PRIME or OFF for Petcocks??

[Locomotiveman]

Anyone,
Petcock Question.Is there any reason I can NOT run my XS1100 Special with both Petcocks in the PRIME position...and then always turn them to OFF every time I shut off the bike? Or will excess gasoline flow freely when at Stoplights etc. Gasoline is leaking back thru the Airbox now if I don't turn them to OFF.
Why do I ask?? I wanna ride the bike before I rebuild the Petcocks, look at the needles and floats etc. etc.

Now for dumb questions:1 Engineering-wise...Why did YAMAHA have a 2 petcocks tank configuration?
2 Is it practical to re-do it all with simple ON/OFF valves and an in-line fuel filter? Thanks in advance for any and all thoughts. LOCOMOTIVEMAN

[jetmechmarty]

If you have gasoline in the airbox, the petcocks ARE NOT the issue! One or more of your carburetor float valves is not closing. It could cause you to ruin your engine with gasoline in the oil, or it could even cause a fire, as happened to me.

[fredintoon]

Anyone,
Petcock Question.Is there any reason I can NOT run my XS1100 Special with both Petcocks in the PRIME position...and then always turn them to OFF every time I shut off the bike? Or will excess gasoline flow freely when at Stoplights etc. Gasoline is leaking back thru the Airbox now if I don't turn them to OFF.
Why do I ask?? I wanna ride the bike before I rebuild the Petcocks, look at the needles and floats etc. etc.

Now for dumb questions:1 Engineering-wise...Why did YAMAHA have a 2 petcocks tank configuration?
2 Is it practical to re-do it all with simple ON/OFF valves and an in-line fuel filter? Thanks in advance for any and all thoughts. LOCOMOTIVEMAN

Hi Loco,
running on prime and turning to off when you stop only masks the symptoms.
What you should do is NOT ride the bike until you have fixed the gas leaks; most likely you'll need to replace the needles and seats.
Why 2 petcocks? Mostly to get the last drops of fuel that would otherwise be trapped in the non-petcock side.
Better ask, why that 8-pipe plumber's nightmare of an octopus valve?
Sans the octo the stock petcocks ARE simple on/off valves except they have filters and a reserve setting.

[DGXSER]

Anyone,
Petcock Question.Is there any reason I can NOT run my XS1100 Special with both Petcocks in the PRIME position...and then always turn them to OFF every time I shut off the bike? Or will excess gasoline flow freely when at Stoplights etc. Gasoline is leaking back thru the Airbox now if I don't turn them to OFF.
Why do I ask?? I wanna ride the bike before I rebuild the Petcocks, look at the needles and floats etc. etc.

The PRIME setting is there to bypass the OCTY (Vacuum fuel valve, which nay or may not have been ditched already). One on each side, because the fuel tank has low ends on each side so you can get to all the fuel like Fred stated. Like also stated, if you have gas over flowing itno the air box, then the float needle valves are not working properly and should definitely be fixed before you ride it again, my opinion anyway.

Now for dumb questions:1 Engineering-wise...Why did YAMAHA have a 2 petcocks tank configuration?

One on each side, because the fuel tank has low ends on each side so you can get to all the fuel like Fred stated.

2 Is it practical to re-do it all with simple ON/OFF valves and an in-line fuel filter?

This is a very common way to set up the petcocks. As stated, the ON function of the stock petcock and PRIME become the same once you remove the OCTY. Except the PRIME outlet does not leave any RESERVE in the tank, so if you run on PRIME all the time, and run out, your done, pushing the big fat heavy hog of a bike. So alot of folks remove the OCTY, and run a fuel line from the back port (ON and RESERVE port) of the petcock to a fuel filter and then to one of the carb inlet T's same for the other side. Alot of folks also put a short piece of fuel line ont he front or PRIME outlet of the petcock and plug it so fuel will not run out if you accidentally hit the PRIME position.

[Locomotiveman]

OK..Understood...sort of.

