I'm inexperienced at carb tuning, please help

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MagnumBarracuda

MagnumBarracuda

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Engine: 408 magnum Aluminum heads 10.6 compression single plane intake
Cam: 241 247 @ 50
Carb: 750 Mighty Demon mechanical secondaries
Primary Jet: 72
Secondary Jet: 84
Inner air bleeds: 38
Outer Air bleeds: 93
Power Valve: 5
Annular booster

This is in a 67 barracuda with 4 speed trans

I have installed my wideband on the car and had an issue with the car. I set the idle to 13.5 - 14.5 afr but it would cruise very rich like <10 afr. After confirming that the power valve was not blown, i decided to drill the air bleeds to a 93ish (original were 70). I set the idle to the same 13.5 - 14.5 and confirmed with vacuum gauge. (It pulls around 10 Hg for vacuum at idle.) The screws are only about 3/4 turn out. The cruise is now acceptable at 12.5 - 13.5 afr. Now the when the throttle is pressed at cruise (2500 to 3000 rpm) and sometimes when just starting in 1st gear, the engine goes way lean and bogs, 17+ afr sometimes. When going full throttle through the gears everything appears fine, which makes sense I had it on an engine dyno less than 2 years ago. I tried a 6 and a 7.5 power valve but that does not solve anything, just makes cruise richer with the 7.5. I'm not sure what my next move is. Has anyone had a situation like this?
 
What is your squirter size.
If your going lean on pressing the gas pedal….
 
rumblefish360 said:
What is your squirter size.
If your going lean on pressing the gas pedal….
Click to expand...

If there is a way to easily get that info, i can check it quick. Otherwise, i don't know.

This is the only other info on the website:

Primary Discharge Nozzle Size (in.):
0.031 in.

Secondary Discharge Nozzle Size (in.):
0.031 in.
 
.070 to .093... that's a big jump. Lean off idle, likely the huge air bleed you currently have.

The better approach would be to reduce the size of the idle feed restrictor. I'm not sure what the emulsion pack is on that. If it's a 5 brass... I block 2 and 4 off and try again.

Most of the carbs have a really bad cruise tune in them. Takes some of work to get it right.
 
The pump squirter size (should be known) is one thing and then there is also a cam controlling how the squirter, is squirted.

If everything else is still n targets according to you, then the squirter is the only thing you didn’t touch and probably needs an adjustment somewhere.
 
What is your cruise vacuum? That engine might like a power valve in the 10.5 range. And you’ll be able to drop primary jetting.
 
Basics, basics, basics. That is a sizable cam. Remove the carb & see how much T slot is below the blades. I am betting it is more 0.060".......
 
Carburetor tuning doesn't come from a book or forums or friends. It comes from straight up experience. All the aforementioned things can certainly help, but no one can really teach you how to do it. I mean holy ****, you have a wideband. That's giving it all to you on a silver platter. How much easier do you want it?
 
Get a book such as this:

9536B6BE-ABDD-4BE6-8EE7-7CBF0C254BA3.jpeg


Learn about every circuit, and then how to systematically and methodically tune them in the proper order. Not jumping around fiddling with them out of sequence, no matter how good you think other circuits might be. Knowing what does what and when etc. first, fiddling second.
 
Both are excellent books & should be read by anyone who is attempting 'home' tuning.
It is interesting to note that for a couple of decades or more, Holleys & clones did not have removal emulsion & air bleeds. There is a simple reason for that: it is getting into the realm of carburetor engineering.
And those 'fixed' carbs seemed to run just fine....
 
12many said:
Get a book such as this:

View attachment 1715925449

Learn about every circuit, and then how to systematically and methodically tune them in the proper order. Not jumping around fiddling with them out of sequence, no matter how good you think other circuits might be. Knowing what does what and when etc. first, fiddling second.
Click to expand...
That's a very decent book.
 
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