Don Terrill’s Speed-Talk

BB Mopar- Brand new MSD Pro billet distributor== My dyno has a MSD programmable -7 box and coil that I always run== When I get the timing close and just hold the RPM at 3500, the timing marks slowly moves up and down about 30 degrees with out moving the distributor at all. Anyone ever see anything like that ?




JOE SHERMAN RACING

The two possibilities(not related to the dist itself) are:
-Fore and aft cam movement(does it have a cam thrust button?)
-oil pump drive gear moving up and down(you can put a collar on the dist shaft to limit the amount it can move)

I don't recall how much shorter the shaft for the low deck dist is compared to the tall deck one, but if it had a low deck dist in a tall deck block, and the blade of the dist was just long enough to engage the drive gear....... I can see where unhappy things would happen.

I'm assuming you're not using a dial-back timing light.

Joe, I would hate to send you barking up the wrong tree but what make of oil pump drive are you using? Last BB Mopar I built I had problems getting a shaft/bronze gear that wouldn't eat itself up. I started with a new Milodon shaft/gear assbly, and after a brief run-in I checked the oil and found gold glitter; turns out the bronze gear had wear marks that were running off the teeth.
I marked the teeth with dykem and ran it again and determined the shaft was junk. I then chucked the shaft by the hex end in the lathe and spun it up and to my dismay found about .200 run out end to end.
Ordered another shaft asbly from Milodon, chucked in the lathe and the run-out was almost as bad as the first one.
Finally I dug through boxes of my new old milodon stock and found a new shaft they had manufactured back in the 70's, spun that one up in the lathe and it was dead-nuts-on straight.
I can see where a shaft with a lot of run out might cause strange timing variation???Maybe not.

Re route the pickup wires from the distributor to see if it's getting some kind of interference. Try another timing light to see if it does the same thing. Try another distributor if available. The other distributor doesn't have to be a race quality piece, just something capable of 3500 rpm. Same with a different ignition box. Moving that much I'm inclined to believe the problem is electrical, not mechanical.

This is a drag race engine. At first I ran the stuff he had been running for about ten years-- I noticed the problem-- I got a brand new MSD distributor, and it does the exact SAME thing- I tried reversing the box to distributor wires where they plug in, and it would hardly even run that way I am using the old oil pump drive shaft that he ran for ten years. It has very little wear, almost NONE . I am at my wits end with this thing. I have NEVER seem anything like this in over 60 years of engine work




JOE SHERMAN RACING

Turn the lights out in the dyno room and watch for witchfire on the ignition bits.

Is the timing *really* varying 30 degrees, or is something fooling the timing light?

Is your throttle just a cable or is it some kind of closed loop speed control? If it's just a cable throttle, RPM, O2, and EGT should all wander around with the timing.

I've had this, turned out to be low battery voltage on the dyno - hooked up an alternator to drive the ignition & problem went away.

Well, can you notice a big diffence on how it works with the time changing all over the place???
If don`t, maybe it really does not change 30* on the engine as it appears to be.

Recently ,found myself with a situation like this. Started to doubt about my old thrust timing light, bought another one......and same thing.

It was DAMPER moving around...

With a 30 degree swing in timing wouldn't you notice it just in the tone of the exhaust note?

When mag trigger wires are reversed the timing can change 30^ or more. When they are backwards the timing will vary with RPM If you switch the wires to test you will need to reset the timing.

When mag trigger wires are backwards the timing will be retarded during cranking and at low RPM and advance as RPM increases.

I have seen timing wander when the HV coil wire and/or the orange and black coil wires and the purple and green trigger wires are tie-wrapped together or all tie-wrapped to the same frame tube. Also, in this condition the system will trigger itself and fire continuously with the switch on when the engine isn't running,

2X for damper, especially if the engine speed/note doesn't change.

Like tuner, have seen a similar scenario with wires reversed.

I saw some crazy timing one time when a buddy of mine changed plugs in his car. He dropped one and it landed on the ground strap and smashed the gap shut. He picked it up and never checked it and just put it in. It was popping and backfiring, timing was all over the place.

I know mechanics that will not pick up a dropped spark plug. Closed gaps, cracked insulators..

Yeah, it's usually not a good thing to do. I had a Bosch platinum plug one time, the little platinum piece was loose in the ceramic insulator and slid out and grounded on the ground strap. Brand new plugs and 1st pass a dead miss. Put the old A/C plug back in and finished racing that day. Warranty covered the plug but that was my last set of Bosch plugs. Switched to NGK's and have been pretty happy with their race plugs.