Hi All,

I am trying to tune my 750 CFM demon carburetor on my 68 charger. Engine is a 440 stroker with a 284/484 cam. Transmission is a 727 automatic.

The manual said the idle mixture screws should not need adjusting. Without adjusting them, I needed adjust the idle set screw to get the idle RPMs to 2,000, which would then drop to 1,200 when I went from park to drive. Any less than that and the engine would die when I went from park to drive. That huge drop in RPMs makes me think something is clearly wrong, and 2,000 RPMs while in park is way too high.

So I backed the idle set screw off all the way and started adjusting the idle mixture screws. The engine is happy in park at 1,300 RPMs and is pulling 26" of manifold vacuum. (I'm at sea level by the way) However if I push the brake, or shift into drive the engine dies. I can increase the RPMs a bit via the mixture screws so that it won't die when I press the brake, but it still dies when I shift into drive. To not get it to die while in drive I have to increase the RPMs via the idle set screw and I'm back where I started.

I've checked everywhere for a vacuum leak, and found nothing. 26" at 1,300 RPMs is fairly high though no? Shouldn't the brake booster be able to use a little vacuum without the engine stalling?

Sorry if this post was all over the place. Any advice is appreciated.

Thanks!

you're fine bro & yes that idle is way too high. what is your initial? no vac leaks?

How do you have your vacuum advanced hooked up? Manifold or ported?

You may need a lot more initial advance. If you are running the very light advance springs, a slight drop in RPM means a decrease in advance, followed by more RPM drop, etc. Disconnect and plug you vacuum line, and advance the timing at idle and see if that changes how it responds to the application of brakes or pulling it in gear.

Just for funsies, cap off the vac line to the booster as well. A failing booster can be a huge hidden vacuum leak.

26" seems like a lot. Sure it is not some other units, like cm?

Is this a new carb? Did it run fine before? Or has it always run like this? What has changed?

Agreed, no way a 484 or any cam has 26 in hg. if CM then that is 10 in hg. I had a 484 or 509 (25 yrs ago) & it loped alot but definitely under 1K idle speed. initial. vac leak.

I have never seen any car pull 26" of vacum other then on decell. Either the gauge is off or you are reading it wrong. Ron

No load at 1300 rpm it's not that difficult to pull 26" vacuum.

That said, there's still a big vacuumleak if you can't get it to idle lower.


EDIT:
I want to revise my statement above. My mind was off by 5 inches of vacuum on the vacuum gauge in my car.
The 318 in my daily has very good vacuum during idling and driving, but it can indeed only achieve 26" vacuum upon deceleration.

Also, the high vacuum doesn't quite match with a possible vacuum leak.

Thanks for the replies everyone. I'm still spinning my wheels badly on this issue. I disconnected the brake booster and all unnecessary vacuum lines. No change, and I hunted everywhere (again) for a Vacuum leak and came up with nothing.

I've tried many different things between the idle mixture screws and the idle set screw, and the car always dies when I shift from park to drive. Sometimes it fights for a little, but it always dies and spits gas up through the carb.

I tried making a video with my phone to show the RPMs at idle (I had them about 1,100 during this try) and trying to show the vacuum gauge. You can see it fluctuates a bit between 21-23. Sorry for the poor quality, I'm a newb at shooting videos. Here it is though -

https://m.youtube.com/watch?v=8o4MAzbSxuY

I'm pretty much out of ideas, but was going to pull the distributor cap and measure the resistance at all the contacts. Roughly how many ohms should I see?

Thanks for the help.

By the way, if anyone says "you're right, that video does suck!" I'll gladly shoot another and can try to focus on whatever.

Have you checked your timing as you go from park to drive to see if it stays the same or drops?

as said..you need alot of intial timing ...

You're sure the vacuum gauge is reading in inches of mercury? (symbol is "hg)

What are your timing specs at? Initial/total/vacuum advance?

With a big cam, you'll typically need more initial timing, which depending on your amount of mechanical advance will push you into pinging territory. So then you need to limit the mechanical advance.

The more you advance the initial timing, the more you'll be able to close the throttle, and the more control the idle mixture screws will have.

If you do increase the initial, don't drive it until the total is limited.

i overhauled a engine one time and put in a hotter cam,no checking ,degreeing etc,just put it in,engine wouldnt idle below 1000 and done better at 1100,still when you put it in gear it would kill the engine,soon after i rebuilt the trans and put in a off the shelf reman converter,that fixed the dieing in gear part,i can only presume one converter was "tighter"than the other!

http://www.4secondsflat.com/Mopar%20Mechanical%20timing%20limiter%20plate.htm

to fix the mopar distributor....

This.

And, do you know your timing is accurate?