Briggs v-twin backfiring

don1103 · Aug 19, 2018

I have a JD LA135 22HP BRIGGS V-TWIN model 407777-0188-B1. Initial problem was the fuel pump stopped working and I replaced. After replacing and mowing a few times it then started to pop/backfire. I replaced the carburetor but problem did not change. I have since replaced the gas, the gas cap, spark plugs, checked the compression and valve settings, new air cleaner, all of the gas hoses and filter, One of the two coil/module, checked for shorts, removed the wiring harness with the diodes from the coils thinking the diode was bad but it was not. I at one point pulled one plug wire and started the engine and the popping and backfiring stopped. Did same on other side with same result. So I can run on one cylinder without backfiring but not on two.

I am at a total loss as to what is causing the problem. Any help?

bertsmobile1 · Aug 19, 2018

when you said "with the diode removed" did you remove the cut out wires from each coil to do this, if not try that and let us know what happens.
The chip that fires the coils is very sensitive to ripple which is why the diode got put in there in the first place.
Strait compression readings are not much use if there is a acr fitted but the actual readings would be good and better still would be the wet/dry readings each cylinder.
A bent exhaust push rod would be my next thought or a shifting valve guide.
The valves are the same length so should sit at the same heights, both cylinders both valves.

don1103 · Aug 19, 2018

I disconnected the diode wires from both coil/modules at same time. I can check the valves again, but tell me if I am off base. I can run without any backfiring on one cylinder at a time so I think the valves are correct. If there was a valve issue wouldn't it show up whither running with both spark plugs connected or only one connected?

bertsmobile1 · Aug 19, 2018

The reason I asked about the wires is it is possible to just remove the diode on some configurations while on others the kill wires themselves have to be removed.
To eliminate ripples there must be no wires attached to the coils other than the HT lead.
You have the mower in front of you I have words on a screen.
What you think you are saying and what I think you have done can be quite different.
Normally I would say a faulty coil trigger or blown diode.
However if this is the case then when the 2 coils are totally disconnected they should work properly and according to what I think you have stated, this did not happen.

When both cylinders are firing the engine runs a bit faster that it does when one is dead, unless you removed a plug when you did the individual cylinder tests.
A valve that is leaking can run fine by itself a little slower but cause problems at higher speeds.

don1103 · Aug 19, 2018

Thanks, you understood correctly. When I removed both kill wires it made no difference, still had backfiring and popping As far as when I ran only on one cylinder I varied the speed and it ran normal. This is what has me baffled. When running on both cylinders I have the popping and backfiring at all speeds.

bertsmobile1 · Aug 19, 2018

Thanks for clearing that up
Down side, is like yourself I am out of ideas.
Not a problem that has come into the shop before ,
While I can not see why a valve problem would cause backfiring when both cylinders are firing and not when only one cylinder is firing, checking the valve movements and lash will not hurt not cost anything other than a pair of rocker cover gaskets.
Hopefully one of the master techs or those with a lot more experience than me will pick up on this thread.
In any case I shall be watching the outcomes closely.

Boudreaux In Eunice La. · Aug 20, 2018

Ok I'll jump in here......

Don your issue is more than likely a valve issue...... Usually when a intake valve doesn't work right it will do that.......

How old is your mower/engine ???

Bert is right on taking the valve covers off and pulling the push rods to check for a bent one ..........

Also a partailly sheared flywheel key can do that also, but is not common, but can happen.... When you get a backfire that can shear a key easily....

I assume you have the 2 barrel carb on your mower... The Wide Nikki carb is what I mean....

Let us know Mon Ami ~!~!

don1103 · Aug 23, 2018

If it is a valve issue help me understand why when I pull plug wire from one side it runs perfect with no backfiring or popping? I just cannot get past this. Need to understand. This happens with either side.
Mower is a 2008 with 130 hours.
I will check push rods.
I have checked the flywheel key and no problem.
It is a two barrel carb which I replaced with a new one.

don1103 · Aug 24, 2018

don1103 said:
If it is a valve issue help me understand why when I pull plug wire from one side it runs perfect with no backfiring or popping? I just cannot get past this. Need to understand. This happens with either side.
Mower is a 2008 with 130 hours.
I will check push rods.
I have checked the flywheel key and no problem.
It is a two barrel carb which I replaced with a new one.

Ok I pulled the push rods and they are not bent. The flywheel key is ok.

don1103 · Sep 12, 2018

don1103 said:
Ok I pulled the push rods and they are not bent. The flywheel key is ok.

Well after all of the parts being replaced and checking and re-checking everything I found out the carb supplier shipped me the wrong carb. All the problems were because of carb.

Briggs v-twin backfiring

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