carb vent tube mystery

[bikeboy]

Hi guys. Just buttoning up my restoration project, and I'm a bit flummoxed by three 'vent' tubes coming from the rear (front? airbox-side) of the carb rack. I have a feeling one goes to the vacuum connector on the petcock, and I think the outer two are just vents? There seems to be a 'pulse' in each of them when the bike is running, and blocking the petcock one improves the idle some, but is this the right configuration for these tube? I've refered to basscliff's excellent documents, but they model a different type of carb, and I'm not sure the two are interchangeable? My rack looks like this (don't smirk ):



with the first tube (possibly) being a vent, and the second (the one that heads up and forward) what I think is the petcock connection?

Have I got this right? I would have thought the vacuum pick-up would be on the engine-side of the mechanism? That's where they normally go?

Any help appreciated.

cheers
ian

[TheCafeKid]

The tube coming from the TOP of your number 2 carb (numbered from left to right sitting on the bike 1 2 3 4.) is the PETCOCK VAC LINE. If youre running a NON vaccum type petcock (like a Pingel) youll need to plug it. the other two are indeed vent lines, and should be routed over the top of the air box, and then down to vent free to air.

[bikeboy]

The tube coming from the TOP of your number 2 carb (numbered from left to right sitting on the bike 1 2 3 4.) is the PETCOCK VAC LINE. If youre running a NON vaccum type petcock (like a Pingel) youll need to plug it. the other two are indeed vent lines, and should be routed over the top of the air box, and then down to vent free to air.

Thanks for that. I *think* I'm on the same page? When you say TOP, the tube I'm thinking of exits carb #2 at the same place as the tube in the photo on carb #1. Is that the right one? There doesn't appear to be any other connectors. The thing I can't get my head around is that if all 3 tubes share the same carb circuits, and one is blocked-off for petcock vacuum, the other 2 are just feeding air into the carbs. I thought the vents were to allow pressure out?

Sorry to sound such a dummy. Never was much good at carb-stuff.

thanks
ian

[mike_of_bbg]

They actually feed pressure IN to the carbs, as an atmospheric reference. Venturi's principle says that air speed increases and pressure drops as a fluid passes through a restriction, and your carb uses that pressure drop against the atmosphere to push fuel into the air flow. You want to make sure the vent tube ends are tucked away from turbulence. I've heard stories told of improperly placed vent tubes causing drops in power at inopportune times, like when passing a truck...

[bikeboy]

They actually feed pressure IN to the carbs, as an atmospheric reference. Venturi's principle says that air speed increases and pressure drops as a fluid passes through a restriction, and your carb uses that pressure drop against the atmosphere to push fuel into the air flow. You want to make sure the vent tube ends are tucked away from turbulence. I've heard stories told of improperly placed vent tubes causing drops in power at inopportune times, like when passing a truck...

cool. thanks guys. good information.

cheers
ian

[tkent02]

The tube coming from the TOP of your number 2 carb (numbered from left to right sitting on the bike 1 2 3 4.) is the PETCOCK VAC LINE. If youre running a NON vaccum type petcock (like a Pingel) youll need to plug it. the other two are indeed vent lines, and should be routed over the top of the air box, and then down to vent free to air.

Wrong, those are VM carbs, # 3 is the vacuum line which goes to the vacuum port on the petcock, the tubes on #2 and #4 are the vents. All of the fittings look the same on the carb bodies, but they are drilled differently inside.

On the bikes with CV carbs the vacuum line is on top of the #2 carb.

[bikeboy]

Wrong, those are VM carbs, # 3 is the vacuum line which goes to the vacuum port on the petcock, the tubes on #2 and #4 are the vents. All of the fittings look the same on the carb bodies, but they are drilled differently inside.

On the bikes with CV carbs the vacuum line is on top of the #2 carb.

OK. I'll set them up according to this arrangement then. Thanks for that. I knew the carbs were different to those in Basscliff's documents, but I wasn't sure if the routing was the same.

many thanks
ian

[KEITH KRAUSE]

Just to back up what tkent02 said...
The ports at carbs 2 and 4 (left to right when sitting on the bike) are to vent the float bowls. The vacuum for the petcock is at carb 3.
If using the stock airbox, keep the vent lines attached and route them so they breath easy (no kinks) and they generally lay under the seat. If running pod filters, I recommend removing the vent lines and leaving the ports open to breath. Fuel starvation can/will occur especially in crosswinds, etc, if you run pods and leave the vent lines attached. Dynojet recommends the same thing as per their jetting instructions.