Fine-tuning the Speed Demon

Hi guys, I'm running an 850/mech secondary carb on my 440..and it works great but I always seem to be rich at idle(750 rpm). (DISCLAIMER=doing it old school for now, no A/F meter)
Anyways I can clean the idle up by going in a little on the 4-corner adjustment. However, this causes a loss of power and response from off-idle up to maybe 2500rpm or so. 6.5 PV in it now pulling 15-16 in/hg in gear at idle, PV not opening too soon. Adjust it fat again and it picks back up. Transfer slots and butterflies are ok, float levels are properly adjusted to just about halfway up the sight glass (it's a Demon not a Holley where they need to be just dribbling out the bottom), no hesitations/bogs/burps or any of that as I've spent time dialing that stuff in. So, my thought is either the low-speed air bleeds need to be opened up a tad? Or the IFRs closed a little? These are not adjustable on my model so I don't want to drill something out and have it be the wrong direction. I've turned in the 4-corners and raised the floats to try to compensate, that wasn't the answer. I get that it's more of a race-calibrated carb and I can live with it if need be, but the gas smell can be strong and it dirties up the plugs fast...I guess to sum it up what are your ideas on how to clean up the idle while maintaining the mix for power on the low end? Thanks!

I had almost the exact problem with my 850 Demon. Ran fine just off idle to WOT. I tried everything short of drilling the airbleeds and still would burn my eyes at idle. I'm running a big (260°@0.050, 108 lobe) cam and I'm sure the lack of vacuum didn't help matters. FWIW I changed to a Holley 850 HP and it idled fine (no burning eyes)for a week or so. Now it's doing the same thing as the Demon.
Sorry, not much help.

StreetWedge65 said:

I had almost the exact problem with my 850 Demon. Ran fine just off idle to WOT. I tried everything short of drilling the airbleeds and still would burn my eyes at idle. I'm running a big (260°@0.050, 108 lobe) cam and I'm sure the lack of vacuum doesn't help matters. FWIW I changed to a Holley 850 HP and it idled fine (no burning eyes)for a week or so. Now it's doing the same thing as the Demon.
Sorry, not much help.

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Wow ok, my cam is 230°@ .050 and vacuum is good. I forgot to mention..the throttle shaft got wobbly on the original baseplate so I replaced it awhile back, but there was no base available with the 'idle-eze' feature. That would probably help the issue..

what part of California are you located in, I know of a Dyno tuner in Chatsworth that will fix that, and he's not expensive.

moparcrazy said:

what part of California are you located in, I know of a Dyno tuner in Chatsworth that will fix that, and he's not expensive.

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I'm up near Fresno, a little too far out the neighborhood...thank you though!

How much transfer slot is exposed? The general info says up to 60 thou but I find 20 to 30 is about right.
If your idle air bleeds were adjustable you could try slightly larger ones. The change is then reversible. You need to do all 4 so you adjust the idle mixture in the primary and secondary side.
If the carb is about the right size I rarely change the air bleeds.
I am at a loss explaining why your mixture adjustment causes power loss as the circuit you adjusted only operates primarily at idle and goes to another circuit when you open the throttle.
By 2500 it should be well on to the primary main jets.
I try and get them as lean as I can I have found throttle response far quicker with less gas than to much. I believe rich is to much fuel and the engine has to burn off the excess before it can get busy.

Didn't see anything about initial timing, but you have healthy vacuum. For some applications advancing the timing at idle allows the carb to be set leaner without sacrificing behavior. I always set to highest vacuum at idle on adjusting the idle adjustment screws and then richen 1/8 to 1/4 turn because it wants a little more fuel to accelerate. I set my timing higher 15-22, but that is what the cam and engine likes, and this lets me run a leaner idle setting. Maybe this will work for you. You will need to keep total timing the same.

Jcharger said:

Didn't see anything about initial timing, but you have healthy vacuum. For some applications advancing the timing at idle allows the carb to be set leaner without sacrificing behavior. I always set to highest vacuum at idle on adjusting the idle adjustment screws and then richen 1/8 to 1/4 turn because it wants a little more fuel to accelerate. I set my timing higher 15-22, but that is what the cam and engine likes, and this lets me run a leaner idle setting. Maybe this will work for you. You will need to keep total timing the same.

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Oops sorry... Timed at 20/36..vacuum maxes out and idle rpm levels off at that point. The basics are dialed, pump cams/arm adjustments etc. Jetting is working pretty well, considering those have been tuned only by cutting open and reading plugs and the 'ol seat-of-the-pants feel. It can be refined for sure, when I get around to adding a meter. It runs very well on the street as-is, just need to figure out this last piece of the puzzle...

steve340 said:

How much transfer slot is exposed? The general info says up to 60 thou but I find 20 to 30 is about right.
If your idle air bleeds were adjustable you could try slightly larger ones. The change is then reversible. You need to do all 4 so you adjust the idle mixture in the primary and secondary side.
If the carb is about the right size I rarely change the air bleeds.
I am at a loss explaining why your mixture adjustment causes power loss as the circuit you adjusted only operates primarily at idle and goes to another circuit when you open the throttle.
By 2500 it should be well on to the primary main jets.
I try and get them as lean as I can I have found throttle response far quicker with less gas than to much. I believe rich is to much fuel and the engine has to burn off the excess before it can get busy.

Click to expand...

Transfer slots are at .020, I think maybe an 1/8 of a turn less to keep the idle down at 750. Yes by the time I believe I'm getting into the mains it boogies and the plugs show a good color for the midrange. Lean idle just makes the car feel heavy from off-idle up to that point.