Carb vents


from what i know, it is to limit the effect atmospheric pressure changes have on jetting. so if you are running some sort of scoop for a ram air effect, the increased air pressure at the mouth of the carb will reduce the venturi effect. This is because the float bowl of the carb is still at atmospheric pressure, making it harder for fuel to be pulled up the jet, this causes a lean condition. so to compromise you vent the float bowl to the pressure at the mouth of the carb. it will usually make jetting more consistent.
 
ACTUALLY ON THE VMAX4, IT WAS A RICH CONDITION. THOSE VENT TUBES HUNG BASISLY UNDER THE MOTOR, AND UNDER HOOD PRESSURE AND HEAT AND NOT VENTED CAUSES RICHER CONDITION THROUGHOUT MIDRANGE AND TOPEND. VENT TUBES TO AIR BOX CREATED FREE COOL FLOW OF OUTSIDE AIR DIRECTLY INTO CARBS. THIS WAS FIRST FOUND ON THE 92 VMAX 4 BY C&H DYNO HERE IN NY. 3:16 (yammie tony)
 
There are 2 vents from each carb. One that is long and goes down into engine bay and the other is short and zip tied to the carb rail. Do all 8 of these go into the airbox? How deep into airbox should they go?
 
TAKE THEM ALL AND SHOVE THEM IN AIR BOX. REPLACE SHORT ONES WITH LONGER LINES. THATS WHAT I DID. THEY WILL HIT SHELF IN AIR BOX AND STOP. 3:16 (yammie tony)
 
I can't speak specifically about the VMAX and it's fix for a rich condition, but when you place the vent into the air box, you change the atmospheric pressure within the float bowl thus changing head pressure of the fuel. That's not to say that the underhood atmospheric pressure doesn't change when riding; and you won't see many piston port engines with vents in the airbox due to cyclic pressure pulses, but its typically more consistant than the airbox.

The pressure in the airbox changes dependent on throttle opening with read valve and rotory valve engines. Idle = near atmosphere, WOT = lower than atmosphere (vacuum). This lower pressure at WOT reduces the head pressure of the fuel in the bowl which effectively reduces fuel delivery through the main jet without changing orifice size. IOW - it's harder to suck the same amount of fuel through the same size opening because the vent is sucking back.

If you think in drastic terms -

What happens if you pump shop air into the vent? You would push fuel into the engine through the main jet and flood it quickly. High pressure=higher fuel delivery

Conversly, what happens if you block off the vent? You eventually starve then engine of fuel because you're not replacing the fuel with air. Or even more drastic, place a vacuum pump on the vent and you'll suck the fuel through the vent instead of the main. Low pressure=lower fuel delivery

Pre fuel injection autos used carborators with bowl vent actuators that would change bowl pressure to tweek a/f ratios to meet emissions and although I've not used them, Temperaflow and altitude compensators use bowl pressure instead of jet changes to tweek a/f.
 
great post. it really is. i don,t know how to even answer ya except in 1992, this problem was first found on the 92 vmax 4 at c&h dyno. yamaha itself went there to work with jim to solve this problem. they saw the problem with their own eyes. now what was done to the 93 vmax 4 to solve this i can,t post because i don,t know. this was a real problem. putting the vent tubes in the air box changed alot of stuff. 3:16 (yammie tony)
 
YAMMIEGOD3:16 said:
great post. it really is. i don,t know how to even answer ya except in 1992, this problem was first found on the 92 vmax 4 at c&h dyno. yamaha itself went there to work with jim to solve this problem. they saw the problem with their own eyes. now what was done to the 93 vmax 4 to solve this i can,t post because i don,t know. this was a real problem. putting the vent tubes in the air box changed alot of stuff. 3:16 (yammie tony)
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What's that ya say? I can't hear ya. lol

I'd guess that a fat spot was found in the fuel delivery calibration and using the vent to tweek it was enough to avoid changing needles, main jets or jet mains on sleds sold that model year.

A cheaper and more reliable field fix when you think of having to ask already low margin dealers to have thier "best techs" tear carbs apart.

Back to 1970's-80's auto technology for a bit: Anyone remember the black coffee cans on the inner fenders of Fords of these vintages?

It was the vacuum reservoir that supplied vacuum to the float bowl pressure solenoid and also supplied other vacuum actuated devices.
 
stain said:
The 94 I just bought has all the vents run into the steering post pad????
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This is the way Dynotech recommended to run them in order to avoid the pressures that develope under the hood at high speed.

With stock air boxes in place this is the same approximate location that the carbs get their air from so vent/intake pressure would always be the same.

opsled
 
When Dyno Tech put the early 750 Vmax 4s on their dyno they found they would run extremely lean due to NO underhood pressure richening up the mixture through float bowl pressures as the dyno is obviously stationary.

I would suggest if you are changing the carb venting to under the handle bar pad or into the airbox you will need to richen up your needles and main jet for a safe and power making BSFC.

JM.02c
 
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