IDA tuning help - Lean spike

[markha]

OK... so I got the IDAs on and everything dialing in pretty good but I am getting a couple of lean spikes when going WOT. I am starting in 3rd gear at about 2000 rpm and as you can see from the log, I get a lean spike at 2800 - 3000. Help. What should I trying changing? Here are the specs.

SBF 302 motor that is a 347.
Cam is a roller tappet
L.S. Lobe Separation 112 deg
Exhaust Lift 592 w/1.6 rockers
Exhaust Duration 242
Opens 52
Close 10

Intake lift 576
Intake Duration 234
Opens 6
Close 48

Idle jets = 70
Idle Holders = 120
Idle screw 1 1/8 turn (I know this is too much but exhaust pops otherwise)
Main Jets = 175
Air corrector = 225
Emultion tube = F16
Third progression hole drilled.

Ignition is an MSD 6AL running a new MSD bullit distributor. Initial timing is 14 with total at 35.

Sorry for the long post, but I wanted to make sure I gave enough of the details.

[Gaz64]

Mark,

You haven't listed pump jet, pump bleed or venturi size.

You also have the "dreaded weber gremlin" - lean condition at 2800-3000 rpm.

But from what you've listed and assuming the choke is appropriately sized for the engine I'd say:

Wrong emulsion tube
Main jet too large
Air corrector too large


There are some EXCELLENT threads on the tuning of IDAs on here,
written by members from their own experience, this info you will not find anywhere else.

[priobe]

Originally Posted by markha

I am starting in 3rd gear at about 2000 rpm and as you can see from the log, I get a lean spike at 2800 - 3000. Help. What should I trying changing? .

I would change to a F11 tube.

This would richen up your transition.

But in my opinion you idle circuit maybe too rich. I think your 70 idle jet is way too rich.

I would first concentrate on your idle circuit then transition with main.

too rich on the mains if I am reading the graph correctly.

So technically you are running rich except where you indicated

[markha]

OK... Couple things.
The pump jets are 50s and the Exhaust valves are 0s.
Chokes are 37s

To Gaz64's comment about info on this site... I agree and I have read every post available on the IDAs multiple times over the last few months.

To Gaz64 and Priobe... I was trying for an AFR of about 13 to get the best HP. Is this about right? Also, the idle circuit is rich because I was trying to get it to work with the 3/4 turn of the screw. But every time I would try, I would get popping out the exhaust on deceleration from a modest 2500rpm. If I would go 1 1/2 turn out the popping would clear up. This was true with 65 idle jets. So my thought was to go to a 70 idle jet and decrease to 3/4 turn at idle. This did not work. 3/4 turn with a 70 still puts me at 16 - 17 AFR at idle and anything below 1 turn still produces popping.

The F16 tube was recommended by Chuck at Inglese. Per what I have found in the weber books, this tubes claim to fame is to bring the main circuit transition to lower in the rpm range. What would the advantage of the F11s be?

Thank you in advance for the help.

[Rick Parker]

First of all an engine with a roller cam will be able to use a smaller fuel jet in the idle system because it will create a stronger depression signal in the manifold due to faster valve action. Go to a 60 fuel jet and 100 air holder and reduce the idle mixture screws to 3/4 turn open (max 7/8). After Idle system is resolved Then reduce the Fuel & Air Correctors on the main system to approx 160-165 fuel with 210 or maybe 215 air corrector. Leave the Emulsion tubes as they are F16's. Keep an eye on your plugs. If it spits through the carbs off of idle it's lean. Make all adjustments with it thoroughly warmed up.

[priobe]

markha,

1st thing I have to say is get yourself a note pad and write everything down. I created a document which I used to tune my Webers. If you are not using a note pad I hope you have an excellant memory.

2nd, You are going to receive different opinion here, I am sure all of them are good and valid. However, get the thought out of your mind that what 1 person is running in the same motor will work for you. All the advice you receive will put you tuning in the ball park.

Dont worry too much about the mixture setting being 3/4 1 turn 1 1/2 turn. Get to know your motor.

Study how the emulsion tube works, get a good understanding of this.

Chuck @ Inglese is a great guy but the values he is telling you is to get you started. Not a perfect tune.

