Backfiring through the exhaust

[PTmowerMech]

Here's the video. I'm not touching the throttle. The RPM's went down on their own. A few seconds later, they came back up.

 

[Auto Doc's]

Hello PTmowerMec,

In the video, that sounds like a coil breaking down issue. You would need 2 adjustable air gap spark testers between the plug tips and the coil plug wires. Run the engine while watching the spark jump the gap on each tester.

One coil will spark continuously while the other breaks down and comes and goes intermittently.

The V-twin engines don't shake and rock as people would expect because they are mounted solid.

 

[PTmowerMech]

Hello PTmowerMec,

In the video, that sounds like a coil breaking down issue. You would need 2 adjustable air gap spark testers between the plug tips and the coil plug wires. Run the engine while watching the spark jump the gap on each tester.

One coil will spark continuously while the other breaks down and comes and goes intermittently.

The V-twin engines don't shake and rock as people would expect because they are mounted solid.

With the spark testers I have, I see no formidable differences between either side.
Note: They're not adjustable air gap spark testers. But I they do show when exactly the voltage comes through. Both sides glow bright.

 

[Auto Doc's]

Sorry, you are using junk spark testers.

 

[PTmowerMech]

Sorry, you are using junk spark testers.

True that. But what I'm supposed to be looking at is gaps between sparks, instead of a constant blinking, correct? Even with the junk ones like I have, these gaps are visible.
I did notice some gaps in the spark. But they didn't coincide with the backfiring. As in many times there was several back fires in a row, when the sparks were constant.

Maybe there's more to those that are adjustable than I'm aware of. Above my pay grade, sort of speak.

 

[Hammermechanicman]

Have you done a compression or leakdown test on the engine?
Disconnect one plug and start the engine. How does it run? Reconnect the first plug and Disconnect the other plug and start the engine. How does it run?

 

[Forest#2]

Here is the good inline spark tester.
OREGON SPARK TESTER Repl Briggs 19051 19368

Pull a plug wire one at a time and listed, compare one cylinder to the other.
Appears it's back fires out the EX maybe. Removing spark might give you a clue, no spark=no backfire
I suspect a valve not seating erratically.
If you find the suspect cylinder, pull the valve cover and closely inspect.
BUT:

I've also seen carb do such (spitting and sputtering) on a Briggs twin OHV, especially the Dual throat 2 barrel Nikki's. Each throat and the intake runner are separated from each other, one side of the carb foul and upset just one cylinder. I've even seen one cylinder completely dead due to the carb.
Is your carb the dual or the single throat? (that model engine used both types carbs, just required a different intake manifold is why I ask)

 

[slomo]

You can hear one good cyl and one trying at times. Might have a valve nearly closed, that is stuck open.

Definitely look at ignition and valves/cams. Pop the valve covers and see what you got.

 

[slomo]

Note: They're not adjustable air gap spark testers. But I they do show when exactly the voltage comes through. Both sides glow bright.

Stop it with the inferior spark tools LOL.

Taryl made a video "showing" all us how a coil can show "visible sparking" with the inline Christmas light types. Would blink at low, med and high revs with the bulb type. Gap type showed spark at low revs, nothing at mid and high revs at all. Weak coil all the way.

 

[slomo]

Here is Taryl's video on spark testers. Explanation starts at 4:04 through 7:07. Fast forward to get to the point.