Lean out and miss fire

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Mims Gt

Mims Gt

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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
 
We need to know more.

Engine build (Heads, cam, compression, intake, etc):
Carburetor brand, size and any changes from out of the box:
Initial and total timing:
Fuel System (i.e. what pump, line pressure)
Distributor part number and what ignition system (i.e Stock, MSD, etc)

Condition issue occurs (Cold engine, just after its warms up, etc)
 
locked out for street racing.

Engine was built by unknown builder
Carb is 850 BLP 4150
timing set 34 deg
fuel system is edelbrock mech pump with regulator @ 6psi no return
MSD billet distrib w/ msd digital 6AL
AFR reading is how I know she is leaning out which is weird due to fuel in site glass
 
Check your bulkhead connections. A buddy with a 67 GTX chased same symptoms until he found a burnt connection at the bulkhead connector. It was one of the big wires that go in and out from/to the amp gauge. I think those are in the middle of the block on an Early A, and fit under a screw. Good luck with it. That's one of those things that will make you :BangHead:
 
Mims Gt said:
locked out for street racing.

Engine was built by unknown builder
Carb is 850 BLP 4150
timing set 34 deg
fuel system is edelbrock mech pump with regulator @ 6psi no return
MSD billet distrib w/ msd digital 6AL
AFR reading is how I know she is leaning out which is weird due to fuel in site glass
Click to expand...

Thanks, Based on that what is you idle speed? Have you connected a vacuum gauge to see how much its pulling at idle?

Did this just start or has it always been this way? You state you swapped carbs, what did you swap from and too and have either been tested as good?

Auto or manual trans?
 
I’d be pulling valve covers and looking at the valvetrain.
 
dano said:
Thanks, Based on that what is you idle speed? Have you connected a vacuum gauge to see how much its pulling at idle?

Did this just start or has it always been this way? You state you swapped carbs, what did you swap from and too and have either been tested as good?

Auto or manual trans?
Click to expand...
idle speed about 1000 and yes i did vacuum guage which is what i set my powervalve to
this started when i bought a bigger, better set of wires and yes both carbs came off of running race cars and test good. its been a head scratcher and a wallet squeezer. I think Im going to also look at ground or RFI interference . Im losing it and i know its something stupid im overlooking. I appreciate everyone's input so far im going to keep trouble shooting until I get it figured out
 
How about installed cam timing?

4 to 8° advance, straight up, 4 to 8° retarded?

You have a new front balancer. Is it a 360 that is still externally balanced and is the TDC mark on that particular balancer in the correct spot?

☆☆☆☆☆
 
Compression Leakdown, think about burnt/ stuck/ bent valves or flat cam

Fuel pressure. Get a gauge on it

Rotor phasing. Google that. It's a thing ESPECIALLY with stuff like locked distributors

If you have a "dial back" timing light get yourself an old school simple light, mark the balancer and CHECK it for accuracy (piston stop etc) and be darn sure you know where the timing is.

Look at the plugs Do they LOOK lean?
 
67Dart273 said:
Compression Leakdown, think about burnt/ stuck/ bent valves or flat cam

Fuel pressure. Get a gauge on it

Rotor phasing. Google that. It's a thing ESPECIALLY with stuff like locked distributors

If you have a "dial back" timing light get yourself an old school simple light, mark the balancer and CHECK it for accuracy (piston stop etc) and be darn sure you know where the timing is.

Look at the plugs Do they LOOK lean?
Click to expand...
thanks ill be looking into this. I have both style lights and yes the balancer is marked. Ive done so many things trying to trouble shoot. i may have missed something. thanks
 
I would be checking supply voltage to the 6AL box. Run a jumper from battery [+] terminal to the thin red wire of the 6AL. Original bal res should have been bypassed.
Check air gap on the pick up. Set it to 0.010".
 
Try reversing magnetic pickup polarity. Is the timing mark steady? I always run the twisted pair pickup wires away from any other wires. I'm sure you know solid core ignition wires are a no no with an MSD box
 
If this only started happening after you changed to a new set of wires I would definitely start there. Could be okay idle but can't handle when RPMs are raised. I also have had faulty new wires, even with top brands it happens. After that I would look at fuel. Could be not getting enough fuel?
 
Check plug wire routing for crossfire. Are they a type of wire recommended by MSD? I would CHECK the wires with an ohmeter for opens or high resistance. "New does not mean good."

Check carefully your pickup wire routing. You want the plug wires to cross that area "at a 90" AKA not run parallel, away from the pickup. route the pickup wires down near the block/ body to help act as a shield. Sometimes you need to shield the pickup wires. There are several ways to do that

You can look for and find short pieces of shielded wire on ebay. You can buy shielding sleeve. You can strip a piece of coax (Go find a 2 way shop or local amateur radio op) Or you can also just use a piece of cat-5. Use one pair for the pickup, and pigtail all the other wires together and ground them. Ground the shield at the CPU end ONLY. Don't ground the distributor end. This is called a Faraday shield.
Mims Gt said:
this started when i bought a bigger, better set of wires
Click to expand...
 
Remember that a malfunctioning ignition system can dump a lot of air into the exhaust and give a false lean reading. I'm suspecting the plug wires as they were the last thing changed.
 
Mims Gt said:
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
Click to expand...
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.
 
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