Briggs v-twin backfiring

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Old 08-16-18, 05:51 PM
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Briggs v-twin backfiring

I have a JD LA135 with a BS 22hp v-twin model 407777-0188-B1 code 071019yg that is backfiring. I have replaced the fuel with high octane, the fuel cap, the spark plugs, the carb, the fuel lines and pump, the air cleaner and one of the coils. If I remove the spark plug wire from either side it stops backfiring. It starts with no problem and I have mowed with it but afraid to do to much with it backfiring. Baffled. Need help. Also I did a compression test and it was normal.
 

Last edited by Don1103; 08-16-18 at 06:52 PM. Reason: additional information
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Old 08-16-18, 08:29 PM
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That type of thing shows up here a lot. With twin coils there is a diode in the kill harness to prevent cross over grounding. Your diode probably needs replaced or the whole kill harness.

If you disconnect the kill wires from both coils and it runs well, that diode is likely the problem.
 
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Old 08-17-18, 05:59 AM
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Not sure how the kill circuit diode is wired but it seems with only 1 diode, only one coil is protected from a short to ground in the kill circuit. I would check the spark plug wires from the spark plugs back to the ignition module. In areas where the wires are in close proximity, look for degraded insulation in the same area of each wire where a spark could jump between the wires. You could also run the engine in a dark area (or at night) and look for the spark but possible not visible due to wire routing.
 
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Old 08-17-18, 06:28 AM
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Is it backfiring while it's running, or only when you shut it off?
If it's when you shut it off do one of two things, leave the throttle wide open when you shut it off, or bring it back to idle and wait a few more seconds then you have been before shutting off.
 
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Old 08-17-18, 07:25 AM
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I have disconnected both diode connections to coils with no difference, still back fires. It is backfiring while running. I have checked the wires for shorting.
 
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Old 08-17-18, 09:29 AM
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Did you remove the kill wire at the ignition module or someplace else? How did you check the spark plug wires for shorting?
 
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Old 08-17-18, 09:59 AM
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When is the last time you adjusted the valves?
 
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Old 08-17-18, 11:06 AM
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Disconnected diode wire at each coil/module. I replaced one of the coils so it has new spark plug wire. The other I looked at and ran in dark. Adjusted valves two days ago but they were in spec. What really baffles me is I can run on one/either cylinder without backfiring but not on both at the same time.
This started with the fuel pump going out. After replacing the fuel pump the new one lasted about 30 minutes and stopped working. Second one is working fine but all of a sudden it started popping/backfiring.
 
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Old 08-17-18, 11:27 AM
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The symptoms sound like a bad diode, but what you have done to diagnose would have sorted that out.

Possibly the new fuel pump is not working to capacity and things are leaning out causing pre-ignition, But getting what you're seeing with pulling the plug wires doesn't make sense. That type of thing is usually a bad diode.
 
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Old 08-17-18, 12:15 PM
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I think you're probably running a tad lean. Are the backfires big gunshot types out the exhaust or smaller pops and little bangs sometimes in fairly close succession? If the latter, try applying a bit of choke and see if it helps stop the backfires.
 
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Old 08-17-18, 12:46 PM
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I tried adding a little choke and it seemed to help. The lean fuel makes some sense. Fuel pump is working but maybe not to capacity. Two bad ones in a row, my luck. I will order another one. Does that make the one cylinder at a time make more sense? Enough fuel for one but not two?
 
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Old 08-17-18, 02:38 PM
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Each cylinder would feed itself, for the most part. In over analyzing you could come up with a little different air flow for one that's firing and the other not. But that's really stretching it a little.
 
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Old 08-17-18, 02:47 PM
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You are right. It is just bumming me out that nothing has work so far. I will try another fuel pump and see what happens. Don't know what else to try.
 
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Old 08-17-18, 02:50 PM
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Are you using OEM pumps or aftermarket? I see your first one played out after a few minutes.
 
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Old 08-17-18, 03:00 PM
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After market but just ordered oem. I will post outcome, thanks for the help.
 
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Old 08-17-18, 09:07 PM
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I doubt the pump is the cause, especially since applying partial choke helps it. That means the carb has enough fuel in it, it's just not delivering it. I'm guessing the carb is aftermarket too? Was it replaced to solve this particular issue or another?
 
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Old 08-18-18, 07:13 AM
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Carb is aftermarket. Replaced because as I was checking old carb found the solenoid was not functioning.
 
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Old 08-18-18, 11:35 AM
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Once float bowl is full, the engine should be able to run 3-5 minutes without a fuel pump. Once running and backfiring, disconnect power (assuming it is electrical) to fuel pump. If the backfiring stops, the fuel pump is part of the problem. However backfiring is more of an electrical issue. Hard to understand how a lean mixture could cause backfires.
 
