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2018/05/11 21:33:51
Pain

Craftsman lawn, tractor backfiring help

Hi
I have a Craftsman riding mower Model #917276681
 
The engine is a Briggs & Stratton Model #445677-0413-E1
 
The issue I am having is its backfiring thru the exhaust and the carburetor. Although I think I might have fixed the carb backfire... It still pops out the exhaust at a idle but not real bad and randomly. At full throttle it seems to have a small miss but again not that bad. It smells rich at idle too.
 
Things I have done.
 
  1. New carburetor (The old one was fried anyways)
  2. New coils
  3. New fuel tank, Fuel line, Fuel filter and fuel pump.
  4. New engine wire harness (I think bad didos. stayed running after key turned off.) (FIXED!)
  5. New spark plugs. But not the ones that the manual calls for. So I just ordered the right ones but I doubt that will fix it.
  6. Pulled heads and lapped the valves. Cylinder walls look like new. No lip at top of cylinders
  7. All mew gaskets from the heads up..
  8. Compression test 170 both sides
  9. Checked for bent push rods and pushed up valve guides. No bent valves. All looked good. Valves appear to open fully and close fully. Gapped the vales too.
The only thing I can think of is that the intake manifold is plastic and maybe its got a leak? I would rather have a cast manifold if anyone knows if they will fit? Thanks for any help. I have exhausted all my ideas. Its not a real big deal but I know it should idle smother and not smell so reach. I also just added some "STP Ethanol Fuel System Treatment" maybe after I mow the lawn this weekend it will fix it..
12 comments Leave a comment
Mikel1
How did you fix carb backfire? Did you lap the valves?
2018/05/11 22:24:29
Pain
Not sure how I fixed the carb backfire. Just doesn't seem to do it anymore. Yes as I stated I did lap the valves. Thanks for any advice you can give me in advance.
2018/05/11 22:33:33
Mikel1
Yeah sorry about that, didn't mean to ask that 2nd question. I meant to ask did you replace air filter?
2018/05/12 00:13:57
Pain
Yes air filter, oil filter and fuel filter, fuel pump, fuel line, fuel tank as it was cracked anyways.
2018/05/12 00:50:30
Mikel1
Run any better with air filter off temporarily, non dusty area perferrably.
2018/05/12 10:53:44
Pain
No. Not that I can tell anyways..
2018/05/12 12:04:45
KuckleBuster AL
With everything that you have done to it one of them should have fixed the problem.  Only leaves one thing that hasn't been mentioned that could be causing backfire.  Sheared or partial sheared flywheel key.  Will throw it out of time causing backfire.  Good luck
2018/05/31 10:18:18
Roy
It's highly unusual for a key to shear on a tractor engine unless the flywheel has been removed and not properly torqued when reinstalled.
2018/05/31 11:14:58
Pain
Thanks for the reply. However I've already had the flywheel off and the key is fine. It still seems to backfire at low-to-medium idle not consistent at all very random. The only thing I haven't done is change the intake manifold but I did check for leaks and can't find none. Runs great runs strong just doesn't like the idol.
2018/05/31 11:27:39
KuckleBuster AL
It is not all that uncommon for a key to shear if someone hits something while mowing is why I mentioned it and can cause backfire. Still something wrong in the Fuel, Valve train including the cam, timing, or spark.  Suggest the B&S balance test to see if the backfire changes.  Put a spark tester inline on both plugs.  Start it, the using a screw driver short out one side, see if it makes a change, then the other.  May give an indication on what cylinder is causing the backfire
2018/05/31 16:00:40
Pain
I pulled one plug wire at a time and both sides seem to backfire. Maybe I'll take a video next time I mow and put it up here or a link to it. Is there a chance the plastic intake manifold is cracked? I've seen videos where they warp I believe from over tightening and cause leaks. I ran the propane bottle around checking for leaks and never revved up.
2018/05/31 19:51:39
KuckleBuster AL
I've never seen one that had been cracked or warped but the is always a first.  Checking with propane or carb cleaner not revealing anything as far as speed change then doubt it.  Sounds by what you posted it's running rich Had some ideas to check on the carb but I see you put on a new one.  Slight rich condition may not show a problem under load when it needs more fuel but causes load up at idle. Nikki carbs have a nozzle gasket that the holes swell shut making a rich condition. But again new carb.  Gonna think some more a video would possibly be helpful.  Spark plug color after running might help too
2018/05/31 22:43:50

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