I come today looking for a little bit of reassurance on some timing real tard things with my truck. I have a 1981 F-150 with a 300i6, and have installed a offenhauser c series intake, Holley quick fuel 4 barrel carb, and some efi exhaust manifold and it really woke the truck up. What caused me to do this upgrade was the truck being down due to timing issues, so I decided to wake up the motor a little bit with this.

My question is, at what RPM should mechanical timing advanced “kick in” or start advancing? I have a stumbled when I more then just barely depress the accelerator in the low rpms. I believe this to be accelerator pump related on my carb. But want to be certain. The reason I ask is because I have already put two new distributors in as the one I bought from Napa was bad out of the box with the advance mech locked up. I now have another one in there, just a stock replacement from AutoZone. When I was setting my Timing, I noticed the timing didn’t begin to advance until what I estimate to be around 1500rpm. Being that this truck is what I’m learning everything on, I have no other motors to compare this to.

Could it be a coisidense that this just happens to be about the point in my RPMs that I have a stumble, or could this be timing related. I didn’t mention that the truck runs amazingly above that point of maybe 1500 RPM or so so that’s why I find it strange because it really doesn’t stumble very bad after that when you stab the throttle.

 

First being nothing is stock any more all you can use the stock settings for is a starting place and adjust from there.

As for when it starts you would need to find that information in a book. I also don't know if you can get a spring kit for that dist?

Someone posted about timing advance curves he plotted. Look in here or the 300 motor area for it as it may have so information for you.
I did not read any of the post so can't say what it may have.
Dave ----

 

Are you running it with the vacuum line hooked to the distributor? If you are, disconnect it and then see if your stumble goes away. I am suspecting the timing is suddenly advancing when you open the throttle off idle, and then when you stab the throttle some more, the vacuum in the engine is dropping, and all the timing advance in the distributor is dropping out, causing the stumble. Just a theory that would be easy to try. In my experience, the stock distributors run way too much vacuum advance for a non-stock custom engine setup.

When you are running it hard above 1500 rpm and you say it runs great, your vacuum advance is not doing anything anyway in that scenario, you have very little vacuum in the engine in that mode of operation.

 

Yep, first thing disconnect and plug the vacuum advance port and forget it even exists for a while.

With a remanufactured distributor, who knows what it was cored from - a dump truck or a bus, who knows. I agree most factory stock truck distributors have far too much built in mechanical advance, and usually very slow and "lazy". Sometimes they will still be advancing north of 4000 RPM. In contrast street performance engine builders try to get their distributor timing curve "All In" by about 2500. This is without vacuum advance.

Basically what you're looking for is about as much ignition advance as she will take at all times, under all conditions, just short of engine knock, or ping. This isn't really a performance thing, it's the nature of gasoline engines. You'll have to map out the curve and see what you have, maybe limit the total mechanical, boost the initial timing, install lighter springs etc. The alternative would be to send the distributor off for recurving to someone who has a distributor machine. Take some test drives.

When you get it sorted out satisfactorily, then re-connect the vacuum advance and tune the part throttle portion of the ignition curve. Using a timing light and trying to measure the effect of vacuum advance will confuse a novice because there is absolutely no load on the engine in neutral. He will think it is "too much". Very misunderstood topic.

 

We are on the same page, except there is no real way to tune the amount of vacuum advance unless you hunt up a aftermarket adjustable unit that will fit.

I did have one on a 302 that had a open slot made into the arm of the vacuum advance. This slot was right in the area of the casting of the advance unit. I drilled a hole through the casting, and the hole went right through the slot area of the arm. I then put different size pins through the hole, and this would limit the travel of the vacuum advance arm. My pins were actually different size nails that I cut off to experiment with.

 

Well I disconnected the vacuum advance as suggested and saw no difference. I don’t think I should have either because I have my vacuum advanced hooked up to ported vacuum and not full manifold vacuum so it doesn’t do anything at idle anyway. (Correct me if I’m wrong). I did pull my spark plugs and they did seem to look a little bit black. Being that these plugs have less then 1000 miles on them, I think I can conclude that the motor is running a little rich. Also, when the engine isn’t fully warmed up, but also isn’t cold, if I rev it a little bit and then as the RPMs are dropping blip it again, I do get a little bit of smoke out of the tailpipe. From what I understand this could mean my accelerator pump on the carb is delivering too much fuel. Can anyone confirm that my hunch is logical?

 

Ported vacuum would give timing advance as soon as the throttle came off idle.
But if you did unplug it from vacuum and plugged the line so no leak and it did the same thing then we need to look else where.

Any tuning has to be with the motor up to temp. Anything below that is just a waste of time.
Now the carb could be too rich and then you give more throttle and that is dumping more fuel in you could have a rich bog not a lean bog.

Now carbs had different tuning parts and you have to break down and tune each part, idle / mid or tip in / high speed.
What carb are you running?
Dave ----

 

No, there won't be any vacuum advance at factory idle RPM. But what we're doing first, is checking the mechanical advance, the centrifugal advance. Just as soon as the idle RPM picks up past factory RPM, it starts pulling in more advance, there's some misconceptions about this. Most people like a higher idle RPM, because ... vacuum advance increases the advance, and it smooths out quite a bit.

To check the centrifugal advance, we have to see what it is doing above idle, and all the way up to a few thousand RPM with a timing light without the vacuum advance connected.

