[ROVERNET - UK] Calling Rovernet Brain trust!!!

John Burkhard burkhaj at earthlink.net
Fri Sep 29 18:19:30 BST 2006

Good morning Ben,

Well I will try to scam the illustration but I am having problems with the 
scanner software.  If I fail I will photocopy it and mail it to you.

To help you understand this float bowl vent thing, think of a plastic one 
gallon gasoline can.  On the top there are two openings.  The large opening 
is to put the fuel in and pour it out.  At the other end there is a small 
vent with a cap on it.  If you try pour the gasoline out, with out taking 
the vent cap off the gasoline comes out in a slow "glug glug" intermittent 
stream.  Remove the vent cap and the fuel comes out of the gasoline can in 
a smooth, fast and steady stream.  This "air" or " atmospheric pressure" 
coming in the vent is preventing a low pressure, or vacuum, from developing 
at the top of the fuel in the can.

The "air" coming into the top of the float bowl has nothing to do with the 
air going into the engine through the carburetor venturi.  Without the 
vent, or putting your finger over the vent in the float bowl, the low 
pressure in the venturi would pull some of the fuel out of the float bowl. 
At some point a low pressure would develop in the space above the gasoline 
at the top of the float bowl and fuel flow would decrease to a point where 
the mixture was too lean to support combustion and the engine would stop 

This explanation is very simplistic but you have proved the main idea when 
you put your finger over the vent and the engine stopped running.  If the 
vent valve is dirty or partly plugged up the engine would idle ok but as 
soon as the is opened the fuel rate from may not be enough to maintain the 
correct air-fuel-ratio and the engine would die.

I hope all this helps you better understand the need for a float bowl vent. 
As for the operation of the vent valve, it works with intake manifold 
pressure, vacuum, but dose not introduce manifold pressure, vacuum, in to 
the float bowl.

Big picture I think that by cleaning this float bowl vent system form end 
to end, that is to say the pipe from the throttle body to the hose to the 
air cleaner, should help if not totally take care of the problem.  let me 
know what you find.  jb
----- Original Message ----- 
From: "Ben Rodgers" <irishrover at netscape.ca>
To: <rovernet at lyris.ccdata.com>
Sent: Friday, September 29, 2006 4:14 AM
Subject: Re: [ROVERNET - UK] Calling Rovernet Brain trust!!!

> Hi John
>           Your suggestion about the economiser would seem to make the 
> most
> sense, it is certainly the next thing to check. Is there any way you can
> scan the diagram from the Wade book and email to me. I'm trying to
> understand how this item works. Why does the engine stall when I put my
> finger over the pipe going into the economiser, why wouldn't the carb 
> draw
> enough air thru the main venturi to keep the engine running????. Anyway 
> I'll
> keep everyone informed as I progress, if I progress that is!
>                              Ben.
> born in Belfast, Northern Ireland,
> Author of "lily and me" a great book and a great read .
> Order on line at amazon.com. Book # ISBN1-55430-019-3
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