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As Spock once said "when one has tried all the logical explanations, one must consider the illogical ones", LOL. You were dead on sir, the sheer pin failed on mine, big time!!! It finally sheered completely and I could spin the top of the dizzy rotor/assy.
The new one has very little slop or rotational play at all. Changing the dizzy got it running again. But it went back to a back fire like when this all started at 3000 rpm, and shake at idle it did not have before. I eye balled the indexing (cut one ear off so I could rotate it), and the back fire is gone, I got it up to 3800 rpm with no problem. Did not push my luck any higher yet. Now I need to solve the mini shake at idle, which is new. I may cut up a cap and fine tune the location in case I went a 1/16" or 1/32" too far and caused the idle miss. The timing is 15 degrees at idle right now and got up 38 degrees at high rpm per the MT-2500. God I love that tool. I also did one last super sealing job on the MAP sensor vacuum line at the throttle body, as I did not trust that I had an adequate, perfect seal there yet.
I did some thinking on this and with 6 cylinders and 360 degrees, 360/6 = 60 degrees. So if the ECU can advance the spark timing from say 14 degrees to 38 degrees (maybe more) 38 - 14 = 24 degrees of change.
60-24 = 36 degrees of room for error in the Dizzy indexing. Assuming a cap pin to pin diameter of 2-7/8"" (?) I get 9" circumference. 9/360 = .025"/degree
24 degrees.... .025"/degree x 24 degrees = .60" of space between the centers of each cap pin, which is about the length of the rotor tip, maybe less?.
Then with that huge rotor tip, and the gap closed up some by the radius of the two pins in the cap the chances of miss firing go up very fast as the ECU plays with the timing of the spark when you get on the edge where the tip is equidistant from both cap pins. So I can now see the need for precision location of the dizzy, as the ECU has a wide range of degrees (at least 14-38 that I saw today on my MT-2500) to advance the spark limiting any possible slop in the rotor alignment to the cap pin.
But I still have a mystery as to why the last distributor worked for 70,000 miles with out indexing by cutting the ears and rotating the dizzy, as I installed both, and I used the standard tooth guide that leaves the rotor aimed dead on at the #1 pin at TDC. The Cruiser54 method that came from Jeep, has the end of the rotor just barely at the end of the #1 pin at TDC. Which is about 20-30 degrees or so by my estimates.
Perhaps I had the right combination of other defects in mine over the years to get away with it?
The new one has very little slop or rotational play at all. Changing the dizzy got it running again. But it went back to a back fire like when this all started at 3000 rpm, and shake at idle it did not have before. I eye balled the indexing (cut one ear off so I could rotate it), and the back fire is gone, I got it up to 3800 rpm with no problem. Did not push my luck any higher yet. Now I need to solve the mini shake at idle, which is new. I may cut up a cap and fine tune the location in case I went a 1/16" or 1/32" too far and caused the idle miss. The timing is 15 degrees at idle right now and got up 38 degrees at high rpm per the MT-2500. God I love that tool. I also did one last super sealing job on the MAP sensor vacuum line at the throttle body, as I did not trust that I had an adequate, perfect seal there yet.
I did some thinking on this and with 6 cylinders and 360 degrees, 360/6 = 60 degrees. So if the ECU can advance the spark timing from say 14 degrees to 38 degrees (maybe more) 38 - 14 = 24 degrees of change.
60-24 = 36 degrees of room for error in the Dizzy indexing. Assuming a cap pin to pin diameter of 2-7/8"" (?) I get 9" circumference. 9/360 = .025"/degree
24 degrees.... .025"/degree x 24 degrees = .60" of space between the centers of each cap pin, which is about the length of the rotor tip, maybe less?.
Then with that huge rotor tip, and the gap closed up some by the radius of the two pins in the cap the chances of miss firing go up very fast as the ECU plays with the timing of the spark when you get on the edge where the tip is equidistant from both cap pins. So I can now see the need for precision location of the dizzy, as the ECU has a wide range of degrees (at least 14-38 that I saw today on my MT-2500) to advance the spark limiting any possible slop in the rotor alignment to the cap pin.
But I still have a mystery as to why the last distributor worked for 70,000 miles with out indexing by cutting the ears and rotating the dizzy, as I installed both, and I used the standard tooth guide that leaves the rotor aimed dead on at the #1 pin at TDC. The Cruiser54 method that came from Jeep, has the end of the rotor just barely at the end of the #1 pin at TDC. Which is about 20-30 degrees or so by my estimates.
Perhaps I had the right combination of other defects in mine over the years to get away with it?
One thing most everybody forgets about is that distributor shaft shear pin. It doesn't have to break completely. It can break and the sharp pin ends can kind of catch again fairly solidly, the rotor has moved from initial setup position but you may never know unless you look (check the indexing). The wifes 87 had a CPS issue, something weird happened, one cold morning it flashed back through the TB, maybe ran in reverse a little and blew the check valve cover off the booster. Idled like crap afterwords, I initially thought it had damaged something internally in the engine. On a hunch and having learned on old Dodge and Chevy carburetor motors, where shaft pin breakage was fairly common, I grabbed the rotor and twisted hard, the pin had snapped but caught and jammed again almost solid.
If it isn't something common and normal it is time to start looking at the odd and unusual.