I had my truck on the dyno and the guy tuning it said that the Duraspark box was advancing the timing to much so he took off the vacuum advance to the carb. Any thoughts about this?

Can I get a kit to adjust the distributor so it doesnt advance it as much?

 

Yes, I think crane sells one. Go look at jegs or summit racing. I think one or both of them carry it.

I have had this same problem before. He must be running the initial timing more advanced than stock, which gave me good low end throttle response, but made it ping a little bit on light acceleration up hills.

If you are not worried about gas mileage, you could just leave it off. That is all the vacuum advance is for. Under full power, you have no vacuum, so it plays no part at all in full power output. And if you have the advance hooked to a ported vacuum source, it plays no part during idle either.

 

This is my Sunday cruiser, gas mileage is not really a concern, but smileage is

I was told it might ping if I run it at high RPM's lots. It'll do for now I guess, I was planning on a full MSD ignition in the future!

 

That's funny. I didn't think the duraspark box advanced the timing AT ALL. Are you sure the dyno monkey knows what he's talking about?

 

That's right. The Duraspark is simply an ignition module. Takes the place of the points in the points system. All advance is handled by the vacuum and mechanical advance in the distributor itself. The mechanical is strictly RPM based, while the vacuum advance is for light throttle, higher load situations. If you are getting spark knock at light throttle, say pulling a hill and kinda lugging it in a higher gear, then you have either too much advance from the vacuum portion of the mechanism, too much initial timing dialed in, or simply too low of an octane gas. (Or maybe just lugging it too much). If it's only at high rpm, WOT, then it's too much mechanical advance, too much initial, or once again, low octane gas. If the dizzy is in good mechanical shape, initial timing right, and gas OK, then there is the possibility of too high of a static compression ratio or possibly carbon buildup on the pistons or in the chambers. As far as dialing in the dizzy, especially on a modified motor, you may want to hit your nearest speed shop. I'm still relatively new to Fords, but for a GM HEI I picked up an adjustable vacuum advance and a re-curve spring and weight kit for chump change. I'd say both should be readily available for a DS-II and cheap too. Dial in the mechanical advance first (no vacuum line on the vac advance) and get the timing for WOT set right first, then hook up the line and adjust the vacuum unit for light throttle conditions afterward. It's a good day's worth of tinkering to get it all right, but the results can be surprising, for cheap. You'll get better throttle response, more overall power, MAYBE better mileage and get rid of spark knock all at the same time, and remember: If you hear pinging, you are doing damage to the engine. It may take a while to accumulate, but it's happening. An afternoon and $30-$40 dollars may save you hundreds later.

 

Technically, the Duraspark 'box' wasn't advancing the timing too much. (The 'box' doesn't advance the timing.) The weights and springs under the distributor base plate control the mechanical advance, and the vacuum can controls the vacuum advance.

At heavy to wide-open throttle, there is usually no vacuum available at the vacuum can. So, was the dyno guy implying that you have too much advance when under heavy load (then you need to change the base timing and/or the mechanical advance curve), or under light load (then you need to trim the vacuum advance curve), or both heavy and light load (you may have to change all three (base timing/mechanical advance/vacuum advance)?

The stock Duraspark II distributor has quite a bit of adjustment in it. If you take the distributor cap off and look down inside the distributor, you'll see a square (IIRC) hole in the base plate. If you turn the engine over slowly until you see the advance spring achor under the hole, you can insert a flat blade screwdriver in the square hole and 'carefully' bend the achor to increase or decrease the tension on the advance springs.

And most Duraspark II advance cans can be trimmed by pulling the vacuum hose off the advance can and inserting an allen wrench in the port where the vacuum hose connects. Turning the allen wrench one way increases spring tension on the diaphram and turning the other way decreases spring tension on the diaphram. This will allow you to change the amount of timing advance for a given amount of vacuum (i.e, engine load).

You can also dismantle the distributor, and under the base plate you will find the advance plate limiter.It will be stamped with the total advance (in distributor degrees) that the advance plate limiter allows. (i.e, if its stamped '15' it allows 30 crankshaft degrees of advance, and if your initial (static) timing is set at say, 8 degrees, that gives you a total timing of 38 degrees. If you flip the advance plate limiter over, it might be stamped with a '10', which if installed that side up, will allow 20 crankshaft degrees of advance, and if you have your static timing set at, say 12 degrees, you'll end up with 32 degrees total timing. (Just curious, but what engine and mods are you running?)

