Forum - Briggs & Stratton Racing
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gary edgar

Registered: 06/06
Posts: 3
Reply with quote  #1 
When viewing the Walbro PZ26 on the World Formula from the front, on the opposite side to the idle screw there's a small "vent". Nobody here in Australia seems to know what it's purpose is. Can someone enlighten me?
I was once accused of spraying fuel on a following kart and one WF owner suggested that it probably came from this "vent" under extreme cornering and that the fix was to run a tube from it to the chassis so any vented fuel would hit the track and basically evaporate. I did so and it seems to have worked, but I would still like to know the purpose of that vent.
BriggsRacing1

Registered: 06/22/11
Posts: 188
Reply with quote  #2 
CHEATER!!!!  lol

There are two vents on the carburetor.  The bowl vent that runs out of the bottom of the fuel bowl that prevents the fuel level in the bowl from exceeding a critical level.  It works as most bathroom sink emergency by-passes, if someone forgets to turn the sink off it doesn't spill and flood your bathroom.

The second vent is an atmosphere vent.  It equalizes the pressure within the carburetor to that of the air around you.  Negative or positive pressure in extreme cases would either prevent fuel or push too much fuel into the system causing your engine not to function correctly.  I would caution you to make sure you are NOT shutting off this vent.  It will likely impact engine performance and/or push fuel out your bowl vent as the pressure HAS to go somewhere.

I would suggest lowering your float level as this is helping fuel migration.  Correct this and there should be no need to take other actions.


gary edgar

Registered: 06/06
Posts: 3
Reply with quote  #3 
Many thanks Briggs! Accurate WF knowledge can sometimes be hard to come by Down Under and I'd drawn a blank on this one. Your explanation of the atmos vent function clears that up and also suggests to me why that on a couple of occasions when track direction has been reversed, that on two different racks in a single hairpin in each instance, I experienced what I can only describe as "fuel starvation" causing the engine to miss (even stall) then recover after a dozen or so metres. I'll do as you suggest and remove the tube I'd attached to that vent and if needs be alter my float level.
J3S4

Avatar / Picture

Registered: 08/01
Posts: 2
Reply with quote  #4 
Just to confirm, are we referring to the part pointed out in red arrow below?

DSC_0236.jpg


Close up photo below.

DSC0237.jpg


If so, then mine is not vented from what I can see. There is no hole going through. But a friends WF with a PZ26 does have a hole/vent. This is a brand new engine. I also have another WF that is the same as above. That is, it has no vent. Curious if I have got the correct part, and if so, why are some PZ26 carb's vented, and some not if it is as has been said, critical to performance?
briggsracing

Moderator
Registered: 05/16/11
Posts: 50
Reply with quote  #5 
This is a bowl vent.  In the photo you uploaded I seem to see the hole underneath in its normal position?  Am I wrong in seeing this? The bowl vent functioning to maintain atmospheric pressure inside of the bowl.  Pressure also naturally equalizes from several parts of the system including the idle and main jet circuits and even from the fuel feed itself.  This is just a secondary vent and sometimes racers will run a rubber tube over this to reduce misting when really they simply need to get their float height correct (reduce the fuel level).  As far as consequences or performance impact there shouldn't be any given the fuel requirement of the engine and the secondary methods of equalization BUT we do recommend that this vent be properly working and not be blocked off.

Please look your carburetor over to make sure the vent hole isn't drilled completely.  If it isn't and you would like to exchange it we would be glad to work with you. 


J3S4

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Registered: 08/01
Posts: 2
Reply with quote  #6 
Thanks for the reply. You are correct. It does have a hole underneath which I did not see ([rolleyes]). What threw me was my friends WF PZ26 has the vent hole through the centre, not at the bottom. It was purchased brand new. Not sure why this is so.

Would you be able to tell me what the float height should be if you were using a 10deg mount? From factory it's set at 14mm. Would you go higher say.. 15mm or lower towards 13mm?
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