Aux bowl vent screw

Splitlog

Splitlog

Well-Known Member
Have seen plenty of posts. Also instructions from S and S to remove the auxiliary bowl vent screw. But have not found a why?? Just curious. Anyone know. I just changed my jets. I now have no baffles and I stalled a spike air cleaner.
Jets are 78/33. I did not remove vent screw yet?? I had put it all back together before I saw to remove it.

Thanks in advance
 
ringo

ringo

Active Member
5. No air cleaner used or air cleaner used is brand other than S&S.

Some air cleaner designs restrict air flow so that carb cannot draw
air freely as needed. Also, other air cleaners may obstruct bowl
vent hole on inlet end of carb and change bowl air pressure.
NOTE - Bowl vent hole passageway leads to cavity above fuel in bowl
and is designed to equalize bowl pressure and atmospheric pressure. If
high or low bowl pressure relative to atmospheric pressure develops,
engine may run erratically.
6. Air horn used without shrouding bowl vent on end of carburetor body.
NOTE - Bowl vent should be shrouded to insure uniform atmospheric air
pressure between cavity above fuel level in bowl and area outside
carburetor body. If high or low bowl pressure relative to atmospheric
pressure develops, engine may run erratically.
 
francoblay1

francoblay1

The Spaniard
SVEN where are you? I did ask this same question in one of my posts and Sven did answered it pretty well (I had to read it twice but I got it) he also posted a picture of the passage hole which gets obstructed by an aftermarket air cleaner.... I will post it here if I find it.

I did remove mine and it made a lot of difference... I run a Spike.
 
S

Sven

Well-Known Member
francoblay1 said:
SVEN where are you? ... (I had to read it twice but I got it)...
Click to expand...
I am velocity. I can move a lot of air for the street. I am more or less a simple made racing carb. I can get away with a small hole because I just putt along.

But if I open it up, demand a lot of gas, i.e., along with that vacuum, then I'd have to neutralize that kind of velocity in the carb bowl. It is trying to bring back 14.7 psi into the bowl on top of the gas level. You can look at me as if I am, 'for every vacuum pull [of fuel out the jet hole] there is an opposite and equal pressure [over the gas level] to equalize that 1 atmosphere back to a balance: so flow is uninterrupted.

If that balance is interrupted by more velocity [or the pulling] of that gas thru that hole, it will cause a vacuum lock at the top of the fuel level. Instead, the air vent channeling into the top of the gas in the carb bowl needs help. That hole used is too small for the new demand. So a simple expansion of another hole to compensate is also designed into the carb.
 
JimNubs

JimNubs

Member
One more agonizing bowl vent screw question.

Sven said:
I am velocity. I can move a lot of air for the street. I am more or less a simple made racing carb. I can get away with a small hole because I just putt along.

But if I open it up, demand a lot of gas, i.e., along with that vacuum, then I'd have to neutralize that kind of velocity in the carb bowl. It is trying to bring back 14.7 psi into the bowl on top of the gas level. You can look at me as if I am, 'for every vacuum pull [of fuel out the jet hole] there is an opposite and equal pressure [over the gas level] to equalize that 1 atmosphere back to a balance: so flow is uninterrupted.

If that balance is interrupted by more velocity [or the pulling] of that gas thru that hole, it will cause a vacuum lock at the top of the fuel level. Instead, the air vent channeling into the top of the gas in the carb bowl needs help. That hole used is too small for the new demand. So a simple expansion of another hole to compensate is also designed into the carb.
Click to expand...
I printed out the picture from the S&S web site on the carburetor. I had pictured in my mind that the vent screw would be small diameter in size. When I found the vent bowl screw, I think it is the right one, it is about 3/16" diameter and takes a 5/32" alan whrench to remove. Is this correct? I removed it and then got nervous and put it back in for now.
 
JimNubs

JimNubs

Member
I printed out the picture from the S&S web site on the carburetor. I had pictured in my mind that the vent screw would be small diameter in size. When I found the vent bowl screw, I think it is the right one, it is about 3/16" diameter and takes a 5/32" alan whrench to remove. Is this correct? I removed it and then got nervous and put it back in for now.

I know it is on the bottom left side of the bowl. Is it the alan screw that takesa 5/32" alan wrench to remove? I think it will leave a 3/16" hole in the bottom of the carburetor housing.

I'm just not that confident in doing this and I worry that gas might drip out or something.... OK Franco quit laughing.
 
mleach72

mleach72

Well-Known Member
Derf Diggler said:
So if my bike runs pretty good with it in is there a need to take it out? Or can it only run better?
Click to expand...
If your bike runs well with it in, your air cleaner probably has a recess for the main vent. In the tuning manual, S&S says to take the plug out when installing an aftermarket ac, just to cover all the bases. Some ac's have the recess. Some don't. You can remove the ac and see if it is covering the main bowl vent on the carb. The vent allows air to enter and exit the carb as fuel rises and falls in the bowl. If the vent is covered up, fuel can't enter the bowl because the air can't escape. If the vent is covered, you would know it. Especially during heavy throttle.
 
D

Derf Diggler

New Member
mleach72 said:
If your bike runs well with it in, your air cleaner probably has a recess for the main vent. In the tuning manual, S&S says to take the plug out when installing an aftermarket ac, just to cover all the bases. Some ac's have the recess. Some don't. You can remove the ac and see if it is covering the main bowl vent on the carb. The vent allows air to enter and exit the carb as fuel rises and falls in the bowl. If the vent is covered up, fuel can't enter the bowl because the air can't escape. If the vent is covered, you would know it. Especially during heavy throttle.
Click to expand...

Awesome, thanks brother!
 
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