Too much Vacuum advance?
 

I have a new to me 78 F250 supercab with a 460. D3V heads. Duraspark II dist. While checking things out discovered the mechanical advance was frozen about mid way through the advance. Truck had been timed that way and was working on vac advance only.

I cleaned it up and got things moving again and re timed it (vac disconnected and plugged). It now runs crappy at throttle like it's missing or stumbling. I checked the timing with vac advance and its really high. I tried using a allen wrench to adjust vac advance but doesn't seem to be any thing to engage with. Seems to run good without vac advance. Do I loose anything other than fuel economy without it? Is there some other way to adjust the vac advance?

vac advance is connected to timed vac port on holley duel fuel line carb. I posted question on 460 forum, but it's a little quiet over there.

Most replacement distributors come with a non-adjustable vacuum canister, unlike the original equipment..... they're generally fixed somewhere between 10-15 degrees of advance, depending on application.

Where is your total timing set? initial and mechanical?

At 600 rpms it's set about 12 deg. It will advance to about 32 deg at 3K. It's stamped 16L on the slot, so I should get 32 deg of mechanical advance for a total of 44deg, but it seems to level off at 32 deg at 3k rpms. If I add the vac advance, it is well past the timing marks but in the 40's. That would line up with the 10- 15 degrees you mentioned, but it's really not happy with the vac advance. It doesn't knock, but runs rough like it has weak spark or something.

Correct, 16L=32 degrees, your springs are too stiff to allow full advance by 3K, so you're only seeing 20 degrees mechanical advance.

Try bumping up your initial to 14-16-18 degrees and adjust your idle speed down accordingly, just be sure to listen for detonation (pinging), if your hear it back of the initial a few degrees.

Bumped it up a little and took it on a test drive. I drove it both with and without the vac advance. What I notice just a little is that with the vac advance connected and I'm just cruzing with little load it runs rough. Same when I'm in the drive way and just rev the engine. Is that because I'm getting vac advance based on rpm whether the engine is under load or not?

Also, do you think I should get a re-curve kit to fix the spring problem?

Suggest you get the mechanical dialed in first. If you are running a stock or mild cam: 12 degrees at idle and 35'ish at high RPM with vacuum line disconnected and plugged. If springs are too stiff, as suggested above, softer springs that get the mechanical all in by 3000 RPM should help.

Then add the vacuum. 10-15 degrees additional advance up to 45 or 50 total or even a bit more under moderate load help the the engine run cleaner and last longer. An advance type timing light will help.

Its also worth checking other parts. Cap, rotor, wires, plugs. My dent really responded to a fresh set.

As you know vacuum advance is load dependent, unlike mechanical advance that's rpm dependent, so vacuum advance (if adjustable) can't be set (dialed in) with a timing light, it's a trial and error process through many test runs.

Crane sells a recurve kit with an adjustable canister and light/med/heavy springs part #99607-1 (about $45), or if you want just the springs Mr. Gasket sells a spring kit part #925D (about $5).

Just keep in mind if you swap out the heavy springs for lighter springs you'll then see your full 32 degrees advance so you'll need to set the initial back down to 4-6 degrees, as not to exceed 36-38 total, unless you swap out to a smaller reluctor arm e.g. 13L or 10L

Here's an excellent write-up on timing...

FORDMUSCLE webmagazine: Timing is Everything - Distributor Curving for Maximum Power

Thanks everyone. I think I have too much vac advance. I'll play with it somemore and maybe try the crane kit.

It's nice to have an adjustable vacuum can, otherwise you're forced to dial back on the initial/mechanical to compensate for an overly aggressive vacuum advance, then performance suffers.

An adjustable can is a must to "dial-in" your timing curve for optimum performance.

I suspect I also have too much vacuum advance, engine lugs and *pops* under load, carb rebuild w/ new accelerator pump diaphragm didn't fix. Is it fair to test by removing the vac advance line from the distributor and driving?
If problem goes away and my vacuum can isn't adjustable what would be my next step?
If problem doesn't go away what then?

thanks in advance,
Mike

Yes you can run it without the vac advance to see if it's a vac advace problem. I really suggest you read the link provided by Montana HB. If you're connected to manifold vacuum, you have vac advance at idle and cruzing, but not under load/heavy throttle. I was connect to ported vac (above the butterfly) which worked great when the mechanical advance was stuck but created problems after I fixed it. I have the Crane adjustable kit now, but haven't had time to install yet. If you go to summit racing you can find the Crane kit. The online instructions for the kit are also good information on the mechanism.

Thanks March I'll give that a try. I actually have read that link several times because Montana is a patient soul and posts it all the time when vacuum question comes up. I just have a hard time with the manifold vs ported with reference to my stock carburetor since that article refers to aftermarket carbs. I'll try my buddy Google, but if you can make it easier on me that would be awesome. Motorcraft 2150 2V.

Thanks1

I pretty sure your carb does not have a ported vac connection. Ported vac comes off the carb above the throttle plate and the vac increases as the rpms increase. typically found in aftermarket carbs. Vac below the the throttle plates (manifold vac) should run steady at 20 psi or so until you hit the throttle, then it drops. That's the set up you should have. I probably threw you off talking about my problems. But, It makes me think your problem is not vac advance. It's still easy to disconnect and check though.

Have you checked the mechanical advance (vac advance disconnected) with a timing light?

I would also check all your plug wires to make sure you have good solid connections. The problem you describe could be weak spark that fails to ignite the heavier fuel to air mixture under load. Wouldn't hurt to also check plugs, rotor and cap.

Appreciate your thoughts. I tried w/o vacuum connected and problem was same and in fact it died at idle. I'll check plugs wires and coil next. If it's timing I would be surprised since truck passed smog in april with flying colors and runs great at any other point except under load when you punch the throttle. what you say about spark under dense fuel air ratio makes sense. Thank you!!

March, wanted to say thanks for nudging me in the right direction. New plug and wires and my girl runs like a whole new truck. No hesitation at all through full rpm range. Thank you!