I have a 74 360 in which the vacuum advance diaphram is busted. I took the diaphram off of an old dist. I have laying around from a 70' 390. I put it on my 360. I left the vacuume advance unplugged and set the timimg to ~12 deg. when I give it gas it will centrifically advance to about 18-20 deg. then when I plug in the advance it pulls it to about 20 deg. but when I give it gas it advances to a whopping 40+deg. then when it idels back down it will kill the engine. unplugg it and it is fine runs at 12 deg. plugg it in and in about 5 seconds it will kill motor.

would these two advances interchange. the 390 looks to have a little bigger body than my old one. could the diaphram just be too big or too stiff for my engine. or any other suggestions??

 

What’s your vacuum source, it sounds like full manifold but should be ported (will have to duck under a rock now). If you don’t mind waiting ‘till I get home, I can look up the specs on the two diaphragms. Seem to remember that the 360 diaphragm actually pulls more advance across the board compared to the 390 unit. Yes, at part throttle with no load the vacuum will pull a lot of advance, it’s normal.

Barry

 

my vacuum source is comming from the left side of my carb.(as looking from front of truck) it feels like a pretty strong vacuum. The only other vacuum port on my carb is in the front on the driver side under the accel. pump. This one is hooked tru a temp valve and then to my egr valve. It has a pretty weak vacuum. are theese correct. that is just the way that they were when I bought the truck

 

Thanks for waiting, now here’s my dilemma. I have the specs for all truck distributors but no application data. I need to know the number on the side of the dizzy to match it up. For the cars I have the application data so I can cross them over. If you can get the numbers off the side of the 360 and 390 dizzys, I can get you the curves. Another hitch, if your dizzys are rebuilt then all bets are off. The rebuilders put in lame advance curves to fit a wider application of vehicles so they have less to inventory.

On your 390 vacuum can, was the actuating arm the same length and bent the same way as the 360 unit?

Back to your truck, sound like the hoses are hooked up right. The EGR port is on the driver’s side and the spark port is on the passenger side. Hook up a vacuum gauge to the spark port and see what it’s reading at idle, it should be very little or none. The vacuum advance should only pull an extra degree or two with 5” or less of vacuum applied. Some vacuum cans are adjustable, try sticking an allen wrench into the vacuum port. Turning the screw CCW slows down the rate. I have a hunch you’re pulling too much vacuum advance too early.

A little off the subject but are you still running points? If so, the best bang for your buck is to convert it with a Pertronics unit and have a shop curve it for you. Tell them all the details on your truck and intended use. I curved mine, it’s nice to have your own distributor machine to play with. Just as an example, used the 13L slot for a max. centrifugal of 26 degrees. Put an extra 12 initial on the crank for a total of 38 degrees. The slope depends on your engine, vehicle and driving conditions but I like to have all the advance in by 3000 rpm if it can handle it. On the vacuum can I slowed down the rate and limited the total to 14 degrees @ 20” vacuum so it won’t ping at part throttle. The early vacuum cans can be taken apart and different springs and spacers can be swapped in to change the total limit and the rate too, pretty slick.

Here’s a couple of links on the vacuum advance:

https://www.ford-trucks.com/dcforum/61_79/5178.html
https://www.ford-trucks.com/dcforum/61_79/4777.html

I'd search for more but I have to do some work right now, sheesh!

Barry

 

Well I went out and looked at the number and they are both 12127 they must be from different time periods though. one has a ford logo to the right of the #'s and the other one has "motorcraft". Yes they are both crummy point-type dist. I am unsure as to weather they have been rebuilt. As for the vacuume, well I don't have a vacuum tester. sorry. The one thing that I can tell you though is that the vaccume line does have a vaccum at idle. and it has a hellashious vacuume when you give it gas. I looked on the carb and I don't see any type of adjustment for it. The vacuum port is on the left side of the carb down at the base. BTW the carb is a motorcraft 2bbl (2100 I believe)I tried hooking it up to the other vacuum port(which has no vacuum at idle)and it has a very bad hesitation when you give it gas. theese are the only vacuum ports on the carb.
p.s. when you give it gas, should the timing advance any at all if the vacuum line isn't hooked up. I know it doesn't with electronic ignition, but what about theese.
thanks for the help