1986 F150 with 4.9L and SROD - [history following]

Due to state of disrepair when purchased, feedback carburetor and distributor replaced with "junkyard" 1982 units. The EGR was removed, but catalytic convertors were left in place. A MSD 6A ignition box was available and was used as a "stand alone" ignition box. The spark plugs were gapped @ .050".

The engine fired up immediately and timing was set. Using the vacuum method, initially timing was set at 19.5" of vacuum [maximum vacuum was 20.5"]. When the vacuum line was reattached to the distributor, everything seemed fine at idle and reving the engine. However, just as soon as I tried to drive, it bucked viciously. With the truck parked, I raised the engine speed to ~1700 - 2000 RPMs [steady] and the engine would misfire, almost as if a rev limiter was kicking in. Only at steady RPMs would this occur. Even if missing and then reved, the missing would dissapear.

Next, I placed a restrictor in the distributor vacuum line, but this did not eliminate the steady state missing.

I capped the vacuum port and drove without vacuum advance and had good overall results, but knew [from reading] that I was not getting maximum fuel economy. Everything is relative and I was actually getting okay mileage. This has been reported in an earlier thread.

After some time, I began to have variations in idle speed and discovered that the throttle shaft was worn. I purchased a remanufactured Carter YF from a 1970 F350 and decided to replace the distributor with a new Duraspark unit from Performance Distributors in Memphis. The distributor was "recurved" to take into account not having EGR.

I was in hope that my vacuum advance situation would be corrected, but it was essentially the same. I am very pleased with the setup other than that I cannot use vacuum advance. Overall, the truck starts, runs and idles great. I have previously posted gas mileage results and routinely get ~17 mpg back and forth to work and 22+ on the highway @ 55-60 mph.

This weekend, I checked the vacuum and the maximum is 20.5" @ ~650 RPMs. With the recurved replacement distributor, I have been driving with vacuum @ 19.5" and have no pinging. I experimented by retarding to 18.5" and tested the vacuum advance at steady raised RPMs and again the missing occured. I intentionally retarded timing to 14 -15" vacuum [engine would hardly idle] and after attaching vacuum advance, tried again and still the missing occured, however it seemed less violent. So, I reset the vacuum to where I have had good overall results.

I suspect the problem is that with vacuum advance, the timing is being advanced too far. Some will say to adjust the advance cannister, but I wonder if enough adjustment is available given the fact that I experimented by retarding an extreme amount and still the cutting out would occur. Also, as I think back, this problem existed with two different distributors and two different carburetors.

Is it possible that this has to do with using the MSD 6A box with its multi-spark discharge, since it was being used from the beginning?

One final observation is that where the 1982 Carter had no vacuum at the port until the throttle plate was moved, the 1970 version draws 5" at idle and quickly increases as throttle is opened.

I would appreciate your input as although I am overall very pleased with the performance of my truck, I can't help but think that I am missing untapped potential.

By the way, all ignition related components are in good shape. I am looking forward to your combined input. Thanks!

 

I am surely no expert on this, but it has always been my understanding that your vacuum advance should pull NO vacuum at idle, and then increase as you open the throttle. I am not sure about the msd box though.

 

Did the guys you bought the distributor from tell/advertise how much centrifugal advance the distributor is capable of adding and did they advertise the advance as being "all-in" by a certain rpm?

I took my factory Duraspark II distributor appart and the slot being used on the reluctor arm was "21L" meaning that it was capable of adding 42 degrees of ignition timing on top of the 10 degrees base timing. The vacuum advance arm was stamped "14", therefore being able to add up to 28 degrees of timing at the crank.

The factory curves use heavy springs, so you would never see that much timing.. unless lighter springs were used without limiting the slot or changing it.

 

I have never heard of timing a engine with vacuum. I have heard of tuning the mixture using this method, but I believe your method of timing it is causing the problem. I believe you are correct, the timing is advanced too much. You must have the stock vacuum advance also. If you want to use that, you are going to have to keep the timing down in the single digits like the factory had it set. If you are going to have a custom timing setting, then you will have to get an adjustable vacuum advance.

1st thing to do; Get a timing light.

 

The 300-6 had several types of vacuum advance.

One where it used ported vacuum at the carb and had 0 at idle, and increases with. This is the most comon.

Another used ported and manifold vacuum. The vacuum advance diaphram would have two ports.

