1986 F150 with 4.9L engine converted to Duraspark with MSD 6A instead of normal ignition module. Fuel injected specification [long reach] copper Motorcraft plugs gapped @ .050". Feedback carburetor [Carter] was replaced with one from 1980; 1982 distributor using ported vacuum. This combination ran well and returned good gas mileage. Timing set by using vacuum gauge [20.5" max@ idle; set @ 18.5"].

One day, I tried using manifold vacuum and as soon as I attached the vacuum hose to the distributor, the engine ran horribly, so I went back to ported and all was okay, but I did wonder what caused that?

Later, I chose to replace the 1980 carburetor [worn throttle shaft/housing] with one from a 1970 F350 [Carter YF] to gain the pre-emmission calibration. Again, the distributor was hooked to ported vacuum and the idle was great, but when I reved the engine and held it steady, it began to miss badly as if a rev limiter was kicking in. It would rev well until allowed to level out and certainly could not be driven [I tried]. I blocked off the vacuum port and the truck ran well and returned good mileage figures, but still I was running without the benefit of vacuum advance.

I had toyed with the idea of having a distributor recurved since I no longer have EGR and all the time thinking the vacuum diaphram was the culprit. Yesterday, the new distributor was installed and initial timing set @ 12 degrees BTDC. It sounded great. Vacuum advance was hooked up and the idle was smooth. When reved and held steady @ 1700 RPM, the missing again was heard, although not quite as bad as I remembered from the previous stock distributor. As soon as the vacuum was removed and port sealed, smooth running under all conditions.

Watching the timing light, the timing was all over the place while the engine was missing, yet was very steady and progressed as expected when the distributor was not hooked up to vacuum.

Unfortunately, I did not have my vacuum gauge available, but watched the arm on the vacuum canister and saw no irregular or jerky movement during the missing phase, so I do not believe the missing is vacuum generated.

Since this phenonomen has occured with two carburetors and now two distributors, I feel that it is ignition related.

My friend asked if I was sure that the 1st carburetor actually had vacuum at the port? Of course one expects it to, but I cannot recall actually checking it and do remember that the brass tube would come out of the carburetor housing if pulled, so it is quite possible that it did not have vacuum.

If true, that there was no ported vacuum on the 1st carburetor, then this problem might have been there since the ignition MSD was installed [remember the problem when I hooked to manifold vacuum; 2nd paragraph].

I do not understand fully what the MSD or standard ignition module does and so cannot grasp what could cause this condition which occurs at high vacuum when the vacuum canister adds additional advance beyond what the initial and centrifugal attain [remember, even @ idle speeds when on manifold vacuum].

Again, the engine runs great at all times as long as the vacuum advance is not in the picture. I am reasonably sure there are no vacuum leaks.

I eagerly await some collective insight!!

 

Using manifold vacuum to the vac advance will cause the timing to advance at idle, which explains the symptom occuring at idle.
As for the missing/misfiring symptom itself, could be the nature of the beast. MSD stands for "Multiple Spark Discharge". It fires the coil several times per cyl, on every combustion cycle, rather than just once. A weak coil, or one that is not quite capable of dealing with the MSD system, can cause a lot of issues. Same goes for a faulty MSD box.

 

Your problem is simple, I have had the same problem before when hopping up a stock engine. The vacuum advance is advancing the timing too much. You can set the timing back to about 2-4 degrees, it will run pretty doggy as far as throttle response, but I bet your problem will go away and you can leave the advance hooked up.

What you really need is a adjustable vacuum advance. On the engine I was experimenting with, I made my own (kind of a rig but it worked). I would set my timing around 12 to 14 degrees initial, and had the same problem as you. I took the stock advance off, and noticed the arm as it went from the diaphragm area out, had a open rectangle slot in the middle. What I did was drill a hole through the cast white metal on the advance housing, so the drill bit went through the top part of the housing, through the slot in the arm, and then drilled through the white metal underneath.

What I found by doing this, I could take different sized nails or cotter pins, and slip them through the holes I drilled, and the different diameters would change the total travel of the advance arm. That way I could fudge how much timing the advance actually added in, and it worked pretty good.

 

Thank you, Rogue_Wulff and Franklin2 for your responses!

If the timing is being advanced too far, as it seems to be, help me understand this [and I know my figures will not be exact or absolute]... initial timing set at 10-14 degrees BTDC + additional centrifugal advance [?, maybe 20-24 degrees] equals a total of 30-38 degrees when the vacuum advance is disabled. With that combination, the engine runs without a hint of missing.

