Push mower backfiring

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  #1  
Old 07-31-15, 01:30 PM
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Push mower backfiring

I have a Craftsman push mower prob about 10 years old or so.

Recently I have been hearing the whole motor sound different. It's hard to explain but sounds different than it used to.
I hear soft pops every once in a while 1-2 a minute or so, that actually sound like it is coming from a different place outside far away. (but it is the motor) then recently if I shut it off and re-pull start it within a few seconds, the first pull it will backfire a normal sounding backfire not like the little baby pops I have been hearing for a month or so. I think the baby pops is a backfire too just extremely small but not sure.


I replaced the spark plug, cleaned the air filter, don't see a fuel filter, I think it might be in the tank (Need to investigate that more)

I do have a new carburetor coming in on Sunday (Thanks Amazon)

In the last few years we moved and the lawnmower has been eating tree roots (not purposely)
In the last few years I replaced 2-3 blades because it bent the blade.

This is a new blade (not bent yet)

I know this is real bad for the lawnmower. I think in the last few years it actually stopped the engine 3-4 times I think.

I tried to move the shaft and don't feel any excessive movement.

Is there a way to tell if the motor is starting to go, If the engine is forced to stop like that what happens internally, Can stuff get bent etc.

Or does it just sound like a carb issue?
 
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  #2  
Old 07-31-15, 01:46 PM
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One of the safety devices against engine damage with a sudden stop like that is the flywheel key shearing. The flywheel will turn on the crank and the shock to the engine stops there. The timing is off and running is affected, but you replace the flywheel key and you're good for the next round.

Sometimes, however, with a few years of corrosion to the flywheel/crank end, the blade stops suddenly and the flywheel doesn't turn on the crank. That transfers enough shock to the crankshaft to bend it. On the higher dollar engines you fix the crank. On the lower dollar ones you look for another mower.
 
  #3  
Old 07-31-15, 04:13 PM
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Sounds to me like you have a partially sheared flywheel key, not a carb issue, just remove the flywheel and check the key, you can't really check the key without removing the flywheel as the shear could be below your visual inspection. The engine probably jerks back on your arm sometimes when you are starting, this is another sign of a sheared key. Have a good one. Geo
 
  #4  
Old 08-01-15, 05:38 PM
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Yep, its the flywheel key, I can see it without removing the flywheel. The flywheel is the thickness of the key away,(not aligned up) but now turned into a disaster.

I got the nut off, and needed to tap for the puller. I guess some come already threaded but this one did not. (or use the self tapping screws)

Tapped it, put on the puller and the threads pulled out of the aluminum and then cracked the aluminum around the hole pretty big.

So now in the process of cutting the flywheel off with a cut off.
Not sure how to get up to the shaft with cutting into it though yet. Probably need to keep nipping at it with pliers-vise grips until I can get it break around the shaft.
 
  #5  
Old 08-01-15, 05:56 PM
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If you've ruined the flywheel you'll have to replace it. Chunks out of it can throw it out of balance. However if it is still in good enough shape and as a reference for future removals, just wedge a couple of pieces of wood in from both sides under the flywheel to pry some pressure up from the bottom. Then tap the side of the flywheel with some sharp raps from a metal hammer. That will usually pop them off.
 
  #6  
Old 08-02-15, 04:38 PM
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I did get the flywheel off.
I am getting a replacement Monday but wanted to see if someone has the torque spec for the flywheel nut. The engine is a Briggs & Stratton 122T05-4846-B1 07011857 There was also a sticker that said 6 ??? Series. After the 6 it is not readable until Series.

Also picked up 222698s Keys will it only fit one direction or do I need to make sure I put it in a certain way?

Thanks
 
  #7  
Old 08-03-15, 01:00 PM
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It looks like 55 ft lbs.
http://www4.briggsandstratton.com/mi...art_ms3992.pdf

The link is a great reference for all briggs specs. I keep a laminated one on my wall.. The key will fit either way... Roger
 
  #8  
Old 08-03-15, 05:43 PM
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Thanks for the torque spec,
I put the new flywheel on and it definitely sounds better. It sounded tinny before. but...

I still have a miss, every 1-3 seconds or so, a hiccup in the RPMS. I haven't heard the the tiny pops I had before or the backfire out of the exhaust the one time I did before.

The carburetor & spark pug was replaced.

The air filter is dirty, I have a new on on its way.

I did notice the spark plug wire near the ignition coil where it gets attached to the coil, the outer casing of the wire is broken-frayed etc. I am not sure if it is just the outer coating.
Maybe it is arcing there?
Is the wire removable on this type of coil? It is part number 802574.
I did not want to start twisting it and really break it if it is not removable


I haven't checked the fuel filter in the tank but when I disconnect the line it comes out pretty good. And the old carburetor was surprisingly clean in the bowl part at least.


Another thing is this lawnmower would always start on 1st or second pull when cold (even over winter break) but always was a pain to get started when hot. Over the last week or go it got a harder to start when cold
After I put it together today it was hard to start but I thought because it was a new carburetor and needed to get fuel in the bowl. It took about 20 pulls to get it started. I only had it running about 1 minute or so.

Went back out a half hour-45 minutes later and it was a pain to get started again.

Any ideas what to check or do next?

Thanks again
 

Last edited by michaelrj9; 08-03-15 at 06:24 PM.
  #9  
Old 08-03-15, 06:52 PM
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While the engine is running spray/dribble carb/brake parts cleaner behind the carb, if the engine tempo changes you have an air leak, the "O" ring on the intake could be failing, also it it is hard to start the gasket between the air filter housing and carb might be failing as it is part of the prime circuit, sometimes you must use 2 because the housing is warped. Have a good one. Geo
 
  #10  
Old 08-05-15, 01:43 PM
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I did spray in back of the carb and it is leaking there. The o-ring is new as it came with the new carb.

Not sure why its leaking there.
 

Last edited by michaelrj9; 08-05-15 at 02:14 PM.
  #11  
Old 08-05-15, 04:07 PM
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Sometimes the o-ring pops out of place. Also often the tube is cracked.
 
  #12  
Old 08-14-15, 02:10 PM
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I ended up replacing the intake tube & gasket & o-ring for the carburetor trying to fix the leak in back of the carburetor.

I also picked up the control bracket (what the carb mounts to) thinking it might of got bent and the o-ring is not seated squarely to make a good seal. Instead of trying to bend it (Don't know if it is even bent) I picked up a new one but...


The new one is a new design compared to the old one. Original part number is 791128 and the current number is 793754. The new one gets mounted to one of the head bolts and on the side of the engine next to the carburetor. The original one had 2 mounting points on the side next to the carb.

The new bracket don't have threads in it for the carb so it needs to be tapped. I don't think they are self taping (but will double check before trying to tap it

Anyone know what size tap it should be? I don't have a die that will fit the bolt to figure it out. If not I will have to take it to the store and find out that way.

How far should the intake tube stick through the control bracket opening? When I mounted the new bracket on the engine and put the new new o-ring on, it was just enough to get the o-ring on. With the old bracket the tube sticks out 1/4" or so. So the tube is sticking out of the bracket more than the new one does.

Anyone know what ones seems correct? The new one or the old one?

Thanks
 
  #13  
Old 08-14-15, 08:04 PM
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A 1/4"-20 tap should do it.

The tube sticking out 1/4" seems a bit much. I don't think it should be that far out.
 
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