Demon carb setup help needed.

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67Dart440GT

67Dart440GT

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I picked up a 70 Swinger with a 318 that is bored and stroked to 390. The car has a Demon 650 carb on it. Throttle response is really good, stomp the throttle and it bakes the tires off. My problem is that it seems to be running very rich, especially at idle. I checked the timing and it is about 12 degrees advanced. I set the floats to the middle of the sights on the front and back. I turned the mixture screws in and then out 1.5 turns. The car didn't start too well on that setting so I moved them out another 1/4 turn. The car runs really rich and you can just smell it when driving. Has anyone had any experience with these carbs and maybe an idea on where to start? This has an electric choke as well.

Another thing I noticed is the throttle seems to "pulse" while driving. Keeping your foot steady it seems to go up and down a bit in rpms. This is very slight. Idle is about 800 to 850.

What are your thoughts?

Thanks in advance.
 
I would lower floats to bottom of sight gauge
If that does not fix it I would inspect and replace
needles and seats if needed.
Sound like a float/needle and seat issue to me.
 
Could be many things. Remove carb & check where the T slot is. More than 0.060" showing above the blades will give rich mixture. Another clue to the T slot position is that mixture screws have little affect on idle. Screwing mixture screws all the way IN should kill the engine.
 
Bewy said:
Could be many things. Remove carb & check where the T slot is. More than 0.060" showing above the blades will give rich mixture. Another clue to the T slot position is that mixture screws have little affect on idle. Screwing mixture screws all the way IN should kill the engine.
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Thanks for the tips. I checked on the Demon manual and it says just a small square of the t slot showing is where to start. I pulled the carb and it appeared to be too much. I found that the screw for the electric choke was holding them open too far even when the idle screw was backed out fully. I turned out the choke screw and set the blades to the recommended setting. After installing again the car won't start or run until I crank a bunch more timing into it. It would fire idle and run at about 20 degrees so I backed that back down and opened the blades again. I read there is another screw that some people adjust in the secondaries to allow it to idle. Any thoughts?
 
GTX JOHN said:
I would lower floats to bottom of sight gauge
If that does not fix it I would inspect and replace
needles and seats if needed.
Sound like a float/needle and seat issue to me.
Click to expand...
I have both set to the lower line now.
 
67Dart440GT said:
I read there is another screw that some people adjust in the secondaries to allow it to idle
Click to expand...
Flip the carb over an look for a tiny screw around the rear throttle plate. I've had good luck with that in the past, tried it again recently and got a backfire in the exhaust system. Closed it back down, problem gone.

You probably need more timing. There's nothing wrong with 20* initial if that's what the engine wants, and it sounds to me like it does.
 
Uncle Bob said:
Flip the carb over an look for a tiny screw around the rear throttle plate. I've had good luck with that in the past, tried it again recently and got a backfire in the exhaust system. Closed it back down, problem gone.

You probably need more timing. There's nothing wrong with 20* initial if that's what the engine wants, and it sounds to me like it does.
Click to expand...
Unless I am blind which in this case I probably am... I did not see this screw. I read that some people will change this for an allen key so it can be adjusted while the carb is installed. I didn't see that screw. I will check again and see. It did seem to want more timing, but when I added some rpm the total was at 40 degrees or more.
 
Here is a Holley Avenger I had on my 340 and the second is the Demon carb.

20220710_182212.jpg


20220710_182221.jpg
 
Sounds like the timing is your issue. When you pull timing in it it seems to allow you to get the t slots correct. I would work on limiting total timing with initial at around 20 and see what it runs like.
 
I ended up setting my timing at 20 initial. Threw on a Holley Street Avenger carb I had and it purrs like a kitten now.... I have a small fuel leak that I need to fix so I have not road tested it yet but does not stink rich like that Demon carb did. Anyone want to buy a Demon carb? HA
 
I bought a demon carb.....woe is me! Thing ran but not real good. Tried all the tricks. Square transfer ports,. Cracking the secondaries with said hidden screw.. does it have a vent in the air cleaner stud boss? Thing would flood every other day. Realized there was a bit of sliced O-ring hanging up the needle in a random manner. It was still connected so it just floated around the seat until it would finally catch it open. Tried 3 different needle seat o-rings and all of them were sliced when I took them back out. Suspected a defect in the seat groove depth or something as they all sliced in the same manner. Put a holley DP spreadbore 650 back on it and it ran way better with no more flooding. Get those transfer slots square, about all you can do with them. Then monkey with the secondary to get a clean idle. There are good catn guys in here, I think one symptom is if you don't get any odle results with the mixture screws, there is another circuit issue.
 
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