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Old 03-26-2007, 01:17 PM
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Tuning 2 Demon Carbs on a Hemi

Okay, my Hemi is up and running and I'm in the process of doing some carburetor tuning. Please bear with me, I know it's a long post, but I'm trying to include all pertinent information. This is my first time tuning 2 4-bbl's with a blower on a Hemi!
For details of my engine setup, please go here:

Early Hemi blower motor carbs- Demon or Edelbrock

This is where I got the recommendation to use two 650 cfm Mighty Demon blower carbs.
Here's the setup on the carbs:
(2) Mighty Demon blower carbs #5282010BC. 650 cfm.
Main jets primary: #71, Main jets secondary #79
Power valve 6.5 primary, secondary plugged
Squirter size: 31
Idle air bleeds: .070
H.S. air bleeds .041
Idle feed restrictor: .029

Now the problem. Like some other posts I've read here, I'm having an over-rich and a fast- idle condition that I cannot seem to tune away. My basic setup before starting was per the Demon instructions: Both primary and secondary idle screws were adjusted to give the correct .020" of visible transfer slot under the butterflies. The idle mixture screws were set at 1 1/2 turns out from seated. Idle-eze screws were set 1 1/2 turns out.
I'm using a Carter electric fuel pump putting out 6 psi, and the float level in all 4 bowls is set 1/3 the way up in the sight glass, even with the bottom mark on the bowl.
I also have a wide band A/F meter hooked up. Keep in mind that this engine is currently on a test stand, so I am only preliminarily tuning for idle mix, idle speed and vacuum. I'm not able to do any road test at this time (the street rod isn't finished yet!)
The engine fires right up and goes into a very fast idle, around 1500 rpm. If I don't close down the secondary idle screws to close up the transfer slots, I start getting into the mechanical advance range of the distributor, and rpm's increase even further.
So, with the secondary idle screws backed out so that the butterflies are closed, my idle will stay around 1450 rpm. Initial timing is set at 12* BTDC. Vacuum is about 16-17 in, the A/F ratio is around 10.8:1, very rich...
After warm-up and with engine at operating temperature, my idle is still around 1450, A/F ratio is at 11.2 and I have a steady vacuum of 19".I can get the idle down to around 1300 rpm by backing out the primary idle screws until the butterflies are closed, but then the transfer slots are covered up. A/F ratio does not change.
Next I start to turn in the idle mixture screws by 1/4 turn amounts (all 8 screws)...Not too much difference until I'm down to about 3/4 turn total out on all screws. That's about as lean as I can go and keep the engine from stalling out. I've gotten it so that another small turn on any one of the mixture screws will get the engine stumbling and ready to quit. With the screws in as far as I can go and still keep the Hemi running, the A/f ratio still is in the 11's, the rpms are up over 1300, and my vacuum is still at 19".
Next I tried turning the idle-eze screws. First I went in all of the way with no effect, and then progressively turned them out to over 4 turns. No change at all in A/F ratio, or vacuum. Rpm's increase slightly with over 4 turns out on the screws, but that's it...
My distributor has about 26* built in, and it starts to come in at 1500 rpm and is all in by 2250 rpm.
So there I am. I'm looking to have an idle around 1000, and I think with my setup, I can achieve that. Before I put the blower on, I had the engine running quite nicely at 950 rpm on a single 4-bbl.
The other thing is the richness. I can't seem to get any leaner than an A/F ratio in the 11's?.What's my next step? And if anybody has some general hints about tuning 2 4-bbls, let me know. Tech@BG?? What do you think?



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