Car is suddenly sputtering- stumbling from low rpm

[Biglablock]

Hello I have run into a problem that has me stumped …I’ve been working on this all day and I’ve gotten nowhere. I was driving to work this morning and the car ran excellent over the San Mateo bridge into Hayward there was no issues with the car in fact it was seem to be The best it had run since I Swapped it into my dart a few weeks ago. Then suddenly and without any indication that I can detect it began sputtering from low RPM when I’m trying to takeoff from a light or whatever and then it begins to grab in about 22 or 2500 RPM and then above that it it’s running strong but I can tell that it’s missing in at least one or more cylinders. Now, I know my timing hasn’t slipped, Distributor is locked down tight. I’m running an MSD 6a, And I figured if it weren’t working at work it wouldn’t work at all it wouldn’t just give me a spark here and there. I looked over my wires, My distributor cap and rotor and everything appears normal. So I checked the carburetor which is an edlebrock 1407. The symptoms sounded to me like he could’ve been a failed accelerator pump so I bought one and stuck it in. The metering rods and step-up springs are in good shape and normal. The fuel filter is fine, no issues. Went back to the fuel pump pulled it down from underneath the back where I’ve got it mounted double and triple check the wiring we did a couple things back there made sure that my lines were not pinched in anyway and when I turned it on it sounded normal although It doesn’t seem to be changing with the RPM anymore and I’m not sure if that’s me trippin, though. It seems like it’s running lean and I don’t know why and I don’t know how do identify the source. But if any of y’all have experienced this and know what I’m trying to deal with please advise. And thank you in advance

 

[Joey4speed]

I figured you're running an electric fuel pump, but what engine?
Car Make, model?
Modifications?
More information would be helpful.

 

[66fyssh]

How do the spark plugs look? Any fouling?

 

[Professor Fate]

Could use a lot more info on your setup, but a few thoughts:
What kind of shape is your gas tank in? Any rust/dirt in the tank will plug up the filter sock on the pickup tube- and fall off after it stops running, causing an intermittent issue. Add the fact that electric pumps have a notoriously weak draw (electrics aren't designed for suction, but make good pressure) makes it hard to keep fuel flowing once the sock starts silting up. And speaking of pressure, you do have it regulated down to 6 or 7 pounds, right? And it is mounted at or below the lowest part of the tank?
You say you just swapped the motor in, but how fresh is it? Later small blocks (factory) had nylon coated cam gears that really don't age well- they start shedding teeth and can give you all kinds of grief... Again, more info would be helpful.
If all else fails, I'd begin to suspect the MSD- double and triple check your connections and wiring to make sure everything is correct. I've been hearing an increasing amount of dissatisfaction with them lately. Just sayin'.

 

[Gator]

I had basically the same problem and after checking everything, timing, plugs, fuel filter, etc.. and all was ok and a friend told me I prolly had some dirt in my carb so we tore it apart and sure enough in the float bowl their was some fine dirt particles that would partially plug the jets. We totally clean the carb and I put it back on and BINGO that was the problem. Car has been running fine since....

 

[Biglablock]

Well my car is a 73 dart swinger it has a 360 that a young friend of mine had rebuilt assuming that it was stock crank he did rings and bearings but as it turned out the crank had been turned 10 under from the factory so he sold me the motor for 600 bucks and I redid the bearings and put in a different cam and fired it up and it runs great makes good oil pressure everything can’t complain.It has just some 596 heads regular 360 valves, comp 901 Springs, In Elgin cam is it 223 447 on a 112, with s weiand stealth intake and edlebrock carb. He has a Mopar magnetic pick up distributor with MSD 6a, some 7 mm accel wires, Champion Spark Plug‘s, Sanderson DD seven headers full duel exhaust with pypes race pro mufflers, Tail pipes all the way out the back, Hi volume oil pump with a kevco gated oil pan, bolted to a 904 that has been built, 2500 stall hughes converter, TCI outlaw, back to an 8.25 with unknown gearing, but its right at 70mph at 3000 rpms. It needs a new harness, that has Been the weak point for a while, and I am doing the suspension now, I got some bell tech nitro drop II’s to put in front, aint chose the back ones yet. Plan was to buy a usct relo/mini-tub kit and swap in a 9” with 4.11’s and discs. I like cal-tracs and will be running those as well. That’s my set up, hope that’s enough info

 

[Biglablock]

Oh yeah my fuel pump is mounted below the lowest part of the tank and it’s regulated about 10 inches in front of the carburetor I have it set at 5 -5 1/2 pounds, It’s a 90 gallon per hour unit from speed master and as far as I know that’s the only foreign object on my car as far as I know anyway ha ha

 

[Biglablock]

 

[Bewy]

I doubt it is fuel related because in post #1 you say it runs strong above 2500 rpm. More fuel would be used as rpm increases above 2500, so I would expect the spluttering to increase, not improve, above 2500 if it was fuel related.
The MSD 6A could be the culprit, hard to know. If it has some amount of misfire at all rpms, could be cam lobes going bad, incorrect lifter adjustment.

 

[Bewy]

Just thought of something...
[1] Make sure the connections of the heavy red & black wires to the MSD are good, not corroded.
[2] If vac adv is being used & dist has high mileage, the pick up wires inside the dist can break from flexing. Result is no/intermittent spark. To test: disconnect p/up plug at dist & measure p/up resistance. Should be 100-500 ohms. Flex the wires while measuring.

 

[Gator]

I doubt it is fuel related because in post #1 you say it runs strong above 2500 rpm. More fuel would be used as rpm increases above 2500, so I would expect the spluttering to increase, not improve, above 2500 if it was fuel related.
The MSD 6A could be the culprit, hard to know. If it has some amount of misfire at all rpms, could be cam lobes going bad, incorrect lifter adjustment.

I tend to highly disagree with this. My car would not idle and had to feather the gas pedal to keep it running but with my rpms at around 2500 the car would idle. So any RPM's below 2000 the car would run ruff and once up to and past 2500rpm the car would run fine. Tearing the carb down and cleaning it completely eliminated the fuel issue. Take this from someone who has experienced that.

 

[Bewy]

Gator,
My comment in post #9 about 'fuel related' was about fuel delivery to the carb, not a carb that has blocked jets/passages. I was referring to the OP comments on fuel filter, fuel pump, which sounded like he suspected the carb was running out of fuel.

Your issue sounds like blocked jet/passages.