Z560 backfire

[Dkite26]

My z560 backfires but only when it’s not cutting. When the blades are engaged and the engine is under a load cutting, the backfires stop. When the blades are disengaged and just driving, it backfires. I’ve replaced all filters and the filter housing and spark plugs. I don’t see any cracks on the exhaust. Kinda hard to drop $200+ on coil packs on a chance that solves it

 

[Mark H_NO]

Is it backfiring through carb, or exhaust? What are the mower and engine model #s? May need serial also. How long has it been since the carb was cleaned? How much work do you do with it?

 

[Dkite26]

Is it backfiring through carb, or exhaust? What are the mower and engine model #s? May need serial also. How long has it been since the carb was cleaned? How much work do you do with it?

Thru the exhaust. It’s a z560 with a Yamaha 825v. Carb hasn’t been cleaned any. It runs several times a week except for the winter but we’ve never had this issue in the past 5 years after sitting for the winter. Always non ethanol gas in it

 

[sgkent]

what brand plugs and where did you get them? When is the last time the valves were adjusted?

What you are describing can have many causes. Basically you have a problem either with ignition, compression or mixture.

If the ignition fails on one cylinder, say a bad counterfeit plug you got on say online, that unburned air and fuel mixture gets pushed out into the exhaust where the hot gases from the cylinder that fired ignite it in the muffler - creating a backfire.

If you have tight valves from many hours of use and them not being adjusted, the mixture flowing into the cylinder can escape out the exhaust valve at low speed and ignite in the muffler, or if it is the intake that is tight, the mixture can revert and push back into the intake manifold causing the cylinder to misfire, again causing an unburned mixture to flow into the muffler to be ignited by the next charge or the other cylinder's exhaust.

If you have two carbs, or a carb with two venturi's, one side can have the idle circuit partially plugged where the mixture is so lean it can't burn. In that case it escapes into the muffler unburned where it goes off. Another cause can be an intake manifold leak, or crack where at idle the air being drawn in is enough to cause a lean misfire, and that charge then ignites in the muffler. A partially blown head gasket can also affect a mixture and cause this. At speed the carb is using a different jet / circuit to run. Also at speed the mixture has much more velocity so it can overcome leaks / tight valves etc., that cause a misfire at idle.

I would probably run a compression test on both cylinders, and see how they compare. Then use an IR gun to see which exhaust is cooler while it is backfiring. If one is substantially cooler then swap the spark plugs and see if the results change. If they are even, I would hold part of my hand over each side of the carb to see if changing the mixture (your hand is working as a choke) changes anything. You can use some Berryman's B12 to make very small sprays along the manifold etc to see it there is an intake leak. Or use a propane torch turned on a little and run it around to look for leaks. You don't want to add so much fuel or propane a fire starts. Also some mechanics use a smoke machine to test for leaks. They are easy to make or buy and can often find leaks one would miss otherwise.

I would do all of this testing before buying any parts with the exception of replacing the plugs if they were bought online where a lot of counterfeits exist, or are Torch brand.

This also assumes that the fuel is fresh and has not been sitting all winter and spring.

 

[Mark H_NO]

So, it runs several times a week, other than winter. I'm assuming it has been running this year, and doesn't have winter fuel still in it. I agree with sgkent, don't start buying a bunch of parts, other than plugs if they are questionable. I've had decent luck running Seafoam to clear up minor carb gunk, though it sounds like its getting run enough to not have the fuel system crud that machines that only run every month or so have.

When you stop the blades and it starts backfiring, is it going to idle, or still running at speed? Definitely look for intake leaks, and compression test is also good info.

 

[Dkite26]

So, it runs several times a week, other than winter. I'm assuming it has been running this year, and doesn't have winter fuel still in it. I agree with sgkent, don't start buying a bunch of parts, other than plugs if they are questionable. I've had decent luck running Seafoam to clear up minor carb gunk, though it sounds like its getting run enough to not have the fuel system crud that machines that only run every month or so have.

When you stop the blades and it starts backfiring, is it going to idle, or still running at speed? Definitely look for intake leaks, and compression test is also good info.

When it starts backfiring, the throttle is still at full speed but it begins to hunt. Engaged, full throttle and no backfiring/hunting. It still hunts when we lower the throttle. It leads me to believe that it’s in the carb because the throttle is constantly hunting. The plugs were new yesterday. NGK’s from the local parts store.

 

[Auto Doc's]

My z560 backfires but only when it’s not cutting. When the blades are engaged and the engine is under a load cutting, the backfires stop. When the blades are disengaged and just driving, it backfires. I’ve replaced all filters and the filter housing and spark plugs. I don’t see any cracks on the exhaust. Kinda hard to drop $200+ on coil packs on a chance that solves it

Hello D.,

I don't think this issue is coil related but more of a fuel delivery problem such as the carburetor needs a good cleaning internally.

 

[Mark H_NO]

Cleaning the carb is always a good idea. If it's been run for 5 years with no carb cleaning, start there. As part of the cleaning, test the fuel flow. Drain into a jar, let it sit and see if there's any water.