Petcock Questions

[SVTim]

A friend and I are troubleshooting some fuel/carb problems on his '94 Katana and I have questions about the petcock posistions. I read the sticky above, but can't seem to make sense of it.

On my g/f's Ninja 250, it is very clear- "ON" "OFF" & "RES". However, in the sticky above it mentions "ON" "RES" & "PRI" for the Katana's. Are the positions different between years of Kat's? My friend's bike was repainted, so the label is not there. What does "PRI" do? How does that differ from "ON"? Is there an "OFF" position at all? I am asking that because I've *heard* that the fuel should be cut off during long storage times to prevent fuel pressure from being placed on the needle valves.

TIA

[tzortn]

There is no "off" position on a Katana (at least for 98+) petcock. My understanding is that because it is a vacuum feed system you do not need it. "Pri" is short for prime. It allows an open flow of fuel to the carbs. I was told that it should be used in the bike has been sitting for more than a week which would mean that the carbs are dry and cold weather starts. Leaving the petcock in prime will cause your carbs to flood so you should turn the petcock back to on once the bike is running.

[Yellow]

what he said... and it applies to the pre98 petcocks too.

[SVTim]

Thank for the reply...

What do y'all think the problem could be if the petcock is turned 'ON' and it stumbles like it isn't getting enough fuel? When left on the 'PRI' position it runs and drives fine, but is definately getting too much fuel.

The only thing I can think of is there is a vacuum leak somewhere if it uses vacuum to get gas when the petcock is on the 'ON' position. Any ideas?

[md86]

Yeah , vacumme leak or bad petcock . Or maybe the vacumme line's bad ...

[SVTim]

Yeah , vacumme leak or bad petcock . Or maybe the vacumme line's bad ...

Unfortunately, I am troubleshooting this bike from memory and over IM, so I am not in front of it. Where is there a vacuum line? The only thing I can think of is the seal around carb/airbox and/or carb/head is not tight and leaking. Is there another hose that comes off of the engine into the carbs or petcock?

[md86]

Line from carbs to petcock is your vacumme line .

[SVTim]

Guess I am really stupid, but I thought that BOTH of the lines out of the petcock were fuel lines? Am I wrong? I thought each line fed 2 carbs. We will check both lines ASAP to make sure it is not cracked anywhere. Thanks.

[tzortn]

I would say that unless the hoses are obviously pretty new it would be a good idea to replace them.

According to the micro fiche at Ron Ayers there are two hoses that go to the petcock and one that goes to the fuel tank. I assume that the larger hose in the petcock delivers fuel, the small hose in the pet cock is the vacuum line and the hose to the tank is a vent. On the other hand I really have not idea. I need to check out my set up one of these days.

[md86]

Well , us older Kats have three hoses off the petcock , 2 fuel , 1 smaller one that's vacumme , so ....

[Yellow]

FWIW the 88-89 600 Kats have one fuel line and one vacuum line... but if it's a 94 with the later petcock I understand that it has fuel delivery problems if only the two fuel hoses are connected

[The CyberPoet]

There is no "off" position on a Katana (at least for 98+) petcock. My understanding is that because it is a vacuum feed system you do not need it. "Pri" is short for prime. It allows an open flow of fuel to the carbs. I was told that it should be used in the bike has been sitting for more than a week which would mean that the carbs are dry and cold weather starts. Leaving the petcock in prime will cause your carbs to flood so you should turn the petcock back to on once the bike is running.

Very close...

Inside the tank, there are two pick-up lines, one shorter than the other. The shorter one feeds the RES position (as well as the PRIME position), while the taller pick-up feeds the ON position.

The petcock contains a spring-loaded vacuum-pulled rubber diaphram. When the vacuum is applied in sufficient quantity, the diaphram gets sucked away from the passage (overcoming the force of the spring), and permits fuel to flow through it (occurs in the RES and ON positions).

The PRIME position defeats the diaphram by using a different fuel routing that bypasses the vacuum diaphram system entirely, while drawing from the lower (RES) tap in the tank.

There is indeed no "OFF" position on the petcock selector; both RES and ON work as "OFF" when the engine isn't running, because there is no vacuum to pull the vacuum diaphram open. This is a common design and has been for about 25 years, since it reduces the chances of a bike fire in case of an accident (where shutting the engine down or having it conk out shuts down fuel flow).

The problems start to occur as the bike ages -- either the spring gets weak (so it doesn't close the diaphram properly), or more often, the vacuum diaphram gets hard or tears, so it won't move sufficiently under a vacuum. For this reason, there is a rebuild kit available to replace the diaphram & spring portion of the petcock (although some users simply replace the entire assembly instead). Additionally, if the vacuum line (which routes from a nipple on the top of right-most carb normally) is in bad shape, insufficient vacuum could be getting to the diaphram.

A carb float is effectively like a toilet bowl float -- when it drops, it permits more fuel to flow into the carb. If the float sticks or is misadjusted, fuel will continue to flow into the carb in excess of what can be consumed. In the PRIME position, fuel is constantly fed to the carbs, which is problematic only if there is also a simultaneous issue with one or more of the carb floats pulling fuel -- fuel can pour through the carbs and run into the cylinder (where it washes away the oil at the rings and then sinks off into the oil galley; the next time the engine starts, those rings will slide up and down with no lubrication for a while, doing excessive damage in short order). For this reason, you should never park the bike in PRIME -- just in case the carbs have an issue.

Finally:
The bike uses gravity feed for fuel supply, and if the fuel line(s) and/or fuel filter don't route downhill the whole way, the carbs can be starved for fuel, presenting the appearance of a bad petcock diaphram without that actually being the cause.

Cheers
=-= The CyberPoet

[SVTim]

CyberPoet- Thanks for the excellent reply!

Well, my friend and forgot to reconnect the vacuum line when the tank was removed. So thanks to everybody who replied. As as we heard there were 3 lines going to the petcock, we promptly remembered we only connected 2 . So he connected the vacuum line (which we both thought was a drain hose of some sort) and the bike is absolutely 100%. It idles and drives perfectly (no stalling, surging, backfiring, etc).

[md86]

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