As Rick mentioned, you can stay with the F16 and tune around it or you can try and see what the motor is asking for at different stages idle, transition, main.

I would forget about the main stack right now and concentrate on the idle circuit. Get this right. I think once you see how the emlsion tube operates you will see that it makes a huge difference with normal driviablility.

Let start off with this:

List all the jets you have so we know what we can test to see if we are heading in the right direction.

As for idle, I think you will find that the 60 idle fuel jet will be the 1 one you will need, as suggested by Rick. You can then get creative with the holder and etube to maintain your 13.5 ratio.

Just to give you an idea I am running 15.5 @ idle (standing still) and 13 .2 off idle. My idle circuit is 60 fuel and 70 holder with 40 chokes on a BB.

When jumping from 60 holder to 65 and then 70, these were huges changes.

Good luck

[DMXF]

Originally Posted by priobe

markha,

Just to give you an idea I am running 15.5 @ idle (standing still) and 13 .2 off idle. My idle circuit is 60 fuel and 70 holder with 40 chokes on a BB.

When jumping from 60 holder to 65 and then 70, these were huges changes.

Good luck

priobe - I'm curious as to the idle jet holder you recommend being in the range of 60-70, as all the big block (and most small block) setups I've heard about over the years are in the range of 120 down to 100. How specifically do you believe going from a 100 down to a 70 impact the fuel curve?

[priobe]

Dcmgt,

I am alittle confused as to your question. I am not recomending the 70 idle jet holder. Just simply stating that this is my configuration.

Also, many of the other users with BB are still not dialed in according to them. When running a 120 holder this leans out my mixture too much. There are alot of variables with motors such as head porting, valve size camshaft etc.

This is what works for me in Miami, FL

[DMXF]

Can you explain more specifically where the larger idle jet holder makes it lean? Tied into that, how does the effect compare with where the idle jet change has an impact? Thanks

[priobe]

Well,

When taking a fixed number on your idle fuel jet, say for example 60 (fuel) you will have a leaner mixture with a 120 holder ( that introduces air) versus a 100 or 70.

Also remember that the at idle you can have a good fule ratio but once the throttle plates open you will receive additional fuel from the pump jets for a specific amount of time. If you are driving with a steady pedal you will not receive the additional fuel from the pump jets. Most people like to run 00 exhaust valves to dump alot of fuel with the intial depress of the pedal. I would rather tune with out the aid (if possible ) of the exhaust valve. This is why my exhaust valve is set to 55.
I hope this answers your question.

[DMXF]

I'm thinking more of what rpm range does the idle jet holder effect relative to the idle jet. I researched this years ago, but as a refresher, doesn't the idle jet effect primarily just off idle to maybe 1500-2000 rpm and then the idle jet holder has the largest effect from there on up to where the mains kick in, like maybe 3k?

[priobe]

In my findings yes you are right, however the way I see the operation is this:

The idle jet holder and idle jet fuel, combined give you a mixture ratio according to you motor. This combination carries the motor to about 2800 rpm or so. Maybe even 3200 rpm. You can remove the main stack and see where your rpm or motor stops running. The idea here is as the idle circuit combination carries the motor up to these rpm range the signal starts to become weak somewhere around 2200 rpm. This is where the emulsion tube plays it roll. The main jet is begining to engage probably around 2500 rpm. Now with the etube and main jet at this low rpm ranges the signal is weak but cuzing an effect to the mixture.

The mixture screws which I like to call volume screw control the amount of mixture to the motor with the throttle plates shut. regardless of the rpm. This is why when you crank open the "mix screws" volume screws on deceleration the back firing tends to go away. Your introducing more of the mixture

[Gaz64]

Mark,

Can I ask whether you have all the runners plumbed together with a "vacuum tree"?

How far open are the butterflies at idle, and what is the idle speed?

Just for your info as well, I run a 38mm choke in my quad 44 IDF carbs on only a baby 308 cuber.

[markha]

Originally Posted by Gaz64

Can I ask whether you have all the runners plumbed together with a "vacuum tree"?

How far open are the butterflies at idle, and what is the idle speed?

Just for your info as well, I run a 38mm choke in my quad 44 IDF carbs on only a baby 308 cuber.

