Technical 40 plymouth hesitation

So I have been fighting a hesitation for a month now. In the last month i have rebuilt the carb, installed a vacuum guage( runs 18hg at idle), replaced the mech fuel pump and installed a pressure guage (hold steady at 4-5). Bumped the timing up to 10.

Even after all of this the truck will start to surge when it gets above 45 in third or 25 in second. It will pull strong up to this point and then starts bucking. The vacuum will slowly drop from 18 to 5... When you let off the throttle the vacuum shoots back to 20+.. the fuel pressure never drops

In neutral you can run the rpm high and it never stumbles and the vacuum idle hold mostly steady.

I'm now wondering if the carb is at fault, it has a d6g1 carb and it is a 1948 dodge 230 engine. Is the carb wrong for the block? What else could it be? Weak springs would show up while racing in neutral, right? The needle may flutter 1/2 hg in each direction when it is racing but that is all.

I'm at a loss? So are my local car guys.... any thoughts?



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Right after I adjusted the valves & tested compression, I'd start with copper plug wires and some non-resistor Autolite plugs. Both a weak condenser or coil will act as you describe. Also make sure the exhaust & air cleaner are not restricted. Heat riser frozen shut ?
You might also pull the top off the carb to check for debris broken loose when the fuel lines were opened. We normally put an inline filter before the pump.

 

Have you tested the actual fuel volume you pump is providing? Take the line off the carb and direct into a soda bottle or similar. Pull the coil wire so it won't start and have a helper crank the starter while you watch and count them pulses from the pump. For 12 pulses you should have 8 ounces give or take in the bottle. If not you have a restriction on the suction side. If the volume is OK check the needle and seat and float level. One culprit is the hose between the pump and the frame that allows for engine movement. They can look fine but collapse under high suction. There is also an oilite flyer around the tank pick up. Blow some air ( 15 psi) back from the suction side of the pump back toward the tank. You want to assure you have good air flow and be able to hear a good release of bubbles at the gas filler.

 

To me, a hesitation is when it will not accelerate with gas pedal movement. What you are describing is more of a load based fall-off.
I'd check the electrical system to make sure it's providing power, then as 302GMC says, check the spark under load. Also check total advance during load. If it's too much it will fall off.
I'd also check fuel volume as plym_46 mentioned. Reving in neutral does not require as much fuel.
When you think of the basics, the compression isn't dropping, so it's either A/F or spark.
If everything checks okay, check for plugged exhaust. Not nearly as much of a problem on older stuff, but will behave as described too.

 

Inside the distributor, there is a thin flexible wire that connects the could wire terminal to the points. This wire flexes as the breaker plate moves as the advance mechanisms work. Failing insulation can cause inadvertantly grounds if the wire contacts the dist body. The wire can also fail from flexing within the insulation causing broken connections. Check it. They are hard to find because you need the flex, and can't use regular insulated wire. I found mine with failed insulation. I wrapped it with thin gotten thread, then coated it with thinned rubber cement. 12 years and still serving. Also note that your car should be set up for positive ground. The wire from ign to coil should go to negative coil terminal, positive term to distributor. Some times an old coil will not be tolerant swapped wires and break down sooner when wired up with reversed polarity.

Oops! Just saw it was a forty. That coil goes through the firewall making reversed polarity between coil and points less likely.

 

Guys I'm happy to say after 2 months of fighting this.... Saturday I changed out the coil, condensor and points (all of which were less than 3 months old) and it works great. Was able to cruise at 55 and it still was . pulling.

Don't know which one of the 3 it was, but on don't care as long as it runs..



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