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 Modify HEI distributor
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Miles
Sitting Bull

Canada
142 Posts

Posted - 30 Jul 2013 :  08:28:22 AM  Show Profile  Reply with Quote
I want to limit the mechanical advance on an HEI. The rate of advance is good but I want to limit the amount. Anyone know how to do it or have any tips for me?
Thanks,
Al

Phil
The Great White Buffalo

USA
7222 Posts

Posted - 30 Jul 2013 :  09:27:59 AM  Show Profile  Reply with Quote
Most of the time you can simply adjust the initial a few degrees in order to dial in the total advance you desire. Or you can modify the center plate and/or flyweight stops but that is more involved/trial and error.

Bowties are for Pee-wee Herman. "Chevy": even the name sounds cheap, but not as cheap as your Pontiac will be with an LS transplant.
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Bill Boyle
Horse Feathers (Charter Member)

USA
4797 Posts

Posted - 30 Jul 2013 :  11:19:41 AM  Show Profile  Visit Bill Boyle's Homepage  Reply with Quote
Yes--I have experience doing it.

Typically a GM HEI provides about 20 degrees of mechanical advance. The springs in them are tight and do most of the limiting. Once you start trying different springs, anything goes and the limit is anyone's guess. When other springs are used to yield a faster curve, one of the slots where the spring pin is located needs to be altered stopping the max travel. Epoxy resin can be used to do it. (Try quick setting JB Weld). It doesn't take much material. However, the amount of material will need to be shaped once it cures. Even on a distributor machine it takes some time do get it close. A Dremel is helpful to use in the scuffing of the slot and later the shaping process. If you get it close to 20 degrees leave it there. Adjust from initial setting to get that one or two degrees you may need.

Bill

"Dedicated to keeping the classic Pontiac engine alive."

----
400 bored +.030, forged TRW pistons, ported 62 heads, Hedman headers, 2.5 SS dual exhaust X Pypes, Comp 276AH10 cam, Scorpion 1.65 RR, 850 Q-jet, stock intake & tuned HEI; original owner.
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Miles
Sitting Bull

Canada
142 Posts

Posted - 31 Jul 2013 :  10:41:07 PM  Show Profile  Reply with Quote
Thanks both for that. I have 29 deg as a target for total advance, as that's where I got the best power and torque on the dyno. If I set the total to 29 I only get nine initial which is way too low, I want 16-18. I'm looking to stop the mechanical at about 10 deg. As the flyweights move out they reach max advance when the tail of the weights hits the arm that the springs are attached to. What your suggesting is I close up the hole/holes in the plate on the top of the shaft [with the teflon pads on] so the bottom of the spring posts makes contact and thus stopping movement? Please excuse me if that question sounded rather daft. I'm looking at the distributor as I post this and it makes sense but I want to be sure I got what you meant.
Al
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Miles
Sitting Bull

Canada
142 Posts

Posted - 02 Aug 2013 :  09:06:37 AM  Show Profile  Reply with Quote
Ok after looking closer it seems the springs control the rate of advance and the flyweights stop the advance when the tails of the weights are flat against the advance cam. I've read the amount of advance can be changed by grinding the weights. Would that would only increase the amount, or can the weights be shaped to flatten against the cam earlier?
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cortcomp
Coyote

USA
5338 Posts

Posted - 02 Aug 2013 :  09:15:36 AM  Show Profile  Visit cortcomp's Homepage  Reply with Quote
You add material like epoxy and that stops them too early, then you test and remove material until you get what you want.
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Bill Boyle
Horse Feathers (Charter Member)

USA
4797 Posts

Posted - 02 Aug 2013 :  09:52:19 AM  Show Profile  Visit Bill Boyle's Homepage  Reply with Quote
If you want to limit the travel of the weights I would not tamper with the shape of the center plate or the weights. Meaning don't try to reshape them.

If you engine performs best with only 29 degrees of total mechanical advance based on dyno runs, then your HEI must be tailored accordingly. I have to ask these questions.

Where does this engine idle best? At what RPM? What is the initial advance set to at this best idle rpm?

Do you plan to use vacuum advance as well?

"Dedicated to keeping the classic Pontiac engine alive."

----
400 bored +.030, forged TRW pistons, ported 62 heads, Hedman headers, 2.5 SS dual exhaust X Pypes, Comp 276AH10 cam, Scorpion 1.65 RR, 850 Q-jet, stock intake & tuned HEI; original owner.
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Miles
Sitting Bull

Canada
142 Posts

Posted - 03 Aug 2013 :  09:41:42 AM  Show Profile  Reply with Quote
Thanks again.
I would like to limit without filling the slots in the plate if I can, I will do it if there is not another way.
My engine idles best at 650 to 700 rpm. When I have the total to 29 deg I have only 9 at idle. In order to get it to idle with only 9 deg I have to adjust the idle speed screw which moves the carb out of the idle circuit and into the transition circuit. This causes run-on at shutdown and stinky exhaust. If I increase the idle timing to 12 degrees I can turn out the idle speen screw, it idles a lot better and the run-on and stinky exhaust goes away BUT the total advance is at 31 deg. It idles even better at 16 deg but I'm getting detonation at that high an advance. I have a 68 428 [464] with fairly high compression ratio.
I'd like to set the initial at 16-18, the total at 29 and use a vacuum advance limited to only 10 deg of additional advance. If not then 18-20 initial, 29 total and no vacuum advance.
If any engine specs are relevant to this discussion some are:
68 Pontiac 428 block, 4.185 bore, 4.210 stroke, 10.22 deck height, flat top pistons, zero decked, hydraulic roller, 112 LSA, 230/236 at 50, 108 ICL, 1.65/1 roller rockers, aluminium heads with 85cc fast burn chambers, 2.11/1.77 ss valves, intake ports 204CC, flow 305/220 cfm at 55 lift, 10.5 static c.r., 8.1 dynamic c.r.
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