Engine timing, vacuum, mech advance, air fuel ratio, etc....

[340_6pak]

Hi Guys and Gals,

The issue today concerns a 73 challenger 340 bored to .060, Holley 6 pack carbs, and a Lunati cam 235/285. It has ceramic coated long tube headers and the other day I attached air/fuel sensors in both collectors and adjusted the carbs to get 13.5 to 14.2 readings, so it runs sorta ok, but not really.

I got out the timing light and the vacuum gauge tonight and got some rather interesting readings. At idle, about 1050 rpm, I get an initial timing reading of 30 degrees. The dist vacuum says no vacuum.

I crank up the revs to 3000, now I get a total advance of 70, 50 at the crank and 20 vacuum. When I hook up all of the vacuum lines like they should be, I get a total advance of 70 at 3000 rpm.

So I break out the calculator, pound a few buttons and I think I get 70 total advance at 3000, with 40 of that being mechanical from the stock Mopar distributor. I do not have any MSD units or anything like that in the mix. The electronic module is black with a gold heat sink and has a mopar part number. If this is important, I can get the part number from the module.

I have read that Mother Mopar says the advance should be about 35 degrees. I have also read that Chryco hates vacuum advance. I have no problems capping the distributor vacuum port on the center carb. And the outboard cards, well never felt them kick in.

So my question now is what to do next.  If I want to get the advance down to 35, I can turn the distributor and lower the initial advance. The cap the vacuum and leave the mechanical alone, meaning no spring changes. When I get it to this state, I then break out the air/fuel sensors and get as close as I can to 14.7? Or do I just turn the dist until it 'feels' right?

Am I on the right track?  Comments/suggestions please.

[femtnmax]

the timing sounds like kinda a mess.  Many ways to tackle the problem.  One suggestion is take the distributor to someone that has a distributor machine, and knows how to use it.  Verify the vacuum and mechanical advance are working correctly.  Then reinstall the distributor and set timing to factory specs.  then start tweeking for that extra performance. 
BE aware that maybe your timing light is haywire nfg.  Also maybe your crankshaft vibration damper outer pulley with timing mark may have slipped on the rubber collar.  Need to check/verify that the timing mark is at zero degrees advance at #1 piston TDC.

Or take the car to someone like chryco that can check and tune it correctly.

With timing set correctly, then work on air fuel ratio.  I use an edelbrock O2 sensor, nothing fancy, just follow the booklet included with the kit.  Its relatively inexpensive, yet accurate enough to tune every circuit in the carb including the idle transfer slots.

[Roppa440]

First find TDC exactly and check that your timing marks are correct.

Small blocks come with two different timing covers and dampers. One or the other may be wrong. Also dampers can fail and twist around just a little before they fail completely.

[Chryco Psycho]

I agree that you need to make sure the timing marks are right , this is why I use my ear not a light to tune , with the rpm around 2800 RPM I advance the timing until the rpm quits climbing , at that point you can advance the timing a few degress with no change & if you keep advancing you will hear it starting to pop out the exhaust . so what I do is to find the point where the RPM quits climbing & levels off then retard a couple of degrees until I hear it just start to drop RPM & tighten it down there  , this seems to yield the top HP on the dyno & should be around 38* 

[Roppa440]

I do exactly that to find best initial timing.

But with a big cam that can be upwards of 20-25 degrees so total is going to be insane. So I always check total with a light and remove timing from the distributor to suit. But you have to KNOW the timing marks are good and don't let a multi spark ignition confuse the timing issue.

[340_6pak]

Thanks guys. I am going to verify the numbers I got last night, and then use Chrycos method to start. That will eliminate issues with the degree marks being off, and the top dead center issue. If I cannot get it adjusted right, I can always put it back. One thing I forgot to mention is that the heads are Eldelbrock Performer RPM. Those, combined with the cam may make my numbers a little funky.  Anyway I will report back when I am through, and thanks again guys!