Picked up a used 750 Mighty Demon... where to start?

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MopaR&D

MopaR&D

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Just got back from the Randy Coy & Sons swap meet at Bandimere Speedway today where I think I got a pretty good deal on a carb. I was looking for a 750 Holley DP for my 360 and after an hour or so of browsing around a guy at the very end of the swap meet area had one carb sitting on his tiny bench... Went over to check it out, it was a 750 Mighty Demon that was previously run on a 440 in an Imperial for demo derby (!!!!). He had it advertised for $300, I told him I only had 200 and he cut me a deal and I walked away with the carb in hand for 200 bucks. Mind you I did talk with the guy for a bit and he seemed like a legit dude so I don't feel like he would have tried to screw me over.

I know the first thing to do is pull the carb apart and clean it out as it obviously hasn't been used in a while. No corrosion or sticking linkages but it's a bit dusty and the seller said it could probably use a cleaning.

Now my question is, where should I start in regards to jets, power valves, etc? I've read up a bit on these carbs and general consensus is they are a proper race-oriented carb so they are very sensitive to proper tuning. I'm OK with that as I am going to keep my Carter AFB 625 cfm to use for street driving and swap on the Mighty Demon when I hit the drag strip.

My engine/drivetrain: .060"-over 360 LA block (stock crank & rods), KB hyper pistons for measured 10.42:1 static comp, untouched factory Magnum heads w/ 1.6 rockers, Voodoo 256/262* cam, Air-Gap intake, Hedman shorty headers and 2.5" duals with X-pipe. Trans is a 904 with stock converter going to an 8 3/4" with 4.10 gears and sure-grip. Tires are 275/60-15 Firehawk street radials so no it doesn't hook LOL.

EDIT: IMPORTANT forgot to mention I am at around 6000 ft. above sea level
 
Pulled the bowls off, it has #76 jets front and back. Only took off one metering block (primaries) I could see a tiny '4' on the power valve. The carb also had 1:1 secondary linkage which I bent and switched to progressive. Acc. pump cams look to be pink on both sides.
 
Find out what the original calibration was for your model carb and begin there.

People do a lot strange things to carbs so getting back to its original calibration will make it easier to know where to go from its basic tune.

Hysteric
 
Just going off the Summit page it says 75/83 jets, 6.5" primary power valve and the secondary PV plugged. Will check tomorrow if the secondary plug was replaced I'm going to get a rebuild kit with new power valves anyway.

I understand the jets are rated above those of Holleys, also the carbs are underrated I read somewhere mine flows similar to an 830 Holley. I also understand basic carb principles where too big of a carb will run LEAN not rich so my first move after I test the carb as-is on my engine will probably be to jet up and/or get bigger acc. pump squirters... right? lol
 
If its used off a running car and you talked to the guy, you got a deal. So many times guys pick these up off Ebay from a guy that got it in a "trade" or "a bunch of parts" and never ran it. Probably because it didnt run good at all, lots of QC issues. But then you find a peach and all is well. Don at 4secondsflat.com used to sell and support these but BG went under, reorganized and came out with the TQ "cube".
 
Good starting point to get everything set right.
[ame]http://www.demoncarbs.com/Assets/docs/Products/Technical/LIT705rev.pdf[/ame]
 
pishta said:
If its used off a running car and you talked to the guy, you got a deal. So many times guys pick these up off Ebay from a guy that got it in a "trade" or "a bunch of parts" and never ran it. Probably because it didnt run good at all, lots of QC issues. But then you find a peach and all is well. Don at 4secondsflat.com used to sell and support these but BG went under, reorganized and came out with the TQ "cube".
Click to expand...

For sure I don't trust eBay for a few car parts one of those is carbs... I read about the QC issues on these being delivered full of metal shavings and needing some "finishing off" but I can see mine was either good from the start or the PO cleaned it out :cheers:

Thanks for the link TXDart definitely helped with initial settings the throttle blade positions at idle were pretty far off. I just need a gasket/rebuild kit and a dual-feed fuel line and I can try this baby out :glasses7:
 
Got the carb on the engine today and started it up... Ran OK, got it to idle on its own but tons of leaks I need new fittings on the carb and some fiddling with the new fuel line from the "Irish parts store" lol. Seems to be a small leak coming from a plug on the "splitting" block can I use Teflon tape on this or will it dissolve and get trapped in the carb?
 
Well got the leaks fixed, ran the carb first last night on the primaries then hooked up the secondaries today... it runs PERFECT!!! No stumbling, surging, flat spots anything like that it just throws you back in your seat when you stomp on the pedal. Just from the way it feels I'm sure I picked up at least 25 HP and probably the same in torque I'm super impressed with this carb. I was expecting it to behave like it was too much for the engine but nope! It even idles smoother than my Carter did which surprised me, guess the 4-corner idle circuit is doing its job.
 
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