Fuel Petcock question

Just wanted to know if anyone knows what the upper connection on the fuel petcock is? Obviously the bottom one feeds the carbs. Is there a vacc line that attaches?

 

Vac line goes straight to no3 carb.

 

Is that to help pull the fuel down?

 

Fuel will run on prime to fill carb bowls when empty, should then be switched to run or reserve to normal use.

If working correctly test by connecting tube to vac line and switch fuel to on or reserve and then suck on vac pipe fuel should then flow through to carbs.

Engine performs this function when connected to no 3 carb.

 

Great thanks!

 

No Worries,

If you do a search, I think big fitz did a photo shoot of a rebuild.

this explains all the insides of your petcock. well worth a look.

 

No, the vacuum line "operates" the fuel valve inside the petcock, when the bike is running. When there is no vacuum, the petcock should not flow fuel in ON or RES, just on PRI.

The vacuum signal "allows" fuel to flow (if the petcock is operating correctly.)

The write-up you were referred to is here: http://xjbikes.com/Forums/viewtopic/t=25058.html

 

Thanks fitz. Very good information. What kind of problems would I see if that petcock internals were bad? High idle, rich mixture...something like that?

 

Fuel running onto the ground, instead of into the carbs.

Fuel in the vac line (if the vacuum membrane in the petcock is perforated).

Fuel filling up the crankcase if the float needles aren't seating fully, and the petcock is allowing fuel to flow.

Fuel getting on your hand when you switch to prime or reserve if the outer o-ring is failing.