IDF Tuning Help

[jabbott]

Originally Posted by Caprimaniac

First off i wonder if these AF- values have been obtained in a steady state rpm/load, or if these are what you see while opening the throttle in a pull?

Its a wot pull on a mustang chassis dyno.

Originally Posted by Caprimaniac

If it's a pull- meaning acc pump is doing something, then you know its acc pump tuning needed.

With such a low rpm engine, I would guess you'd need some fuel bypass in the acc pump curcuit.
You could also tweek the springs/ rod to move the length of stroke and when it comes in (if I remember correct).

Yes i am going to put some bypass in the pump shot to even out the afr on the initial hit.

Originally Posted by Caprimaniac

First off you should go with the 37 or maybe smaller main venturis and see what they'll do.

I have a 36 on order.

Originally Posted by Caprimaniac

If it still persist, you might need a emulsion tube giving more fuel at the top and then go down on main fuel.

Couldn't I just keep the same tubes and lower the air jet (same thing as a high speed air bleed on a holey) which would also richen up top, then lower the main jets to get the afr I want.

[Gaz64]

Originally Posted by jabbott

Couldn't I just keep the same tubes and lower the air jet (same thing as a high speed air bleed on a holey) which would also richen up top, then lower the main jets to get the afr I want.

Yes, you can do that. Main jet ( and to a certain extent, the emulsion tube ) determines A/F ratio at low rpm, air corrector at high rpm.

Emulsion tube affects the "curve" of the A/F ratio between low and high rpm during acceleration.

Theoretically you could nearly obtain the the correct A/F ratio throughout the throttle range with the correct main/emulsion tube/air jet, and then less accelerator pump shot required.

One book of mine states to run air jets in the 60-80 range, then pick the main jet, then choose the emulsion tube. This is all after the correct venturi is chosen for the application.

[wolf k]

QUOTE< Emulsion tube affects the "curve" of the A/F ratio between low and high rpm during acceleration.

Yes this is correct, the emulsion tube has a very pronounced effect on part-throttle driveability. Finding the correct one is the interesting part.

Besides, the over-rich condition throughout the range, are you having any stumbling, popping, reversion?

[jabbott]

There is stumbling below 2500 which i think is cause by it being to rich. I'm hoping between correcting the pump shot and having the afr around 12.5 through the whole rpm range will fix it.
Also keep in mind the rich condition is only at wot. The car cruises and at around 13.3 afr.

I do have some reversion in the lower rpm when backing off the gas. I was under the impression that there will always be some kind of reversion.

[jabbott]

OK, did some more tuning today.
I had ordered 36mm chokes, but unfortunately they didn't send them. So this is with 40mm chokes.


I soldered over the holes and re drilled them.
the air jet is now about a 110
the main jet is now about a 150
and the same f11 tubes
idle screw 1 1/4 turns
60 pump bypass

As you can see by lowering the air bleeds it brought the upper rpm down in afr closer to the mid-range. It still goes leaner at about 4600 but its much better. I can totally see how in one of the books you have it says 60 - 80 because that's probably what would have made it close to perfect.
I also lowered the main jet back down to lean the whole thing out a bit.
and i drilled a bypass in the pump jet to get rid of the massive rich condition on the initial hit.
Also i turned the idle screws out a little bit richer and it idles around 13 afr.
between all those mods the car drives good. No hesitation or bog at any throttle or rpm.

[Gaz64]

Looks like you have a good result, although going from 0 to 60 bypass is a big jump.

I still think you should try a 35 bypass.

[jabbott]

ye i agree a 35 probably would work well. Just figured it was so rich on the initial hit before. It went down the 9.0 afr on the hit.