IDF Tuning Help

[jabbott]

I have a gt40 kit car with a 302 with 48idf carbs on it I am trying to tune.

Trying to get it tuned and this is where im at.
40mm choke
165 main jet
180 air corrector
F11 emulsion tube
0 pump bypass (changing to fix overly rich tip in)
52 idle jet
idle screw 1 turn out

Right now the idle is pretty good but its pretty rough between 2000 - 2500 rpm part and full throttle.

Here is a copy of the last dyno I did with the afr on it.

I know I need less pump shot, so don't even take that into consideration.
Its rich down low and as the rpms go up it gets leaner. So I was thinking of lowering the air jet to make it richer up top which should flatten the afr curve across the rpm band and then going down on the fuel jet to get it back in the mid 12's.
Also I have no idea what is causing the lean blip at around 4000 rpm.

Thanks, any suggestions would be great

[Skuzzy]

Your problem area is occurring at the transition RPM from the idle to main circuits. Seems it is either the idle circuit falling off too early, causing a lean gap between the transition, or your emulsion tube may be incorrect.

If you change emulsion tubes and the bad spot is still there, then it might be time to go after the air bleeders, mixture screws, and/or choke size. I would not change jets, right off the bat.

Hope that helps.

[wolf k]

Welcome to the world of weber tuning. Had a similar problem to yours, had to modify the F11 tubes to rectify the issue. See the thread below yours labeled "Any advice with my tuning."

[Gaz64]

I'd change the choke (venturi) down to 37/38 and appropriate main jet change.

How much work is done to your 302?
How much power at what rpm?

[jabbott]

Thanks for the replies

Originally Posted by Gaz64

I'd change the choke (venturi) down to 37/38 and appropriate main jet change.

How much work is done to your 302?
How much power at what rpm?

I had ordered 37mm chokes with the tubes and jets, but there was a mix up and they sent me the wrong chokes so i decided to go ahead and try and tune it. I have the chokes coming in next week.

As far as what is in the engine, I have no idea! Its a customers car and he bought it like this. It can't be anything to crazy as it idles pretty smooth and only makes power till about 5400 rpm.
As far as how much power and rpm. Its in the dyno graph pic in the first post with afr.

[Gaz64]

Yes, I see it tops out in the early mid 5000 mark.

Must be virtually stock heads and cam or very mild at least.

The 48s are way too big for your engine combo.

Run the 37s, might even be need to go to 36mm.

48s come with a F2 e-tube ex factory, 44s comes with F11.

[jabbott]

Originally Posted by Gaz64

I'd change the choke (venturi) down to 37/38 and appropriate main jet change.

How much work is done to your 302?
How much power at what rpm?

Originally Posted by Gaz64

Yes, I see it tops out in the early mid 5000 mark.

Must be virtually stock heads and cam or very mild at least.

The 48s are way too big for your engine combo.

Run the 37s, might even be need to go to 36mm.

48s come with a F2 e-tube ex factory, 44s comes with F11.

Yea, Its definitely a mild setup.

I know 48's come with f2's i took them out to put the f11's in.

[Rick Parker]

The 40 mm chokes on a 302 are best utilized with high compression and a lot of camshaft and RPM's such as experienced at the track. If it's a street car with about 10:1compression with less than an all out camshaft, you'll have a more flexible motor with much better throttle response, by going to somewhat smaller chokes and you'll use less fuel, and it will become easier to tune.

[wolf k]

Yea , but doesn't the A/F chart look pretty good with exception of the transition zone? How does it run otherwise outside the transition zone?

[jabbott]

Originally Posted by wolf k

Yea , but doesn't the A/F chart look pretty good with exception of the transition zone? How does it run otherwise outside the transition zone?

No, its 11.0 at 3000 rpm and 12.8 at around 5000 rpm. That's terrible. It has almost a 2 point afr spread from mid range to top end at wot.
Just leaning out the mid range to around 12.5 would net at least 15 torque and 15 hp.

That should be relatively flat across the rpm range at full throttle.

[Caprimaniac]

Originally Posted by jabbott

No, its 11.0 at 3000 rpm and 12.8 at around 5000 rpm.

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?

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).

If it's steady state AF numbers, there are other things you need to adress, like the main venturi, air/ fuel and emulsion.

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

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

With the IDF's on my 347 there was also a rich- spot in the transition. It made the engine a little lazy out of the corners.

You can check in the Weber jetting file in another thread and see what air and fuel tubes used in the 347 (up to 8000rpm) and compare, for what it gives....

RS

[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]

I guess what I was trying to say is your A/F graph is fairly flat, considering Webers don't have power valves, or metering rods. A simple jet change should lean out your mixture across the board. Now your transition zone is going to require something else.

[Gaz64]

As you have said, you have an overrich condition from the accelerator pump.

I'd fit 35 pump bypass valves and re-evaluate.

When you fit the 37 chokes, go smaller on the main jet, about 145-150 and smaller on the air corrector, about 160 as a start.

[wolf k]

[quote=Gaz64;1208269]As you have said, you have an overrich condition from the accelerator pump.

Gary, wouldn't over rich accel pump give only a momentary rich condition, not across the entire range? I agree he needs smaller main jets. I would love to see if he could tune this car without changing the chokes, just to see if it could be done.

[Gaz64]

The engine is recovering momentarily from excessive pump shot volume.

The entire fuel curve from the main circuit is a tad rich in the middle.

He could leave the 40mm chokes in, run a 35 bypass, drop the main to 160 to start, and possibly drop the air corrector.

Myself, I'd drop the main to 150 and the air to 150 and see where the AFRs go, number drill up from there, then buy the right jets.

[jabbott]

Originally Posted by Gaz64

Myself, I'd drop the main to 150 and the air to 150 and see where the AFRs go, number drill up from there, then buy the right jets.

That's pretty much what I was thinking.

Thanks for the help guys. I will be tuning it again some time next week and let you know how it goes.

[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.