Any advice with my tuning

[wolf k]

44 IDF's on 331 small block with mild cam, 212 duration, 112 lobe separation. 20 deg initial timing, 140 mains, 175 airs, 55 idles, 65 pump bypass, F11 tubes. 12.6-13.6 air-fuel ratio through all ranges of operation. Runs great, plugs look great.
My issue is, while moderately accelerating in 1st gear from a stop, I get some minor stumbling and fluttering from the exhaust in the transition zone, 2800-3000 rpm. If I accelerate slowly, no problem. If I accelerate hard, no problem again.
My AFR meter is not indicating if this is a lean or rich stumble, it stays in the 11-13 range through idle to full throttle.
The Pierce Manifold tuning guide states as follows; the emulsion tube influence is more marked at small and average throttle opening angles and during accelerations. This sounds like where my problem may be.
I already have gone through many jets and different combinations to get where I am now. It looks like I need to start guessing at emulsion tubes. At $12 a tube x 8, I would like to start guessing in the right direction. Any advice on which one?

[Gaz64]

I run a F2 tube on a smaller V8 with a wilder cam than yours.

Refer to this thread:

Lets talk Emulsion tubes

I think you could try F16 or F2. There are others who have modified existing tubes.

[*13*]

Jets clean & seated? Have you bumped the idle mixture screws up a bit to see if it makes any difference? Damn jets will break ya!

[big deuce]

That sounds like a great combo, which chokes are you using?

[STL Mark]

PM Sent.

[Caprimaniac]

If you disregard the O2- reading on your AFR- meter, what does your plugs look like?

With warm engine: Bottom the throttle to get the engine to stumble, stop , turn off engine and check the plugs.

I'd also checked the plugs at warm idle. 55 idle screws might be on the large side. I have 50's in a 347 with a much hotter cam. (If a little sooty at idle/ low revs your transition gets stumbly when plugs won't fire properly.)

A AFR- meter does not alway show you what it is supposed to. Where and how are the sensor mounted? Have you calibrated the gauge???

Also check your timing curve. Max timing? When?

I guess you should be fine with the F11's. The rest of the jetting sound OK too. Although your 65 pump bypass might, but I think not, cause some trouble while accelerating.

I have 48 IDFs and plugs are light chocolate brown in all kinds of conditions. No obvious stumbles. The jetting are on the list.

There was a guy posting a "How to install & adjust a 44 IDF- setup" here once. I think it was a PDF. I looked for it the other day, but could not find it... Maybe something for you it if someone is able to find it....

[zimmy]

Possibly think about needing a slightly larger accel pump squirter. The new fuels seem to require a richer mixture than the past. I think that mild accel needs a bit more fuel than in the past and the squirter may have to be a bit larger. Just a thought. The pump valve controls the amount (duration) but the squirter jet controls the volume(I believe).

Just a thought.

[wolf k]

Thanks for the responses
Gaz64; F2, F16- leaner low throttle response than F11?

13; Oh yes, jets cleaned and seated many, many times. Amazing what happens when a fleck of tiny debris finds itself in one idle jet.

big deuce; chokes are 36

Caprimaniac; plugs look really good, medium grey with midwestern fuel. Will check them as you stated. AFR sensors are located just after header collector. FAST meter, do not know about calibrating, will read up. Timing is 36 deg @ 2800 rpm, 20 deg initial. Was that IDF tuning pdf on a GT40? if so, his major issue was linkage. I had the same problems with loose linkage arms, which have been corrected.

zimmy; have not messed with the squirter size, but have gone up in pump bypass size. In my case, less pump fuel, better results. I am at 65 bypass now.

[Gaz64]

Originally Posted by zimmy

Possibly think about needing a slightly larger accel pump squirter. The new fuels seem to require a richer mixture than the past. I think that mild accel needs a bit more fuel than in the past and the squirter may have to be a bit larger. Just a thought. The pump valve controls the amount (duration) but the squirter jet controls the volume(I believe).

Just a thought.

Bypass is volume and squirter is duration.

[zimmy]

I think we are saying the same thing 2 ways. With the larger squirter you will get more gas in the shot (out of the squirter) and the bypass controls the total amount of fuel available. So with the larger squirter, the entire accel pump will last a shorter time (i.e. - duration).
who knows - maybe I'm totally off. Just my idle ramblings.

When in doubt - take your car to Jim Inglese and have him tune it. He did wonders for me. Runs great after 7 years of fiddling myself. I got pretty close, but he got it right.

[wolf k]

Finally resolved my transition zone issue. I knew my problem would be solved by an emulsion tube change, but with all the internet reading, there was no real consensus with which e-tube would solve my problem. I did not want to purchase handfuls of e-tubes and experiment.

I started by enlarging my air correctors by drilling, this was going with the assumption that my main circuit was starting too early. With each progression in enlarging the air correctors, the results got better. Once I got past 2.20 on the air corrector, anything larger had no more effect, all air bleed restriction was now being handled by the e-tube itself. So I simply removed the air corrector jets.

However, I still had a slight occasional flutter occurring in the transition zone. Finally, drilled two additional holes in the e-tubes second row from the top. Problem solved, car runs like a modern fuel injected V-8. A/F ratio consistent in 13's, 12's when under load. In my case, the air correctors, did not have a very big impact on A/F ratio, only in the way the e-tubes performed.

[big deuce]

Cheers Wolf, I love it when things work out. Thanks for sharing your experience!