1968 340 timing

I am setting my 340 up,I have bnoticed that it doesnt "like" the 5 BTDC initial setting, at frist I thought my damper had slipped,but i verified TDC and the mark lines up. My engine seems to like 10 or 15 btdc. It has been converted to MOPAR electronic igintion with Orange box. It will run at 5 btdc,eith 16 inches of mercury on a vac guage, but it doesnt start smoothly,it long cranks. I am thinking it needs more advance.

My suggestion is to measure timing from the lowest rpm to the highest timing/rpm you can measure with whatever you have on hand.

Write down each measurement and plot on the timing graph. Use the 273 non-emissions graph as a reference.

Since it was converted to an ECU, clearly the distributor is not the original.
So the only way to know what the advance looks like is to measure and plot.

You can also look to see if the distributor still has its tag. The tags are held onto the side with a screw like this.
upload_2020-8-17_18-29-10.png

MP ones are often colored green.

One final thought. Initial should always be measured with the vacuum advance line disconnected and plugged. You prob know this but a lot of people don't. Golf tee is a good hose plug.