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I just purchased a mighty demon 650 on ebay and im not sure if ive got a raw deal. It was damaged during shipping and one of the vent tubes was broken off so I jb welded back in will this affect any performance? Then I noticed that there are no vacuum ports on the baseplate is this something that can be changed or possibly drilled? I would like to run dist vacuum adv. Second I could not get it to idle at all, I broke it down and cleaned the circuits and it solved the problem but now I have a serious stumble just off idle that wont adjust out by adjusting idle mixture screws or anything else. Any recommendations?
my combo: 355ci w/ edelbrock e street heads, thumpr cam, edelbrock rpm intake, 4speed nova
 

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I just purchased a mighty demon 650 on ebay and im not sure if ive got a raw deal. It was damaged during shipping and one of the vent tubes was broken off so I jb welded back in will this affect any performance? Then I noticed that there are no vacuum ports on the baseplate is this something that can be changed or possibly drilled? I would like to run dist vacuum adv. Second I could not get it to idle at all, I broke it down and cleaned the circuits and it solved the problem but now I have a serious stumble just off idle that wont adjust out by adjusting idle mixture screws or anything else. Any recommendations?
my combo: 355ci w/ edelbrock e street heads, thumpr cam, edelbrock rpm intake, 4speed nova
What's your timing look like? Initial and total?
 

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Need to make sure your timing and curve is correct before making carb adjustments. Then, make sure your accelerator pumps are adjusted correctly - and that they are not torn. What color cams are on it and in what position?
 

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On the Demon carbs website it states your carb has vacuum ports.

650 CFM Mighty Demon - Mechanical Secondary
Part# 5282010GC


Features
Billet Baseplate with Idle-Eze™
Billet Metering Blocks
Patented Concentric Venturi
No choke horn
Replaceable Air Bleeds
Replaceable Idle feed restrictors
Mechanical Secondaries
Patented Large View Sight windows
Dual inlets
Metering blocks with tuneable jets (Primary and Secondary circuits)
4 Corner idle adjustability
Built-in Power Valve blow out protection
Vacuum ports — 2 constant, 1 ported
Most common tuning parts interchange from other manufacturers (Jets, Power Valves, Squirters, etc.)
 

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If that carb has vacuum ports then it takes a special metering block gasket. The idle circuit feed/crossover is machined in a bad location and if you use a normal metering block gasket the idle circuit will leak raw fuel into the vacuum port dribbling into the engine below the thrttleblades.
the pics show the problem, the 1st pic (i hope the order didn't get mixed up) shows the early billet plates and a holley plate, look at the idle circuit feed at about 5 & 7 o'clock on each plate & the billets' is much lower.
The 2nd pic show a typical metering block gasket and the probe is adjacent to the hole for vacuum.
the 3rd pic shows the probe inserted into the vacuum port hole and it goes directly into the idle feed machining.
The 4th pic shows the orginal gasket taht was on that metering block and you can see the idle feed machining relieved into the vacuum port.
The 5th pic is a cheap 'fix' where you can just put a piece of tape to seal off the idle circuit feed.
The proper gasket for that metering block has a round hole for the power valve, the gaskets shown are the normal ones that have a tombstone cutout for the powervalve.
I have the other gaskets (the round ones) etc for that carb, if you need them shoot me a pm and i'll fix you up. Take it apart first to verify, it may not have vacuum and then you don't need these gaskets.
 

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Discussion Starter #6
well my timing now is now 18 initial and 32 at 2500 but Ive played with it and that seems to be best. Well I just realized somethin looking at the pics posted by oj, there are no power valves or vacuum ports my baseplate because I think its a blower carb. I finally got it idling and taking the gas pretty good by adjusting the pump arm but theres still a little stumble so I will change the squirters and cam(orange) but it runs like a scalled cat on top end 74 primary and sec jets, Ill keep ya posted Thanks
 

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If it were a blower carb it'd be pig rich at idle. The race type demons have no provision for vacuum. the idle richness screws (ea side of the metering blocks, front and back) should be out only about 3/4ish of a turn, generally 1 full or less of a turn.
Off idle stumble are generally due to have too much of primary blade angle - you have the idle too high. The throttle blades are over-exposing the 'transition' slot (do a search and you'll probably find all kinds of explanations of the 'transition slot' on the primary throttle blades. A cheap way to fix that type of stumble is to open the secondaries a little more and then you'll be able to close down the primaries to restrict the transition slot opening. To open the secondaries flip the carb over and you'll see a lttle setscrew at the accelerator pump side of the secondaries. Run this screw in until you'll see an opening appear alongside of the throttleblade - that will be the secondaries' transition slot and you just want it to see a little vacuum. Then you can close down the primaries - you'll see the transition slot above the primary blades and when all is set proper the transition slot will make a small square with the blades closed against the adjusting screw. Anything more than a square and you'll have a stumble.
Changing squirters, pump cams etc are tuning, you have yet to get to where you can 'tune'. I bet you bought the carb off ebay and are now finding out why it was up for sale. Nobody ever takes a perfectly good carb off an engine and sells it. Just don't happen. That is a good carb, you'll have to get the original factory specs and return it to the way it was originally designed and then it'll be fine.
 

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Discussion Starter #9
Thanks for the info its not idling fast at all now that I got it to idle. I will try to adjust the secondaries a lil better like you suggest. Hell for 200$ I didnt care what was wrong with it and would try to fix it., So far it seems to be working pretty damn good just havent played with demons or holleys too much just edelbrocks and rochesters but it definitely gives me better performance over the edelbrock 750:spank: I had on it previously.
 

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Glad it is coming around for you. they are good carbs, i especially liked the idle ckt - very well designed.
I try not to tune hotrods in the winter, you'll go thru a bunch of work getting it just right and have to change everything in July. Thats the nature of hotrods.
 

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Discussion Starter #11
Thanks yea ur right abt the weather espeicially since it doesnt have a choke:nono: I think I have fouled the plugs already, its just not as crisp as it was earlier when the weather was nice.
 
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