At idle speeds with the same initial timing of 10-14 degrees BTDC + vacuum advance hooked to manifold vacuum and the engine misfires immediately. This would not involve centrifugal advance, so does the vacuum advance take the total timing to greater than the 30-38 degrees contributed by the initial and centrifugal described in the previous paragraph?

 

Most vacuum advance units have a number stamped on the arm, and that number is the degrees of advance at the dizzy, so double it for crank timing. Anyway, there are advances that have 12/24 degrees, which when added to the initial timing gets to the range of missing on your engine.

 

Yes, you do not figure the vacuum advance into the "total" advance calculations. You only use the initial and the centrifugal for the calculation. The vacuum advance adds timing over and above the calculated advance.

With a light load cruising down the highway, you may be running upwards of 50 degrees advance with the vacuum advance connected. The engine can stand this, and run more efficiently, because there is hardly any load on it. As soon as you put a load on the engine, you are going to push the pedal down, that's going to open the butterflies in the carb up, and the vacuum is going to drop. This will let the advance unit diaphragm relax and retard the timing back to the initial + centrifugal.

You can experiment also with the port or manifold vacuum. I have found I cannot get most engines to run correctly with the manifold vacuum, especially automatic tranny vehicles. With the manifold vacuum, the idle goes up way high and you have to reset the carb and it's adjustments. Usually when I do this with a automatic, when I go to put it in drive it wants to stall. If I turn the idle up high enough while it's in drive, then in park the idle is way to high and it wants to "bang" when you put it back in drive.

 

Amen! I agree completely on manifold vs ported vacuum. I get far better results with ported, especially on automatics, than with manifold. With ported I get the same idle every time, and almost the same speed in gear as in neutral.

 

Your Plugs are gapped too wide. This has a effect of changing your timing as well.

Stock gap for the plugs is .044 optimum. service range .042 min /.046 max

Also your initial base timing is too great for the stock timing curve of the distributor. It's advancing too much. 10 degrees BTDC is standard for the 300. There are exceptions. My 300 in my truck has to be set at 6 degrees BTDC.

The 1970 carb the ported vacuum port is designed for breaker type ignition systems. The spark is weaker and not as quick with points and condenser type systems, so the plugs are gapped narrower, and the engine advances more to make up the shortcomings. The breakerless Duraspark system has a stronger faster spark, therefore the gaps are larger and the advance curve isn't as high as consequence.

Put a 1980 carb exactly like the one you had and those problems should go away.

Not sure about the MSD ignition, but that can throw something in the mix, I'm not aware of.

If you want to run your engine with the 70's carb, and the plug gap at .050, recurving the distributor is the only real way you are going to accomplish this.

EDIT: Ford used Vacuum Restricters and Vacuum Delay Valves in the advance line. One of these may also solve your problem.

 

Thanks again, guys!

I will regap the plugs and try that. The opened gap was suggested with using the MSD.

Interesting information, 81-F-150-Explorer re: the 1970 carburetor. The ported vacuum port has ~5" Hg @ idle, where as the 1980 carburetor had 0" Hg @ idle.

I did try a vacuum restrictor without success.

Thanks for the very good suggestions, all of you! Have a nice weekend!

 

I agree that the dist is not designed for the high initial advance. That's his main problem, he has modified the engine, and is tuning it by feel and ear, which is all you can do when changing things around. For a long time I ran my engine around 14 degrees initial, and just left the vacuum advance disconnected. It ran so much better like this. The factory runs little initial, since they want the carb to have to be opened up to get a decent idle. This adds air, which is all for emissions.

You can run wider gaps on your plugs, but you just have to change them more often. Plug gaps are a compromise and the factory spec gives a good spark and long life. A narrow gap gives a lousy spark but very long life, and wide gap gives a nice spark but a short life before it will start miss-firing. Wide gaps also put more strain on the wires and the ignition system, but the MSD is a hotter system so it's designed for it. Of course they recommend the expensive wires to go along with all this.

 

What cap and rotor combination are you using? Ford went to a large diameter cap to eliminate cross firing at high advance settings. Just for grins, stick a DurasparkII box on it and see if it runs better. Some Ford and aftermarket vacuum advance units have an adjustment screw (1/8" allen) in the diaphram. These usually have a hex shape to the diaphram housing at the vacuum nipple. I have never been a big fan of MSD systems due to the multiple sparks, they're hard to time and will cross fire at high levels of advance. If the Duraspark works, try cutting the end of the rotor to a point, rather than the wide tip tha Ford used. We used to do this on the older small caps and almost all Chryslers.