Demon carb help
Hey guys. I could use some help here on my 6-71 blown small block I just finished. It has two boost referenced Demon 650BC carbs on it. It is extremely lean at idle even with all the mixture screws out 2.5 turns. I test drove it and it wants to stall when I pull up to a traffic light, then it recovers from 500rpm back to 1100rpm where it idles. It's a good thing I have a loose converter as I am having to drive it with two feet! I got it back home and covered the idle air bleeds and the rpm increases and smooths right out. When I uncover them the engine rocks and has a random misfire. Tapping the accelerator pumps also smooths things out so I know she is real lean on the idle circuit. When I accelerate it bogs then takes off hard from 1/3 throttle up. So the main circuit is close. The pump shot is also correct because if I wing the throttle is reponds RIGHT NOW.
Any help on the idle circuit?
You can take some small diameter wire (like bell wire) and temporarily place it in the idle air bleeds to partially restrict them and go for a test ride. If it improves it is an indication your idle jet is too small. You will then need to determine what size the idle jet (IFR) is in the metering block and enlarge it. Be careful. A small increase in diameter makes a big difference in the amount of fuel it will pass. Go up .002 at a time and test. When you get it close you can fine tune with the idle air bleed size if the are removable.
A wide band O2 meter takes the guess work out of this process.
I tore everything down and check for a vacuum leak. I put the 1" plastic spacers on the formica counter top and they rocked. BINGO! Sanded them flat top and bottom. Next, took the throttle plates off and noticed a different gasket on each carb. One was correct, one was a Holley for two corner idle! Fixed. Cleaned all orifices and found one of the boost referenced power valve fitting had NO flow. Fixed.
Set the idle screws out 2 turns this time and fired it up. HUGE difference and it idled cold just fine. But now it smells rich. I set all four throttle screws at .020" of the transfer slot but it idled at 1500RPM. I am shooting for 1000-1100. Backed the screws down equally and now it has a 500rpm blower roll I do not want. Tried leaning the idle screws all the way to one turn out and it runs great EXCEPT the roll, and in gear it is even worse and undriveable.
I thought about what the engine was doing and why it was rich/lean/rich/lean ect. Woke up at 3:30am with the solution! Couldn't start car because of the neighbors so had lots of coffee.
Here's what I did:
Fixed! Posting this so I can help the next guy that does an internet search. Here's what I did:
Started off with the idle screws out 1 1/2 turns, had a 500rpm blower surge when engine is hot.
Ended up with idle screws at 1/2 turn each, and closed primary throttle blades 1/4 turn. OPENED secondary throttle blades 1/4 turn.
Engine now idles at 1200rpm with maybe a 50rpm surge or less. No smoke, no miss. Drop it into gear and it idles smoothly at 1100rpm.
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