Craftsman Push Mower Misfiring

[Merkava_4]

Craftsman Push Mower
Model: 907.388920
Serial: 112905M023952
Engine:10M902 0489 E2 05101754

Not misfiring violently like a car does, but rather a series of quiet muffled pops from the exhaust in between an otherwise perfectly running engine.

I've had the same issue before on an entirely different Briggs & Stratton engine that was a horizontal shaft. I never did find the cause of that one either.

This muffled pops syndrome seems to be much more difficult to pinpoint than your typical surging issue. A surging issue is always caused by fuel starvation.

The other day I was walking out of the VA hospital. A construction crew had a generator running there. That generator had the exact same muffled pop syndrome as my push mower is experiencing right now. The generator at the VA will be the 3rd small engine that I've encountered with this mystery issue in my lifetime.

I'm thinking it's probably unmetered air getting in somewhere. I've got the new diaphragm and tank gasket installed, along with a new primer bulb.

The gasket surface of both the tank and the carburetor are flat. I checked that with the straight edge of a steel ruler.

Everything is there where it's supposed to be except for the throttle shaft seal. Those throttle shaft seals are always made out of a light foam material and they always disintegrate in short order. I'm hoping you guys will tell me the little throttle shaft seal is more of a dust seal than an air seal.

Thoughts?

[bgsengine]

I'm hoping you guys will tell me the little throttle shaft seal is more of a dust seal than an air seal.

Thoughts?

It is. (dust seal, that is - Intended more to filter dust away from building up on the shaft and causing shaft to bore wear, but can often be a source of wear themselves if they get oil soaked for example and COLLECT dust instead)

The misfiring you are seeing is quite common, really - can often be due to a little too rich mixture, can be caused by poor fuel, water in fuel, spark plug issues, carbon build-up, pinhole perforations in coil wire, internally broken coil wire, ignition coil problems, electronics (or sticky breaker points) issues, bad condenser... many many little things or a combination of them can make this one of the biggest PITAs to diagnose correctly - quite often it was hit and miss after a tune-up and maintenance, and fine-tuning any adjustments, the engine was running fine OTHERWISE and was just left to "live with it" - I have had a few to diagnose in my many years by OCD type customers that just had to have things perfect (Including me) so a few of them finally got fixed by discovering or replacing one or more of the above mentioned little things , so you could replace ignition coil (or swap out), replace plug (making sure correct type, heat range, and gap - preferrably a solid brand like Bosch, NGK, Denso, if not an OEM original - if using Champions, Autolite or off-brands, never assume - always try 2-3 new plugs if nothing changes) , swap out carb, pop head off and de-carbon (and do a leakdown test to check valves seating, usually BEFORE pulling head, and again after putting it back together), make sure valves are in spec, change out the fuel and/or swap carburetor with known good, inspect control and governor linkages, and so on.... Just doing one thing at a time and re-test - such engines do often take quite some time if insisting on diagnosing the issue, so for a pro shop, they are often not worth the trouble (or the customer is willing to pay several hundreds of dollars in labor & diagnostic time)

[KE4AVB]

Don't work on walk behind mower much anymore as those just don't come in for repairs much here like when I first started repairing equipment.

Kinda sounds like a worn exhaust valve guide since the engine is a 2005 built unit. But can be just be valve clearance issue since the clearances tend to close up over time.

I hate having to pull the head as it always means a new head gasket just to check this but it is the only way to check the guide and the only way to set the clearances.

At least you don't need the valve guide tool kit for this exhaust guide as it appear the splinter bronze bushing which extracted and the new one driven in.

[Merkava_4]

Today I tried a different carburetor, different tank, different, coil, and different spark plug. No change. Engine still fires up on the first pull. Runs beautifully except for the random muffled pops out the exhaust. I can't stand listening to those pops, so I'm not going to use the mower until I get this figured out.

I haven't tried a brand new carburetor yet, but I will if someone tells me it's a sure bet. My gut feeling tells me it's still getting unmetered air into it somehow. Do you guys think a cracked rubber vent tube elbow would be the cause of unmetered air? The engine runs the same with or without the air cleaner on.

