Stumble, backfire when I hit the gas

J

Joe D.

New member
The Scout in question is a '77 with a 345, Holley distributor with the gold box, and a 2210 carb. The coil and the gold box are fairly new. We rebuilt the carb, using the correct gasket that doesn't cause a vacuum leak, and it was starting and running sweet for over a year.

Then one day it started missing when climbing hills. Soon it became undrivable. Can barely drive down a flat road, misses and backfires on acceleration. Took the carb apart, it's still spotless inside, and everything was in good working order. Accel pump squirts fuel immediately. Took the dizzy apart, cleaned everything, and replaced the broken advance bushing. Now the mechanical advance is smooth as butter. Vacuum advance has no leaks and full range of travel. Cap, rotor, wires and plugs all look great. Starts easy, but drivability has not improved. Playing with timing doesn't help.

Here's my last ideas:
1.coil has failed - but I would expect it to run worse as the engine warms, instead it is equally bad.
2.the pickup in the dizzy has failed - but then why would it idle perfectly?
3.the gold box is failing - could this cause a weak spark that would ignite idle mixture but not a richer acceleration mixture?

I'm not sure what else to check. Any ideas?
 
Since it's easy to check the coil, pull a wire put a widely gapped plug in it and see if you get a nice hot blue spark. You might also check and see what the voltage to the coil is with it running.
 
Joe d. said:
The gold box is failing - could this cause a weak spark that would ignite idle mixture but not a richer acceleration mixture?
Click to expand...

Don't discount what has been recommended already but see below for another good possibility.

The rule of thumb is that the more dense the air / fuel mixture, the more voltage it takes to ionize the air and jump the gap.

In the case of idling, that is when the cylinder pressure is the lowest and jumping the gap is the easiest. As you place the engine under a load and open the throttle, the cylinder pressure increases and thus making jumping the gap more difficult to do. A compromised ignition coil.

The problem is usually the coil insulation breaking down and the spark jumping to ground inside a compromised coil. Once the spark goes to ground inside the coil the first time, a carbon / conductive track is created in the coil where the spark can more easily get to ground. Bad plug wires can increase the spark voltage and weaken the coil and or add to the secondary coil voltage requirements until the onset of coil failure.

The same thing that I wrote about the coil can occur on the cap. Once the carbon tracking happens the cap is done.
 
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My money's on the ghost box. Been down this road many times, suffering the exact symptoms. Hope I'm wrong and it turns out to be something simple. But if not....
If you want easy but not super cheap, there's a drop in module made in san dimas, CA that works fine. If you are handy with tools, check out bill hamilton's hei conversions (look in the efi forum here). Gives you the performance of modern ignition with the ability to replace any (or all) of the conversion pieces at any parts store. Plus its half the price of the other thing.
Whichever route you go, make sure to match the coil to the application. It matters!
 
The bushings were replaced first thing.
The carb was rebuilt and is still perfectly clean.
The coil was also replaced and just about to try replacing the gold box as that's the last piece of the ignition system to test.
Spark is strong and no sparking at night.

The Scout Joe mentions is my Scout. So that's why I'm posting these updates

I will post an update upon further testing
 
I know it has been awhile since any updates so here's what going on. Reset timing again. Tested new coil and gold box with same issue. No arcing at night. So it really only leaves the carb. The carb looks fine and has been running fine for years so not sure what happened to it. Trying to adjust the idle screws doesn't work on it. So I'm sending it to Holley to be remanned
 
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