leaky petcock - please explain

[beechfront]

in the petcock tech tip, it says "The big danger of a leaking petcock is that fuel will overflow the carbs"

what I don't understand is how this can be an issue? the petcock does nothing to monitor fuel level in the carbs. Or is it just saying that if you have carb problems already - and you shut the petcock off (and it is leaking) it will overflow the carbs because they aren't doing their job.

[beechfront]

if i am correct in my thinking, if the carbs are working properly then would there be much need to rebuild the petcocks? if you don't shut them off when you park the bike what difference would it make?

[Snow]

hmmm...that does seem rather odd. I left my petcocks on prime for about two days straight a couple days ago without a problem (was having problems after incorrectly rebuilding them) without a problem...at least I thought without a prob.

[RiskyBusiness]

Originally posted by beechfront
if i am correct in my thinking, if the carbs are working properly then would there be much need to rebuild the petcocks? if you don't shut them off when you park the bike what difference would it make?

I suspect that the reality is most people's carbs don't totally shut off fuel flow like they should. I know if I leave mine parked for a few days with the petcocks open I have a nice little fuel puddle. I suspect the cause is twofold: 1) my petcocks need rebuilding 2) my carbs don't regulate fuel level well.

I agree that if the carbs are doing what they are supposed to be doing then wide open petcocks would not be a problem.

-J

[jasonwatkins]

I leave mine on all the time as when I do move them that is when they leak gas onto my hands.
Jason.

[xssiveone]

Originally posted by beechfront
if i am correct in my thinking, if the carbs are working properly then would there be much need to rebuild the petcocks? if you don't shut them off when you park the bike what difference would it make?

Yes, you would be correct in that assumption.. in a perfect world. However, if a peice of dirt or rust were to lodge on your float valve, stopping it from sealing, and your petcocks didn't shut off the fuel... darn gas gets in the air box, on the garage floor, and also in the oil. Gas thins oil big time resulting in low viscosity and possible bearing damage.

IMHO, the vacuum petcocks are a back up for the float valves, and vice-versa. If one fails, then the other is there to take up the slack. I think it's well worth the time and money to ensure both are functioning as designed.

[beechfront]

i was trying to think of some sort of 'backup petcock' but i was thinking more along the lines of an electronic valve of some sort. the vacuum valve sounds like a much better idea. do you know anyplace that sells them?

[xssiveone]

Originally posted by beechfront
i was trying to think of some sort of 'backup petcock' but i was thinking more along the lines of an electronic valve of some sort. the vacuum valve sounds like a much better idea. do you know anyplace that sells them?

Yes. Yamaha. In fact, they came as standard equipment on every XS Standard model made. The Special models used a vacuum control valve (affectionately called "the octopus") mounted between the petcocks and the carbs. Rebuild kits might still be available from Yamaha, if not, then I am pretty sure that Dennis Kirk still sells aftermarket kits.

[beechfront]

if i am thinking of the right piece - i believe mine has the octopus thing you're talking about. now if i understand you right - it should shut off flow to the carbs when the engine is off? if so, mine is apparently not working.

[Jerry]

Petcock theory

Petcocks were designed to perform several functions. First, they shut off gas flow when you want to unhook the gas lines to remove the tank or carbs. (Off function.) Second, as a safety feature, they shut off gas flow when the engine is not running, preventing gas from flowing out of the tank in case of an accident. (On function.) Third, they provide a bypass operation to allow fuel to flow at any time, ususlly used to fill the carbs after a cleaning or service (Prime function.) Forth, they allow you to access the reserve fuel in the tank when you start to run out of gas. (Reserve function.)

Off, On, Prime, Reserve: four settings on most XS and XJ petcocks.

On some XS and XJs, the 'octopus' works with the petcock. In either case, octopus or petcock, engine vacuum opens a diaphram that allows fuel to flow. When the engine is off, the diaphram closes, shutting off fuel flow. (Petcock in ON position. PRIME bypasses the diaphram.)

The relationship between petcock/octopus and carb flodding is this: If the fuel flow does not shut off due to inoperable diaphrams, AND the inlet floats on your carbs are worn or leaking, fuel will overflow the carbs and you will get gas coming out of your air cleaner and in your oil.

Some people depend on shutting off the petcocks manually to prevent flooding. Not a problem, but constant turning of the petcock levers can wear or deform a small spring washer in the petcock assembly, leading to eventual leakage. Cleaning this washer, polishing it with fine abrasive, and rebending it to original shape will usually put it back in working condition. Polishing the seat it sits on doesn't hurt, either.

If your inlet needles and seats are worn enough to leak, chances are your engine isn't running as well as it should, as the fuel levels in the carbs will not be held at a constant level. Using the petcocks to eliminate flooding when standing still is, at best, only a stop-gap solution. Use the Off position for long term storage or when servicing the carbs or removing the tank.

Options:
Repair petcock/octopus and replace inlet needle valves. (Most common approach.)
Replumb the gas lines without the octopus and use the petcock Off/On function to control gas flow. (2nd most common approach.)
Replace stock petcocks with after-market units, either straight mechanical or vacuum-operated. While this last option has been discussed, it is a hassle due to petcock mounting dimensions and Reserve gas settings.

If the petcock/octopus diaphrams are working, and the inlet needle/seats are OK, the stock system works quite well.

[nashville_bill]

I am currently running the octopus and all is well, but it has too many hoses and with an additional two fuel filters, it is very cramped. Does anyone know if the standard, vacuum operated, petcocks will fit the special tanks? In other words, can I replace my petcocks with the vacuum operated type and do away with the octopus?

[mitch]

so long octopus

Bill, i took mine off and run a gas line across the front two taps on my petcocks to equlize them and then run a direct fuel line of the rear two taps to my carbs works great just got to remember to shut the fuel off when parked for over nite etc . doug m

[RiskyBusiness]

Re: Petcock theory

Originally posted by Jerry
Petcocks were designed to perform several functions.

... clip...

If the petcock/octopus diaphrams are working, and the inlet needle/seats are OK, the stock system works quite well.

Jerry, thanks for that overview. I understand things better than I did before.

[Jerry]

Couple quick addendums

Leaving the bike on the side stand may cause the fuel inlet needles not to seat tight enough to prevent over-filling, particularly if the side stand sinks into soft ground, increasing the lean angle. Use the center stand when you can.

Adding in-line fiters is recommended. As the bikes get older rust and/or sediment has built up in the bottom of the tanks and this could get into the fuel lines, particularly if you have to run on reserve at some point. Ask the folks who went to Spearfish 2002 about this.....particularly Dale.

Vacuum and fuel lines should be replaced periodically. Heat/weathering can degrade them, and leaks in vacuum connections will make you bike run poorly, no matter what you do to the carbs.

Finally, make sure the vacuum lines are routed properly. Particularly on Specials and XJs with the octopus it is easy to get a line hooked up wrong. Check the manual, or on the XJ there is a decal on the bottom of the seat pan that shows the correct routing.

[beechfront]

thanks a bunch - i was wondering why there were two lines on the petcocks - and when i turned fuel on gas only ran out of one - i guess the other is for vacuum.


where can i find a petcock rebuild kit? i've been looking all over with no luck. all i've found were on partsnmore and won't fit my 79 special.