Runs then dies
My ‘72 340 Demon ( stock except ‘ mild’ cam ?? ) just started this problem . It starts fine then after warming up 10-20 minutes it struggles and dies driving or at idle ( ambient temperature was 80’ ) No backfire. Will restart immediately but does the same. Spraying fuel in carb does not help when struggling.
My tests and new and near new parts-
fuel tank ,lines ,filter ( it stays full) , fuel cap vent ok & air filter
Thermoquad rebuilt
Fuel pump- 6 psi, 10 inches vacuum , 7 oz delivery in 10 sec of cranking
Coil and ECU - both tested at parts store recently
Dual ballast , magnetic pickup, spark plugs
Timing (initial) 30’ BTC
I use non alcohol fuel
Coolant temperature normal
I am thinking this has to be simple but I am confused as is a local mechanic . I believe one of you could diagnose this in 15 minutes if you were here. I would appreciate any ideas. Thanks.
Yote
Here's my interpretation of what you wrote:
The 6 psi, 10" vac, 7 oz deliver were all fuel pump tests.
Here you were trying to determine if the pump is the problem.
Coil and ECU
Here you were trying to determine if the ignition was the problem.
Timing of 30' BTC.
Not really sure what you were checking here, other than major issues in the timing chain/gears or the mechanic changed something.
What is needed is the rpm, and whether the vacuum advance was disconnected and source plugged. 30* could be fine or way off depending on the other info.
Considering it starts, runs, drives and doesn't backfire, timing doesn't seem way off.
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My suggestion to find the problem.
A. What was changed?
if nothing that you know of
B. Remove at least one spark plug and see if there are any obvious clues.
C. When it dies or is dying, look directly at whether its fuel or spark. For now, we'll assume compression is not an issue because it starts and runs.
When it dies, remove the air cleaner lid.
a. Look to see if choke is open.
b. Look for clues that the bowls are over filling (dripping boosters, everything looks wet) or too low. Clues for too low can be done by working the throttle by hand to be see if the accelerator pumps have fuel in them. If the choke closes, hold it open so you can see. If you have the skill, check the bowls directly to see they have fuel.
Air cleaner on.
c. Install an in-line spark tester. Try to start it up and watch. If its not sparking, you'll see it, and you'll know the problem is in the ignition not the fuel. Its possible the problem occurs only when the ECU or coil gets hot. Or it simply could be a loose or poor connection in the ignition circuit.
Junk or gunk in the carb. Yes a possibility. You need to know a little bit about the carb circuits. I've had carbs get gunked up with charcoal dust from a vapor collection canister. Hard to figure out at first but one clue of gunked up was turning the idle mix screws did nothing (at slow idle).
With the carb off, you can often see dirt, varnish etc in the bowl etc. Blow out each passage or system (wear goggles) with spray to be sure they are all clear. Its good way to learn the different circuits.