And I just remembered that I changed the pump diaphragms to the black rubber and enlarged the pump jets. I get a long squirt, I measured if but I only recall that it was more than sufficient. On hard acceleration my AFR usually go rich to 10 ish.

Thanks Shane.

I’m all the way backed out. 1 or 2 threads showing.

My guess is my float bowl check valve has a larger bleed hole which is reducing the squirt. If correct, I'm going to try solder it up tomorrow.

Don’t suppose you happen to know where to source the replacement diaphragm?

I bought the black rubber diaphragms from a long time eBay seller, it currently shows out of stock. I think I changed the bowl bleeds as well but I don’t remember if that made as much of a difference. The original diaphragms were worn so I think changing those made the biggest difference. http://forums.pelicanparts.com/uploa...1755981521.jpg

Thanks Shane. Much appreciated.

So I’m a little stumped. Swapped out the float bowl bypass valves from a 0.45 to a 0.00. No bleed hole on the 0.00 means more squirt and did deliver 0.8-0.9cc except for #2 and #5 (which I believe are always a little less on carbs). Car is blowing dark smoke so doesn’t look lean.

I filmed the bores under throttle blip to see direction of jet (into throttle or into wall of auxiliary venturi). Into bottom of the wall but there’s enough velocity for it to fine its way into throttle.

I did see what looked like air “coming back out”. That’s the best I can explain it but I’ll post a video.

Could it be the 1.75 M&K primaries and GT3 exhaust?

That squirt looks more than sufficient. If you hold the throttle open it will keep squirting a bit longer while accelerating.

I think what you are showing is typical AFR’s (15-17) during acceleration. On my gauge that’s what I’m seeing. Is it causing a stumble or hesitation?

Pretty frustrating day.

Putting the airbox on richened up part throttle significantly. Weirdly also going lean above 5000RPM. But I’m parking that for the moment. I’m confident I can resolve idle and main circuit tendencies and see that as different to main problem ..,

… The real quandary is lean under accel. Dangerously lean and doesn’t seem to clear ie. Can’t accelerate through it and allow vacuum signal to catch up. Something else is at play …

What else could it be?

If left to my own devices, I’m starting to think exhaust. If headers are oversized, could that cause a sudden jump to persistent evil lean under hard acceleration? Or would that show up across the curve.

Exhaust / PMO Carbs
 

I had similar issues configuring & tuning my PMO's to perform well across RPM's with my 3.2L (in 2014...https://forums.pelicanparts.com/911-engine-rebuilding-forum/821976-gordos-pmo-carb-tuning.html.

I was able to discuss the matter with Richard Parr (PMO designer). He strongly encouraged me to replace my 1-3/4" B&B headers with 1-5/8" (SSI / OE).

I continued to try to configure and tune out the problem and at some point, caved and bought a new (used), SSI & muffler.

I was changing quite a few things at the time (venturi, jets, correctors, hatchets...) - but the lean problem seemed to resolve with the exhaust change, or I was at least able to find an acceptable idle & transition to main setup.

I never went back to the B&B system. - but that was my engine. You gotta give them what they want to make them perform & no 2 are exactly alike.

Good luck, keep at it and you will get there.

Gordo

You mentioned you are using a stock CIS fuel pump, how are you regulating the pressure?

Gordo, your thread is an ornament. Thank you. It’s been extremely helpful.

I re-read it yesterday evening and you seemingly were able to tune your carburetors to eliminate the lean spike at acceleration with the 1.75”. Not optimally, but acceptably.

If I recall it was more main jet and/or smaller air correctors … by comparison, I’m running 185 main jets and 160 fuel jets. AFRs still 17+ under hard acceleration.

If you were able to make the 1.75” work on a 3.2SS, it stands to reason that I should be able to do the same on a 3.4L. Now, I also have a GT3 muffler which is essentially a straight through exhaust.

One other thought … could it be a leak at base gasket?

I ended up swapping that out for a low pressure pump. Much better. The CIS pump pressure could be metered down, but it was not stable.

Thanks - great to hear the old carb tuning thread is helpful.

