My experience was that I made a lot of tests changing the IFR, then the IAB, then the IFR, then the IAB until the low speed driving seemed the best.
That didn't help me understand the relationships, but some stuff posted by Shrinker and Mark did.
I generally don't worry about how far in or out the idle mix screws are unless it is extreme. At least a half turn out and less than two turns out (on most carbs) is the range I'm confortable with. I think Tuner is too but you can check with him on that. But mark and Shrinker say what you guys say. Try to get it in more in the 1 to 1.5 range. So rereading this I learn more. :) See snips below
Here's one more IFR 'test' but its also done in neutral so like the one I posted before (which comes from Urich's book) its seems only useful identifying if the IFR is way off.
"raise the idle speed two or three hundred rpm with the idle speed screw, turn in the idle mixture screw 1/2 turn. If the engine speeds up, the mixture was rich, if the engine slows, it may be lean.
Now, turn the mixture screw out 1/2 turn, if the engine speeds up, the mixture was lean, if the engine slows, it may be rich." That's from Troy Patterson (TMP carbs) who doesn't always know what he's talking about. But this seems to be a reasonable test. Another one is to increase the throttle opening (off idle position) and block or partially block the IABs to see how the engine reacts. These are quick tests to get a sense of what is going on and what directions to explore for improvements.
Idle Bleed - Idle Jet Relationship
jmarkaudio Fri Oct 17, 2014 10:25 pm"Idle Circuit Function"
Smaller idle jet/bleed will provide fuel farther in the throttle opening than a larger jet/bleed combination that provides roughly the same amount of fuel.
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The smaller bleed will maintain a signal at a larger throttle opening than the larger bleed, hence the name air bleed. Larger bleed bleeds the signal sooner.
08-13-2007, 06:42 AM
shrinker
The idle mixture screw should normally be around 1 to 1.5 turns out and the t-slot maybe .020" exposed up to .040", depends. If the IFR is too large and the idle air bleed too large then you have to restrict the idle mixture screw to get it correct at idle in neutral. When you put it into gear the revs drop a little and then the vac drops and then the flow restriction of the idle mixture screw has too much influence and it leans out and the engine dies. The idle screw restriction needs to be so that it doesn't restrict it too hard and early as the revs slightly. The t-slot comes on very slowly at first and the idle has to make up the difference so that the mixture is consistent. If the mixture is set correct without using as much idle screw restriction the mixture from neutral to in gear will be more consistent.
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If the IFR and air bleed are too big then the fuel condition exiting the slot at low openings will be blobs. Thats no good for distribution. If distribution is poor then you have some lean cylinders and the engine has no power. Cylinder power is affected more by being lean than it is by being rich.
That's the shortened version from Shrinker.
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