Thread: 496 Running Rich

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My big block has been bored .060 and has a 4.25 stroke. It has 781 heads that have been heavily ported and flow tested. The compression is at 9.5:1. The cam is a Comp solid roller with 242 intake and 248 exhaust duration at .050. .650 lift at valve. The carb is an early 850 Mighty Demon on top of an Edelbrock RPM airgap. . It idles at 1000 rpm pulling 11 inches of vacuum. Timing is 12 advanced at idle – 39 full, no vaccum advance. I know it should have vacuum advance but the Mallory distributer I have doesn’t have it. The plugs are always sooty and dark brown. When I’m driving, if I open the throttle hard it sputters then clears out and moves. Then it runs good if I floor it again. I shift at 6500. The plugs have run dark since I installed this carb. Floats are set right and fuel pressure is right. I’ve heard the early Demon carbs were jetted rich. Would a vacuum advance clear this up or should I concentrate on the carb?

I'd concentrate on the carb. Sounds like you just need to drop down on your main jets & metering rods to pull back the fuel. Since it runs good after you've cleared it out it sounds like the accelerator pump is doing OK, you're just running rich, IMO.

Check the power valve

I'll check powervalve. Should I just drop the main jets on the primary side?

Originally Posted by BigBlockBird

I'll check powervalve. Should I just drop the main jets on the primary side?

Since it seems to "clean out" well, I'd concentrate on the primary jets & metering rods. Drop down a size or two and see how it does, cruising for some miles then shutting it off and checking plugs before it idles around a bunch. Keep iterating in on the primaries, and then if you find a need play with secondaries.

The early Demon carbs were notoriously hard to tune. They were missing an adjustment (Idle Eze) that was added after Barry Grant started getting complaints. Some changes were made in the Demon carbs, which made them perform better, but by then, the damage was done, and Grant filed bankruptcy. I finally dumped my street demon and installed a Holley Street Dominator.

I know those Demons CAN be tuned, and they CAN run great. Mine just wore me out.

Link to an article on the Idle Eze

OK----------theres no metering rods in Holley type carbs


With 11 inches vacume you need to use a 4.5 power valve


Carb upside down adjust the throlle blades to where the plate covers all but a little of the transfer slots-it should be to where the amount of the slot that is visabl looks like a tiny square-both primary and secondary


Adjust idle mixture screws to about 1 3/4 turns from full in-----be careful not to overtighten them to bottom out as you can disform the carb body seat hole /passage for the needle tip on the screws


You need to adjust timing curve in your distributor to have 15-16 degrees timing at idle (which should work out around 850) and no more than 34 or so at anything over 2500


Idle ease if you have it is adjusted from the top of carb thru the air cleaner stud hole with a fairly narrow straight blade screw driver-------start of by bottoming out the adjustment and then opening a couple of turns-count the half turns on screw driver----get it where it will run and then work on the mixture screws and keep adjusting the idle eze to a smooth idle speed-----if it idles too high rpm you will start to get timing advance and thusly will probably retard the timing to set your desired numbers-----fuel pressure not more that 5 1/2 lbs---have fire extingser and escape plan (especially if electric fuel pump)

Originally Posted by jerry clayton

have fire extingser and escape plan (especially if electric fuel pump)

These are words to live by, literally..

Those transfer slots usually (every one I seen) have a burr rolled over from machining the slot and it is stuck in the slot transfer holes in the base plate------plus sometimes for a period of time the Demons all had about a tea bag size group of random chips that evidently no one assembling them on line ever blew off with an air gun--------

BigBlockBird, I did a little bit more "Google Research" and realized that my reference to metering rods was not right - no metering rods on this style carb, which was generally a copy of the Holley approach. Sorry for any confusion I may have caused you. More to the point, from what I've seen the Mighty Demon line is really intended for race only applications, and is not particularly street friendly. This clip from Summit summarizes the three different models pretty well - https://www.youtube.com/watch?v=-NN9X76ZM2g

So--he was here at 3 :48 but no post?

Thanks for the info. I was wondering about the metering rods. In the past, I used the same carb adjustment instructions as Jerry wrote about to get the idle down to 1000. I bet if I set the initial timing at 15, I could get it to run leaner. With the 3.73 gears and automatic trans its probably hardly on the main circuit while cruising. To bad this distributor isn't like a GM where everything is on top and easy to get at. This is off topic but I wonder if Mallory is going to keep making distributors. I was on their site and noticed that all off their distributors are listed as out of stock.

I've tuned lots of Holley and Demon carbs----------you need to check if in fact yours has the ideeze in the middle and get a 4.5 powervalve, some jets and a holley blue gasket rebuild kit---for starters going down 3 or 4 jet sizes on the primaries and leave the secondaries alone. I'd be happy to guide you thru this but won't do it with any other posters involved. I'll help you on the club Hotrod site but just so you know you I am and what I've done, check around and then let me know


We won't be doing any Google search or talking to Summits tech line or cataloque

Originally Posted by BigBlockBird

This is off topic but I wonder if Mallory is going to keep making distributors. I was on their site and noticed that all off their distributors are listed as out of stock.

The scuttle I heard was MSD bought Mallory and is phasing the Mallory line out.

Originally Posted by jerry clayton

I've tuned lots of Holley and Demon carbs----------you need to check if in fact yours has the ideeze in the middle and get a 4.5 powervalve, some jets and a holley blue gasket rebuild kit---for starters going down 3 or 4 jet sizes on the primaries and leave the secondaries alone. I'd be happy to guide you thru this but won't do it with any other posters involved. I'll help you on the club Hotrod site but just so you know you I am and what I've done, check around and then let me know


We won't be doing any Google search or talking to Summits tech line or cataloque

My carb does have the idle eas. I've adjusted it before. I've had the blue gasket holly rebuild kit sitting around for a few months. I've got to get a 4.5 power valve and some 80 and 78 jets. It has 83s and a 6.5 power valve in it now. Nobody in town carries many Holley parts anymore so I'll have to order them.