazy steve]

The fix for the 'short' piston in the petcocks is to stretch the spring a bit....

[Titbit21]

A quick sanity check is to run the bike on "prime". That way, the fuel flows, and does NOT stop. IF it runs well then, you know it's the petcock. Also, when you sync the carbs with a gauge, make sure you have a fan running to cool the engine. You CAN overheat the engine very fast while trying to sync...

I'm pretty sure I have done that when i had it running last to see if anything changed. That failed. I was thinking if I switched sides the fuel is coming from to see if the same problem happens to the 3&4 cylinders instead of 1&2. That is the only thing I haven't tried.

[Titbit21]

MEASUREMENTS:
I'm looking at the old diaphragm that came out. It measures 35mm x 35mm
The seal "O" ring is 8mm OD. On the opposite side, the disk is 25mm diameter across.
Piston sticks 10mm OUT from the rubber seal and has a diameter at the base of 10mm across.
Base of piston is 5mm OUT X 10mm across then the piston narrows to 8mm diameter for the remaining 5mm.

Also thanks Jeff for the measurements. I will be checking these as well.

[petejw]

i know its not really fixing the cause but
u could try linking the 2 fuel lines
together with some 't' pieces.

[GLoweVA]

I saw that you checked the vent T's, but what about where the vents go to.
If that is plugged, then those 2 bowls won't vent well and then they won't fill up correctly.

Another thing you can check, with the tank off and a suitable container, try testing your petcocks then.
prime, fuel flows, on and res, no flow, apply a little vac to the petcock and see if you get good flow then.

[motoman]

Might wanna actually check, with magnifying glass if necessary to verify if ALL the jets, air and fuel have the Mikuni logo engraved into them. Just cause they have the same sizing number doesn't mean they are Mikuni. Aftermarket(K&L,RD, etc.) are NOT hole sized the same even though having equivelant size numbering as Genuine Mikuni jets.

[LoHo]

Another thought (seeing Diver Ray on made me think of this), make sure the vacuum line to the petcock is not leaking air, although the "prime" test should have eliminated this possibility.

[motoman]

Another thought (seeing Diver Ray on made me think of this), make sure the vacuum line to the petcock is not leaking air, although the "prime" test should have eliminated this possibility.

.........or sucking fuel past diaphram via the vacuum hose.....JAT, as I've heard this happening once before here.......