I had that problem with my 351W when I first got it! had too much advance on tip on with light throttle taking off slowly. New reman distributor, with adjustable Motorcraft advance pot. Backed it off a little and it's been fine for 2.5 years now.

 

Ok, so to clarify, yes the bucking was as soon as the throttle was applied.

Heres what I've done...

Vac Advance disconnected and plugged, timmed at 10 degrees advanced. Vac gauge in cab reads 20hg. 650 RPM smooth idle, smooth revs with a crack of the throttle. Drivable, however not much pull. I had more with the other carb. no ping.

Again, with the advance hooked up, its undrivable. If I attempt to hook up the advance to manifold vac at the tree, pull off plug, engine idle increases. before I can even get the hose pushed on, shes choked herself out.

So obviously to much advance to soon.

Dizzy has roughly 2500 miles on it. Do I have to pull it from the motor to check the wires?

 

The issue with pulling the cap on the tree and having the engine die has nothing to do with timing. You haven't even gotten the advance hooked up then. It says your air/fuel ratio is so lean that the introduction of a vacuum leak makes it too lean to run the engine.

I don't think you have an ignition problem, but an air/fuel ratio problem. Perhaps the float level in the new carb is too low. Perhaps you have a bad vacuum leak that is leaning out the AFR. But whatever it is you need to work on the fuel system, not ignition.

 

your misunderstanding.

the engine doesnt die when I pull the plug.

it dies as im plugging in the vac advance.

 

I could probably drive the truck all day with the plug removed.

 

I'm just reading what you type. I cannot infer things, all I can do is to read exactly what you say. And you said "If I attempt to hook up the advance to manifold vac at the tree, pull off plug, engine idle increases. before I can even get the hose pushed on, shes choked herself out." So that says to me that the vacuum advance is not involved.

Apparently that is not what you meant to say. So please take your time to explain exactly what is happening. You have to be very precise with technical issues as things are easily lost in translation.

 

agreed.

what I was saying is that the moment the slightest amount of vacuum is applied to the hose hooked up to the advance, the engine starts to stumble and shake. if I push the hose completly on to the nipple, the engine will die.

If I hook the advance up to "ported" vacuum, when I accelerate, I get the same result.

 

Now that makes sense. Thanks. And, I now agree with you - too much advance. Or, maybe it is bad wiring in the dizzy that is grounding as soon as the advance moves, as was suggested. So pull the cap and the rotor and you can see the pickup and its wires. Make sure they aren't bare and aren't touching.

But, if that doesn't produce anything then we need to step back and think it through. You said once, I think, what your initial timing is. How much is that? If the vacuum advance is really giving the engine more advance than it can take then either the initial is too much or the vacuum advance unit is defective. And the initial lead can be more than you think if the harmonic balancer has slipped - and many of them have on these old trucks.

Or, you have the dizzy set up such that the rotor isn't pointing to the cylinder it is supposed to fire. In that case a lot of advance will move the rotor to point at the next cylinder and the engine will buck and die. So, while you have the cap off, turn the engine to TDC on #1 as shown on the balancer - we'll assume it is ok for now. Then look at where the rotor is with respect to the #1 terminal. It should point right at it or maybe a bit behind it.

 

Distributor was installed the way you described when new, could someting have moved since?

Don't forget, the truck was runing fine before the carb replacement. not perfect, but running fine. even with the advance hooked to ported vacuum.

I'll have a look under the cap tomorrow and see if I can find any wire issues.

Someone mentioned earlier about the vacuum port on the carb. I've been using the port in the bottom of the carb. I have a port near the top of the carb closer to the choke butterfly. Will the upper port allow a different rate of vacuum?

Suppose I'll try that tomorrow too.

 

I doubt anything has changed in the dizzy since installation, although it is worth checking out when you have the cap off. And, I remember this has come up seemingly as a consequence of the carb change. But, the symptoms don't fit the cause very well, so it pays to consider all possibilities rather than just focus on the carb.

Speaking of that, how hard would it be to swap back to the other carb? If the symptoms go away we know it really is something to do with the carb. But if the symptoms persist then it is elsewhere.