DGXSER, Fredintune, and...
Thanks. Much appreciated. A coupla more questions:
1. IF the petcocks are A-OK and it's purely Float/Needles problem will the Fuel Flow be so excessive that the bike may not start after stalling, at say a Stoplight??
2. If the Petcocks are A-OK...how is it that fuel flows thru to the carbs when the bike is Parked/Not Running?? It doesn't flow when the Petcocks are in the OFF position while parked...but it does in other positions.
3. So, on a XS1100 Special....it's OK to have 1 petcock shut OFF? No parasitic vacuum issues, or any such thing. I understand the Fuel Line 'Check Valves' on my Twin Engine boat....this Yamaha thing?? Not really. And thanks again. Helpful. LOCOMOTIVEMAN

[DGXSER]

1. IF the petcocks are A-OK and it's purely Float/Needles problem will the Fuel Flow be so excessive that the bike may not start after stalling, at say a Stoplight??

Very easily would. Basically, you have one carb or more that essentially has no float valve, it just keeps getting gas loaded itno it as logn the petcock lets the fuel flow. Normally, the float in the bowl raises up and pushes the valve shut stopping fuel from flowing into the carb when it reaches the appropriate level. Since your float valve is not shutting it keeps loading up with fuel and will run VERY VERY Rich and would easily flood out that cylinder when stopped.

I like to explain the fuel system like a kitchen sink. Your float needle valves are like your faucet. They control fuel flow under normal operation. Your petcocks are alot like the shut off valves under the sink. They only shut off fuel flow when you leave the biek for a period of time, or need to work on the fuel system.

2. If the Petcocks are A-OK...how is it that fuel flows thru to the carbs when the bike is Parked/Not Running?? It doesn't flow when the Petcocks are in the OFF position while parked...but it does in other positions.

If you leave the fuel in the ON position and someone has removed the factory supplied vacuum operated fuel valve ( the OCTY), then fuel will flow into the bowls until the float needle valves shut the flow off. Just liek when the bike is running. If one of them is not shutting off properly, then fuel will continue to flow and pour out the carb mouth into the air box or worse ito the engine filling your oil with gas. And yes, it can do this while the biek is running also!! which can thin the oil and ruin the bearings causing major engine damage!!

If your petcocks are operating properly, when in the off position, no fuel can flow to the carbs from the tank period. Just like when you shut off the valves under the sink, nothing come sout the faucet no matter how you turn the handles.

PRIME was put in to let you over ride the OCTY when it was in place, without the OCTY it ha sno purpose. And Reserve obviously lets you use the last bits of gas in the tank, you have to turn it there so you know your low and should get to the closest station NOW!!

3. So, on a XS1100 Special....it's OK to have 1 petcock shut OFF? No parasitic vacuum issues, or any such thing. I understand the Fuel Line 'Check Valves' on my Twin Engine boat....this Yamaha thing?? Not really.

No, the way the fuel lines are configured in stock piping, you need both petcocks on as one feeds two carbs, the other feeds the other two. Now, if you still have the OCTY, then essentually, one petcock can feed all four carbs because the valve is designed to let both inlets feed one chamber that feeds both outlets to the carbs.

Now you can run both petcoks to a T, and then run that outlet into a T and let it feed both carb inlet Ts. With this piping set-up you can run one petcock and feed all four carbs.

Hope this all clears it up just a bit. Look over the factory fuel diagrams. Your special model will have one under the left side cover that shows the OCTY fuel line diagram.

[Locomotiveman]

OK...NOW I 'Get it"

DGXSER,
Thanks much.
For now, I'll rev the bike while whacking the carbs with a rubber-faced whatever; Run some Sea-foam in the fuel and hope I get the needles to seat better. Buy 2 Petcock Repair Kits. Turn BOTH petcocks to OFF upon shutdown. Then hope for the best....but expect the worst. So far my $535 Dollar investment has been outstanding....Sturgis, Canada, etc etc.

ps Luckily, Oles Custom Cycles (The XS110 guru), is just up the Road.
Thanks again. LOCOMOTIVEMAN