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40mph Stumbles then stalls out


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Edelbrock carburetor stumbles then stalls out around 40mph.  Replaced carb with a Summit Racing M2008 Series, 600 cfm, square 4 barrel, electric choke... same issues.  Mechanics have tune it at least 3 times already and after about 20 minutes of driving, trying to go over 40mph is impossible.  Truck stumbles/hesitates, I let off gas.. dropping down to about 25mph, give it gas and then it just stumbles out and stalls.  Sometimes I have to wait at least 5 minutes for the truck to start back up.. other times, longer because the truck won’t even crank.. so something is needing to cool down to even start it again.  I’ve replaced the fuel pump, added a clear fuel filter and nothing is gumming up in it. New plastic 33 gallon gas tank, fuel sender is good.. used Lucas fuel treatment.. checked wires, hoses, spark plugs.. everything is good.  I can’t think of anything else to try except to take out the carburetor and try a Fitech Fuel Injector System.

ANY THOUGHTS & or SUGGESTIONS PRETTY PLEASE!!!!

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What kind of car, what engine, what ignition system? More details will help us nail it down. A good number of carburetor problems turn out to be ignition problems, and if you've had three mechanics try and fail to fix the carburetor, my guess would be that you're looking in the wrong place. Coil? Points? Plug gaps too wide? Wires breaking down and cross-arcing? On many Fords, the ignition system runs on 8 volts with a resistor wire from the ignition switch to the starter solenoid that drops the voltage when the engine is running. That wire can go bad, the ignition switch can go bad, the distributor might have a loose wire that's causing an intermittent ground, anything like that. My wife's Mustang had an untraceable stalling problem that would cause it to stutter and stop running without any warning. We replaced everything in the fuel and ignition systems without any success. Turned out to be a faulty starter solenoid that was bleeding off enough energy from the ignition to cause it to slowly stop firing the plugs. 

 

If it's not fuel, it must be something else. Ignition is where I would be looking at this point...

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On 2/26/2019 at 8:38 PM, 79fordgirl said:

other times, longer because the truck won’t even crank

 I'm always confused by what people mean "engine won't crank". To me it is simple, just like the hand crank, if the engine does not crank then the crankshaft IS NOT moving. i.e. the starter is not engaging the engine, if the starter is even turning at all. 

 

An engine that does not start running when the key is turned or the hand crank is spun, but the crankshaft is turning, is an engine that is not starting. It IS cranking.

 

Rant off.

 

BYTW, what did YOU mean by your statement?

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Verify fuel delivery. There was an account of an early sixties Cadillac that had a speed issue similar to this one. After many, many , many months of diagnoses and parts replacement, it was determined to be two small dents in the fuel line. The spacing of those dents and the timing of draw for each stroke the fuel pump, interrupted fuel at the high speed. So verify fuel delivery, especially given a new plastic gas tank. Perhaps the pick up portion of the new tank was not drilled out enough? And is the new tank properly vented?

 

Could this be an ignition problem? Coil getting hot? Original Poster doesn't provide enough info on the vehicle configuration.

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Lots of guessing......I recommend a good technician and a proper diagnostic technique. Fuel, ignition, and compression are all it takes. Bad coils and condensers get hot and short out. Fuel pressure AND volume check. Too many possibilities with no information. I would run a manual fuel guage and tape it to the windshield, ignition is simple if you carry the proper tools, it’s not rocket science........it’s going to be something simple, just as all running problems are. Don’t guess, diagnostic techniques to prove good/failure and a process of elimination. It shouldn’t take more than twenty minutes to figure it out.

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11 hours ago, edinmass said:

Lots of guessing......I recommend a good technician and a proper diagnostic technique. Fuel, ignition, and compression are all it takes. Bad coils and condensers get hot and short out. Fuel pressure AND volume check. Too many possibilities with no information. I would run a manual fuel guage and tape it to the windshield, ignition is simple if you carry the proper tools, it’s not rocket science........it’s going to be something simple, just as all running problems are. Don’t guess, diagnostic techniques to prove good/failure and a process of elimination. It shouldn’t take more than twenty minutes to figure it out.

 

 

Notice I said........... fuel pressure AND volume..............been there done that. Glad you got it figured out. Happy Motoring! Ed

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