Weber Needle Valve Differential Sizing?
 

I've got Weber 46s on my 9.8:1 single plug (electromotive) 3.0L racecar. Currently tuned with 160 air correctors, 195 mains, F24 Etubes and 55 idles. Pump jets are 50 size and the main venturis/chokes and secondaries (4.5) appear unmolested. Trans is '84 915 close-ish ratios = stock 3.18 1st, 1.68 2nd, 1.32 3rd, 1.13 4th and 1.0 5th and 8:31 R&P

Ran pretty good my first event out in June on hot humid days, other than some popping at idle. Seemed to pulled strong and revved quickly & smoothly compared to what i've been used to before- mildly tuned 3.2 Motronic in my '87. Although the A/F gauge showed it to be well rich at WOT throughout the RPM range, which I thought should be improved with some jet tuning. No plug fouling though, which I checked at the track during my next outing when a problem arose.

Fast forward to a few weeks ago I developed a bogging/hesitation problem where the engine is strained to pull strong at high rpms- pulls okay but not normal up to 4500 and after that it just doesn't want to rev quickly at all. Cleanliness is said to be paramount with the Webers, so I started there. While cleaning the 1-2-3 carb entirely to learn more (Webers are a new thing for me) and to check for clogged idle jets which are said to cause popping, I found that I have two different float needle valves in my carb assy. The forward valve body contained a 200 and the rearward has a 175. This seemed odd to me..... I have a ton of extra new & old 175 valves in my awesome collection of Weber tuning/spares the previous owner gave me, but I have ZERO 200 valves. So that's what lead me to believe the 200 is improper?

Car ran fine/strong up until this bogging/starvation issue came up recently. So i'm starting first with the carbs to ensure they're clean and then i'll check the fuel pumps (two Holley Reds in parallel with pressure regulator set at 5-6 psi on VDO mechnical pressure gauge in the dash). So i'm not saying my bogging problem is on account of the float valves. Just wondering if this is an acceptable means of tuning the carb.

I did not check the other carb (yet) and thought i'd check here first. Obviously this mix of needle valves functions because the car ran strong before the problem, but is it proper or just a fix for something else- correction for the famous high g-load stumbling? Plus the brass valve caps are notorious for being stubborn and one of mine is already booged-up. PO re-sized the wrench hex to accept a smaller wrench on account of rounding it off. Well the resize didn't work and I had no choice but to use the dreaded vise grip to get it off........:mad: That's how I discovered the mix of needle valves.

I also have a strong & consistent pop from the left bank of cylinders (I can isolate my problem since I have a non-merged exhaust system with independent tail pipes) and I believe I can trace this to the cyl #2 idle air adjustment and mixture adjustment screws being turned all the way in/fully closed. So hopefully I solved that problem. I cleaned the idle jets so i'm hoping I will have corrected the popping with the cleaning and re-adjust the idle behavior on #2 with the air and mix screws set to a proper setting.

Haven't had a chance to fire it up yet as i'm still buttoning the fuel system up after the 1-2-3 carb rebuild/cleaning and installation of some new inline filters ahead of the carbs. Just trying to get a feel from the experienced here on the seemingly odd 200-175 needle valve combo before I start it up (and fill the carb with fuel) and begin the tuning process. Sorry for the long winded post.

Thanks,
Kevin

Kevin,

As you can well imagine, this is a complex issue but I'll offer that all four needle valves should be the same and the fuel pressure must never exceed 4 psi. This is the very foundation for all your jetting & tuning adjustments.

On race cars, I've also had excellent success by looping the fuel lines between the carbs to ensure that each fuel inlet "sees" the same fuel volume and pressure: this means installing an additional connection between them over the fan shroud using double-ended banjo fittings at the rear fuel inlets. Naturally, this is the time to ensure the inlet screens are nice and clean.

Thanks for the response Steve. I do appreciate the pressure setting recommendation and the looping advice.

I was told 6 to 6.5 psi was the shop & PO's preferred setting, however I completely realize that 3-4 is the recommended max. Only reason I can think they recommended 6 psi is due to the long run the gauge feed line makes from the dash to the distribution block/wye fitting just upstream of the two mini 3/8 can filters and the carb inlets. I've been running closer to 5 on the gauge. What's the problem with overpressure- float levers can't close the needle valve if too much psi?

Thanks!

Speaking of inlet plumbing, I have the PMO blocks. So I assume the screen filters are at the banjos fixing the blocks in place? I was expecting to find screens within the needle valve caps but then realized no when looking at my Weber diagrams.

Yessir, that pressure drives the needles right off their seats and can flood the engine. I've seen a few engines that hydrolocked from this. Needless to say, with the engine getting far too much fuel, its a real challenge to get consistency from proper adjustments and jetting.

Weber screen filters surround the banjo bolts and act as the final filter to keep stuff out of the float chambers.

You can see them in this illustration: p. 4 photo - Ernie Wilberg photos at pbase.com

Thanks for the tips on the pressures Steve. I think i'll disconnect the dash gauge and put a gauge in the engine bay right atop the distribution block. Since I have to be in the engine bay anyway to adjust the pressure on the regulator, might as well have the gauge in close view vs. being afar in the dash where the factory fuel/oil level gauge normally resides.

Thanks also for the link to the Weber manual. I have that and forgot to refer to it, since I have the Weber Tech Manual by Bob Tomlinson

Carburetor Books Page

Nice manual i've been working with, which gives some nice troubleshooting tips and how-to information on how the carbs work. Has good practical information on a lot of different Nice substitute for the Weber factory manual.

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