So it's been a while with a hectic work schedule since spring and a move over the summer. Finally back to the 914... Forgot how much fun she can be.

I left off last spring fighting a stumble with pulling away and with partial throttle acceleration.

My engine is a 2056, weber 40 IDFs running 28 vent, 130 main, 50 idle, F11 emulsion. Timing is set for 27 deg at 3500, Pertronix SVDA dizzy (had an 009 with worse results), Webcam 86b cam, stock heads.

I installed an AEM A/F gauge last weekend - numbers look decent with idle, wide open, cruise (most of the time), but partial throttle (perhaps 1/4 - 1/3 pedal travel) leans out to 15-17 and stays there until cruise I can throttle back or open the throttle up. While cruising slight increases in throttle for a hill or small speed increase leans out from 12.5-13.5 range to 15-17 and stays there once again until I can throttle back or open up more. Seems to be dependent on throttle position, not RPM.

Carbs are synched using a snail gauge checking at idle, 2,000 RPM and 3,000 RPM.

Any thoughts/guidance from the carb gurus out there?

Thanks!

If the throttle is just slightly cracked then you are still running on the idle jets and transition circuit. The idle mixture screws shouldn't have any effect except at idle.

There's a test for idle jet size in the Braden Weber Carbs book that I've been meaning to post for a while. It's on page 39.

Click to view attachment

I was actually working on this today and as I slowly moved from 700 rpm to 2000 rpm it was pretty clear that I was way too lean in the 1300 range - reading 16-17 on my wideband AEM AFR. I changed from 55 to 57 idle jets, reset the idle mixture, and repeated the test. Got it down to 14-15 just with that small change. I'm going to try some 60 idle jets next but may have to go to 62's or 65's to get it perfect.

I'm running 48 IDF's on a 2316 Raby Engine, BTW.

What size are your airs. Try 55 idles then readjust air fuel mixture . Test. If that doesn't work move up to 57 then 60 . Do you see your accelerator pumps squirting nicely.

Thanks guys.

As for the test, I haven't run this one but read it the other night. My assumption is that idles were too small, but I've also read conflicting info that RPM plays a roll on the progression circuit transition to mains which doesn't seem to be the case, therefore throw it out for the more knowledgeable crowd.

As for airs, I'd have to pull them and check.

Physics dictates that everything works together in terms of the progression circuit. RPMs of course play a part, but so does throttle position and to a lesser extent even ambient air temp and fuel density (specific gravity). The beauty of modern ECUs and fuel injection is that all of this is taken into account. For carbs... well at some point it is a little bit of trial and error.

If your carbs have adjustable accel pump positions, you can try that first (see my thread in signature - not all of them have them). But from what you're saying, it's not the throttle tip-in that's causing it to go lean, it's constant part-throttle, which wouldn't be solved by adjusting the accel pump shot.

You can also try E7 emulsion tubes - some people have success with them, as the progression holes within the tubes (note: different than the progression circuit in the carb body, but same principles) allow the mains to take effect a little sooner than E11s.

A lot of people think that Webers have a progression circuit that isn't suited for our cars. The reality is that no one carb will fit every application perfectly out of the box. The adjustment methods are there, but the process can take some time (and sometimes money).

Repeating what PotterPorsche already said...

I would bump up to 55 idle jets as suggested, and then reset the idle mixture screws (for highest idle). Do the slow increase in rpm test with the idle adjustment screw and see if it still goes lean when the transition circuit is play (between 1100 and 1500 rpm, most likely). If still going lean, try 57 or 60 idle jets and do the same.

How many turns out are your idle mixture screws with 50's? What about the other idle jet sizes - advise when you test.

I would get the idle jets sized correctly first and then work on the accelerator pump to be sure it kicks in correctly when you punch it.

Nothing wrong with running lean at part throttle. 16-17 under light load will yield good fuel mileage.

Ordered a set of 55 idle jets and will go from there. The accelerator pump definitely squirts on all four barrels, timing and volume appear to be in the same ballpark based on casual observation.

Also, I have rebuilt the carbs so everything is clean and floats adjusted - no change in running before/after.

As for AFR numbers, I've read threads saying over 15 except for a short transition is bad. So if 16-17 under a light load is OK, how long can you run like this? That is if it goes lean to come up a few MPH under cruise, that might be 10-20 seconds. Going up a slight grade requiring a little more throttle might have me sitting there for a minute or more. When should I be concerned about lean running?

Once you have the 55's installed, I would be interested to know if you have a flat spot as you accelerate - is there still an off idle stumble? The goal should be to eliminate it, whether that means an AFR of 15 or 17.

Got it. I'll follow up once I get the 55s installed.

The AFR range to generally avoid is around stoichiometric. (14.7:1 for straight gasoline)
Lean (16-17) at part throttle is good and runs cool. However, even close to stoichiometric isn't harmful at light throttle settings.
Rich (12.8-13.2) at WOT is required at any rpm. Richer will work fine up to a point where power loss is noticeable, but it wastes fuel.
Off throttle may be very rich at some rpm but not much fuel is being used so it doesn't really matter.

Installed 55 idle jets and retuned/synched carbs. Great improvement - still a little off throttle stumble but significantly reduced. AFR shows lean conditions (15-17) under partial throttle but not as lean and for shorter durations on the real lean side, so based on Chris' input I'm not worried with these numbers/duration.

Guess it's time to move up to 57.5 or 60 idles to see if I can eliminate the remnants of the off throttle stumble.

Before going larger on the idle jets, try a bigger air corrector (+.15) and a slightly bigger main (+.05).
That might move the transition rpm down enough to eliminate the flat spot.

At least I was half on the same page as Chris . I must be retaining something in my tiny noggin afterall.

Will do guys. Thanks for the re-education as I haven't messed with carbs since, well, let's just say it's been a while... Man I'm feeling old...