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Old 08-18-18, 06:38 PM
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is it backfiring out of the exhaust?
extremely lean mixture may put more hydro carbons in the exhaust but I would probably suspect ignition issue or mechanical issue first as a slightly lean mixture or even an engine starving for fuel do to a bad pump usually does not cause backfiring.
 
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Old 08-19-18, 02:54 AM
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Lean condition can cause backfiring through the intake and through the exhaust. Partial choke reduced the backfiring, confirming it is the culprit. I've had this problem with aftermartket carbs in the past.
 
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Old 08-19-18, 10:54 AM
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I am not sure of this, but I had the similar problem with a Suzuki engine last winter on a snow blower, I think Susuki builds the V twin Vanguard. Mine was not backfiring but I could not stop the governor from excessive "hunting" without running almost full choke, THAT eventually fouled the plug and stalled it, I could just let it run, and hunt, You may have to bore out main jet, tricky stuff but doable.
 
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Old 08-19-18, 01:34 PM
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When you disconnected the kill wire at the ignition module(not anyplace else), did you disconnect both ignition modules simultaneously and still got backfiring?
 
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Old 08-20-18, 07:47 AM
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It is backfiring out both exhaust and carb. I did disconnect both ignition modules at same time and still had backfiring.
 
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Old 08-20-18, 10:04 AM
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Looking for a logic on the results he's getting by disconnecting plug wires. That still points to ignition.

Did anything else happen when it started or it just went from running well to the backfiring?
 
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Old 08-20-18, 11:24 AM
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would recheck the valve adjustment and check for a flat cam turn the engine over by hand make sure all the valves are opening up about the same, your compression seems good so the valves adjustment is probably ok but this engine is known for some issues with the valvetrain like valve guides coming out and bending pushrods.
as per the backfiring getting better with plug wire removed or choke on really may not point to anything usually an engine running lean you would see a significant rpm increase when choke was applied it could help with the backfiring cause the mixture is to rich to ignite.
 
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Old 08-20-18, 05:28 PM
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Today I pulled carb again and made sure surfaces were clean and no obstructions in intake, etc. Put all new gaskets and reinstalled carb. This eliminated the backfiring thru carb so I only have backfiring thru exhaust. Did not find any obstruction but must not have tightened correctly or something else.
As far as what happened when it started. The fuel pump stopped working and I replaced. New fuel pump lasted 1/2 hour and went out. Replaced again and mowed for a total of about 1 hour and it started backfiring/popping. I have a oem fuel pump coming should be here 8/22.
Just to be clear, when I disconnect one of the plug wires the popping/backfiring completely goes away. Engine runs smooth. Maybe I am wrong but it seems if it were a valve problem it would happen whither running on one or both cylinders.
When I applied the choke a little to see what effect it had it backfired less but began racing.
If the oem fuel pump does not solve it I will start checking everything again from the beginning.
Thanks for the help I will post update.
 
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Old 08-22-18, 11:33 AM
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Ok the latest. I replaced the aftermarket fuel pump with oem and it made no difference. I when thru all of the checks again that I have been thru before and still found nothing that solves the popping/backfiring. I thought the backfiring thru carb had been solved but it still does it once in awhile. The popping is all of the time.
Can timing be the issue? I checked the flywheel key no problem. Timing gear? Bur if it is then why does it run on one cylinder without problem? The running on one cylinder with no problem is what I keep coming back to. I hat to open the engine up unless I can come up with an answer to the running on one cylinder.
 
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Old 08-22-18, 06:43 PM
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Since all efforts have failed to date, remove the flywheel and replace the key.. Also check the flywheel for loose magnets.
 
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Old 08-24-18, 07:48 AM
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Nothing has worked. Let you know if I ever find the answer.
 
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Old 09-12-18, 02:13 PM
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Well after all the parts replaced and checking and re-checking I found out the carb supplier shipped me the wrong carb. All of the problems were caused by carb.
 
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Old 09-12-18, 05:34 PM
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Glad you found the problem. However, I am of the opinion that the carb, whether the correct or incorrect one, operates independent whether both plug wires are connected or either one is disconnected. The carb has no interface with the plug wires. Can anyone connect the dots for me for the backfiring scenario presented herein with the incorrect carb?
 
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Old 09-12-18, 09:44 PM
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Connecting the dots

I think the gentleman said that he got a backfire with both plug wires disconnected. Unless I have had too much to drink tonight, I believe that is impossible. I have a 16 hp kohler that was , and still will, backfire. I eliminated a lot of the backfire by re adjusting the float in the carburetor that was not set at the correct attitude, allowing way too much fuel to collect in the float bowl. So I can believe that the addition of the new (correct) carburetor fixed his problem, but if he disconnects both plug wires, he won't have a backfire, or for that matter, a frontfire…..
 
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