Just purchased a 650 Mighty Demon

Its going to be here on Wednesday so i would like some advice before i put it on.
My combo:

DSS 306
9.5 compression
victor jr. intake
fully polished non-emission AFR 185's
1 5/8 long tubes.
Mac 2 1/2 full exhaust
Z-303 camshaft
1.6 roller rockers
3:55 gears at the moment. Might swap rearends to make it 4:30s
T-5 tranny

The car pulls 10 lbs of vacuum at idle
Around 37 degrees timing full advanced

I have read in several post that:
"that going from a 70/78 (out of the box jets) to 76/83 responds extremely well on the 650 Demons."
With 10 lbs of vacuum what power valve would be correct?
I have read subtract 2 from your idle vacuum and i have read divide by two for the correct power valve.

All advice welcome!!!!!!

What's the duration @ .050 with your Z-303 cam?? The mighty demon is a step up from the speed demon. Anyways check to see what your jet sizes are but 76/83 would be optimum for the 650 Demon carbs. Sounds like you got a good combo.

Duration at .050 is 228.

Im having trouble with the holley 4150 750DP being fat at idle no matter what i do.
I was hoping the Idle-Eze option on the mighty demon would help solve that porblem.

That's 10 inches of mercury, not 10 pounds. I'd use a 4.5" power valve with that reading.

Run the Demon with the stock jetting. Making it way fat will likely hurt things. Jet up only after you have a base line, then change one end at a time two jet sizes. What might have worked for X and Z will most likely not work for you.

Spali32 what power valve should i use with 10 lb of vacuum?
I have read two different ways of doing it.
One says subtract 2 from what i get at idle.
The other says divide your vacuum at idle in half.

Thanks again

4.5" power valve sounds like "the one" to get.

I just got the demon at work just now.
I noticed it had 6 /5 power valves in it. I'll keep those in and see how it does.


I did notice when looking at the bottom of the carb that the idle transfer slots on the primaries are not visible. The butterflies barely cover them up.
The transfer slots on the secondaries do show the square pattern.

Cant wait to get home to see what the innovate A/F meter shows.
Thanks for the help guys

Reading the demon manual says you must have equal transfer slots all the way around.

Checked again, the secondaries have the square appearance.
Primaries show ZERO slot. A minor turn of the idle screw will correct the problem.


Makes me wonder what else they didnt check before shipping.

Seems to me you should of gotten a speed demon vs the mighty demon according to Barry Grant's chart and you only having a 228 @ .050 duration. We'll cross our fingers and hope the mighty demon works out for ya the way you want it

Demon Selection Guide

Well spali you were right about the jets being off from factory.
It had a huge lean stumble and was lean everywhere.
When i rev it up at idle it would hesitate really bad! Even worse when under a load.
I noticed the shooters had a weird spray pattern. Took the shooters off
and couldn't believe what i found. It looked like a kid drilled the holes. They had burrs and weren't uniform in anyway. They looked nothing like my holley.
Here are some pics:
douglas387 - Photobucket - Video and Image Hosting

I took the ones off my holley and that fixed my stumble.
I took the carb apart to change the jets and i found fine shavings when removed a plate.
What a nitemare!

My job is quality control for an aircraft company. Finding this kind of quality in such a high priced part drives me nuts.