I should post a pic of the setup. They are on an Inglese intake that has a vacuum runner cast into the intake. So yes. And then running a PCV.
Butterflys are completely closed at idle and it runs at about 1000rpm. I actually like the idle a little lower but can not acomplish that without backing off the timing. The initial timing is already a little retarded for that cam.

I am going to try a few things this weekend and will post my findings. I am going to start by going back down to a 60 or 65 idle jet and pull the mains to make sure they are not influencing anything to start. My rpm pickup for the LM1 died, so I may not get much useful info logged until it gets repaired.

I will let you guys know. Thanks again for the help.

[Gaz64]

Originally Posted by markha

I should post a pic of the setup. They are on an Inglese intake that has a vacuum runner cast into the intake. So yes. And then running a PCV.
Butterflys are completely closed at idle and it runs at about 1000rpm. I actually like the idle a little lower but can not acomplish that without backing off the timing. The initial timing is already a little retarded for that cam.

I am going to try a few things this weekend and will post my findings. I am going to start by going back down to a 60 or 65 idle jet and pull the mains to make sure they are not influencing anything to start. My rpm pickup for the LM1 died, so I may not get much useful info logged until it gets repaired.

I will let you guys know. Thanks again for the help.

Mark,

Although your engine is 347 cubes, the cast vacuum runner may contribute to your idle speed.

Each cylinder can draw more easily from its respective runner AND the volume contained in the vacuum runner, each cylinder can effectively draw through 8 barrels at idle hence why the butterflies are shut and your need for "retarted" timing.

8 little tubes of 3/16 can still supply enough vacuum for brakes etc, without "overbalancing".
I'd be looking at a smaller cross sectional balance setup, which means a manifold change.

Then you can have the butterflies control the majority of air PER cylinder, and run more timing at idle with less in the distributor.

As others have said you should concentrate on the idle/progression of the motor up to 2500, before you worry about the rest.

[markha]

Originally Posted by Gaz64

Mark,

Although your engine is 347 cubes, the cast vacuum runner may contribute to your idle speed.

Interesting thought. I had not thought about that, but you are probably right. It is creating a small shared plenum.

[markha]

OK... So I call me crazy, but I could not resist trying some 44 mm chokes today. I know... major changes. But the idle circuit mixture did not change much. A little leaner... I assume that is do to flowing more volume which seems a little strange at idle. I tried leaving the mains out and running on the idle stack. Everything looked pretty good. Right about 13 - 14 AFR. Then I through in the mains, and I tried a bunch of different main jets but could never get the thing to even run on the main circuit. Always way to lean. Any ideas for a starting point for 44 mm chokes for mains and air correctors?

Thanks,
Mark

[Gaz64]

Originally Posted by markha

OK... So I call me crazy, but I could not resist trying some 44 mm chokes today. I know... major changes. But the idle circuit mixture did not change much. A little leaner... I assume that is do to flowing more volume which seems a little strange at idle. I tried leaving the mains out and running on the idle stack. Everything looked pretty good. Right about 13 - 14 AFR. Then I through in the mains, and I tried a bunch of different main jets but could never get the thing to even run on the main circuit. Always way to lean. Any ideas for a starting point for 44 mm chokes for mains and air correctors?

Thanks,
Mark

Mark,

Temptation is hard to resist sometimes,

A venturi change would not affect the airflow your engine uses from idle to 2000 rpm.

Jumping from 37mm to 44mm will certainly affect the main system startup hence your lean condition.

I wouldn't be staying with a choke of 44mm personally, you may find that you'll never get around a lean condition between the end of transition to start of main, irrespective of what emulsion tube/main/air corrector you use.

A 2mm jump in venturi diameter would be considered the norm, then 1mm steps to zero in on optimum diameter for driving style.

Here is my suggestion to you:

60F10 idle, 100 holder
55 pump jet
35-50 pump bleed
F7 emulsion tube
160 Main
180 - 190 Air
38 - 40mm choke.

What's your rev range?

What's your timing at idle and 3000 rpm?

If you block off a choke completely to one cylinder does it still idle ok? or does that cylinder go away? (overrich condition)

In other words, is that cylinder receiving more than 50% of its air from the "plenum"?