And yet another uses semi-ported vacuum that has vacuum at idle, and the vacuum increases with engine RPM. The vacuum is retarded by a EGR Solenoid. My 1981 uses this setup.

Make sure what you got at the vacuum advance nipple at the carb...

I do believe your problem is with either your timing method, as the 300-6 has a timing range of 6 degrees to 14 degrees BTDC and/or the MSD box, and/or with the spark plugs gapped at .050. The correct spark gap is around .044 depending on engine calibration.

To adjust the timing by the book, you need a timing light and adjust the engine to the timing marks. That adjustment may or may not be at highest vacuum as with using the vacuum method. The cutting out you are getting at lower timing, may be lean misfire and the carb needs to be richened up, or the plugs gapped too wide.

However, since you are getting 17-22 MPG, you are right there at the nominal MPGs for this engine That MPGs is about what you would get with the DS-II ignition and working advance with the spark plugs at .044... I doubt you will get anymore MPG than that, infact swapping over or changing things may make it worse.

I'd leave it alone really, especially if there is no predetonation...

 

Factory timing settings are generally set with a "safe for everyone" mentality.
Setting timing with a vacuum gauge takes into account your engine, your elevation, your weather, temperature, atmospheric pressure, gas grades, etc. etc. etc.

However, when it's all done, it's still good to check it with a timing light to see where it's at.

I would recommend mapping the timing curve of your engine. You'll need to do four things.

First, you'll need to know what RPMs the engine is at. If you don't have a tach, you'll need something you can hook to the engine that'll tell you.
Second, get a timing light.
Third, get a hand-held vacuum pump (like a little Mighty Vac or something).
Finally, add extra timing marks to your harmonic balancer so you can see timing all the way up to 60°. Unfortunately, timing tape doesn't fit onto the small lip of the 300's harmonic balancer, so you'll need to do it another way.

I did it by turning the engine until the timing mark lined up exactly with the 0 mark. Then, I used the file to put another mark on the harmonic balancer at the 10 line.
Rotated the engine again until my new 10 nick was lined up with the 0 line, and used my file to put another nick at the 10 line. Basically, I was using the timing marks to accurately mark every 10 degrees. Worked really well, and was really simple. Then, filled each nick with chalk. If you have different colored chalk it's even better.


Now, to map your engine.
Disconnect the vacuum advance and cap it off at the carb/intake.
Start the engine and bring it up to temp.
Write down on a piece of paper your RPM range skipping every 250 RPMs, like:
750
1000
1250
1500
etc...

Then, use your idle RPM screw to slowly bring the RPMs up. Check your timing every 250 RPMs and write it down. Mine went something like this:

750 10°
1000 10°
1250 12°
1500 17°
1750 22°
etc.

Eventually, the mechanical advance in the distributor will hit it's limiter and won't advance anymore. After you've hit that, you're done. Bring your RPMs back down to normal.


Next, hook your hand pump up to a vacuum line running directly to the vacuum advance.
On your piece of paper, write 0 through 15 (hg of vacuum).
Slowly pump up the vacuum pump 1hg at a time and note where the timing is at each vacuum level and write it down minus your base timing. Go until the vacuum advance canister maxes out.
Mine was something like this:
0 0°
1 0°
2 0°
3 0°
4 1°
5 4°
6 8°
etc.

That's how much timing your vacuum advance adds while you're driving. If you have a huge spike at the beginning (my old canister jumped up 10° in timing between 2 and 3 hg. Not good!!)


Now you know exactly what timing your engine is at at any given RPM and condition. Drive with a vacuum gauge in the cab and see when the engine starts to buck. What RPM is it at? What vacuum is the engine at? Look at your chart and see what timing that'd be at. For example, if you're at 1750 RPMs and your hg is 6, you know you have 30° timing (22 + 8 from the example above).


However, after all that, I would definitely agree that if you like the way the truck drives AND you're getting 17 to 22mpg without the vacuum advance, there's no point in hooking it back up!

 

Wow. Thanks, AbandonedBronco!

I remember you detailing doing this exercise, previously, on your Broncos, but now it makes more sense to me.

One final question and if reread, the original post details this, what is the likelyhood that two different carburetor and distributor setups would react the same? And, on the original Duraspark 2 conversion, the timing was set by timing light @ 10 BTDC.

Thanks again, all who have shared your muchly appreciated wisdom, knowledge and experience!