Backfiring through the exhaust

[Dpmulvan]

How many hours on the mower, what year is it? Is it parked outdoors, any sign of vermin? Sounds like an mis to me not a backfire. Does it get worse when it’s warmed up? If so probably a coil. Start easy,
obvious, before chasing your tail. I’d type more but the pop ups on this site make it ridiculous sometimes.
very annoying.

 

[pull-do]

My 407777 was doing the same symptoms, took the Nikki off and cleaned it, still acted up,, (lean backfire) I thought. Took the carb back off and went through it again thoroughly with the new O-rings,,, runs good now.

 

[Stainless69]

407777 0188 B1

On a service call, started out, this thing ran like crap. Lot's of back firing through the exhaust. After about 10 minutes of it running & mowing, it seemed to clear up. It was still backfiring, but in spurts, it would completely clear up for several seconds. Maybe even a couple of minutes.

So I thought maybe it was just some junk in the carb that made it's way through. A couple of weeks later, customer calls and says it still running bad and that it wouldn't even stay running. So I picked it up and brought it home. (like a dummy, I didn't even try to crank it there)

At home, I crank it, and it's still got a little back fire issue. But nothing major, like it was the first time I cranked it. I throw on a set of new plugs (RC12YC). Of course that didn't change anything. But it eliminated the possibility of a bad plug. I ran it for about 10 minutes and the plugs were spotless.

So I guess the question is what could make it run like crap one minute. I don't want to do nothing to it, return it to the customer, only for to start doing it again.
Like I said the only real symptoms now is the slight exhaust backfire. I remover the air filter so I could see down in there, and didn't see any carburetor back fire.

Mine did this before. I had to adjust the valves. It was pretty easy to do after watching a few youtube vids. The worst part was cleaning the old gasket off of the valve cover.

 

[PTmowerMech]

Mine did this before. I had to adjust the valves. It was pretty easy to do after watching a few youtube vids. The worst part was cleaning the old gasket off of the valve cover.

I checked the valves, they were at .06. Close enough.

 

[PTmowerMech]

How many hours on the mower, what year is it? Is it parked outdoors, any sign of vermin? Sounds like an mis to me not a backfire. Does it get worse when it’s warmed up? If so probably a coil. Start easy,
obvious, before chasing your tail. I’d type more but the pop ups on this site make it ridiculous sometimes.
very annoying.

I got the new tester in (after I returned it). When I get back over there, I'll see what the spark looks like. Customer says it's funning fine now. But I'm thinking she just can't hear what I hear.

 

[PTmowerMech]

We have a brand new John Deere Z950M Zero Turn at the park where I work since retiring! With only 50.4 hours on it, it started backfiring through the exhaust. Long story short, It turned out to be Water in the fuel. Since it was brand new, we took it back to the dealership to let them work on it. They drained all of the gas, blew out or replaced the fuel lines and put on a new fuel filter, fuel pump and replaced the spark plugs. Today it has 75 hours on it and it is running flawlessly again!! So, check closely for water in the fuel system!!
Good Luck.

There was no water in the carb. It's kept under a car port. But here in Arkansas, that doesn't mean much, as we have high humidity all year long

 

[PTmowerMech]

So you cleaned out the jets and passageways, not just looked?

Yes, sprayed carb cleaner through every hole, keeping an eye on the stream that comes out the other end. Especially to two ports that come out into the venturi. Ran the guitar strings and probes through the holes and the jets. Replaced one intake mounting gasket. It was very clean.

 

[Forest#2]

My 407777 was doing the same symptoms, took the Nikki off and cleaned it, still acted up,, (lean backfire) I thought. Took the carb back off and went through it again thoroughly with the new O-rings,,, runs good now.

Quite often with the dual throat Nikki's I can get a good run leaving the carb mounted to the engine and remove enough stuff that I can remove the 3 screws in the top plate, carefully remove the plate and then use low pressure air, about 20 to 30#'s and blow air across the top of the little channels. Do not direct inject air, keep the air nozzle back away from the channels. This is a jet area and the fixed jets are smaller than a tag wire.
Wear eye protection because the air will backflush fuel out of the carb bowl upwards also. Apply a lube to the flat gasket or remove the gasket and turn it upside down with lube for a seal. Start the engine and test.
If no go do as pull-do indicates.
Main thing is do not throw your NIKKI away expecting a China clone replacement to run ok. Try to repair your NIKKI.
These Nikkis'a have a learning curve and lots of info on-line about proper ways to repair them.