Hi I am having a hard time working out my vacuum advance on my 340. I set the initial timing with a vacuum gauge hooked up to the manifold source, and rotated the dist until I achieved highest reading ( approx 15" ). I have a mopar performance distributor which is stock, motor is basically stock with a rv cam. Car runs fine with the vac advance disconnected, but once hooked up the car surges and bucks around the 2000rpm range. I adjusted the the allen key in the can 1 turn at a time but by the time I did 5 full turns the problem just seems to move to around the 3000 rpm range. What am I doing wrong here? Thanks for the advice

Try using ported not manifold vacuum, or adjust the vacuum advance can, an allen wrench in the nipple will raise or lower the vacuum level at which it starts to work.

So I set the initial timing using ported vac? (metering block on carb)

That's how the factory had it setup.

I would set the initial by idling in gear/ebrake on (or neutral if a stick) & plug the dist into manifold like you did & keep advancing the timing till you reach the highest vac # (you got ~15) all the while reducing the idle speed to your lowest in gear RPM that it will stably idle at & take off good from the stoplight then back the TIMING off till the VACUUM drops 1 in hg (~14) & recheck/reset your idle speed as needed then plug the can into ported. let it get good and hot & shut it off & if the starter dont kickback you are set on initial but if it does, reduce the TIMING (1) deg & retry. I would purchase the FBO $22 plate to set your mechanical (best bang for the buck by far) & pick the slots that will give you 35 total (initial+mech) such as the (10) pair of slots in that plate (10+14). then toss the heavy spring with the elongated loop on one end & sub in a mr gasket or MP light spring in its place for a start & mix and match spring so you are just under the pinging point at WOT up thru the gears on your hottest/driest (most likely to ping) day. there is silent ping & gas ain't getting any better so give your self a cushion then plug in the can to ported & adj its slope with a 4/32 alen wrench, again staying just under the pinging point in everyday driving under varying load/RPM conditions. CCW slows the rate. Do these IN ORDER. wouldn't hurt to reduce lower shaft axial play to .005" & check/correct rotor phasing & reduce rotor tip to cap clearance to .015" EDIT 3/32 allen wrench

Welcome to the world of engine timing. Some say one thing, others disagree. The bucking and surging you are experiencing is most likely due to too much total timing.
Manifold vacuum or ported for a vacuum advance is a hotly disputed subject. Myself, I have always used initial ( another debated subject) and mechanical to achieve the numbers the individual motor is happy with.

And No I am not interested in a P'ing match

you might check what the timing is at the 2K/3K RPM points you mentioned and drill a 1/2" hole in the dist cap top flat area 2/3 of the way from the center cap post to the #1 cap post (closer to the #1 cap) & shine your light in there & see what the phasing looks like at 2K/3k with the can adjusted those 5 turns.

This may help some also,on a Holley (if you are running one) you can adjust the secondary throttle blades to expose the idle/off idle transfer slot a bit.This will help set the idle speed w/o too much throttle opening on the primary side.The only problem with this is the idle screw is on the gasket side of the base plate,requiring carb removal to set it.

Publicbottle,
Every combination is a bit different.
Knowing the year, vehicle, transmission, exact cam or cam specs, and any emissions equipment will help.

I agree with TJP's assesment that too much part throttle timing is the reason for the trailer hitching.
The reason you can't change it with the adjustment screw is that controls the amount of vacuum to cause advance, not the total vacuum advance. To limit the total vacuum advance wrap the advance arm with sheet metal butterfly or piano wire.
Shown in this RFS post about vacuum advance.

Alternatively, with an RV cam and that distributor try setting the timing more like a late 60s stock 340. Use very little initial advance, something closer to 0-5 degrees BTC at 600 or 700 rpm. Then experiment with manifold vac vs. ported vac at idle. With that little initial, it will probably prefer manifold vac.

If you can, measure the timing all the way up to 2800rpm or whenever it stops advancing. (This requires timing tape or a dial back timing light.) The max mechanical (initial plus advance, no vacuum advance) needs to be between 32 - 36 degrees for a typical iron headed engine or you'll notice a drop in throttle response and max power.
A 0-5 degree initial may not get you there, or it might, depending on the advance springs and advance slots.

As mentioned above, the MP distributor with Mallory guts is different than the Chrysler built versions. The ones with Mallory guts have an bronze upper shaft (remove the rotor to check) and you can usually see the GM style advance system if you look carefully.

Set Initial Timing with the vacuum advance source capped or plugged.

Not all factory timing used ported advance for timing, some used manifold vac. It depended on the specific year, engine+car, and emissions goals. Some vehicles used both with the help of a mixing valve and various temperature controlled valves.
The difference between the two is only when the throttle is closed. It will make no difference to part throttle, moderate load situation.
Which will work best with your setup will depend mostly on the intial timing.
>If you're using an initial of 15-20 at 800 rpm, then ported makes more sense - especially with a cam that generates a lot vacuum at idle.
>If your using initial at 5 or less, then manifold is probably going to be better - depending on which emissions controls if any.
>5-15 depends on how much vacuum advance for vacuum at idle - test methodically to figure out which works best for the specifics.

Example: Initial is set at 8 deg BTC at 700 rpm and lets say this results 12" Hg. And for simplicity, lets say it also provides in 32 at 2800 rpm, which is what is wanted.

If the vacuum pod adds 15 deg at 12"Hg, then using a manifold vacuum source will result in timing of 8 + 15 = 23 degrees at 700 rpm. That's probably too much, and when you put the car in gear its going to be a bit weak.

Continuing with the example, if the vacuum canister can be dialed back so it adds only 6-10 degrees at 12"Hg, then it might be worth trying with manifold vacuum.
If it can't, ported will probably be the better choice in this example.