Distributor curve kit

[juggernaught]

The dist. I have in my '78 360 is relativelly stock but is pointless. Did the original dist. have points? I am still running a vacuum advance and I am sure there are weights also if I looked hard enough. I want to get more advance faster, right from off idle to 2000rpm. Thought that a new set of weights might be the ticket. Who sells a dist. curve kit and has anyone else used one? I do NOT want to go to an high energy system at this point.

[tufcj]

Is it Prestolite ot Motorcraft? Prestolite has a plastic vacuum advance. Not much aftermarket support for Prestolite, but Motorcraft will use Ford parts to recurve.

Bob
tufcj

[Old Rugged Crosser]

I take it that you realize that there are two different advance mechanisms in the dissy; vacuum and centrifugal ? I will include below a short article on this issue. It may help you or others.

"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."

Hope this helps advert stumbling around in the dark.

[juggernaught]

Thanks for the specific info, but I need to carry it one more step. It makes since that under accel. the engine vacuum drops. I believe this is causing my engine to hesitate. I still have a hesitation between1,000-2,000rpm. Without going into great detail, I have pretty well ruled out all of the carb. related causes. So let's focus on the dist. adv.
I believe that recurving my dist. will both allow the timing to begin more advanced and to reach total advance earlier. I have been told that the recurve kits will allowtotal advance by 2,000-2,800rpm. Can you tell me by what rpm the stock dist. reaches total adv?

[ironman_gq]

what carb do you have? Your secondaries might be opening up too early and causing the hesitation or your enrichment system might not be opening up

[Old Rugged Crosser]

Here is some more info that may be helpful:

if you are running the Duraspark ignition, the vacuum advance can cause a stumble from too much advance at idle or mid-throttle cruise. This vacuum advance curve is tunable via the allen head screw inside bellows. Experiment. I hate the Duraspark ignition system precisely because of the stupid, smog- friendly advance curves built into these distributors. I run more centrifugal and less vacuum advance in my Delco system then the Duraspark will allow.

Check the timing, vac advance detached, vacuum line to distributor closed. If the mark jumps around, the chain is loose and therefore spark timing will be erratic.

[Old Rugged Crosser]

If you are using the motorcraft dissy check out these springs: Distributor Centrifugal Advance Springs;
FROM: Aftermarket Supplier. Mr. Gasket P/N 925D (Around $4.00)

[juggernaught]

As my sign. says I run a truck avenger 670....secondaries are not opening early...........this is not a engine load problem...........enrichment is working properly.......I do not have Duraspark.........chain is a new steel double roller, timing mark is rock solid.......Motorcraft springs are on order. I want to tackle the distributor/timing side of this issue. At this point the carb has been dialed in and seems to be working well. Can anyone tell me the rpm/adv. curve for a stock '78 Motorcraft dist? At what rpm does it reach max. adv. I am hoping the adv. curve kit will solve this problem. Thanks to everyone for all of the quick responses.

[msalaba]

For AMC's with the Motorcraft dizzy:

There are several timing curves used throught the years and depending if it is CA or 50 state (Federal) emissions legal. There is a chart of the various curves in the factory AMC service manual. There seems to be many discrepancies between this set of charts and what I've seen in the forums and my own personal toys.

Any tuning information that applies to Ford V8 applications from the late 70's applies to the AMC application. As far as I'm aware the only difference between the Ford dizzy and AMC dizzy is the body and main shaft. The reluctor, pickup weights and springs all cross.

The weights have a number stamped on them that indicates total centrifical advance in camshaft degrees. For example, my dizzy is from a 1978 360 from a MN J20. The weights are stamped 13L. I run a 4* initial and have a 30* full centrifical advance. (I run propane so my curve is a little different than gasoline)

The advance springs control when the centrifical advance starts to "come in" and when full centrifical is achieved. This part is still a mystery to me. It seems that a distributor machine is required to accurately set the proper curve "timing." I have the previously mentioned Mr. Gasket advance spring set. I used the small spring that was factory in the dizzy and the small spring from the Mr. Gasket kit. From what I can tell with an advance timing light, my curve starts @ 900 RPM and is all in by 2500 RPM. (again I'm running propance so my curve is a little different as propane wants less total advance in sooner)

As ORC mentioned, you can adjust the vacuum advance can with an allen wrench. From what I found is this only tunes the amount of vacuum required to start to acctuate the vacuum advance can. i.e. you install a cam with a weaker vacuum signal. Be carefull when adjusting the vacuum advance can as you can back the screw out too far and render the can useless. (Don't ask me how I know this!)

I run manifold vacuum to the vacuum advance. I found that the can applied too much advance for my application so I fabricated a stop to limit the amount of vacuum advance applied to 8*. This allows me to "cheat" the system and have a 4* initial for easy hot restarts and have the 12* @ idle that propane wants.

I hope this info helps someone down the road as I'm still convinced there's some black magic in these dizzys.

[wyrmrider]

It's been a really long time but on the Delco "window adjust" points distributor I think we used an Isky kit