It helps to have flourescent timing tape applied to the balancer, or highlight the marks with a china marker etc. Most any OHV engine will be in the mid 30s when spooled up without vacuum advance. What we want to check for is that the mechanical advance operates smoothly (and quickly) up to where it should, and back down down again.

 

I recommend quick an dirty testing to narrow down what's causing it. You disconnected the vacuum advance, no change, so we know that is not causing it.

If you are still concerned about the timing and the timing curves, I would take a timing light and check where you are at now, and then just turn the dist advanced a little bit and then try it retarded. If this makes any difference with your problem, I would dig deeper in the timing. Otherwise I would turn toward the carb.

If you want to do some experimenting with the carb, and you think it's rich, you can turn the idle mixture screw in as far as you can and it still runs, and then try that. You said it happens around 1500 rpm, if that is a estimate, that is very close to off-idle for the carb circuits. The idle mixture screw still may have some affect at that lower rpm.

Just quick rough messing around to try to narrow the problem down.

 

I’m really starting to think that this isn’t a timing related issue because I had spent a lot of time trying to get everything set properly a month or so ago. I also have a friend who isn’t a ford guy, but knows a lot about older trucks, who tells me with the vacuum advance disconnected, I shouldn’t have timing advance right off of idle. It should take a few hundred RPMs before it starts to advance. I don’t know how accurate this is, but based on how well all his vehicles run, I’ll take his word. LOL.

The carb I am running is a Holley Quick Fuel Slayer series 450 CFM Vacuum secondary carb. I made a phone call to Holley and talked to one of their techs and described my problems and they tell me it’s sounds like I have an undersized power valve. I’m pulling about 19 inches of Vacuum at idle, but the carb comes with a 6.5 power valve.

I also noticed that my issue is actually sustained throughout the RPMs. I typically drive the truck fairly easy and don’t take it over about half throttle or rev it out much passed 2500 RPM or so with my day to day driving, but I made some adjustments to the carb prior to my call to Holley and decided to go out for a ride and run the truck hard in various situations. Full throttle from down low, full throttle from light acceleration, slowly rolling into the throttle, ect.

My observations were once above about 1500 rpm or so, if I rolled into the throttle it ran great, unless I put the pedal completely to the mat, then it would bog slightly like it was starving for more fuel. Also, if I really aggressively stepped on it even to 3/4 or so it bogged and then took off. This happened throughout the RPM ranges. Hopefully I’ll have the power valve early next week, but I’m not sure if Holley works the Friday after thanksgiving, so it may not ship until Monday.

 

This 450 carb I take it has vacuum secondaries?
In stall the heaves spring so they don't open and tune the timing & carb to run as a v2 carb first.
Once that is running properly then sneak up on the secondaries opening with a little lighter spring and see how it runs then a little lighter till you get the bog and go back to the heavier that had no bog.
I think you have the secondaries opening to fast causing a lean bog, no pump shot on vacuum secondaries and why you have to tune it as above.
Dave ----

 

Yes it is a Vacuum secondary carb. If the power valve doesn’t completely fix my issue, I will definitely give this a shot next.

 

Do it before the PV swap as the heavy spring will keep the secondaries from opening and may be the bog is gone?
You still need them closed to do any carb tuning.
Again you tune the carb & timing with the carb running as a v2 carb not the v4 you have now.
Once running good as a v2 then you can work the v4 side.
Dave ----

 

That's not how it works. It's called "vacuum advance". Your vacuum is highest at idle and low rpms. The more you open the carb up, the less vacuum you have, so the less vacuum advance you have. When you use ported vacuum for the advance hose, the port is actually being covered up at idle, so there is no vacuum at idle when using a ported vacuum port. As soon as you move the throttle open just a llittle bit, that port is uncovered and all the manifold vacuum is sent to the distributor.

So with the vacuum advance connected, your timing is being pushed and pulled all over the place depending on the throttle position and the rpm. The weights inside the distributor are straight advance depending in rpm. But the vacuum is adding or subtracting to this all the time depending on the load on the engine. When you are having weird problems, it's good to take it out of the picture temporarily and just use the straight rpm advance from the weights. And you can add or subract from the weight timing (centrifugal) by turning the dist advance or retard a little bit for experimentation purposes.

 

@Reel News OCMD Have you disconnected and plugged the vacuum advance and taken a test drive as suggested? It's so simple a test, maybe that's why nobody can be bothered to try it, I dunno.

I'd encourage anyone interested in engine tuning and distributor timing and power valves and the rest of it to connect a large face vacuum gauge to the manifold and run it into the cabin so you can see it, and what manifold vacuum level is actually achieved under different driving conditions.

Manifold vacuum does drop out instantly under load, esp. at anything just above idle when accelerating. Anytime the go pedal is pressed. But if you watch the gauge, it also comes back just as instantly. It is constantly making wild swings except at steady cruise.

The engine cannot possibly know or care where the ignition timing advance or retard comes from. It is what it is.

Mechanical distributor advance is strictly RPM based. Vacuum advance is strictly load based. They more or less work at cross purposes to each other most of the time. The mechanic doesn't need to necessarily get in there and recurve the distributor, but he does need to verify that it is working correctly according to minimum specs throughout the operating range under all conditions.

Your friend knows how to tune a distributor for performance, but this is not by any stretch a common skill. A lot of people just want to argue over the internet about their pet theories on vacuum advance compared to actually getting out there and directly observing how it really works. Then you will actually understand it, and won't need to ask anybody.