Idle drops 400 rpm in drive.

Hi, Mustangs are very popular in Holland to.

It's about my 302 mustang from 68. It has a mild grind schneider camshaft.
I'm looking for an answer on the following questions:

After a drive, when the engine/transmission is at operating temperature, idle rpm drops from 1000 to 600 when shifting from neutral to drive.
When the engine is relatively cool (just started,choke fully open)idle rpm drops from 800 to 600. The temperature of the transmission fluid has an influence i think.
A brand new carb. didn't change anything. The mixture is set right, and there are no vacuum leaks.

Where to look for now?

I like to convert the ignition system into an electronic one. Any sugestion what to buy and what not ?

Best regards Ton.  

Your warm engine out of gear idle speed seems high with just a mild cam?
It should be more like 750-800 rpm and drop less than 200 rpm when put into gear.
Try adjusting your carb idle mixture screws and idle speed when warm to get it to idle well at a lower out of gear speed. Make sure you aren't compensating for a vacuum leak with the high idle speed even though you think there aren't any. Make sure you have the right jetting and that the secondaries are shut during idle. Be sure your float level is good and your choke high speed idle screw/cam is out of the way when @ warm idle.
What is your torque converter stall speed?
I use a Pertronix ignitor module conversion and Flamethrower coil with the otherwise stock distributor with very good reliability and performance for street use.  

Thanks J for the quick reply.
to be honnest, the old obselete Holley is gone, it's replaced by a new carb. specially designed for liquified petrolium gas.( petrol costs almost $9.- a gallon overhere.) The mixture is checked and the engine is performing very well. Despite the new carb. the problem did not change.
Can you explain a bit more about this convertor stall speed ?  

I understand about your fuel costs and switch to LPG. See if you can reduce the idle speed down to solve the problem and check to be sure you don't have a manifold to cylinder head vacuum leak or one anywhere else like under your LPG carb for that matter.
If you have a real tight stock replacement torque converter with a low side of spec stall speed and a cam that wants to idle high then the toque converter can tend to drag the engine idle speed down with a pre-load when put in gear on an automatic trans. You should shoot for less than a 200 rpm drop by reducing your idle speed as much as possible.
That is why a lot of cam manufacturers recommend a higher stall speed converter with big street cams.
Most mild street cams can be used with the stock Mustang converter.
If you have a tachometer take the car out in an open parking lot and put the trans in first gear and put your left foot hard on the brake then slowly give it throttle with your other foot at the same time (with the brake on) and see when the car begins to move forward. That is your approximate stall speed. Should be between 1700 and 1900 rpm. If 1700 or less your converter is a tight one and rquires a slower engine idle speed. The pre-load condition will also tend to make your car want to overheat when stopped while in gear.