petcock

[dodjh]

What is the function of prime vs. on . Off and on seem self explanatory---but when would you put the lever on prime position?

dodjh

[TomB]

On the Standards 'ON' is when the petcocks are turned on by the vacuum from the engine inlet manifold. Hence "Vacuum Petcocks"

'Prime' turns the cocks on to fill the carbs without the engine running, which is useful if the bike has not been run for a while and the carbs have dried out.

Dunno what the Specials are like with the octopus pipes.

[AlanB]

Hi Dodjh,

Special works the same way; "On" and "Reserve" are routed through the octopus and are controlled by the bikes vacuum; If it is all working properly, the fuel cannot flow unless the engine is running.

This means that if the carbs are empty, the bike cannot start 'cos it cannot get fuel UNTIL it has started!

"Prime" bypasses the vac system and puts fuel direct from the tank to carbs.

When you stop the bike (especially if you live somewhere warm!) heat from the engine will evaporate fuel from the float bowls; Many owners have found that their bikes will start more easily on "Prime" than "On" since the float bowls will be FULL, rather than just having the un-evaporated remains.

This problem was made worse on on the FJ11/12s by the fairing retaining heat, so Yamaha fitted a heat-shield between the carbs and engine to help.

AlanB

[John]

Originally posted by AlanB
[B

This problem was made worse on on the FJ11/12s by the fairing retaining heat, so Yamaha fitted a heat-shield between the carbs and engine to help.

AlanB [/B]

That combined with the fact that the petcock is located under the tank and cannot be accessed unless you remove the seat and lift the tank, just to make a selection on the petcock. What a Royal PITA!

[dodjh]

petcock positions

Thanks to all of you for the replys. After the carbs are filled via
primary position and bike is cranked, do you move the selector to on or can you just leave it in the primary position? ( I know if your planning to let it sit up awhile you'd run fuel out and put in off position.)

dodjh

[fredintoon]

Gotta Octopus?

Hi Dodjh,
a PO binned my Special's Octopus so I run with the Prime nozzles blanked off so my Prime position is another Off position.
But I gotta remember to turn the gas off or it stays turned On and relies on the carburettor float valves to keep the gas out of the crankcase.
If your Special still has it's Octopus, the vacuum diaphragm inside it will automatically turn the gas Off when the engine stops and turn it On again next start-up. That's if you leave the gas tap turned to On or to Res. If you turn the tap to Prime the Octopus gets bypassed and fuel runs directly to the carbs. Consider Prime to be a permanent On and only use it to let the carbs refill after a long stop and switch back to ON immediately the engine starts.
Fred Hill, S'toon.
Keeryst! if it's that complicated to explain, I'm happy mine is gone & I don't have to fiddle with it.
F.

[dodjh]

petcock

Thanks for the feed back fredintoon I'll operate it in that manner.

dodjh