IDF dead spot at 1/4 throttle

[ebrown0104]

I installed a set of IDF 40's on my car about two months ago, and the difference between them and the 32/36 DFEV is amazing, especially over 4000rpms. My car dyno-ed at 108whp/108wtq with the DFEV, and I can almost guarantee when I run it again it's gonna pick up another 10hp/10tq.

I have one small hiccup that's happening with them. At about 1/4 throttle, which is where I'm assuming the transition occurs bewteen idle and main circuits, the A/f ratio leans out. I have a wideband O2 gauge set up, and the AFR will jump from 12.5 to 16-17 at that point. Also, the car will lightly cough until you either add more or take away throttle. If you keep the throttle below 1/4, it runs fine because it's running on the idle circuit only. If you accelerate briskly and activate the accelerator pump it runs fine, because the pump adds enough fuel to overcome to lean spot. There's just that one spot where it leans out, and unfortunately under normal 40-50mph driving that's where you spend a fair amount of time.

My current set-up is this:

F7 emulsion tubes
4.5 aux venturis
32mm chokes
140 main jets
200 air corrector jets
45 idle jets
1 turn out on idle screws
200 needle valves
000 pump exhaust jets
55 pump jets
10mm open/32mm closed floats

I originally had F11 emulsion tubes but switched to F7's because they are supposed to richen up the lower end of the main circuit and lean out the higher compared to the F11. It helped a little bit, but it still happens. I've tried 135 and 145 mains, and neither really make a huge difference (135 is best for full throttle at about 11.5-12 AFR). It seems like the more timing advance I give the less that the stumble happens, but if I set my timing any further my idle speed is too high. I haven't tried smaller air correctors yet, but I'm thinking that will richen up the main circuit too much where it's not needed (mid-to-high RPM's). I tried 47 idle jets, but the car loads up while it's at idle and the idle screws are only turned 1/2 turn out. I've even tried tightening the accelerator pump rod nuts to try and get the pumps to spray easier, but that doesn't seem to affect it much either.

I'm at a point right now where I'm stumped. Anybody ever heard of this problem before? Better yet, is there anyone near Pittsburgh PA who's a Weber tuning expert (Fiat or otherwise)? As always, thanks for any info

[baltobernie]

First off, I'm not the Weber expert you seek, nor do I have any experience with IDF's. But I've experienced the same problem you have when I installed a 38DGS on my 1800. The 38DGS carb does not have a primary/secondary like your DFEV. I could not get this carb to work despite a large number of jet combos. The symptoms were exactly as you describe with your IDF's, a "lean miss". I finally gave up and got a 36DCD, which is a progressive carb, and although I'm giving away some HP at the top end, the driveability is great.

According to one Weber book I have, the problem lies in the progression circuit. First thing I'd check is throttle plate location. With the throttle closed, does the plate exactly cover the first progression hole? You undoubtedly know about this, but for others following this thread ....



.... here is a photo with the throttle open slightly, exposing the progression holes. You kinda need X-ray vision, but with the throttle closed, can you imagine the throttle plate thickness being exactly equal with, and perfectly aligned with, the progression hole diameter?

If so, I'd move on to testing the idle jet size. I can pdf you the section of the book I've referenced. PM me your e-mail address.

As a last resort, I might try F10 emulsion tubes, which are the same diameter, and have the same size holes as the F11, but lower in the assembly. "Theoretically" this would bring in the main circuit earlier.

[ebrown0104]

So I've been messing around some more, and I got it tuned in a little bit closer to being "right."

I bought a GM HEI distributor timing curve kit at the auto parts store and put the lightest springs into my distibutor. I reset the timing to 35 degrees at 3000rpm which puts it at around 8 degrees at idle. Having the advance come in really early helped to lessen the hesitation a lot. I also went to a 47 idle jet with the screws set at 3/4 of a turn out. This helped, too, so I tried the 50's at 1/2 turn, but it's too rich right off idle so I went back to the 47's.

The only other thing I might try is to go to a vacuum advance distributor, since I'm currently using a Plex setup (mechanical advance only) or F10 emulsion tubes. But, where it's at right now, the very minor hiccup I'm having is well worth the trade-off of smoother idling, better performance, better gas mileage, and the cool-factor when you pop the hood.

[sptcoupe]

Just some thoughts.

F7s are very rich in the mid range and very lean at the top. You will definitely have transition issues with that set up. Looking at your jetting, I would try the following changes:

F9s,
52 idles
190 airs
35 pump exhaust jets

Also, be sure your carbs are balanced. They will never transition correctly if the bores on the carbs are not drawing equally.

Finally, on your idle mixture screws, the goal is not to have them set equally, but to use them to get the "best lean" condition on each bore. Turn them out about 1/14 turns, and with the engine fully warmed up, screw them in slowly, one at a time, until the engine just begins to run roughly. At that point back them out slowly until the engine smooths out again (usually no mote than 1/8 turn). Do that process several times, then re-synch the carbs using the inter carb linkage. That will give you a smooth isle, easy starts and excellent transition.

What other mods have you made? Cams, CR, header?