Timing / Idle Mixture / Vacuum question

[bswilson80]

Seems like my comment went away. But I switched the vac advance to full vacuum and the popping went away. Thanks for everyone's help.

[travisr1988]

Changing the vacuum port shouldn't change the timing at idle because you are supposed to set the timing at idle with the vacuum line to the distributor disconnected!  If you do not disconnect the vacuum line to the distributor vacuum advance when setting your ignition timing, the ignition timing will be WAY off.  I would set the timing to 16* BTDC with the vacuum line disconnected, and then make sure you don't hear any detonation when, say, flooring the engine from a slow speed while going up a hill.  If you do hear detonation, back it off to 14* BTDC and test again.  If you need to back off again, 12* BTDC should be safe for just about any engine setup.

I did set the timing with the vacuum hose disconnected and plugged, when I put it back on non-ported the timing advances, on ported it doesn't advance at idle.

[rich weyand]

Yup, because with the vacuum advance line above the throttle plate (ported vacuum), when the throttle plate is closed you get zero advance.  With "non-ported" vacuum from below the throttle plate, the high vacuum at idle will pull in the vacuum advance.  The lean mixture at idle burns slower, and so needs more advance, but the pollution control weenies moved the vacuum advance above the throttle plate to pass idle emissions tests.  Which didn't really help real emissions, but there you go, they were playing the game by its own rules.  That's fine, but it makes the engine run crappy, as you found out.  Prior to the emission controls era, the vacuum advance was ALWAYS taken from below the throttle plate.

On ported vacuum, the idle will be rougher, throttle applications from idle will catch the distributor in the wrong position (fully retarded) causing a transition stumble until the advance catches up, and the advance will be all wrong when decelerating with your foot completely off the throttle, causing incomplete burning and ejection of unburnt mixture into the manifold, hence the popping.

Or something like that.  That's my story and I'm sticking to it.  Anyway, pulling vacuum advance from below the throttle plate, as all cars did before they started gaming the emissions regime, is the way to go.

Thanks for letting us know that solved the problem!

[74 C-10 Shorty]

Here's an interesting read about timing and vacuum advance...

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.

[rich weyand]

Uh, yeah.  Like I said.  :-)  34-36 degrees of total timing (set to 14-16 degrees at idle with a 20*-centrifugal-advance stock distributor) and vacuum advance connected to manifold vacuum.  Yoda says: "Run like a top it will."

[travisr1988]

Thank you 74 C-10 Shorty, that was very helpful and a great read, I'll switch to a non-ported for my advance now

[74 C-10 Shorty]

Thank you 74 C-10 Shorty, that was very helpful and a great read, I'll switch to a non-ported for my advance now

You're quite welcome, glad to be of help.

[bswilson80]

seems that I may have spoken too soon. I don't have a permanent tach, so I threw on a temp one yesterday because after switching ports to full vacuum the truck sounded like it was idling too high. It went from about 850 in park (ported vac) to about 1200 (full manifold), and then I checked the timing advance, and in park at idle the advance was up around 55-60, this seems high?

When I lowered idle back down to 850 it ran a little rough, so I tried to smooth it out with the air/fuel mix, it's pretty smooth now, but the timing at idle is still up around 50 degrees and the popping is back, not nearly as frequently as before, but in back nonetheless.

Also, the truck revs really high before violently kicking into the next gear, this started after I switched to full manifold vac.

Without the vacuum advance my timing is 14 degrees at 850 rpm and all-in at 3000rpm with 38 degrees.

[rich weyand]

Distributor centrifugal advance sounds modified; the stock one is 20 degrees all in and you have 24.  That said, 14* BTDC should be OK.  You might back it off to 12* BTDC so that it is 36* BTDC when the centrifugal advance is all in.

A fellow in an adjacent thread found his carburetor step-up pistons and rods were gummed up, affecting performance.  Pull the rods in the Edelbrock and make sure the step-up pistons, the cylinders they sit in, and the rods are all clean.  They gum up within about a year of normal operation.

While you have the rods out, use a magnifying glass and a strong light to read the tiny numbers stamped on the shaft.  Should be something like "6852".

[bswilson80]

Distributor centrifugal advance sounds modified; the stock one is 20 degrees all in and you have 24.  That said, 14* BTDC should be OK.  You might back it off to 12* BTDC so that it is 36* BTDC when the centrifugal advance is all in.

A fellow in an adjacent thread found his carburetor step-up pistons and rods were gummed up, affecting performance.  Pull the rods in the Edelbrock and make sure the step-up pistons, the cylinders they sit in, and the rods are all clean.  They gum up within about a year of normal operation.

