My 351 with unknown miles started running like crap last week, Brand new Re build Eddy, running fine for 2 weeks, then last Monday, it started spitting and back firing through the carb, loss of power and the tachometer was jumping from 0 to 4K, just all over the place.

I have a 1-wire GM style HEI dizzy, and called the manufacturer and they suggested a new ignition module, Got that today and runs just like the first module.
I did notice after I marked the crank pulley I can rotate the crank 1.25 inches before the rotor moves, this is quite repeatable...
But and the timing chain make the tach. jump?????
Thanks

 

The tach is entirely electrical.
If you have huge spark scatter it could jump around, but I'm more familiar with DSII where it does this when the coil fails.

Timing chain slap is a real thing.

Have you pulled the distributor and looked at the cam gear?

 

Gear looks good, looking down the shaft....
No noisy slap up front....

 

The tach is entirely electrical, but it gets its signal from the distributor which has a lot of mechanical parts, and is driven by the entirely mechanical camshaft / timing chain. If the timing chain and distributor drive have so much slop that the crank pulley can move 1.25" before the rotor moves, yes, that's a lot of slop in the timing. If the crank pulley is 4" in diameter (just a guess), that's 36 degrees of slop! (if the pulley is smaller it's more degrees, larger is less).

Would that result in the tach jumping around? Hard to say. First of all, just because there's that much slop doesn't mean even that the timing is bouncing around. It might be that all of the slop tends to be taken out in one direction, so it stays pretty consistent.

Also, even if the slop was causing the timing to jump around, on average it won't affect the tach. Sure maybe on one revolution it will think the engine is turning 1800 rpm and the next is 2200, but on average it will read whatever the engine is really turning. The response time of the tach will affect whether you can actually see the fluctuations or not.

However, 36 degrees of slop in the cam / distributor drive is a problem! And you won't fix it by replacing an ignition module or coil. You need to figure out what's worn out. It could be the timing chain, but I don't think a worn timing chain would give you that much slop. I'm guessing the distributor drive gear, and the mating gear on the camshaft are more likely culprits.

 

Reverse your rotation and see how far the crank moves before the rotor goes.

I would look at replacing the timing set.

The usual symptoms are a wandering idle caused by wandering timing. The idle will in some cases deteriorate to nearly a stall, then snap back to "normal" or a little higher. This is because the cam and in turn ignition timing are moving around.

However, that should not cause a spike at the tach. I think you have some other electrical issue causing that.

I'd fix the timing set first.

 

Tach jumping from 0-4K is not a loose chain.
Since the truck is doing the same thing and you've already replaced the module, I would check all connections and ohm out the coil.

HEI's are one power wire (and in your case a tach)
How does this thing ground?
is the distributor shaft free of binding and slop?

 

One wire from relay so it gets direct 12V, the other is a tac lead.
Must ground through the shaft/body - to- block.
Block ground is good, (and added 2 more grounds)

 

In my experience, a bouncing needle like that indicates an ignition-system problem.

 

I'm guessing the magnetic pickup inside the distributor.

But you could also try disconnecting the tach wire all together and seeing what happens. Maybe the tach wire is grounding out somewhere, or you've got a bad tach and its shorting the ignition.

 

Pulled the tach wire, And no difference in the problem, At lease We know the tach system is not the problem,,,,

 

This is like saying you unplugged the living room lamp from the wall but it can still be turned on...

In the words of one of my favorite Microsoft quotes, I need to ask:

Are you sure?

 

The problem is the truck bucking and stumbling.

I would ohm both primary and secondary sides of the coil.

 

Are you sure you unplugged the Tach? ... Yes.
Are you sure it still runs like poo with the Tach lead unplugged? Yes....

 

I'm not saying there isn't an electrical problem. But if there's this much mechanical slop in it...

... I would get that figured out before I threw anything else at it. There have got to be some mechanical parts that are SHOT. Worst case I could see the timing chain and sprockets, the camshaft (and therefore the lifters) and the distributor shaft all needing to be replaced. Maybe it's not that bad, but I'd want to know how much of that work / expense was going to be needed before I put any time or money into the ignition system.

 

OIC, the bouncing tach is given as a symptom and isn't the primary problem, thanks.

A problem like this that begins "all of a sudden" isn't the result of normal engine wear that's encountered slowly over time, something like this is usually a bad electrical connection or failed component somewhere.