Plugged hose.....

[Hugh Jass]

I was putting my carbs back in when I noticed that there is a plugged fitting coming out of the rear manifold that was plugged by the previous owner.

It looks to me like there were one hose coming out of each manifold that converged into one hose going to the fuel pump.  But the owner broke the 2 into one fitting and just plugged one and ran a hose to the other.

Is my detective work solid?  Should there be 2 hoses, 1 from each manifold that converge into one hose leading to the fuel pump?  

Please help shed some light on this mystery........

[RobTx]

Only one hose.  The rear should be capped off.  Use it for carb syncing.  The front manifold has a hose to the petcock and one to the fuel pump, the rear has one fitting and it's capped off.  To sync the carbs use the capped off fitting in the rear and the one to the petcock on the front cylinder.  Be sure to set the petcock to "Prime" so the engine will run.  That's the way mine is.
Rob

Rob

[Lucky]

The fuel pump is operated from vacume pulses in the intake.  It sounds like the previous owner thought that double the pulses (from both manifolds) would increase the fuel flow or pressure.  interesting idea, I have no clue if it would work.

Rob, when your petcock is on "prime" it bypasses the fuel pump and depends on gravity to feed the carbs. in most instances this usually makes little difference, but under hard acceleration with a low fuel level in the tank, you could lean out the mixture.

'On" provides vacume to the petcock, opening the passage to the pump only while the bike is running.
'Res' allows fuel to flow from lower in the tank giving you some reserve fuel when you run out.
"prime" is a direct gravity feed for filling the carbs when the tank is run dry, or the bike has sat for a while & fuel has evaporated from the bowls.

I suspect you already know this, so i'm surprised you allways keep it on prime, unless I misunderstood.
--Lucky

[RobTx]

Lucky,
I only meant leave it on prime when you are syncing the carbs so you can use that vacuum fitting for your gauges.  I ride in the on position.  I don't think prime bypasses the fuel pump, as there is only one hose that carries fuel and it goes to the fuel pump no matter what position the lever is on.  The only ways to bypass the fuel pump is to disconnect the vacuum line for it or run fuel lines directly to the carbs from the petcock.  I don't think that would be a good idea.
Rob

[glennw]

I have a comment along these lines, after I rebuilt my petcock my bike was a real bitch to start cold. So one day
I looked at my clear filter and realized it had no gas in it. Not enough vacuum to make it work? I turned it to prime, waited about 20 seconds twited the gas and it fired right up. It's worked great for cold starts every since. One it's running I switch off of prime. Am I stupid... or is this how it suppose to work?

[Brian Moffet]

Glenn,

That's exactly how it supposed to work.  Prime causes the fuel to flow even without the vacuum of the engine.  It doesn't bypass the fuel pump, but the fuel will flow from the tank.  When the petcock is on "On" or "Res", the fuel will not flow until there is enough vacuum to turn the petcock on.  

The difference between reserve and on is left as an exercise for the reader :-)

I occasionally run on Prime, but mostly because I forget.  I also occasionally run on Reserve when the tank is full.  Also because I forget. Sometime during the ride, I'll remember I left it on Prime or Reserve.  I only recall leaving it on Prime in my garage once or twice.  No harmful effects seems to have taken place...


Brian