As for the other vacuum port, it is probable that will give a different amount of vacuum. However, that may only mask the issue as you probably won't have as much advance and then the economy may be gone. Plus, it will introduce a new set of considerations. So, I'm not in favor of doing that as we don't know what vacuum that port gives and won't know how to interpret the results.

 

Got to spend the morning in the garage, and on the road and heres what I was able to do... Forgive me if it gets long.

Cool here in TN, roughly 35 degrees out, set choke with pedal, fires right up, good idle, choke a bit long to come off, likely due to loose conection at the heat pipe, it needs a new fitting.
Bring her up to temp, check timing, still set at 10degrees advance. Vac advance is disconnected, everything plugged up. Lets go for a spin...

Smooth acc through the gears, no noticeable miss or stumble. pulls well through first and second, third is my cruising gear around town, typicaly 35 to 45, if I need some umph I drop to second. inclines still a challenge, due to no advance? I typicaly shift at about 2000 RPM. After about 15 minutes I pull over and hook up advance, no go, disconnect advance and hook up vacuum gauge to ported vacuum from bottom of the carb. This port sees 0hg at idle 5-8hg under normal throttle and up to 18hg if I push on the pedal enough. Hooking the vacuum gauge to the port at the top of the carb has no vacuum at all, idle, full throttle, or anywhere in between, 0hg steady. No difference in drivability while switching around vac lines. so, back home.

Back to old carb!

Tossed the old(2yrs) carb back on, fires right up, reminded of how rich that carb seems to be, let choke open drive down the road and pull over to check timing, still at 10 degrees. Drivabilty is as expected with the old carb, while still drivable, good pull, decent acc through the RPMs, just a bit more slugish about it the the new carb. Carb from the 70 f350 is noticably more "pep" for lack of better word. even given the advance issue, the truck seems healthier with the new carb.
SO.....right, old carb currently installed, hook up vacuum advance to port at the bottom of the carb, no change, truck drives the same, hook vacuum advance to port at the top of the carb, no change, runs fine. Interesting. well, both ports on the old carb have no vacuum!! Go figure. Back home.

New carb back on, runs just as it did first trip this morning.

Obviously an issue with the advance, or vacuum pod on the dizzy.
I'd like to note, that after pulling the rubber cap off the bottom vac port on the OLD carb, it was full of gas. Not the case on the new carb, or atleast not enough to notice. Thoughts on this???
Another note, Vacuum gauge reads 20-21hg at idle, seems a bit high, though appears to run well there. The new carb, out of the box, had the throttle stop screw all the way out, and the mixture screw 3 and 1/8 turns out. Before install I set throttle stop so butterfly was open roughly 1/8 of an inch, left the mixture screw alone, installed and she fired right up. I ended up backing off the stop screw about an 1/8 of a turn and leaning up the mixture screw one full turn.

I'll look under the distributer shortly. Thing I might borrow a vacuum gauge from autozone for a few days.

Thanks for the help!!

 

So, some observations:

1. The truck runs as well, if not better, with the new carb as with the old
2. The vacuum advance, although hooked up with the old carb, hasn't been working for some time as neither vacuum port on the old carb works
3. Adding vacuum advance with the new carb kills the engine, and we don't know what it does with the old carb because you didn't hook it to manifold vacuum with the old carb on.
4. The engine runs at 20-21" of vacuum at idle, which is very good

Given that, it looks to me like the issue is one of two things:

• Too much advance: We don't really know what your timing is set to as we don't know if the harmonic balancer has slipped. Many have at this stage of the game, so you may well have a bunch of timing dialed in and the addition of another 10 or more degrees with the vacuum advance is just too much.
• Bad wiring: Or, the wiring may well be bad such that it touches when the advance moves.

One way to test the balancer slip theory is to tune the timing for maximum vacuum and see what you get. If another 10+ degrees is enough to kill the engine I would think that you are already advanced past the max vacuum point w/o the advance. So, put the vacuum gauge on manifold vacuum, loosen the dizzy's clamp bolt, and turn it to wherever you get max vacuum. Then check the timing.

