Lean out and miss fire
Hey guys I have a 66 DART GT w/ 71' LA 408 stroker I have a new balancer, new digital distributor locked out , new wires,coil, plugs, I have check compression and spayed intake and carb for leaks. I cannot get it to run under a load few hundred rpm above idle before it starts to lean and miss and doesnt clear up during driving either. I swapped BLP carb no change and changed MSD no change. It is something we havent been able to diagnos . everything that has been done has resulted no change. weird part it idles perfect. thanks
What triggers the MSD?
Is it a magnetic Pick-up?
I have experienced exactly this, if the pick-up is wired in reverse polarity. If you change the polarity, you will have to reset the base-timing. There are threads here on FABO explaining how this works.
Another thing that happens with so much idle-timing, is that the throttles will be way down on the Transfer slots. In compensation for the lack of transfer fuel, the mixture screws will have been cranked out. Almost without exception, this causes a tip-in sag, as the transfers try to come up to speed. The transfer-slot exposure under the primaries needs to be a lil taller than square, and the mixture screws cranked back to normal; then the timing adjusted to deliver an acceptable idle-rpm.
Then you fix the timing curve to deliver adequate timing from stall to in the window of about 3200 to 3600, depending on the stall.
Because of how locking out the distributor, affects the idle fueling, locking out the distributor for street use, is, IMO, not a good idea.
Happy HotRodding
Edit
as to the reversed polarity, and using a Non-MSD amp, my experience is this;
My car quit on the road, so I pulled over and diagnosed that the pick-up had quit.
I installed my spare, started the car, and set the idle-timing. I noticed that I had to move the distributor quite far, but didn't make much of it.
Then I wanted to check the power-timing. I began to open the throttle, and the engine began to run stupid. I checked the strobe-action of my timing-light, and saw that the timing was jumping all over the place; backwards/forwards/double-strobes/and even missing strobes....... but it idled just fine.
I didn't know what was going on (first experience), but I knew that a half hour ago the engine was not doing this. Then I recalled something; warning, long story;
I have been a Motorcycle Mechanic most of the years post 1994. I was doing a pretty good job working for the local Honda dealer. One day my boss brought me a special project. A nearly new 6-cylinder that had been in a minor fender bender. Another shop had brought it back to serviceable, but the damn thing would not rev up. It started fine and idled fine, but by 2500 was acting the same stupid as my SBM now was. It had been to several shops, none of whom was able to get to the bottom of it.
My boss made the guy a deal; he would have his mechanic look at it (which was me), and if I could fix it, the customer was willing to cough up $1500. But if I couldn't fix it, there would be no-charge. The customer agreed.
Then the boss told me what the deal was.
Oh crap, said I, tell me again who all has worked on this thing? I mean, this thing had already been to all the best shops around!
Well I spent a few hours going over it, cuz the frame had been changed because of the fender bender, and even the entire wiring harness, because the best shops couldn't find the problem. Well, I checked everything, and found NOTHING wrong..... I prayed about it, then pushed it into a corner. (this was winter work, so I had other catch-up work to be doing).
Now I know what yur thinking; you what about it?
I said, I prayed about it.
You see, I am not a particularly smart man, nor a particularly smart mechanic, but I have this invisible friend that knows everything.
So about a week later, a block of time becomes available, and I push that monster back into my stall. I start it up and look at that stinking stoopid strobe, again.
Out of the blue,
a thought comes to me; I know a little about electricity, so I wondered, could a reverse-polarity pick-up cause this? So I look on the paperwork, and see that one of those other shops must have already thought of this, cuz sure enough, it's been changed.
So I went back to my boss, and told him what I was thinking. After a short conversation, I got the go-ahead. So I cut those brand new pick-up wires and spliced them back up, and hit the key. Badaboom! Just like new. Except the base timing was out-to-lunch. No problem; that I know how to fix. So then, after all is said and done, we have one very happy customer.
And I sit down and have a little one-on-one time with Jesus.
Ok, back to my roadside problem.
I remembered that experience and promptly reversed my pick-up wires. With same success, and same out-to-lunch base timing. So I wind the D back into place (remember I had noticed the change earlier), and BadaBoom!, she's back in action.
Now, as it turns out, the pick-ups are directional! If you compare pick-ups between those designed for CW rotation like is in the SBMs, and those designed for CCW rotation, like in 440s, and I think slantys, you will find the wire colors are different, and that is how I recognize them now.
As I recall,
the pick-ups for SBMs have one Orange and One violet wire, Not sure.
and the others have one gray and one black, IIRC. I mean it's been ~20 years since I looked at mine, so don't quote me on the colors.
Okso I just remembered how this works; this is how it was explained to me.
When a vane of the reluctor passes by the magnetic pole-piece, it generates a tiny voltage that increases and then decreases, as the vane passes by. The amp senses that, and converts it into a on-off signal, then amplifies it up into something that the coil can work with. Well, that's how it is supposed to work.
But if the pick-up has been installed with reverse polarity, then it does not signal the amp properly. Instead of the amp triggering on the rising side of the voltage, it triggers on the falling side. Of course the amp knows nothing about that and at idle dutifully fires the coil . But as soon as you rev it up, the pick-up-signal goes into a tizzy, and the amp just keeps on doing what amps do, which the timing-lite sees as random firings, and the engine responds to be running stoopid.
That's how it was explained to me; or something like it.
The proof of reverse polarity is your random strobing/extra strobes/and missing strobes. If you don't have that, then you don't have a reversed polarity pick-up.
If you do, then either replace the pick-up with a proper one or do what I did; cut and splice.
Ok so now that you know this, make sure the spare in your glovebox is coded the same as what works under the hood. Nothing is quite as exciting as being 500 miles from home, and you got this stoopid-running engine to deal with.
