Just bought a Duster that has a 340 and was trying to use a vacuum gauge to set timing. I am getting the most vacuum 18 inches with timing at 27 btc at idle total is about 52. When setting timing at 8 btc I am only getting around 10 inches.Don't know much about cam other than it is purple shaft specs unknown. The engine seems to Rev slow. What is best way to gethe timing set on this engine. At 27 it doesn't want to start. With timing set at 34 total the timing set at 8 btc and engine seems sluggish.

Temporarily disconnect the vacuum advance. Back the timing off from 27 until it does start OK when hot. Whatever number that is, probably around 18 degrees, is where your initial needs to go.
You probably have a stock distributor with way too much mechanical advance if it goes to 52 with the vac advance plugged.

Since your total timing still needs to be 34, that means you have to modify the distributor so its total advance is 34-18 or only 16 degrees.
I recommend one of these:

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

Far easier than welding up and filing the slots.

So 18 is not too much initial timing ? I get around 15 inches on vacuum gauge. Did not know i could run that much timing.

Nah, I ran 12* initial with stock-cammed 383s, no problem even in hot weather. Current BB is at 18* initial with a bigger cam, no problems.
Bigger cams tend to need more initial timing.
Most of the time, 34-36 is the max mechanical a Mopar wants. Vac adv will add some to that.

I would get max manifold vacuum all the while reducing your idle speed to the lowest stable hot idle speed in gear (which is what you will see stoplight to stoplight) or neutral if a stick, that you can achieve then reduce the INITIAL till the VACUUM drops 1 in hg and at that vacuum point your initial timing is ideal. if it will not crank when hot then reduce the INITIAL (1) degree & retry, then another degree if needed OR keep it where it was first at & wire in a NC momentary closed contact switch to the ign switch "ign2" terminal/circuit & push the button then get it cranking with the ign key then release the button while still cranking. As said get the FBO $22 plate, it is the best deal going by far & use it to set the total (initial+slots) & I would set the total to 35 with open chambered heads and 31 with mag heads which will be very close to ideal. I had 18 on a bone stock 80's 318 2bbl. holler what you end up with. EDIT then work with springs then plug in work with the can (if used). you want the springs to start advancing ~100-200 RPM above the hot in gear idle speed you have

Should I still run vacuum advance on the distributor or block it off. Just ordered plate from Mancini for 19.95

street, street/strip or strip

We see this over and over on big cammed engines. The engine wants timing at idle to be somewhere north of 20 deg but the engine will not start.

So the timing is pulled back to allow the engine to start but the torque at tip in is reduced, it doesn't idle as good, throttle response is reduced and the mileage falls off.

Vacuum advance or computerized ignition control is the only way to have both.

For ignition timing you need to control at least 4 things, timing at cranking, idle, cruise and wide open throttle.

You need lighter advance springs. The timing is lower when cranking, but once running, the springs are light enough to allow mechanical advance to kick in at idle. Since mechanical is already operating at idle, there is less available at higher rpm, so your total won't be so high.

http://www.manciniracing.com/malidiadspse.html

The plan is to drive the car on the street . The car had been sitting in a garage from 1994 until now. When I brought it home and started it up it had little to no oil pressure I drop the pan and changed oil pump and the engine looked brand new inside. Started it up again and still no oil pressure finally found that a oil gallery plug at the end of the lifter valley was missing pulled intake found purple shaft cam every things looks new. Started it up 70 pounds oil pressure. So I guess the engine was built but was never run because of oil pressure and the torque converter was rubbing on the dust shield. They must have thought the bottom end was about to come loose. The owner passed away twenty years ago he was older guy who like the drag strip. Found time slips in glove box mid 15 times.I guess he was wanting to go faster. What I have is 73 340 factory head did not see any port work on intake side and looks to have the factory brown valve spring,some type of purple shaft,dual plane Mopar aluminum intake 340 hp exhaust manifolds, Chrysler distributor with orange box and a holley 750 3310 carburetor. With a/c and power steering. I think it will be a fun car to drive but need to get this timing setup for the best around performance . As of right now my plains are going with limit plate to set total to 34 and running vacuum advance , And to get the vacuum up at idle

I would set the initial then limit the total (initial+slots) to 35 with that plate then toss the heavy spring with the elongated loop & sub in a mr gasket or MP light spring in its place for a start & mix and match springs so you are just under the pinging point at WOT up thru the gears on your hottest/driest day (dont crowd that line too close but you want to be fairly close to it & any ping you can hear is WAY too much) then adj the can with a 3/32 allen wrench, same deal staying under the pinging point in everyday driving under varying load (in hg) and RPM conditions. CCW slows the rate. also on the springs you dont want it to start advancing any less than 1-200RPM below your hot idle speed is (no mech adv at idle), IE idle at 700, advance starts at 850

Thanks for the advice, ordered plate last night and springs today I will try and let you know how it turns out.

He'll never get a stable idle if he does that, unless the advance is all in at idle (which is no good for the street). If the advance is already operating, more timing = faster idle, more timing... put it in gear, slower idle, less timing, slower idle, etc.

Vaccumn guage readings are often much more impacted by other than ignition timing,,,vacuum leaks, defective carbs, carb settings, leaky cylinders, plug wires, idle speed, the list goes on.

It is a very rough, but complementary troubleshooting and tuning tool.

Relying on a vacuum guage to set timing is not the method that I would subscribe to. I prefer a digital set back timing light at a minimum.

The point being that you may have issues other than timing that you are dealing with that too must be explored.