Timing Advance With Petronix In V6

I've been going over the final prep on the jeep for the rubicon. (plenty ahead of time!)
After installing a new carb adapter (seen in build thread) over the weekend, changing oil, checking spark plugs ect...... went ahead and threw the timing light on for a checkup. I found the timing to be right at 5 degrees @ 650 rpm, vac adv disconnected. Revved up to 3K rpm I only get up to about 15 degrees advance. So 10 degrees of mechanical advance is all I'm getting.
I hooked the vac adv back up to the carb and noticed that there was no change in timing advance at idle.
I hooked it up to manifold vac and still no change in advance.
I pulled the distributor cap and verified that the distributor weights are functional and that the vac advance plate/arm moves with a screw driver. I ordered a new vac advance can.
When I checked the spark plugs I did notice that they looked a little rich. Not bad, but rich none the less.
My jeep doesn't get very good mileage. Pretty much 10 mpg even when I had the overdrive. It doesn't seem to me like it is hurting for power and runs good but I want to make it even better.
The lack of increase in advance at idle when the distributor is hooked to manifold or carb vacuum, combined with the rich condition/ poor mileage; seems to point toward the vac advance can.
This is a delco distributor with petronix points eliminator. I have been running it for several years.
My question is, how much mechanical advance should I expect to have?

 

You have a picture of your weights? Specifically the beaks that rub on the football shaped cam that is brazed to top of rotor shaft. Sometimes they become badly worn and the action of swinging outward no longer results in the desired advance on the breaker plate.

You should be getting so much advance that you need a dial-back light to find the timing mark.

Timing and Vacuum Advance 101 | 1973 & 1978 Corvette "Stingray" Restoration

 

You sure your breaker plate isnt rusted to the shaft? Cant get past that you are getting neither mechanical nor vacuum advance. Both mechanisms rely on the breaker plate to rotate about the shaft on which it is centered. When you pull the ends of weights outward does the plate rotate a bit? Does it snap right back?

Or maybe your springs are so weak that at idle your mechanical advance is all-in so has nowhere left to go. You have both springs installed?

Dont sweat vacuum until you have mechanical advance sorted out. Its important but irrelevant if mechanical adv isnt working.

Really would lile a photo of your distributor with cap off. Swing out a weight and take a look at the surfaces of cam and beaks.

 

The timing and advance 101 is a great write up that is always my go to. Napa just dropped off a new vacuum canister. Sure enough mine is bad.
I'll dig deeper into the distributor this evening and post pics.

 

I re-read and dont know how I missed it, but now see that you are getting 10-deg mech adv. So plate isnt locked up, and springs are apparently enough that you arent full advance at idle. But you are getting 10-deg. Some people put a limiter bushing on the travel limiter pin. Look under the football shaped cam for a stubby pin that rides in a slot in the breaker plate. You might have an advance limiter bushing on that stem. If you want more mech adv remove that little bushing.

For performance engines the mech adv system may deliver too much advance because the driver wants a lot of advance at idle without excessive advance at WOT. A shorter slot, or a bushing on the limiter stem is the way.

 

Tore the distributor out last night and cleaned it up. Before pulling it I verified that the advance weights were functioning properly.





From there I yanked the distributor and tore it down for cleaning. It was dirtier than I remember leaving it, and the advance plate movement was not as smooth as it could be. I was able to verify that my old vacuum advance canister was bad. I picked up an Echlin VC680 advance canister from Napa.
All of the hard parts got an hour soak in solvent before wire brushing and rinsing with brake parts cleaner.





Mechanical advance limiter





I will re stab this evening and see what happens. I can now suck on the vacuum advance can and watch the advance plate rotate.

 

If you cant get as much idle advance without too much total advance you might try putting a bushing over the limiter pin.

 

Your weights are totally shot. Worn half way through the beaks. Thats likely a good bit of your problem. One of your springs is upside down too. Its the spring that rubs on the weights.

 

Thank you for chiming in. I spent an hour or so reading your thread from a couple years back.
My engine is really the only part of my jeep that I don't know what parts are in it. Previous owner said it was rebuilt. It came painted pontiac blue. Oil pressure is 60psi cold 20psi hot idle, cylinders are all pretty close to 120 psi on compression. I cut my filters open every oil change and they have always been clean. I do not know if the cam is stock or performance. I'm assuming its stock.
I will hook everything back up, check timing/advance tonight and report back. I do now know that my vacuum advance can was bad and the internals of the distributor were not as clean/ free as I would like. I used grease where appropriate and some dry spray lube in the other areas.
The rubicon trip should be a blast! We are finishing up steve's 71's winch mount tonight!

 

I just got done timing mine. Set initial timing at 5 degrees @600 rpm vacuum advance disconnected and plugged. Mechanical advance @ 3k rpm is 19 degrees.
According to the timing and advance 101 write up, manifold vacuum produces the best result. I went ahead and tested that for my self.
Next I connected the vacuum advance to the carb port. At idle advance stays the same. When revved to 3k rpm total advance is 32 degrees.
Next I swapped the vacuum from the carb to the manifold. This brings the advance up to 21 degrees at idle and 35 degrees total advance @ 3k rpm.
35 is just under the generally accepted max advance of 36 degrees. I am interested in seeing what this does for my mileage.

 

Took the jeep out for a test drive last night.
I should start off by noting that my jeep always ran really good. I never came up short on power. Lots of people even commented that for a carbed vehicle it was easy to drive and fired up without fiddling with the throttle. Put it this way, year before last I let my wife take it on her own for a 10 day hunting trip. All she knows is how to do is put gas in it and engage the 4wd......
Now with the vacuum advance working properly the lower rpm's are crisper. I wouldn't say that there is any noticeable Hp increase. The engine does seem to be able to lugg down lower and I can now start out from a dead stop in 3rd gear just by feathering the clutch. Now I just need to go on a long trip to see if the mileage has improved.

 

No noticeable operating temp change?

 

No noticeable change in engine operating temperature. I am running a 3 core aluminum radiator, custom fabricated shroud and a big blade flex fan. Jeep basically runs 185 all the time. The fan shroud and flex fan are recent additions. For several years I ran a 6 blade solid steel fan with the aluminum radiator and no shroud. The temp would get to maybe 200 wile crawling on a 100 degree day but would cool down as soon as speed increased. I never had an issue, the steel fan however was too close to the radiator for comfort and made a ton of noise.

 

Just to follow up with this thread. I picked up some new advance weights when I started this thread. My old wights were worn and the pivot holes were egg shaped.
This kit comes with 3 different sets of springs. I used the middle set which felt the closest in weight to the stock springs. Maybe the engine is even crisper? Maybe its placebo effect?



The new Moroso weights are slightly heavier than the stock weights. Tapping them on the bench sounds like they are similar hardness.



Overall it runs good time will tell as far as mileage goes.

 

Worn weights will limit total advance and cause the advance to come on later. So new weights probably did result in an actual improvement.

Lighter springs will allow the advance to come on earlier. Springs need not match, so you could experiment with changing one spring to one lighter.