First off, our patient: my 1980 F-150, stock 300 engine.

Now, my problem: Over the last couple of months, I've noticed the quality of the idle at stoplights declining. Last week it actually stalled on me sitting at a light. This is Not Acceptable, so I decided to start tuning things up. Easy stuff first...it had been a while since the last refresh, so I replaced the distributor cap (which had some pitting inside), rotor, spark plugs (Motorcraft BSF-42 as specified, gapped to spec), and plug wires. No change in behavior.

Yesterday I decided to check the ignition timing. This is the 300 engine, and so has two sets of timing indicators. I found two V-notches in the pulley, and marked both with a paint pen. Putting the timing light on, I saw that one of them was pretty close to 10* BTDC on the passenger side indicator (the sawtooth thing bolted to the timing cover), as the emissions label says it should be. Engine vacuum at idle was 18"-19", so I am assuming that the correct notch on the pulley is being used.

Since I already had the vacuum gauge hooked up, I decided to see if I could use that to tune it a little better. After a bit of fiddling with the distributor, I settled on about 20" as a better reading, and the idle did seem to be a bit smoother. I tightened the distributor back down, and went to the timing light again.

This time, the indicated timing was at 12*. OK, that's pretty close, good enough for me. This is when things start to get weird. I reattached the vacuum advance line to the port on the carburetor, and the idle roughened significantly. I had failed to notice earlier, but the idle had in fact smoothed out a bit when I first disconnected the line. Weird observation #2: after uncapping the carburetor port, but before putting the line on (that is, with an active vacuum leak) the idle remained pretty smooth. I would expect this to cause some kind of problem with idle, if everything is adjusted correctly.

Weird observation #3: Sucking on the vacuum line (I don't have a vacuum pump) produced no observable change in idle speed or timing. It didn't feel like I was getting air through the line, but it's pretty hard to tell. Weird observation #4: with the line reattached to the carburetor, revving the engine produced no significant change in timing, as indicated by the light. The mark seemed to jitter a bit, but remained centered around 12* BTDC.

Weird observation #5: Leaving the vacuum advance disconnected, and with the carburetor port capped, I took the truck for a test drive. Stoplight idle was PERFECT. Even turning the A/C on produced no noticeable change in idle performance - and running the A/C was what caused the truck to stall at the light previously. Even getting on the highway, the truck drove as it always had.

I realize the above is pretty long, so let me summarize my questions here:

1. Am I using the correct mark on the pulley to measure timing? I am assuming using the wrong mark would throw things off significantly.

2. Why did the idle not roughen when I had the vacuum port uncapped? Does this imply that my carburetor is set too rich?

3. Why *did* the idle roughen when I attached the advance line? As I understand it, the usual failure mode is for the diaphragm in the advance module to fail, resulting in a vacuum leak. But an actual vacuum leak produced no problem, so what is going on here? Is my advance module shot?

4. Even if the vacuum advance module is shot, should not revving the engine have caused the timing to advance, due to the mechanical advance system in the distributor? If so, does this *also* suggest that my mechanical advance is shot as well?

5. Driving the truck with the vacuum advance disconnected, should not my highway performance have been degraded (loss of power, etc)? Does this mean that I may have been driving the truck the last five years with significant problems in ignition timing, and have never experienced proper performance?

6. Does the picture below show the correct port (with the blue cap on it) to connect the vacuum advance to? Remember, this is a Carter YFA carburetor:






As I said in the subject line...school me, please!

 

Hi Dorsai,

I have a few suggestions.

1) Yes that blue cap looks like the correct vacuum port for the vacuum advance line. It is the lowest port on the passenger side of the carb (the only port below the throttle plate). It should have a low vacuum present at idle (around 7" of Hg), but increase with RPM up to the maximum vacuum (~21"" of Hg).

2. It is odd that your timing did not advance with RPM. That suggests to me that your centrifugal advance (weights and springs in the distributor) is not functioning. Timing as observed with the timing light should advance significantly with increasing RPMs.

3. You can check the vacuum advance module manually. Look at where the vacuum advance module connects to the distributor. There is a visible flat-edged metal rod/actuator that extends from the advance vacuum canister into the distributor. If you suck on the vacuum line, that rod should visibly move.

