Backfire Under Load.

This issue has nothing to do with vacuum.

 

I just went through this with F-134 in the snow plow jeep, the rotor was dirty, replaced it and problem solved

 

I’m thinking electrical or possibly float issues. I confirmed I had good grounds from batt to block to frame and firewall.
Here’s my test drive technique:
Start it in the driveway and let it get to operation temp. Then I drive down to the main road (couple of miles) and it runs great. Turn on the main road and WOT and it takes off and is pulling hard till 30-35 mph then it just dies. No bog just quite, like you turned the key off. If you put it in neutral and turn the key off then back on it will start right up and run great.
Thinking electric because if the float was set wrong it would bog out not just turn off. I don’t know….

 

Checked it and feels fine. No marks on the inside of the rotor from rubbing.

 

The Hei power is on a relay that is triggered by the key. That was my first thought. I was thinking drop in voltage and the coil was dropping out.

 

Or like the HEI is fugged. I hate electronic stuff, failure behaviors are not subject to roadside logic.

How many pins is your module? Does it have the "knock-sensor" fifth pin module? Wondering since it dies when pulling hard.

 

Weak/bad battery?

 

4 pin

 

Your problems all seem to point to an electrical (ignition) issue. Does your advance mechanism in the distributor move freely? Assuming it has a vacuum pod on the side, can you test that if you apply vacuum, the advance operates? Is the movable plate where to ignition module is located grounded properly?
Is your ignition switch bad? When driving, wiggle the key a bit to see if this duplicates your issue.
-Donny

 

So I checked all advance parts and that are working great.
For the ignition question I rigged up my volt meter so I could drive and watch the supply voltage to the HEI coil relay. It was 14v the whole time but the jeep did die again and the voltage did not drop. My next thought was a bad ground. Checked the module and tightened and cleaned the ground points. I even went as far as to sand the and clean the distributor hold down clamp. It’s still doing it!!!!
I’m stumped!!! It acts just like when you turn the key off when you’re driving down the road.

 

Just thinking here, may not apply to the wiring of a HEI dist, I do not know. However I had 74 ford F-350 truck with a 390 and a points/condenser distributor. It used a resistor to lower the voltage to the points. It also used a fender well mounted starter solenoid with the extra wire that would send a full 12 volts to the points when cranking the engine. I was having intermittent problems like you, the engine would die for a couple seconds and then restart. The cylinders and exhaust would fill quickly with raw gas fumes and would backfire when the ignition came back on, even if it was off for only a couple seconds. It would usually do this when the engine was pulling a grade, when the accelerator was pressed down and loading the cylinders with raw gas quickly. After weeks of frustration I finally found the problem when it finally shut off and did not restart. What was happening was the starter solenoid had shorted out inside and would temporally and occasionally ground the wire to the distributor killing the voltage to the points. New solenoid, problem gone. Like I said it may not apply to a HEI system..

 

If it died while you were watching the meter and it was still 14v at the coil, this wouldn't apply. But I had a similar issue, and after hot wiring it to narrow down the cause, it was the ignition switch, and since it was intermitent, it took a little while to get it.

 

Two other non-ignition things that could cause similar symptoms are:

• Float level too low. This can lean out under load. I had this issue after rebuilding the Edelbrock carb on my '71. I added spring loaded needles and it wasn't clear if you set them with the spring compressed or not compressed so I set them not compressed. The engine would run fine at light load but would pop and lean backfire under load even dying sometimes. Setting the float level with the needle springs compressed raised the floats about 1/8" and solved the issue.
  
• Flat exhaust lobe on the cam. Since the exhaust can't get out of the cylinder the normal way, it backs up into the intake and wrecks the carburetor airflow causing the engine to lean out.

Float level is easy to check. You can check the cam by pulling the valve covers and looking at the rockers while cranking it over. All the rockers should move about the same amount.

All that said since it immediately starts right up afterwards it seems like it might be ignition. I would swap a new quality module into the HEI and make sure you use heat conductive paste when you do.

It would also be interesting to have your timing light in the cab when under load to see if it stops flashing when you have the issue. You could try it on every cylinder. That would at least give you some insight if it's air or fuel.

 

All these guys are way smarter and more experienced than I am but maybe something simple stupid. Have you checked your carb mounting bolts?
I had slightly similar problem after a carb rebuild. The new gasket compressed after initial installation creating a loose mount.

 

It could still be the module. I converted my TR6 to a GM based fuel injection a few years ago. One of the problems I initially had was it would run and drive fine for a short period and then quit. If I let it cool down, it would start and repeat the cycle. I tracked it down to the ignition module. If you decide to swap out the module, get a good quality one like an AC Delco and be sure to use the heat sink paste that should come with it. Good luck.

Tim

 

just may be a loose connection on the ignition switch.. Only take a few seconds to drop switch and check for loose connection

 

Try removing the vacuum hose to the advance and plug it and drive it. This will eliminate any movement of the pickup coil to the module. HTH.

 

I checked the float and changed base gasket but no change. Still dies!
Bought a GM module and will install it tomorrow and see there is any change.
Thanks for all the input so far!!!

 

New GM module did the trick. It had white heat gel and the other had clean. There might be something to the white gel??
Thanks for everyone’s input!!!!

 

The white gel is for enhancing heat transfer. Without it, the module will eventually overheat.