Problem with Mighty Demon 750

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the stock emulsion bleeds in the metering block were .031 drilled. You can drill and tap for 6/32 brass.
 
That is good.... So in the metering block, are those 2 sets of 3 vertical holes wide open and tapped with threads? Or are there some inserts in the holes that have very small holes in them? If these holes are wide open, then they should be closed up with emulsion tube air bleeds like 340mderoy describes (or the homemade ones made from set screws as he suggests).

Does the front metering block's same 2 sets of 3 holes have small brass inserts in them with small or no holes?

340mderoy, where are you getting the bleed and jet info for this? Just curious....
 
I dont recall them having any. I will pull it apart again tonight and be totally sure. I'll snap some pics as well.
 
nm9stheham said:
That is good.... So in the metering block, are those 2 sets of 3 vertical holes wide open and tapped with threads? Or are there some inserts in the holes that have very small holes in them? If these holes are wide open, then they should be closed up with emulsion tube air bleeds like 340mderoy describes (or the homemade ones made from set screws as he suggests).

Does the front metering block's same 2 sets of 3 holes have small brass inserts in them with small or no holes?

340mderoy, where are you getting the bleed and jet info for this? Just curious....
Click to expand...

I have a 650 and 750 demon I run on my car.
All the info used to be on the demon website but since holley took them over it is gone.
I have drilled and tapped the emulsion, IFR, PVCR for 6/32 x 1/8 brass set screws and tuned both of my carbs.

The 650 and 750 metering blocks are the same and have 3 .031 drilled holes for each tube.
IFR is .031
PVCR .059

Demons I have run rich on the idle circuit (idle - approx 2500 rpm) After 2500 the Primary Jet comes in. I always go smaller on the IFR to lean the idle circuit, smaller PMJ to lean highway cruise, and bigger on the PVCR to richen WOT when the PV is open up front.
 
mderoy340 said:
Demons I have run rich on the idle circuit (idle - approx 2500 rpm) After 2500 the Primary Jet comes in. I always go smaller on the IFR to lean the idle circuit, smaller PMJ to lean highway cruise, and bigger on the PVCR to richen WOT when the PV is open up front.
Click to expand...
OK, tnx, good info. My son and I are going to be working on his 670 Avenger for fuel mileage and the data points are good. That is, after we try my old 6619 to see how it fares on his engine for mileage.
 
OK. Here ya go. No inserts. But they are definitely equal size holes. They definitely came like this. There's no threads in the block or anything for jets to screw into.

20160606_183433.jpg
20160606_183520.jpg
 
OK, those look like the original restrictions...not drilled out and threaded for modified restrictions... that is what you need to start. Sorry, I could not see in there before. If you have a piece of 20 ga stripped solid wire it would juuuust barely not quite fit into a .031" diameter hole. A piece of 22 ga stripped solid wire would slip in easily. Or try a 1/32" drill bit.
 
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71valiant said:
And the outer air bleeds are #50 drill bits. The inners are #51 drill bits.
Click to expand...
Yes, the inners looked abnormally big. Sounds like you are pretty close to a good baseline now. When I saw the inners drilled out, and the rather largish air holes in the primary throttle blades, my reaction was 'uh-oh, what else has been changed'.....

With the blocks out, it would be a good time to check the power valve restrictions (the 2 holes in the power valve seat area) per the info supplied.
 
mderoy340 said:
I have a 650 and 750 demon I run on my car.
All the info used to be on the demon website but since holley took them over it is gone.
I have drilled and tapped the emulsion, IFR, PVCR for 6/32 x 1/8 brass set screws and tuned both of my carbs.

The 650 and 750 metering blocks are the same and have 3 .031 drilled holes for each tube.
IFR is .031
PVCR .059

Demons I have run rich on the idle circuit (idle - approx 2500 rpm) After 2500 the Primary Jet comes in. I always go smaller on the IFR to lean the idle circuit, smaller PMJ to lean highway cruise, and bigger on the PVCR to richen WOT when the PV is open up front.
Click to expand...
****. Can I send you mine. You seem to know what the hell youre doing. Lol
 
71valiant said:
And the outer air bleeds are #50 drill bits. The inners are #51 drill bits.
Click to expand...

The #51 .067" is way to large for a MAB and explains why the PMJ and SMJ are so small. The previous owner made the mistake of trying to lean a 750 to work on a mild engine.

This is what I would do:
- Get your ignition curve set. Your combo needs more initial timing to clean up idle and throttle response. Get initial around 26* and total mech 34*.
- Move the #51 .067" air bleeds to the outer holes (IAB)
- Put the new .039 air bleeds you bought in the inner holes (MAB)
- Use a 65 PV up front, leave rear plugged.
- Change primary main jet (PMJ) to 75, change secondary main jet (SMJ) to 83
- Set the throttle blades up front to .020, set rear blades to the point you just see the transfer slot appear.
- 31 accelerator squirters front and rear. You may need a 33 up front.
- If you have an AFR gauge get the motor to idle around 12.9 AFR in neutral 1000 rpm engine fully warmed up.
- Disconnect the secondary linkage and drive the car as a 2 barrel. Tune the front of the carb to get crisp throttle response (you may have to change the ignition curve to really dial in)
- Hook up secondary rod and dial in WOT.
 
mderoy340 said:
The #51 .067" is way to large for a MAB and explains why the PMJ and SMJ are so small. The previous owner made the mistake of trying to lean a 750 to work on a mild engine.

This is what I would do:
- Get your ignition curve set. Your combo needs more initial timing to clean up idle and throttle response. Get initial around 26* and total mech 34*.
- Move the #51 .067" air bleeds to the outer holes (IAB)
- Put the new .039 air bleeds you bought in the inner holes (MAB)
- Use a 65 PV up front, leave rear plugged.
- Change primary main jet (PMJ) to 75, change secondary main jet (SMJ) to 83
- Set the throttle blades up front to .020, set rear blades to the point you just see the transfer slot appear.
- 31 accelerator squirters front and rear. You may need a 33 up front.
- If you have an AFR gauge get the motor to idle around 12.9 AFR in neutral 1000 rpm engine fully warmed up.
- Disconnect the secondary linkage and drive the car as a 2 barrel. Tune the front of the carb to get crisp throttle response (you may have to change the ignition curve to really dial in)
- Hook up secondary rod and dial in WOT.
Click to expand...

I appreciate all the help. I'm grabbing the carb parts after work today and putting it together. I threw the 6.5 PV in last night. I'll get everything together tonight and see how it runs.
 
WELP. Got it thrown together..... Then this.Ordering a new head gasket tomorrow. Lovely....

20160607_210613.jpg
 
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The Mopar thin headgaskets. Putting on Felpro 1008's tonight. A little thicker, but better protection. THIS is what 50* of timing looks like. Long story... Lets just say, when a "old car guy" says, "I'll time it by ear and take her for a spin" Make sure he puts a damn light on it first....
 
Yeeeeah! He brought it back and said.... "May need to turn that timing down, I got some pings"

NOOOO ****?!?!
 
71valiant said:
Yeeeeah! He brought it back and said.... "May need to turn that timing down, I got some pings"

NOOOO ****?!?!
Click to expand...

Jeez you make him pay to fix it?


This thread has some fabulous info I'm subscribed, I also have an old 750 Mighty Demon I bought for racing but haven't had the chance to try tuning it at all. I'll definitely start by putting it back to stock according to a previous post.
 
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