 

Additionally, in both F150 4.9L's that I have worked on, the timing mark on the pulley existed as a faint stamped line on the pulley flat edge, NOT a "v-notch". I have seen those V-notches, but I don't know what they are for.

This timing mark stamped line on the pulley flat-edge can be faint. To find mine, I had to remove spark plug #1 and turn the engine by hand to find TDC of the compression stroke. That is actually easier to do than it might sound. Once the mark is found, paint it white!

 

Does the engine need to be running, or can I do this with the engine off?

I read that in a number of places...but I spent a *lot* of time bumping the engine and inspecting the pulley, and never found anything that looked like a line. Ford seems to have changed the timing indicators several times over the years, and sources weren't always clear about which year used what marks.

If my timing were dramatically off, as one would expect from using the wrong mark, would the engine run as well as it does? It starts easily, idles (mostly) smoothly, holds 19"-20" vacuum at idle, and is perfectly driveable around town and on the highway. MPG averages 14-15...which isn't fantastic for a 300, but seems to be on the low end of average.

 

If the engine vacuum is indicating 19" or 20" of Hg at factory idle the ignition timing is very close to optimum. This is a sea level figure too. Subtract approx. 1" for every 1000' ASL for a healthy stock engine as well.

Disconnect and plug the vacuum advance, the distributor mechanical advance can be observed with a timing light from just above idle RPM all the way up to around 3000 or 4000 RPM. This is why they are marked out to 40 or 50 degrees BTDC. Be careful, stay out of the plane of the fan blades in case it decides to let go.

Usually, a "spark" or timed carburetor port will have no vacuum at all at factory spec idle RPM. If the idle is set too high though, this will confuse things and is probably why all the directions ever provided on setting ignition timing always specify to disconnect and plug vacuum advance. Recall that vacuum advance itself is strictly load dependent, and is completely distinct and separate from mechanical advance utilizing only engine RPM. They complement each other nicely. Real ingenious. Today of course everything is electronically or computer controlled.

Vacuum advance helps with overall drivability and part-throttle acceleration and is mainly a factor at steady cruise. Whenever the accelerator is mashed the vacuum advance goes away, until the speed (vacuum) levels off. It adds about 15 per cent to fuel mileage so unless you're drag racing at full-throttle all the time, you definitely want it hooked up and working correctly.

Find TDC and install fluorescent timing tape. This is a nice little "upgrade" that makes it easier on the tired eyes. All the speed shops have it like JEGS and Speedy Bill and SUMMIT, etc.

 

Hmmm, I have only tried it with the engine running - my system produces around 7" of vacuum at warm idle from the vacuum port I use for vacuum advance. So when I hook this vacuum line back up to the vacuum advance canister with engine running (after checking the base timing), I can see the vacuum advance actuator arm that is external to the distributor move in response to vacuum being applied to it. And my timing advances a little bit accordingly.

I do agree that you can't be too far off with your general vacuum reading.

 

Me, with the engine off I'd pop off the distributor cap, remove the rotor (so I can see the electronic pick up), attach a long piece of vacuum hose to the vacuum can on the distributor, and suck on it.

#1 it must hold vacuum. If it doesn't then the diaphragm is shot and needs to be replaced.

#2 the pick up plate should move several degrees. If it doesn't move it might be stuck and require lubrication. The worlds smallest snap ring holds the vacuum advance arm to the pick up plate and if you remove it you can disconnect the arm from the plate to see how freely it moves. It should be pretty free.

If all that checks out then put the rotor back on the dizzy and give it a little twist to check your mechanical advance and springs. It should rotate a few degrees and then snap back when released. IIRC you can oil the mechanical advance with a couple drops of oil in the very center of the distributor shaft, under the rotor. There might be a little felt puck covering the center of the shaft, lift it up, oil, set it back down.

Ideally what you want at light throttle cruise (50 or 60 mph on a flat road in high gear) is for your EGR to be open and your vacuum fully advanced. This is how these trucks were set up for best mileage.

I've heard of people using lighter springs to bring in the mechanical advance sooner but IMO that's not a good idea for a working truck. Mechanical advance all in by 2,500 RPM will likely cause it to ping, especially if it's loaded.

 

I put my vacuum gauge on the port at the carburetor a little while ago, and only saw 3" of vacuum at warm idle. I don't know if this means anything or not.

