What size jets for my 625 Demon?

[Moabdude]

I have a pretty stock setup. A 400 Small block, stock cam, stock every ting.
I have headers and a high rise intake.
The engine is smoking like hell during startup. you can smell the gas. Is also using A LOT of fuel.
I think the jets are too big. Don't know whats in there now. Will check tomorrow.

What size jets would you advice to take?

[Chevelle598bb]

Does it smoke at full throttle? Is the gas smell mainly at idle? Sound like an idle mixture and possible timing issue than a jet issue. You may have to restrict the idle feed. What's your initial timing set at?

[MonzaRacer]

Sounds like blown power valve or other issue,,,unless you got one of the MAGIC BG crap carbs

[MonzaRacer]

if it has holley jets a 600 holley has 66 front jets but that wont cause start up smoke. Id say 68 to 72 primary 72 74 in back with no secondary powervalve, with one square jetting till you get it nailed,,, also would recomend buying a wideband like Innovate Motorsports LM1 or LC1 one of the wide band for tuning, figure some where in the 14 to 1 AFR at cruise,,, plus or minus, keep adding fuel and timing till it knocks or slows down or loses power on secondaries.

[Moabdude]

I installed the Demon right out of the box. I did not tune it at all.
Now I have been reading the manual and made some changes to the carburetor. I changed the primary butterfly's and adjusted the rod for the accelerator pump. This week I will adjust the timing and the bowl levels. The jets are both 70's. w'll see.

ps I already ordered a Holley......

[Moabdude]

Here is how it looks on the ground after starting the car.

[MonzaRacer]

big thing is it 4 hole idle, ie idle mixture on both primary and secondary,, if so Id start from scratch install.
Close all 4 idle mixture screws, to start Id try 1 1/2 to 2 full turns out all around or in front if only front idle, start with choke on first step if it has one, or idle screw open a bit so you can pull it down and get it to idle.
Also on stone stock 400, you need at least 14 to 18 degrees of initial timing I do this by ,marking my distributor for number one hope fully you ahve 7 or 8 in balancer with the timing tab at about 2 oclock looking from front of engine, if so a normal timing tab the top side nearest 12 oclock is about 16 to 20 degrees. Low compression stock engines can handle quite a bit of timing,,,as long as it doesnt strain or "grunt against the starter your ok, best to have more initial and take some out of the centrifugal or vacuum advance.
Ok get it down to lowest idle possible in gear ( set p brake and put them up against a parking curb or something) with vacuum advance un hooked.
Hook a vacuum gauge up adjust for highest reading thats not pushing car hard, smooth and even.
Then hook up vacuum advance with car in park or neutral, again reset the idle speed to normal range ,generally 750 to 950, what ever will idle in gear. Back in park, reset idle mixture to highest reading on vacuum gauge (this gauge should be hooked to manifold vacuum . Same with vacuum advance , if it works, the engine idles up,, this is normal and way you want it as you readjust with it hooked up.
I always go for lowest idle car can handle due to converter, then at idle I check secondary float level idling, I like to have at least an old towel under carb and extinguisher handy. idling in park or neutral with timing also set right, fuel should just be seen at sight hole or just below or just tipping out,,, 3 ways to have it set, running fast out float too high.
Then reset front float. Recheck idle speed and mixture.
4 hole idle I like to set rears first then as you open up the fronts, lessen the rears. Most times if you can get car to idle well with more rear idle mixture it give less issues to my findings.
Now as idle and timing are set accelerator pump settings by Holley are 0.015 clearance at pump arm,, but a I prefer to use the wiggle method I want just enough looseness at pump arm adjustment to let me wiggle arm front to rear but not squirt an fuel, on secondaries I like to start with just loose at the arm, ie adjusted you can feel but not see the movement too great before arm touches pump diaphram.
for a stock 400, I still like to use stock HEI weights and light springs,, there is a stop under the weights that can be modded.
Initial timing plus centrifugal total should be coming in as quick as possible without spark knock and makes best power. tuning takes a little time and understanding.
With initial + centrifugal= best power. Then add in vacuum advance to advance timing at part throttle cruise, I use as much as engine will take with out surging and rattling at tip in.
I use as much accelerator pump as engine can handle with out blowing large amounts of black smoke or stumbling on take off. Tuning for launch is all part of matching accel. pump shooters , pump cams, and adjustment to make car run best.
If confused just ask.
Oh and if it still pumps out black smoke and you have issues tuning idle mixture, pull float bowl and metering block and install a new power valve,, on a stock engine Id start with around a 6.5 to 8.5. tuning these can be pain. Also some BG carbs had issues that never got resolved,,, got real jaded by them, had Holley issues back when quality issues were large but never had so many issues as the BG carb issues,, too bad I always wanted to try on of the inline 4bbls they made.

[Moabdude]

Whow Lee, thats a lot of information!
Thanks for that.
I'll try to follow your instructions and see what happens.

[Moabdude]

Okay guys, I changed the Demon for a plane and simple Holley.
All problems are solved. The engine runs smooth and does not smoke anymore.

Thanks for all the advices and instructions.