Lotta power and fuel mileage to be found in them thar DuraSpark II distributors!

 

Sorry but ya, the vacuum advance was advancing it to much, not the box...

It runs fine for now, I'll toss the MSD in sooner or later

 

Great! Would you mind sharing with us what you did to solve your problem?

 

I didnt do anything really, it was on the dyno with vacuum advance hooked up and it was pinging a little when approaching 5000 rpms. Pulled it off, reset the timing and it was fine. I was just wondering about driveability with it off and such, just wanted another opinion other than the tuner's.

 

When the carb gets near its flow limit, it is possible to develope some vacuum in the venturi, which could effect your vacuum advance. Use your allen wrench as described above and tune the advance can.

IMHO, running with the vacuum advance hooked up and adjusted correctly will result in crisper throttle response at part throttle situations, increased fuel mileage, and a cooler running engine.

What was the tuner's opinion about driveability with the vacuum advance disconnected?

 

This will happen if the carb is undersized or the air filter doesn't flow enough air, but it is not something that is planned from the factory with any kind of vacuum advance adjustment or performance tuning. In other words, I would not tune the engine at wide open throttle because the carb was too small.

The above is true, and running with the advance hooked up is the best, but if the vacuum can does not have an adjustment screw, I would run the initial timing more advanced and sacrifice the vacuum advance. On a 302 I built with flat-top pistons and small chamber heads, I could either run the static timing at 2 degrees and leave the vacuum can hooked up, or I could run the initial at about 14 degrees and unhook the can. It ran much better at 14 degrees initial (static) timing and leaving the vacuum unhooked. I never got around to buying the adjustable advance can.

 

Agreed. But this doesn't appear to be a factory set up. (Haven't been able to figure out just what engine and mods we're dealing with here.)


All the DuraSpark systems I've worked with came from the factory with adjustable advance cans. (Just gotta figure out what size allen wrench fits the blind adjustment screw :-)

 

I never actually checked for the allen screw. It may have had it. Also I was wondering how it really worked. You said it adjusted spring tension on the diaphram, but I would rather it be a stop adjustment to give a certain amount of advance and then that's it.

I did experiment around a little bit on that 302. The vacuum advance had a slot in the arm right where it comes out of the back of the can. What I did was drill a hole in the boss of the back of the vacuum can. I then stuck different size cotter pins, nails, whatever, in the drilled hole. The cotter pin when installed in the hole I drilled, fell into the middle of the slot in the arm. When the vacuum pulled on the arm, the cotter pin would limit the travel of the arm. A larger diameter pin would limit it more.

It actually worked, you could see the difference on the timing light when I revved the motor. I drove it that way for a couple of months with good results. But then the vacuum can failed. I didn't know if my rig caused it, or it was just old age, but instead of more testing, i just left it unhooked.

 

Turning the allen wrench turns a threaded shaft. THere is a nut on the threaded shaft that is the seat for the internal spring. When the threaded shaft turns, the nut moves in or out depending on which way you turn the shaft. This increases or decreases the spring tension on the diaphram.

If you want a maximum of, say 5 degrees vacuum advance, tighten the spring so that it takes, say 22 inches of vacuum to advance the timing five degrees. If your engine can't generate more than 22" vacuum, you can't get more than 5 degrees advance.

I put a timing tape on my harmonic dampener. To plot the vacuum advance, I disconnect the vacuum line to the can and plug it. THen I connect my mighty-mite vacuum pump to the canister and a timing light to the number one cylinder. With the engine idleing , read the timing (say its on 10 degrees BTDC). Squeeze in some vacuum until the timing reads 12 degrees and read the vacuum and record. Squeeze in some more vacuum until the timing reads 14 degrees and read the vacuum and record. Etc. Get some graph paper (or use your favorite spread sheet) and plot the advance vs. vacuum.

Turn the allen wrench counter clockwise (or clockwise) a turn or two and repeat the above. The different plots will show you the different vacuum advance rates, the amount of vacuum to start advancing, and the maximum advance for different settings of the allen wrench.

Note that tuning the vacuum advance is the icing on the cake, ~after~ you have determined the base timing and tuned the mechanical advance rate.

Ah! If'n you still have the vacuum can, try cutting it open to see just how the adjustment works!

 

I cant beleive the discussion that went on with this. I posted something on Corral and nothing was said!