Also, when I sucked on the vacuum line attached to the vacuum advance module, I could see the actuator arm move a bit. So just for fun I got a screwdriver and used it to push the arm pretty far back along its path of travel, and the engine started to stumble. Of course, I couldn't observe the timing while I was doing this. So I know that vacuum will move the arm, and I know that moving the arm enough will do something to timing...I just don't (yet) know how much it is moving operationally, and when.

 

Now this is excellent troubleshooting advice, thank you! I will try this as soon as I have free time this week - probably Tuesday or Wednesday.

 

Don't discount the possibility that the distributor is simply worn out. When the mechanicals get sloppy, so does the timing, though this was a bigger problem with mechanical points ignition. Distributors usually aren't outrageously expensive either at least for a rebuilt example with core exchange.

 

It means nothing, my idle speed may be higher than yours, causing slightly higher vacuum advance at idle. My vacuum advance is adjusted to near maximum, and the vacuum response to RPMs isn't linear.

Some simple suggested troubleshooting, which overlaps other advice given here.

1. With warm engine running at idle, vacuum advance hose disconnected and plugged, and timing light hooked up, check base timing advance. It should be steady at 10-12 degrees BTC or so.

2. Observe timing advance while increasing RPMs by hand using the throttle lever at the front of the carb. Timing should advance significantly with RPMs. If not, there is likely a problem with the mechanical advance mechanism (it could be sticking).

3. Return to idle, and connect the vacuum advance tube to the vacuum advance dashpot. If there is vacuum present, timing should advance beyond base setting a little bit.

4. Disconnect vacuum advance tube from vacuum advance dashpot and connect this tube to your vacuum gauge. Increase RPMs using the throttle lever in front of the carb. Vacuum should increase with RPMs.

5. Hook vacuum advance tube back up to the vacuum advance dashpot. Observing with timing light, goose the RPMs up and down using the throttle lever. Timing advance should increase significantly more than it did with the vacuum advance disconnected (since both mechanical and vacuum advance should be operating in tandem). In fact, timing advance should increase almost off the bottom (lower edge) of the sawtooth scale at higher RPMs.

 

What's the usual life expectancy of a distributor? Assuming mine is original, the engine only has about 127,000 miles on it, and has never been driven or worked hard.

 

That's a good question, one that I'm not too sure about. As a practical matter, probably something over 100k though, means it's time for a replacement or at least inspection, cleaning, lube etc. But then, how long is an old school V8 good for?

Distributors get both gummed up and rusty/corroded in different sections. Bushings tend to wear quick once they start. This was an issue with point systems, the distributor shaft starts to wobble imperceptibly and makes setting a consistent point gap (dwell) impossible.

Lots of people often comment how much better their engine runs after installing an electronic ignition module in their distributor in lieu of points, for example. I think this is partly or even mostly a result of the distributor itself being worn out as a solid state unit and magnet is a lot more forgiving of any slop than mechanical points. A properly set point system and distributor works just fine at reasonable RPM levels anyway. For a while at least.

 

With one thing and another going on, I never did get around to working on this further, until this past weekend. As it turned out, it seemed that the mechanical advance was seized up...following the advice above, I was able to get it broken loose and lubricated a bit. I don't know if I have the full range of motion in it or not, but it should be working somewhat.

Also, I replaced the vacuum line from the vacuum advance module to the carburetor; it was the original Carter-era line, and was extremely brittle. I don't know for certain there was a leak, but I do know there isn't one now.

Finally, I went through the timing process again, and took the truck out on the road. Results? My idle is pretty good now, and I seem to have a bit more power on the highway:




Previously the fastest my truck has ever gone was 75mph, and that only on a downslope with a tail wind. This time I had it up to 85mph on level ground, plus was able to maintain highway speeds on hills. Definite improvement there.

Assuming my gas mileage holds up, I'm counting this as a win. Thanks all for the good advice.

 

Good deal. Many (most?) Ford distributors have a felt disc underneath the rotor at the center of the distributor shaft, this should be saturated with a drop of engine oil occasionally. I believe it's supposed to keep the breaker plate moving smoothly.

Can kind of see the whole distributor in operation when using a timing light - as engine is revved up, and back down, the light and pointer indications should move smooth, crisp and "lock in" without scattering too much or wandering. Sticking or wallered out distributor components will cause poor idle and it just gets worse from there on out.