I consider myself to be a pretty good small engine mechanic, but I'd be lying to you if I told you I wasn't disappointed in myself.

[bgsengine]

Today I tried a different carburetor, different tank, different, coil, and different spark plug. No change. Engine still fires up on the first pull. Runs beautifully except for the random muffled pops out the exhaust. I can't stand listening to those pops, so I'm not going to use the mower until I get this figured out.

I haven't tried a brand new carburetor yet, but I will if someone tells me it's a sure bet. My gut feeling tells me it's still getting unmetered air into it somehow. Do you guys think a cracked rubber vent tube elbow would be the cause of unmetered air? The engine runs the same with or without the air cleaner on.

I consider myself to be a pretty good small engine mechanic, but I'd be lying to you if I told you I wasn't disappointed in myself.

Have you tried fresh ethanol free gas (not the store bought stuff at wal-mart either which is often questionable as to content) water in suspension in E10 fuels can give some trouble as well as low octane gas (Try 89 or 93 octane?) Also check the muffler itself - missing or bent baffles can lead to unburned fuel building up as well (or if there's way to do so safely try running without muffler at all? - especially in a shaded or darkened area where you'll see the flame from exhaust that might give a clue) Inspect intake port and passages for carbon build up - changes or variations in the fuel/air swirl pattern can cause uneven mixing that shows through on exhaust side (Though that's more of a racecar/performance engine thing, it could still in theory apply to single cylinder engines) Just a few more thoughts....

[Merkava_4]

bgsengine, I have not tried a different fuel yet. It still has Costco 87 octane with some fuel preservative added.

I don't think any place in town carries ethanol free gas. However, it's rumored that 91 octane is ethanol free.

What about that cracked rubber vent tube elbow; can that cause unmetered air to enter the engine?

[KE4AVB]

Even the best mechanics get problems that can drive us nuts at times.

If rubber vent is feeding before the main carburetor butterfly (vane) which most do then no it won't cause un-metered air problem though it can allow dirt or dust to enter.

Let us know you finally find out to be the problem but I am leaning toward an exhaust valve problem myself which of course could be a mislead too.

[KE4AVB]

And here's what happen yesterday on what should been a normal carburetor cleaning. Kawasaki FH680V-AS24. Just show you are not alone with aggravating problems.

Engine just was off idling. Partial choke needed to run right. So I pull the carburetor and cleaned in the USC. Well first a welch plug fell out. No biggie just reinstall it. While putting the carburetor the choke and throttle butterflies were hanging. Someone before installed the engine side insulator upside down and the air cleaner side gasket upside down. Now note there was governor problems before the removal of carburetor. Well I started the engine up and ran fine until went to idle down. It flat refuse to idle down and felt like the throttle cable was hanging up. Now I did just replaced the throttle cable last year according to my records. I just keep checking things until I found the slack take-up spring was in the same hole as the throttle linkage on the governor arm. Reinstalled it correctly and now the engine runs fine. The spring hook must somehow jamming the linkage. A 30 minute job not counting USC time turn into a 3 hr job.

Just shows you things can happen that you don't expect at times especially when someone else has been working on the engine.

[Merkava_4]

I think I may have found the problem. With the carburetor mounted to the tank without a gasket and the screws tightened, I can slide a .006'' feeler gauge in between the mating surfaces on the side that faces the intake manifold tube. With the gasket installed, and the screws tightened, I can slide a .0025'' feeler gauge in between the mating surfaces on that same side. That's the side opposite of the primer bulb. The other three sides aren't quite as bad. The primer bulb side is almost completely flat still. I first started out with a .002'' feeler gauge which is what Briggs & Stratton recommends for checking warpage. After that I tried a .004'' and then onto the .006''. Even though there is no gasket there, I'm pretty sure you're not supposed to be able to slide a .006'' feeler gauge in there. Being that the pickup tube and the emulsion tube tower is in the way, I don't think I will be able to file the surfaces flat.

[SUKI]

Had just about forgot about the plastic carburetor warpage issues. Just don't see that many push mowers anymore.