 

Ok so, under the dizzy cap looks good, clean, wires intact, no odd corrosion, mechanical advance moves freely.

roughly 2 years ago, when I installed the "old" carb, which was a reman at the time, along with the DUI dizzy and eliminating the rest of the feedback stuff, the balancer, timing mark, and piston all matched up, I have what looks like a saw blade with numbers and marks stamped in it, at one end is a hole probably 1/4-3/8 big. at TDC on compression stroke the notch in the balancer is in middle of that hole and piston is at the top. I've had no issues with timing moving around on its own, and with a timing light initial advance is set a t 10 degrees. just to clarify, I hook up my light set it to 10 degrees advanced and turn my dizzy til the notch is showing up at the 10 stamped in the timing plate. Or should it show in the hole at 10 degrees?? On the bottom of the dizzy, etched into the aluminum is 36 degrees total timing, maybe 34 but I'm pretty sure its 36. So mechanical+vacuum advance should yield the number etched on the dizzy no?

We can agree that the vacuum advance wasn't working with the old carb installed, and to clarify, when I hook up advance to manifold vacuum with the OLD carb installed, I get the same result, clearly to much advance and engine will choke out. I apologize, I did forget to mention that earlier.

we can agree, that after installing the new carb(1970 f350)) that the engine sounds, feels, smells healthier. And I am rather pleased with the results.

I've read where the vacuum can on the side of the dizzy may be adjustable. Not to keen on poking around in there with an allen wrench, but is it possible I may need to adjust the rate of the can?

I've done the whole tuning for highest vacuum per the in cab gauge, i cant tune for much above 21hg, which per my light has me at about 12-13 degrees advanced. if I leave it there, its drivable but I'll get a ping. if I leave it at 20hg, which has my timing just above 10 degrees, I get no ping unless I purposely create it with my foot. So, I back it down just a hair to 10 degrees. smooth out the idle and enjoy. I will get a ping if I have too much throttle at a low rpm but I don't typicaly drive like that.

I'd like to get the vacuum advance working properly, I'm sure I'm missing some grunt out of the engine without it, and I havent even clocked MPGs, though it doesn't seem all that bad.
Am I just that out of whack on my initial timing, am I using my timing light bass akwards??

 

I'll try to address your questions/comments one by one:

• So, no bad wires in the dizzy means we must have too much advance when the vacuum is connected.
• I'm going to have to see a picture of the timing indicator as I'm not understanding what you have
• No, the 36 is probably the # of degrees the mechanical advance gives w/o the vacuum advance
• Good - You tried the vacuum advance with the old carb and it died on it as well. More support for the too-much-advance theory
• While the vacuum can may be adjustable lets don't do that just yet.
• If you are able to get pinging w/o the vacuum advance then you have too much advance. Or, maybe I should say you have too much advance to add vacuum to it. In actuality you have just about the right amount of advance to get the best MPG, but probably too much to get the best performance.

Again, I believe you have too much advance already, before the vacuum is added. So, it seems to me that one of three things is going on:

1. You are using the timing indicator incorrectly and getting way too much initial advance.
2. Your damper has slipped so your 10 degree mark is way past that point. However, you said the piston was at TDC when the damper showed TDC a few years ago?
3. Your vacuum advance is giving you waaaaaaay more advance than it should.

Let's test #3. Start the engine with the timing light on it and, using your mouth, start pulling a vacuum on the advance unit while watching the timing. How far does it go before the engine starts to die? Does the engine speed up any with a bit of advance? (Better yet would be to use a Mityvac vacuum pump and note the amount of advance you get with each inch of vacuum.)

 

Thanks so much for timely responses and the plethora of info!

I'll try and get a picture tonight, and I'll try to aply some vacuum with my mouth to see what I get. My timing gauge is on the passenger side of the motor next to the alternator if that helps. I'm sure its factory.

I'm gonna read up a bit on the use of my timing light just to be sure I'm using it correctly.

I have tried backing off the timing quite a bit when I first swapped everything over a few years ago and if I recall, I never got any good results.
Thanks again, I'll report back once I've had time to tinker some more.