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?

[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.

[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.

[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?

[Gaz64]

I've just reread your post.

You say pretty rough between 2000 - 2500 part AND FULL THROTTLE.

So possibly late main system start up due to oversized choke, and/or wrong emulsion tube for the application.

[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.

[jabbott]

Originally Posted by Gaz64

Can you run the engine to the lean point under no load in neutral?

Is the auxiliary venturi discharging any fuel at that point?

Do you mean rev the engine in neutral? And what is the lean point? I don't understand your terminology.

[Gaz64]

Originally Posted by jabbott

Do you mean rev the engine in neutral? And what is the lean point? I don't understand your terminology.

Yes. The lean point on your AFR scale at approx early 2000 rpm zone.

I've editing my post, can you answer my other question?

[jabbott]

Originally Posted by Gaz64

Yes. The lean point on your AFR scale at approx early 2000 rpm zone.

I've editing my post, can you answer my other question?

The engine revs fine in neutral.

The chart starts at 2500 and the car was not wot untill about 2650. What your seeing is just going from a cruise to wot. It cruises at about 13.3 then when it goes wot there is a quick blip up to 14 (that is just a delay till the pump shot kicks in) Then it goes very rich due to it having to much pump shot and then levels out at about 11 afr and slowly rises till it gets to about 12.8 at around 5500.

[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.

[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