Hello guy's,
I have a 66 F100 with a new 300 six, when the distributor's vacuum line is plumbed into to the intake manifold it runs excellent, when I attach the distributor vacuum line to the carbs ported connector it runs good but cuts out under heavy acceleration. any ideas? also can someone tell me the average reading on a Vacuum gauge? I run a reading of 15 on the vacuum gauge at idle and thought it was supposed to be around 20 for optimum performance.

 

Yes 20" of vacuum is optimum.

If your engine is running at intake vacuum then your not getting vacuum advance above idle. Your timing can change the amount of vacuum your engine has.

You need to check your timing and see where it is set. Make sure your vacuum advance is working on the distributor.

 

I prefer to set initial timing with a vacuum gauge. Unhook the vac line to the dizzy and plug it. Hook up a vac gauge to a manifold vac source and turn the dizzy until the highest steady reading is obtained and back off 1-2 hg. Adjust carb the same way then drive and try and adjust from there if needed. IIRC most stock set-ups work on a timed port but try it both ways to see what it likes.

More info here. Reading your vacuum gauge

 

Yes, 20 in. of vacuum is best, but that rule of thumb is for sea level.
It's safe to subtract 1 in. of vacuum for every 1000 ft. of elevation.
Barometric pressure will also have a noticeable effect on your reading.
A high pressure weather system can raise your vacuum reading 2 or 3
inches above that of a low pressure system. I live at an elevation of 850 feet
and my readings vary from 17 to 19, depending on the weather.
What ever the reading, the needle should be steady.

Here's another link to go along with Harte3's.
How to Use and Interpret a Vacuum Gauge

Murph.

 

I did just use a Vacuum gauge to set the timing as recommended by Harte3, I also found the point gap to be a little wide so I adjusted that and reset the carb air/idle mixture, I will see if that fixes the problem, it just was cutting out under high throttle such as when passing and I could feel a little surging when driving. I am at 5500 feet above sea level so I backed out the air/idle screw an extra 1 1/2 turns, it ran great before I changed the distributors vacuum line to a ported outlet on the carb, it also has a 5500rpm cam installed to the stock timing marks are no longer of use. I will post results when I get home from work. I want to thank everyone for their help!

 

High elevation creates it's own challenges for tuning.

Here in Spokane mine pulls just short of 19 hg...2400 ft. Down at Lewiston, ID, 20 hg...740 ft.

 

At around 7500 feet my vac reading is around 15 or so, if it helps. Also, my 300 runs better on full manifold vacuum, but was a b#tch to start, so now it's on ported.

 

That's why I changed to the ported vacuum line as well, on full manifold vacuum it had power up the wazoo but would flood easy and was hard to start sometimes, I used the manifold port for a mileage minder Vacuum gauge that mounts like a tach on the column, I had one years ago and found they still make them. I bought it thru carparts.com, they had the best price and free shipping, item # 73CP5800P cost was $35.00 to my front door. Here's a picture: