Craftsman mower- backfires - no load

I was researching this problem online and found this site... liked what I saw and decided that I might enjoy being a member and might have knowledge that would be helpful to other members, so here I am.
My Mom's mower is a 22hp Briggs&Stratton ohv about 5 years old, 125hrs use, has performed flawlessly until last week. It was hot that day and I had been mowing for almost 2 hours. When I finished and disengaged the blade, the engine was spitting/backfiring. Thought it was just hot and put it in the shed. A couple of days ago I cranked it and it was still doing it so I replaced the plugs and fuel filter. This didn't correct the problem. Here is the thing... Under load/ with blade engaged, it runs fine. No loss of power or spitting but at idle or half throttle it spits/backfires. Cranks and runs fine just spits when it doesn't have a load on the engine. All the troubleshooting sites have plenty of things that could cause an engine to misfire under load but my problem is the opposite. Any ideas?
Also note.... I'm only at my mom's about once a week and may not make it next week since we are going out of town... so I can collect ideas but it may be 2 weeks before I can actually work on the mower again. Thanks and I look forward to being part of the DIY community

 

On those it's a good idea to set the valves, then see where you're at. In analyzing why it would do the same thing under no load as with load, think in terms of same thing just the other end. That simplifies it.

This logic only works if setting the valves fixes the problem.

 

Sounds like a carburetion issue to me. Does applying partial choke smooth it out?

 

I have been thinking carburation or gas quality myself. I'll check the partial choke idea. The gas that I've been using does contain ethanol. I was considering adding gas stabilizer and maybe trying a high octane gas with next fill up. I use high octane for all my gas/oil mixes and haven't had any carb gumming problems with weedeater, chainsaw, etc since going to high octane but I've been using 87 octane in riding mower.

 

If it was a carburetor issue I would think it would show up under load. That usually happens due to the higher head temps with a lean mixture and preignition. When an engine is under load the pressure in the cylinder is higher having to turn against the load on the crank.

When you have no load and that type of problem I would still look at the valve lash.

 

I didn't think I would get back down to mom's this week but it worked out that I had to take her to see an eye specialist today so I played with the mower when we got back from the doctor appointment.

It cranked fine but was popping throughout the throttle range. Slowly pulled the choke knob out.... no effect until I was just about half choke... then the popping got progressively worse. Played around with different throttle and choke positions for a bit then at about 1/4 throttle and the engine popping to beat the band, I slapped the choke knob in. (Normally I have always pushed the choke knob in slowly) It smoothed right out at 1/4 throttle. No popping at all. At about 1/2 throttle and above it pops about once every 5 seconds. (Was doing a pop or more per second throughout the throttle range without load) That tells me the choke flapper is sticking.

Didn't have my carb cleaner with me but that will be my next step, I reckon. Still wondering about going to higher octane/detergent gas?

To Cheese, thanks for the suggestion.

 

I'd leave the high octane alone. If you want something to help clean the fuel system, add some seafoam. The high octane doesn't burn as fast as the regular, and the engine is made for the combustion rate of regular gas.

 

10-4 I have Seafoam Deep Creep spray that I planned to spray in the carb. Do they make it in the form of a fuel additive? Seafoam was the only thing that would clear a tiny clogged hole in the carb of my PAS unit so it is my go-to cleaner.

 

Yes, they make it as a fuel additive.

 

Just wanted to update..... Seafoam got it running smooth again ... Thanks again for the help

 

Thanks for the update! I don't believe in mechanic in a bottle type stuff, but seafoam does a pretty good job when it's just a minor blockage.

 

Did you put it in the gas (what was your concentration?) or did you spray it into the carb throat?

 

I had the spray and gas treatment. I sprayed into carb while working the choke open and closed since the problem seemed to be a sticky choke flapper. Then I let it sit for a couple of hours while I worked on a tree that had fallen in a recent storm. Then I added about a third of a bottle of the gas treatment thinking the whole bottle might be too much in the 3(?) gallon tank of the mower. Don't know the size of the bottle and it's at my mom's. I think spraying the choke probably did the trick but added the treatment since I had it. Blew plenty of white smoke when I cranked it but she runs like she used to. ;-)