my understanding of mechanical advance & vacuum advance, tell me i'm wrong

[vector1]

received my dui dist. back from davis dui people. marked on the bottom of the dist. is 20 deg. at 3000. i'm thinking this is 20 deg. mechanical advance all in at 3000. there is a vacuum advance but it should be on top of the mechanical advance, and mechanical advance checked with vacuum not hooked up.

i can set total mechanical advance at 34 deg. at 3000 rpm and let it idle down and it shows about 22 deg. at idle, or i can set the idle advance at 12 deg and total advance will only go to about 22-24 deg. so i'm thinking there is only 10-12 deg mechanical advance on the dist. i don't know where they are getting the 20 deg. instructions by the way recommend setting idle avance at 12 deg., no other reference to checking total mechanical.



this is pretty much the same way it worked when i sent it in. vacuum canister works well regarding advance.

am i missing something here, if not i'll have to call them monday.

[Excaliber]

You do have the vacuum advance disconnected when your checking the advance I assume? It would be logical to assume the mech advance is indeed 20 at 3000, which would be added to whatever your base timing is.

Before you check base and add advance be sure your idle rpm is consistant, low enough to NOT impact the advance weights/springs.

When you call them to verify it's really 20 max also ask them at what rpm it BEGINS to ramp up.

Sounds like the advance mechanism is sticking or binding, not working smoothly.

[vettestr]

Vector,
I read your method of finding/verifing amount of mechanical advance and I agree. If you set initial at 12 degrees and check total @ crankshaft at 3200 RPM (anything above 3k) to be sure all mechanical is in and find 22 degrees is max, you have 10 degrees mechanical.

Just a thought .... I bet the rotor was not sent to them with the distributor for repair ...so just as a sanity check make sure nothing is haging up on the rotor on the bottom side. I had this happen to me once so now I check when things are not as expected. Also in the mind fart catagory is the fact 20 degrees camshaft is 10 degrees crankshaft.

[Silverback51]

The vacuum advance should always be disconnected when you are setting your timing, otherwise it will affect the reading you are getting.


I also have to correct Jeff. 20 degrees of crankshaft rotation is 10 degrees of camshaft rotation. The crankshaft rotates twice (720 degrees) for every rotation (360 degrees) of the camshaft.

[vector1]

thanks guys, your thoughts pretty much agree with mine, i just cannot figure this out. i checked the mechanical without the vacuum. i told them it would not idle at less then 1200 rpm.

initially the rotor was binding and not advancing correctly which led me to check the timing at advance. this is a brand new $400 distributor that is supposed to be curved on a distributor machine that has been sent back once.

i told them i wanted all the advance in at 2600 which is cruise at 60, and they did change one of the heavier springs for a lighter looking one.

i just don't understand why i don't have 20 deg. advance on the mechanical side and thought maybe i was missing something.

maybe i'm getting some advance at idle so i'll check it again but i had it idling at 1000 and there was no difference i could tell.

also those familiar with the hei's, the post the weights swing on have a groove above the weights for a circlip, there is none, only the springs holding the weights on and i can move the weights up to the springs and they could miss the center stop piece, is this kosher? i put some circips on for peace of mind.

[Anthony]

Make sure the weights/springs are mobile on the mechanical advance, so you know you're getting full mech advance.

You need to know what your total advance should be, ? 32-34 degrees, but whatever it is, you need to set your engine timing at that. With the vacuum advance hose disconnected, hook up your timing light, and gradually rev your motor, watching the timing advance until it stops advancing, and that is your rpm when full advance is in, and adjust your distributor(twisting it), at that rpm, to your target timing setting. Your distributor is now timed correctly.

The vacuum advance is only for cruising, that is, it only kicks in at partial throttle to even further advance the timing, even more, to get better mileage, as under partial throttle, your engine can tolerate more advanced timing for better power,economy. However, under full throttle, the engine vacuum drops, and therefore the vacuum advance drops to zero, as under full throttle, the engine cannot tolerate the "more" advanced setting the vacuum advance will provide, which could result in damage to the engine.

