I need some help. I have a 75 f250 with a 390 in her. I'm working on getting the timing right but having a hard time. I unhook and plug off the vacuum advance line and set the timing right around 12 degrees. When I hook the vacuum advance line back up the old beast stumbles and backfires. Any suggestions why this happens? It also has an Edelbrock 1406 on it that I am fairly certain is way to rich. I live at 5000 feet elevation and haven't the slightest clue where to begin with tuning this carb. Any advice would be very much appreciated. Thanks

 

Sounds like the vacuum advance plate bushings in the dist are worn. This was an issue many years ago, when vacuum pulled the advance plate around it would ether close the points up or open them to wide preventing them from making good contact. We used to be able to purchase a replacement breaker plate but not sure if it's still available.

 

Could be the vacuum advance diaphragm is shot? Sounds like reconnecting the vacuum advance introduces big vacuum leak then. Destroked 450 sounds like he knows what he's talking about though.

 

I have an electronic ignition system in it. So far I have replaced the ignition control module, cap, rotor, stator, and plug wires. When the vacuum advance is disconnected I can feel vacuum pressure on the carb side. With the cap off I can suck on the vacuum advance line and watch the arm on the advance move, which should mean that the diaphragm is not leaky. Could it be that the edelbrock 1406 is just pulling to much vacuum pressure?

 

Not really, any motor at 5k feet is going to pull less, not more, vacuum anyway and they are designed for every bit of it. I'd suggest investigate Destroked450's advice, especially since the vac advance is holding vacuum as you suggest. Either way, a reman distributor shouldn't break the bank.

 

Thanks Destroked 450 and Tedster9. I'll buy a new distributer and start there. Do either of you know where I need to be with the tuning on an edelbrock 1406? Like I said in my original post I think this thing is jetted way to rich. Not only by what the plugs look like but the numbers on the metering rods put it as 6% rich. From what I gather I need to be around 4% lean at 5000 feet elevation.

 

If you've changed it to electronic I'd do some testing first, what kit did you put in the dist or did you swap in an electronic dist. With the engine idling and the vacuum advance not hooked up to the dist does the engine run ok and can you rev it up? On a stock Ford engine timing called for 6* but I found they ran better if set at 9*, the 12* setting is more for a engine with a hot street or mild race cam. The Edelbrock 1406 has two small vacuum ports on the front of the carb, the dist advance port is the one nearest to the electric choke and should not be pulling vacuum at idle providing idle is not set for more than 800 rpm. Have you wired up the electric choke so it will fully open, if not you need to do that before any more tuning can be done.
Adjusting the two fuel mixture screws in the front of the carb is best done with the choke fully open, the engine warmed up and idle set as low as possible (around 600 rpm). turn each screw in slowly (clockwise) one at a time until engine rpms start to drop then turn it back out (counter clockwise) until rpm increases. Turning the screws 1/4 turn at a time has always worked best for me, once I find the spot where the rpms drop when I turn the screw 1/4 turn I back it up that 1/4 turn and then adjust the other one. Once you have it set so that turning ether screw in 1/4 turn changes the rpm and back it up that 1/4 turn the carb idle fuel mixture is set.
After you have the mixtures screws set adjust the idle to 750 rpm for a manual trans or 900 rpm for an auto trans, then reconnect the vacuum advance and see how it runs. If it cuts out and backfires with the advance reconnected you have a problem with the dist.

 

Thanks for the response Destroked 450. I think the tuning tips will help alot. First off let me give you a little of background about the truck. It is a 75 F250, manual transmission, 390. I purchased the truck several months ago, the pervious owner had made all of the engine changes prior to me purchasing. It does have an electric distributor. When I bought the truck it had no spark, I replaced the stator in the dist. as well as the cap and rotor. Still I had no spark, I then replaced the coil and the ignition control module. Still no spark! Finally I found that the ignition control module was wired in wrong. The previous owner had cut the plugs off the harness and used wire splices, when he did this he had it wired wrong. Once I re-wired it properly it fired right up.


This engine is mostly stock, with only a few modifications. It has the Edelbrock 1406 carb, Edelbrock intake, and headers. It will idle fine with the vac. advance hooked up, but fall on it's face when throttle is applied. With the vac. advance disconnected it idles and revs up. The vac. advance port I'm using is the one on the choke side. I will check the choke setting this evening.


