The Dodge Challenger Message Board

After exhausting my limited experience with carburators I am looking for help from the real experts. I have a 73 340 with a Mopar Performance 284-484 cam and automatic transmission. The carburator is an Edlebrock 600 cfm. I have got the car running very smooth with smooth acceleration. My problem is that when the engine has warmed up enough for the choke to kick off the idle rpm has to be set at 1200 and that will drop to about 800 with the car in gear. After driving the car for a few miles and the engine is fully warmed up. the idle rpm will jump to 1500 out of gear but still remain smooth at 800 with the car in gear. I have the vacumn advance hooked up to the manifold port on the carburator.

What is causing the difference in RPM with the car in or out of gear? Carburator settings? timing? vacumn advance? or vacumn leak?

or is this normal for having the larger cam in the car.

Thanks

Your idle speed is way too high. In neutral it should only be 800-900rpm.

You say you HAVE to get the idle rpm up to 1500 for it to idle. This is where you are going wrong. I doubt very much that at 1500rpm you are even on the idle circuits. I bet if you turn the idle mixture screws they have little effect on idle speed or mixture?

The main problem you probably have is not enough air at idle for the cam size. So you are having to open the throttle to let more air in and that is making you run on the main jets instead.

I bet the exhaust stinks.

Timing may also be an issue. How many degrees at idle does the engine see with the vacuum advance hooked up that way? How much timing does the engine get without the vacuum advance both at idle and at 3000rpm?

It is critical to get the timing right first before trying to adjust the carb.

Try reading the various carb tuning guides I have posted on here.

Thanks for the help Dave. I know what the timing should be with a standard cam but have no idea what it should be with the cam that is in it. Where should I start at setting the timing?

Measure it and see what it is first.

The thing is the factory settings are no good with modern fuels. You need more advance at idle.
Then add to that the bigger camshaft which also needs more timing at idle and low rpms and you can see how trying to use factory or close to factory settings will make your engine act as it it is retarded. The exhaust will smell bad and your eyes will water with the fumes.

You will probably need anything between 15 and 25 degrees inital timing at idle and something between 34 and 36 degrees max all in by 3000rpm.

How you get that high initial timing can be done in two ways.

By using manifold vacuum to advance the distributor.
Or by modifying the distributor to limit the amount it can advance and doing without vacuum advance completely.

You can tell if your engine needs more advance. If the rpms increase as you advance the timing with the engine running that is a good sign that the engine "wants" more timing.

Thanks Dave for all of your help. I will give the timing a try as soon as I can get a minute to spend in the garage and let you know how it turned out.