[vector1]

that mimics my thoughts exactly anthony.

i'm going in. i've looked at the msd stuff and it is the same as the hei uses. they have graphs for springs, stop bushings, etc.

i was playing with the weights and there is no way they are getting 20 deg. with their setup. i can swing the weights out and put a protractor on top and it shows little over 10, put the msd weight on, swing it out, and it shows 20 plus. davis dui didn't put any limiting bushings on so i will have to use what msd suggests and check with the timing light but i bet i get the full advance.

i think i know the problem with american business.

i had a laptop cook a hard drive so i took it back to "yangs computer" and it was out of warranty so paid to have the hard drive fixed and they put on a chinese based os complete with back door trojan, took it back and picked it up this afternoon and can't log in because the computer loops between sign in and sign off prob because of the trojan still on there. this is comical though, not dissapointing like the $400 distributor that can't be curved correctly.

[Excaliber]

Is this an MSD distributor? If so a black bushing typically stops the advance at 18 degrees, not 20. Strange that no bushing is apparently installed. Without it you have no way of limiting or adjust max advance.

Total cruise advance with vacuum is around 50 degrees, I wouldn't let it go higher than that under any conditions, by the way...

[vector1]

no, this is a hei clone. got it fixed though, no problems so far.

couldn't use the msd bushing cause the posts have a shoulder, but needed to limit the travel some, so went to hardware store and bought a piece of thin round tubing to slip over the shoulder, bout all that will fit, put weights on from msd unit and used lightest springs. figured between the vacuum advance and light springs advance is going to be all the way in at all rpm's anyway except when starting it is at 12. checked total mechanical timing and it was at 34, go figure (got lucky). advance at idle with vacuum attached is little over 30 so it's prob putting 18-20 in, don't know what they are supposed to be at.

think i'll go have a smoke now, cept i don't smoke.

thanks for the sanity support.

[Excaliber]

So, what was the point in sending out to a shop if you had to fix it yourself?

[vector1]

i called them this morning, and i knew pretty much how this was going to go, but i had to give them "feedback".

i told them i wasn't getting the mechanical advance i was supposed to get even after i had sent it in for correction and they first suspected my timing light but i assured them it was working then there was a long pause as consultation was in order then i was assured there was 22 deg. advance at 2600 rpm ( where i wanted it curved to). i asked them how they set it and he assured me the 1000's of dist. they build are put on the distributor machine.

i told him no way, no how did this dist. have 22 deg advance, and what i had done to correct it, and that i pulled their sticker off the top as i could not advertise for them with this kind of service.

i tried to tell him that their choice of weights wasn't correct for this application but i don't think he was really listening--they have a distributor machine that calibrates all their distributors before they leave.

car runs great if anybodys wondering.

[Argess]

Mechanical advance....what the engine needs when under maximum load.

Vacuum advance (really total advance as it includes mech adavnce).... what the engne needs with no load

Moderate load.... mechanical advance is there, but vacuum advance is less that no-load, but more than at full load.

As vacuum advance is really feedback to the ignition related to load, you car can work pretty well with in-correct timing. But that's not good enough.

As above, you have no idea wether the 20 Deg marked on the dist is cranksahft degrees or dist degrees (1/2 of crankshaft degrees). You'll have to ask them.

As a Cobra is light, and you don't push it hard down low, 20 deg intial and 38 total are good for mech advance only.

My engine (with mech only dist) idles best at 38 Deg (just a fluke the number matches the max advance number), but obviously if i set it there, I will get detonation.

For a decent cam (290 advertised duration), Barry Grant recommends 16-18 degrees. If you set it at 18, and you have 20 mech advance measured at the crank, you get a total of 38 max. I suspect that's what they set it for.

But in such a light car, you can get a bit better idle and low rpm response at 20 or a bit over. Just maintain the 38 max so you don't get engine damage.