What else could be wrong in the Distributor? Could it be that the tuning on the carb is so bad that it is causing all of this?

 

If it will rev without the advance hooked up I'd say the carb is ok. Is it a Ford Dura Spark or aftermarket dist? Dura Spark dist still use an advance plate like points dist had, if the bushing or plastic support pads are bad it will close or open the air gap just like opening and closing points. Connect a small hand vacuum pump to the hose going to the dist advance and with the engine idling advance the timing by pumping the vacuum pump, see if the engine starts stumbling or dies. You can do this by sucking on the hose but make sure it's long enough to get your face and hands away from moving parts (fan blades, pulley's, belts), also a carb backfire in your face is not a good thing (took a while to grow back the brows and stash).
If advancing the timing this way causes the engine to run bad the problem is in the dist. I have seen on a couple of rare times where the advance unit was moving to far over advancing the timing, if the dist is at fault I recommend just getting a complete reman dist.

Edit: Went back and looked at one of my old tuneup books (1977 edition) Ford started using electronic ignition in 1975 so it's factory on your truck, had it in my mind it was 76 when they changed.

 

Thanks for all of the advice Destroked 450. I will start trouble shooting everything soon. I think I'll start with a re-manufactured distributor. The current distributor and ignition system is a duraspark system. I'll post again as soon as I get a chance to go through everything and hopefully find the issue. Thanks!!

 

I just got the new distributor installed. Set the timing to 10* and it runs pretty good. I hook up the vacuum advance and it will rev fine without any backfires or sputters. When I took it out for a drive it is very doggy and weak. I unhooked the vacuum advance and the problem went away. Any suggestions???


Also while at an idle I can hear what sounds like a miss on the passenger side exhaust. I cant hear a lifter or anything pinging on that side of the engine. Wondering if over the course of the years that this thing sat not running, if a valve was opened and the spring is now wore out.


Pulled the carb apart and checked the jets and metering rods, it all looks good and seems to jive with the Edelbrock reference charts for my elevation. Set the choke and adjusted the mix screws.


Why am I still having issues with the advance???

 

Vacuum advance mechanism is just as much a "no vacuum retard" mechanism. It really only comes into play at part throttle cruising steady state speeds. Mash the go pedal - vacuum advance goes away, until the new speed levels off. When you're loafing down the interstate at say 60 miles per hour, it takes very little throttle to keep it there, and only around 50 horsepower. So the engine vacuum comes back up. The fuel to air becomes very lean. This needs lots of advance.

This vacuum characteristic is used to dial in more ignition timing - above and beyond whatever is already there or on top of the mechanical advance governed by RPM. On a V8 it's actually around 50 degrees total BTDC - initial or "crank" timing, mechanical advance, and finally vacuum. I only point all this out, to explain why the vacuum advance per se isn't the problem here.

There is probably some adjustment required in the initial and mechanical . A lot of people will disconnect and plug it, but vacuum advance adds better driveability at part throttle and cruise, plus about 15% better fuel economy, so you ALWAYS want to use it unless you're a WOT drag racer.

 

Forgot. Vacuum advance is adjustable on both factory and aftermarket units. On the latter an Allen wrench - 3/32nds or 2.5mm. Usually 10 to 15 turns total of the can to work with. Try fully clockwise first and check for ping while at steady cruise on the highway. A very slight rattle now and then is OK, but excessive pre-detonation will nuke a piston or rings. If you notice ping or predetonation from a standing start or acceleration though, now that's a different issue, in this case need to adjust the initial crank timing or the mechanical advance inside the distributor. It's a PITA to get the curve dialed in but it's essential for both performance and economy.

 

So at this point I'm very lost.... I tried adjusting the vacuum advance set screw and it didn't help. Figured I'd unplug the vacuum advance and see how much it would advance when given throttle, this is the mechanical advance right? Timing is at 10* when I give it throttle it is retarding the timing several degrees. I may not be the sharpest tool in the shed, but it shouldn't retard the timing, right??? Any suggestions would be greatly appreciated. Thanks

 

Did they sell you a dist that had two vacuum ports on the advance or just one? I've never seen the mechanical advance retard the timing.