Install MP Tach Drive Dist now car wont idle hot

I picked up a tach drive for a small block and installed it Monday. I had a MP vacuum advance dist. in the car and it ran okay but I always admired the tach drive so I found one and swapped it in. Now the engine (Stock stroke 340) wont idle in gear and the in park idle will increase about 200 rpm when it is hot. The engine idles at 1000 rpm in park and 850 in gear when cold. Like I said the car ran fine with the other dist. so I don't think it is a carb issue or a bad coil (I know it sounds like a bad coil). I am going to pick up a cap and rotor for the tach drive because I haven't swapped those yet. Could something inside the tach drive be messed up, though? I pulled a spark plug and it was covered with soot (were a nice tan color with vacuum advance dist).

It probably wants more timing... does that dist have
advance

Assume the tach drive distributor does not have vacuum advance, in which case you just don't have enough advance at idle for the cam you are using. I bet your other dist. had the vacuum can hooked up to manifold vacuum. Only solution is to shorten up the curve in the dist. ( so you can run the same total advance ) and run more advance at idle. Tach drive itself shouldn't have anything to do with it.

The vacuum advance was hooked up to ported vac with the MP vacuum dist. The tach drive is the MP aluminum body "race" unit so no vacuum advance. The vacuum advance dist had 14 degrees at 1000 rpm and the tach drive has 28 at 1000 rpm. Both dist I set the total timing to 32. The engine idles awesome cold in park or drive. Currently, the tach drive has a cap and rotor of unknown vintage.

P.S. Sorry I did not specify the type of distributor I have in the first post.

Since you say it idles fine when cold that tells
me its fat when warmed up... assuming it doesnt have
a choke and a fast idle cam on the carb... check the
cap and rotor first to make sure its getting a nice
spark and check to see if the phasing is correct

Quote:

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The vacuum advance was hooked up to ported vac with the MP vacuum dist. The tach drive is the MP aluminum body "race" unit so no vacuum advance. The vacuum advance dist had 14 degrees at 1000 rpm and the tach drive has 28 at 1000 rpm. Both dist I set the total timing to 32. The engine idles awesome cold in park or drive. Currently, the tach drive has a cap and rotor of unknown vintage.

P.S. Sorry I did not specify the type of distributor I have in the first post.

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From that info I would say 28 degrees is too much advance at 1000 rpm. Depending on the cam you are running, I would shoot for 14 - 18 degrees advance at idle. The more overlap you are running ( LSA in the 108 range ) the more advance you will have to run. In the 110 112 LSA range, advance at idle in the 14 degree or less range. Forget the total for now and just set the initial for best idle - in gear - with a vacuum gauge. Once you are good there, you will need to adjust the dist. curve length ( in degrees) so your total falls in where you want it - 32 degrees I think you said. Example: If your motor likes 17 degrees of timing at idle and it likes 32 degrees total, 32 minus 17 = 15 degrees of timing advance at the crank ( 7 1/2 degrees in the distributor ). Once you have the amount of advance set in your dist, then you need to set the curve rate with the springs. You probably want it all in around 22-2500 engine rpm. BTW, an easy way to set the amount of advance in your dist is with a cheap plastic protractor. Cut off the straight section and just use the curved degreed part. Tape it to the side of your dist. and make a simple pointer with a piece of wire off a rotor hold down screw. Hold the dist shaft and rotate the rotor to see how many degrees advance you have ( again, dist. degrees. - it will be double that at the crank ). Weld up the slot or file it longer to get the amount of advance you are looking for. Hope you can follow this .