I also went to a recirculating low PSI fuel pump - and watched the PMO gauge go from jittery to locked at 3.5 - just one less thing to consider.

Colortune? if not, I highly recommend for troubleshooting - it will show you if your intake has a vacuum leak. Leaking Cylinder will have a white / lean flame - no matter how much you twist the mix screw.

Some combo of configuration made mine way easier to tune, and stay tuned. I always feel I can do better - but remind myself how challenging it was to get it "more friendly".

Keep at it

- Gordo

Thanks Gordo.

I sourced a color tune and will try that. Am also going to try an F3 emulsion tube. Higher up lower holes than F7 and a slightly wider waste (otherwise very similarly). Should lean out bottom end while fueling top end.

Good point on fuel pressure and low pressure pump. 3.5 psi works for some. I have found that dialing in the least amount of pressure you can have while keeping the float bowls filled is best. I set pressure at about 2.6-2.8 and the float bowls levels are exact at the top of the etched circle in the port hole glass. To check that I have the pressure right I use an uphill elevation run, I go flat out for a mile at full throttle (if I don’t see the sheriff). I cut the ignition and check the float bowls and plugs at the top.

I’ve seen a lot of carburetors that run over 3 psi that have weeping fuel stains, and are usually rich at idle. Same symptoms usually are accompanied by high float bowls.

Gordo - which color tune did you buy? I looked them up - thinking the motorcyle one due to how deep the plugs are in the head? Haven't gotten my carbs yet - PP is saying they won't ship till October but trying to get all everything lined up and ready

14mm

Link to video using Colortune on my 911 -

https://youtu.be/j6pam6Ql_JE?si=75_Kz0L8N-AGJuuH

A bit time consuming, and tricky to get a mirror in place to see the flame, but great at ID'ing firing/fuel issues and produces a much better mixture adjustment than I was ever able to achieve by ear.

Great tool.

Good luck,

Cams? What did you set your cam timing at? Porting for I/E? What muffler are you running with 1.75” exhaust? What is the primary length for the 1.75” OD headers? There’s a lot that comes into play with running 42 chokes in 46 bodies with 1.75” OD manifolds and i wouldn’t recommend it to most for a street application.

50 idle jet doesn’t make sense, especially rich at cruise. You are sure that your bowls aren’t flooding? Can you please pull your mixture screws and upload a photo of the tip? Way back Richard used 2 different orings for the mixture screws and idle jets. If you threw a mixture screw oring on an idle jet holder, it was too large by a minuscule amount and it would cause the idle jet to not seal correctly. He went to the smaller oring later for both so that people wouldn’t have this issue.

Pull your needles and valves and look at the mating surface inside the valve. I dug into this before and found The original Weber valves were all metered at the fuel entry up top for 1.75/2.00/2.50 etc and drilled 3.00 at the base and had the same chamfer angle for the viton tip mating surface. This meant the needles all sat at the same depth. The new genuine Weber are drilled for the meter number all the way through and the chamfers are different for each size. This means the needle sits out further for a 175 and way less for a 250. If you set the floats per Richards spec, this changes how much the total travel is for a 175 and a 250, so it’s tough to get the bowls to fill and seal consistently from side to side.

For reference, here’s are two valves. Old vs new. Look at the difference in chamfer angle, as well as depth. The new 175 valves have a chamfer angle that doesn’t even match the viton tip. Introduce some motor vibration and now you have floats that aren’t sealing well.

http://forums.pelicanparts.com/uploa...1756402239.jpg
http://forums.pelicanparts.com/uploa...1756402239.jpg

Saw more of your details. I would change out your solid motor mounts before you go any further. Are you needles annd valves genuine Weber? You should upload a few photos of your valves and take a peek at the sealing surface. Some aftermarket have no chamfer paired with machining burs, loose fitting needle, and will never seal with engine vibrations.

Club sport mounts (dealer sourced only) or Wevo with black pillows. I have run blue and black before. I prefer the soft black.

Is the gt3 muffler running both the loud and quiet entry, just one, or valved?