While you have the rods out, use a magnifying glass and a strong light to read the tiny numbers stamped on the shaft.  Should be something like "6852".

The distributor is just this one: http://www.amazon.com/gp/product/B005GRGOGG/ref=oh_details_o04_s00_i00?ie=UTF8&psc=1 . It's brand new, the only thing I changed was one of the springs for one lighter one as the all-in timing wasn't coming in until too late. Would this cause the extra degrees?

The metering rods are stamped 7547 and everything seems relatively clean in there.

[74 C-10 Shorty]

seems that I may have spoken too soon. I don't have a permanent tach, so I threw on a temp one yesterday because after switching ports to full vacuum the truck sounded like it was idling too high. It went from about 850 in park (ported vac) to about 1200 (full manifold), and then I checked the timing advance, and in park at idle the advance was up around 55-60, this seems high?

That's normal, ported vacuum you have no vacuum at idle, full manifold you have vacuum at idle and it's pulling the vacuum advance at idle, vacuum advance units come in different amount of degrees they pull and they are marked how many degrees on the arm, I like the adjustable ones myself. Go back and read the article I posted it explains how this system works.

[rich weyand]

seems that I may have spoken too soon. I don't have a permanent tach, so I threw on a temp one yesterday because after switching ports to full vacuum the truck sounded like it was idling too high. It went from about 850 in park (ported vac) to about 1200 (full manifold), and then I checked the timing advance, and in park at idle the advance was up around 55-60, this seems high?

That's normal, ported vacuum you have no vacuum at idle, full manifold you have vacuum at idle and it's pulling the vacuum advance at idle, vacuum advance units come in different amount of degrees they pull and they are marked how many degrees on the arm, I like the adjustable ones myself. Go back and read the article I posted it explains how this system works.

Distributor centrifugal advance sounds modified; the stock one is 20 degrees all in and you have 24.  That said, 14* BTDC should be OK.  You might back it off to 12* BTDC so that it is 36* BTDC when the centrifugal advance is all in.

A fellow in an adjacent thread found his carburetor step-up pistons and rods were gummed up, affecting performance.  Pull the rods in the Edelbrock and make sure the step-up pistons, the cylinders they sit in, and the rods are all clean.  They gum up within about a year of normal operation.

While you have the rods out, use a magnifying glass and a strong light to read the tiny numbers stamped on the shaft.  Should be something like "6852".

The distributor is just this one: http://www.amazon.com/gp/product/B005GRGOGG/ref=oh_details_o04_s00_i00?ie=UTF8&psc=1 . It's brand new, the only thing I changed was one of the springs for one lighter one as the all-in timing wasn't coming in until too late. Would this cause the extra degrees?

The metering rods are stamped 7547 and everything seems relatively clean in there.

I can't find the spec on the centrifugal advance on that distributor, but it has an adjustable vacuum advance canister.  Not sure about the vacuum advance arm.

7547 (.075" on cruise and .047" on power) sounds lean on cruise and rich on power to me.  Do you have the stock intake and exhaust manifolds, or is it an aftermarket intake manifold, or headers, or both?

[74 C-10 Shorty]

All "GM" units have a number on the arm(only way to tell how many degrees it pulls), adjustable units don't have a number because, well they're adjustable,lol. The rod is hooked to the diaphragm, by turning the little allen screw puts more or less tension on the diaphragm which extends or reduces travel of the rod and controls the amount of degrees it pulls, I like the adjustables because you change them a degree at a time if you want where the non adjustable I believe are 5*.

[rich weyand]

All "GM" units have a number on the arm(only way to tell how many degrees it pulls), adjustable units don't have a number because, well they're adjustable,lol. The rod is hooked to the diaphragm, by turning the little allen screw puts more or less tension on the diaphragm which extends or reduces travel of the rod and controls the amount of degrees it pulls, I like the adjustables because you change them a degree at a time if you want where the non adjustable I believe are 5*.

GM HEI distributors had a bunch of different vacuum advance curves depending on the year and vehicle.  77-78 light trucks (i.e. mine) had the least at 5 degrees @ 12-14 in Hg.  The most I know of in a stock application is 15 degrees.

[zieg85]

Very interesting read, thanks guys!!  I bet the majority of poor mpg's and performance issues stems from mix-matching parts over the years.  I guess I have never really thought about it and was lucky the combo's I've had or have had the right "stuff" to make them run fine, pass emissions and get acceptable mpg's.  Folks can't understand that I have averaged 15 mpg on my 85 C20 with a 454.  All I can say is that it is untouched, original and that it just does...