Crappy running continues...with a twist now

[Blown4.6]

Okay, I've been going around and around with the stumble on light to moderate acelleration and also cruise forever. Here's an odd twist. Maybe it will shed some light on the problem as I think it's what's causing the problem altogether:

The new twist became obvious today. After work, I started it (it is almost impossible to start when cold....you have to feather it just right and rev the crap out of it or it will die and flood). It barely started, like normal....then ran really, really poorly. I mean, it sounded totally different than usual. It sounded like it was missing on one or more cylinders, then, it was clearly hitting on all cylinders but sounded like a volkswagon not a cam'd 351w. Regardless of RPM, it just sounded like crap and didn't really want to be running. I let it idle and it just died. Then, I was able to restart it immediately. Now, it sounded like a healthy NASCAR???!!!! Great idle complete with lope. When I reved it, it roared like it should.

Based on this, the problem is intermitent; but what changed between the first start and the second? I can't figure it out?? I think this is the clue to what the whole problem is. What could just come and go between starts......kind of like a reboot or something.????

 

[nds03]

whats on the engine, carb? ignition?

 

[65fastback2+2]

Okay, I've been going around and around with the stumble on light to moderate acelleration and also cruise forever.

i know how this junk goes

 

[Blown4.6]

whats on the engine, carb? ignition?

Engine: 351w
Intake: Edelbrock Performer
Carb: Street Demon 525cfm w/electric choke (have
some suspicions about the electric choke)
Ignition: Pertronix Distributor w/flamethrower coil
Cam: 230/230 @.050
Vacuum: 16@idle

It used to drive okay and stumble at 1500 rpm when acellerating from a stop under light-moderate load. Really slow acelleration usually avoided the stumble as did romping on it. This is still the case; but, now it just doesn't want to run most of the time. It will idle fine then just BLIP, shuts off. It also, once warm, just wants to die on the freeway when cruising. If you punch it, it smooths out. Otherwise, it just feels like someone's turning the key on and off while driving.

Like I said, yesterday I started it and it ran like absolute crap. Didn't even sound like my engine. Then, it stalled. I restarted it and it roared like a beast (normal). What happened during those 5 sedconds? What changed?? I think this is the heart of my problem, if I can put my finger on it.

??????

 

[jcode68]

Have you checked the fuel filter for a clog? An inline pressure guage would be ideal, but filters are cheap enough if you haven't replaced it in a while. If you suspect the choke is not working properly, this is another obvious suspect that would cause the symptoms you describe. Hard to tell based on what you described if it's a fuel or spark problem. Keep us posted on any other nuggets that night help solve the puzzle.

 

[2nd Mustang]

How old is the carb? Try disconnecting the wire to the choke, make sure the choke blade is in the upright position and see if the problem persists when engine is warm.

 

[snowball]

Sounds like it's carb related. The accellerator pump may be the problem here. Maybe the carb needs a rebuild.

 

[StangDreamin']

Ignition: Pertronix Distributor w/flamethrower coil

Never heard of it happening to this particular brand; but inside that Pertronix Ignitor module there's either an SCR or an FET (electronic acronyms for "little components filled with smoke") that might very well be on its way out the door. After you've gone all over fuel delivery and vacuum hoses; if nothing else turns up, look at the module - maybe swap it with somebody else's, if you've got a really good friend that's willing to take a chance.
Those particular ("transistors" is a good generic technical term) as a group have be known to once-in-a-while develop a "hole" in their semiconductor layers that will "heal itself" now and then; and once they "heal", you're back in business. This breaking down doesn't happen that often, but it can happen. If this is what you're experiencing right now, you can rest assured that you will finally pinpoint the problem when it will die completely and forever - on the freeway, in the rain, on your way to a job interview or in the bottom of Salt River Canyon in Southern Arizona in mid-August (don't ask how I know - both instances).
It just doesn't happen all that often (given the sheer numbers of Pertronix/MSD/Duraspark/GM HEI boxes that are out there); but if nothing else turns up.... But as I said, I know it can happen. MSD on my (long gone) '72 F100, re-badged Duraspark on my then-girlfriend's AMC Gremlin (apt name for the car).