Backfiring : Honda 5.5 HP Craftsman

[Drosselmier]

The last two times I used the mower it backfired three times total, after a 10 minute break. One of them was a fainter than the other two. I am using 91 Octane fuel. I cleaned the air filter but it was not too dirty. What could cause this ? It has not backfired previously.
Thanks for any suggestions.

 

[eljefino]

sheared flywheel key?

 

[ironman_gq]

by backfire do you mean it popped back through the carb or blasted out the exhaust? A true backfire pops back through the carb and is usually either a sticking or burned valve or improper ignition timing. Popping through the exhaust can be caused by a fuel buildup in the muffler and the rush of fresh air when the engine is shut off causing it to ignite or several other issues.

 

[Drosselmier]

I could not tell where it popped, muffler or carb. I only heard the noise. Mower has been excellent till the popping. Always starts on 1st pull. Maybe lower octane fuel needed ?

 

[Skittles]

Originally Posted By: Drosselmier
The last two times I used the mower it backfired three times total, after a 10 minute break. One of them was a fainter than the other two. I am using 91 Octane fuel. I cleaned the air filter but it was not too dirty. What could cause this ? It has not backfired previously.
Thanks for any suggestions.


91 octane doesn't help in anyway with this engine.

Did you hit anything with the blade? I would check the flywheel key and make sure that the timing is on. I have had these keys only partially shear and barely knock the timing off. This would cause exactly the symptoms you describe.

The carb could be starting to plug up from varnish in the jets.

 

[topbliss]

I have been running 92 octane in all my lawn equipment and they never backfire, so its something else. Not the gas

 

[Rand]

If it only does it on hot restart I'd think it might be a carb issue.

 

[Hokiefyd]

Check for a sticking intake valve. It's not uncommon on these engines -- I fixed one for a friend. It had little compression when I pulled the rope, and told him that it was a mechanical problem and that I may not be able to fix it easily. I tore into it and, sure enough, the intake valve was stuck open. It took some light oil and vice grips on the end of that valve to get it to move again. Poor fuel quality is what cause it in these engines. If the engine is shut down hot with an intake valve open, it'll stick in that position, so the next time the rope is pulled, the intake valve stays open, even off the cam lobe.

 

[JTK]

Originally Posted By: Drosselmier
I could not tell where it popped, muffler or carb. I only heard the noise. Mower has been excellent till the popping. Always starts on 1st pull. Maybe lower octane fuel needed ?



If you heard a loud pop/bang, it came out the exhaust. If it was that loud coming out the intake, it would have blown your air cleaner apart.

 

[Greasymechtech]

Bad or old fuel, dirty carb, iffy spark plug,....

Maintenance history?

 

[Jake54]

exact same issue I had with a mower a couple weeks ago.

 

[Jake54]

this fixed the problem I had.
Originally Posted By: eljefino
sheared flywheel key?

 

[Drosselmier]

The fuel is not old.

 

[Drosselmier]

I started to answer this post. I thought I submitted it but all I see is " the fuel is not old ". I don't know what happened to the rest of the message. Although the spark plug has been in for 3 seasons, it always starts on the first pull. Is there an additive I could clean the carb with ?
I started it twice this afternoon with no popping. I checked the oil and it was down a little so it took about 100 ml. I don't think that would have anything to do with the problem. I have to finish the front lawn this evening. See how it goes.
Thanks, guys for all your advice. Been a big help.

 

[Drosselmier]

Originally Posted By: Jake54
exact same issue I had with a mower a couple weeks ago.


How did you resolve it ?

 

[Jake54]

I changed the sheared flywheel key. Put everything back on and she fired up the first pull

 

[Jake54]

Watch "BRIGGS AND STRATTON LAWN MOWER ENGINE REPAIR : H…" on YouTube
BRIGGS AND STRATTON LAWN MOWER ENGINE REPAIR : H…: https://youtu.be/QrNlnDJoghA

Here's a video that should help

 

[Jake54]

The flywheel key might not be the issue in your case though. Did you run over something by accident?

 

[Skittles]

Originally Posted By: Drosselmier
Is there an additive I could clean the carb with ?


Most likely the carb will need to be pulled and cleaned if it is starting to gum up. Wire needs to be run through all the small passages. I do this on 100's of mowers, snowblowers, atvs, and motorcycles every year. This fixes 99.9% of the starting and running issues I deal with on carbed equipment.

But again, I get the occasional sheared flywheel key too. I have had keys that didn't actually shear either but just bend the aluminum enough that it is off 1/8". This will cause a slight miss and popping while running.

 

[Drosselmier]

Probably no more than a pine cone.