Weber Needle Valve Differential Sizing?

[KTL]

Weber Needle Valve Differential Sizing?

Any help is greatly appreciated from the engine tuning gurus!

Thanks,
Kevin

[Eagledriver]

Normally I agree with everything Steve says. In this case however I think there maybe a reason for the different valves. The Webers I used to have had different valves for each bowl as well. I speculate that the reason for this is that one bowl feeds 2 cylinders and the other bowl only feed one. If the bigger one is on the 2 cylinder bowl then that would make sense as this bowl will need a higher fuel flow to feed 2 cylinders.

For your popping issue all I can add is make sure you have a good seal on the idle mixture screw. Usually these have an o-ring on them to keep air from being sucked in along the threads.

-Andy

[KTL]

Thanks for chiming in Andy. I had the same thought on the different valves, so it's interesting to see this has been done before. The bigger 200 valve is on the forward valve body. I'll have to take a look to see if that inlet/bowl does indeed feed two cylinders. Can't really tell from just the diagrams.

Thanks for the tip on the o-rings. When I pulled all three mixers to clean the port and tips, I did notice the o-rings were well-smushed. So I replaced all three with new orings lubed with Dow 112.

Haven't had a chance to diddle with the car more yet (last night = my night to watch Noah) so i'll mess around tonight and likely put the 200 valve back in. Still have to check the gauze filters at the inlets anyway since I overlooked those. Plus some further searching here indicates that it's not all that uncommon to have a closed air screw to compensate for air already sneaking past the throttle plate. However i'd like to see if I can align the throttle plate to allow the #2 air screw to actually serve its purpose. That being said, I still am bothered by the same #2 cylinder's mixing screw being all the way in/closed.......

[Walt Fricke]

On each bank, there are two inlets and two float chambers. Obviously, one feeds one venturi, and the other two. You can tell by looking at where the main jets are. The one with only one main (and an undrilled casting hole beside it) is the one where the venturi has a whole float chamber all to itself.

I put a larger "gorse" (?) valve in the one which feeds two main jets. Seemed like the logical thing to do.

Which doesn't mean a) I wasted a little money, or b) the motor would have performed a bit better had I kept them equal.

But my motors have always run fairly well, and the plugs always look about the same, so doing this at least didn't seem to cause nasty issues.

Walt

[KTL]

Thanks for sharing Walt. Should be firing up the engine soon when I get this pesky fuel cell buttoned up. I'll switch back the 200 valve and keep going the way it's been thus far.

[KTL]

Was searching for some other stuff and stumbled across this thread from years back. Wanted to follow up by saying I think the different sized inlet valves is a wise idea for a race/track car. I never had problems with my car in terms of float bowl starvation, and my bowls were never modified with the anti-slosh baffle plate trick.

Another point worth mentioning is PMO equips their carbs with different sized valves for the carbs as common practice for every carb installation.

"46 and 50 mm PMO carbs use 1.75 needle valves in the single main jet float bowls and 2.50 needle valves in the
double main jet float bowls. The 40MM PMO carbs use the 1.75 needles and seats in both float bowls."

Taken from PMO's tech update page, noted at the bottom of the page

http://www.pmocarb.com/Images/Instructions-3.pdf