Problem with Mighty Demon 750

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Well, at least the head gasket let go before a piston! I tuned a Vega by ear ONCE upon a time... only ended up at 32 degrees though. Learned a lesson....
 
The old "tune by ear" is a myth that needs to disappear. We have timing lights and other tools so we can do our job accurately. Would you torque down main caps without a torque wrench? Would you torque down head bolts without a torque wrench? No. Because you're not stupid. Don't be stupid with the most important part of that engine. The tune.
 
Well. It's all back together. Right now I've got the E curve locked out at 34*. This thing confuses the **** out of me. I may eventually go back to a standard dizzy and 6AL box. Running pretty well on 34*. Starts a little rough but comes into it.
 
71valiant said:
Well. It's all back together. Right now I've got the E curve locked out at 34*. This thing confuses the **** out of me. I may eventually go back to a standard dizzy and 6AL box. Running pretty well on 34*. Starts a little rough but comes into it.
Click to expand...

Are your EQ heads open or closed chamber? If closed chamber you may want to try a little less when locked out, ex. 32*
I'm at 9.3:1 OEM closed chamber magnum heads 234@50 hyd roller, and 26-28* intial, 34* total 89 octane fuel makes the most power at the track for me.
 
I've got my initial set at 24* and full is 34* @ 2500 rpm. Seems to be good. Still idling rich as hell.
 
Still black smoke, or is it any better? is this in neutral? How far open are the 4 idle mixture screws? Are they open approximately equal amounts? If not, then some work on the primary vs secondary throttle opening at idle is called for.

Is the vacuum level under the carb still 13 in at idle in neutral and 8 to 12 in gear idling?

Any fuel dripping from the aux venturi's at idle? Look closely.

If you feel the idle screw settings are where they should be, try lowering the floats a bit. Idle it for a while to see that it goes down.

This is a pretty simple situation where it is all on the idle circuits. (Assuming the power valves are not open for some reason).
 
No black smoke. Just gas smell. I'm feeling like it's in the floats and idle screws. I can turn the rear idle screws all the way in, and it doesn't seem like it wants to die. Almost like they aren't doing anything. They aren't turned out the same amount as the fronts. They are turned out more if I recall correctly. The vac is still in the 13 range.
 
Do you have the transition slot set on the primary and secondary?
.020 square up front and barely visible in the back. You need to set the idle mixture biased to the front. The idle circuit is what you drive on in a street car to approx 2500 rpm. You will have to lean that circuit with larger idle air bleeds, smaller IFR, or both. IMO with 13" at idle the holes in the butterflies drilled by the PO are not needed. You may want to replace them or close them up with solder or rivets.
I run a 66 IAB front and back with a .028 IFR. Front 7/8 turns, rear 5/8 turn. 12.8 AFR idle in nuetral and 13.5 ish light cruise till the primary jet comes in at 2500 with a 3.91 gear.
 
mderoy340 said:
Do you have the transition slot set on the primary and secondary?
.020 square up front and barely visible in the back. You need to set the idle mixture biased to the front. The idle circuit is what you drive on in a street car to approx 2500 rpm. You will have to lean that circuit with larger idle air bleeds, smaller IFR, or both. IMO with 13" at idle the holes in the butterflies drilled by the PO are not needed. You may want to replace them or close them up with solder or rivets.
I run a 66 IAB front and back with a .028 IFR. Front 7/8 turns, rear 5/8 turn. 12.8 AFR idle in nuetral and 13.5 ish light cruise till the primary jet comes in at 2500 with a 3.91 gear.
Click to expand...

Correction:
Looked at my carb today .061 IAB front and back
 
Yeah, Ive got a 39 as my IAB as per stock spec. So putting a larger one in would lean out the idle circuit?
 
71valiant said:
Yeah, Ive got a 39 as my IAB as per stock spec. So putting a larger one in would lean out the idle circuit?
Click to expand...

The stock IAB (outer air bleed) should be .070
The stock MAB (inner air bleed) should be .039

A larger air bleed allows more air in the emulsion tube leaning the air/fuel mix.
I run a .061 IAB BUT I also reduced the IFR from stock .031 to .028 This change in the idle circuit took care of the rich idle and light cruise up to 2500 rpm I had.
 
I know this is an old thread but I thought I'd chime in.
You HAVE to lean out the IFR's on a Demon carb they are rediculously rich from the factory. Mine are at .028 if memory serves me correctly. Get the IFR's set before anything else. My target is to get the primary butterflies set to 1 turn in by replacing the IFR's to get to this point. I then fine tune from there if needed by altering the IFR air bleeds as required. Take your time, it'll be needed to properly tune the entire carb. If you're like me as you get into it you'll find that you'll get into it more and more and start tuning stuff you never thought you'd get into... it's fun! A O2 sensor is gods gift to carb tuning. No more guessing. The numbers tell you everything you'd want to know. The more you use it the more you understand it and the easier it is to tune.

After the IFR's are dialed in then get your cruise circuit set, check your vacuum at cruise once dialed in then make sure your PV is 2# below your average cruise reading. Then fine tune cruise in with the HSAB's. Then adjust the main jets for max power or some people (I am one of them) adjust the PVRC instead of the main jets at this point to help get a linear (flat) fuel curve across the entire RPM range. Then move on to the emulsion wells to get the fuel curve FLAT. You should only need to adjust the emulsion wells if you have an issue like you are rich down low, then get lean at mid RPM's, and then go back to rich at high RPM's for example though. Typically this is only if you really want to dial in the carb 100%. I'm a gear head so it's what I do. :) It's pain staking and time consuming but the end result is a carb that is unbelievably easy to tweak at the track which is where the remaining tuning that you will need to do needs to be done (main/secondary jets). Set for trap speed NOT ET.

I use to live in Columbus but moved, too bad we could of had a few beers and dialed your car in. :-(

Mopar to you!
 
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