Carb Tech, need help tuning.

ExplodingGopher · Sep 29, 2005

Figured i'd post in here since you guys probably have the most experience with carbs. Motor is a completely built 466 big block, 10.4:1 comp, cam is 234/244 @ .050

Lately i tried adjusting my Demon 850 DP carb and it is running really really rich right now, like burn your eyes and nose at idle kind of rich. The butterflies are set to be open equal amounts, mixture screws are out i think 2+ turns. If i try to lean it out at idle, it idles fine but once the secondaries open, it stumbles/pops through the carb. If i keep it rich, it runs ok but its just killing my already minimal gas mileage. Carb has stock jets in it which i think are 84 in the front and 93 in the back, seems a little big. Any ideas?

10secgoal · Sep 29, 2005

I would guess the butterflies up front are open too far, or the secondaries are open and shouldn't be. The the floats could be too high, or a faulty power valve/gasket. Try turning in the idle mistures screws all the way. If it dies, that's good. If not, you are getting gas from somewhere else.

thehueypilot · Sep 29, 2005

Sounds like you need more air and less fuel. I agree with 10secgoal except you need to open the secondaries (add an adjustable sec. pump cam bracket) to get more air at idle while closing the front primaries to get the idle circuit working. The miss at sudden acceleration means you need a bigger shot from the squirters. The jets seem to be really big for your engine size. I'm running 73/80 on my 850 Vac which gives me 12:1 at the strip. The 650 DP is 66/74 which gives me 12:1 at the strip.

Dark Knight · Sep 29, 2005

Some of the demon carbs have a screw under the air cleaner stud to fine tune the idle..

Backfire through the carb is usually lean, you might want to change the acc pump cam or squirter.

You have checked the float level and tried changing the timing, right?

What's the idle vac. and what power valve do you have?

Might try http://www.carbdford.com/ and
http://www.network54.com/Forum/85220

ExplodingGopher · Sep 29, 2005

So at idle, i should try having the secondaries open more than the primaries to lket more air flow? Or should they be open equal amounts, cause the equal amounts just doesn't work for my car.

To answer a few questions:

the power valve is 6.5

the engine does dies if i even turn one mixture screw all the way in.

I dont have that idle-eze screw bloew the air cleaner stud, my demon was made before they cam out with it.

float level is in the center of the sight glasses.

fuel pressure is at 6 psi steady.

not sure what the stock squirter size is though.

I already posted on the BBF network 54 forums and im just looking for a broad range of suggestions

10secgoal · Sep 29, 2005

The floats are too high. You want it at the bottom of the hole. It's possible the vacuum is drawing the fuel into the motor from the jets at idle which is shouldn't be. If it dies when you screw the idle screws in, then Huey is right. You need a pretty wild cam to need to open the secondaries, or drill holes in the butterflies. I would lower the floats first. I would doubt any major tunning is going to be needed. Check the basics as mentioned. The idle screws having an effect on the idle is a good thing. As long as the motor isn't getting fuel from areas it shouldn't be, then start opening the rear butterflies.

Carb Tech, need help tuning.

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