Craftsman mower backfires thru carb issue

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  #1  
Old 04-16-09, 09:18 PM
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Craftsman mower backfires thru carb issue

I have a used 12 year old Craftsman push mower (917.387151) that appears to be back firing thru the carburetor. I am not the original owner so I don’t know the history of the mower. I am mechanically inclined but still learning about small engines. Below is what I know thus far.

I tried to start the mower with starter fluid but nothing would happen other then a very rarely backfire noise. I took apart the carb and soaked it overnight in carb cleaner and rebuilt it. No change in it’s function. Overall outside appearance of the mower body is good but the carb and gas tank did have water in it but oddly enough no corrosion. Previous owner used a wheel barrow to cover the mower during rain storms as the significant other told me. Anyway, with the carb body removed, I spay a bit of starter fluid in the inlet and try to pull/crack the engine. It backfires thru the inlet port. Also noted was someone had taken a wire to fix the governor lever arm all the way back (connected from lever arm to gas line)

I do a bit of searching online for some trouble shooting ideas and see the possible problems could be a partially sheared key or sticky valve might be an issue.

The blade shows no noticeable damage and the key on the shaft is in perfect condition. (got an excuse to buy a new tool – wheel/gear puller)

(Here is where my lack of experience comes in to play).

With the carb off, I place my thumb over the inlet and spin the engine. I can feel my thumb being sucking in and out from the carb inlet. I’ll have to double check on the timing of the engine as far as TDC/magneto location………….

Next I start running out of ideas and decide to take off the side plat covering the valve’s shaft and also the head. The valves still move up and down freely. I don’t know much about what a proper gap is expected between the valve and body???

What should I try next? Is it still a sticky valve suspect? Maybe the valve clearance???????? How do I check this?
 
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Old 04-16-09, 09:57 PM
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Does it have spark? Have you tried a new plug and visually verified spark? Does it have decent compression?
 
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Old 04-17-09, 06:08 AM
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cheese,

Thanks for the response. It does have spark. I checked this by pulling the spark plug out and connecting it to the spark plug wire. Pull the string and could see the spark arching. I even tried a couple of other spark plugs I had laying around but it still would not turn over.

As for compression, I do have a compression gauge that I hooked up and tried spinning the shaft by hand but it did not register. I’ll try this again once I get the pull string mounted again.

When I took off the head I could see the piston and valves all moving. (FYI – I did damage the head gasket when it was removed). If there is no compression, what could be happening? Visually it looks as if the valves are seating all the way down. I tried using a feeler gauge to measure the gap and could only measure somewhere less than .010”
 
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Old 04-17-09, 06:24 AM
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If you're getting a suction followed by a compression pulse from the intake with your hand over it, you could have a problem with the intake valve = sticking, leaking, or no valve clearance.

If, the more you turn the engine over, the compression feedback is greater and the suction less, you could have a problem in the exhaust valve not opening or an exhaust obstruction.
 
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Old 04-17-09, 06:58 AM
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Marbobj,

Thanks for the tips. I'll need to do a little more troubleshooting / understanding of the positions of the valves when the piston is up or down. I'm still trying to visualize your comment about the intake valve with no valve clearance. Wouldn't this be a good thing so no pressure escapes back thru the intake valve? If the valve is leaking this would correspond with a pressure pulse back thru the carb and possibly backfire. If the valve is sticking open then I there would be basically no compression and a pretty good chance of a backfire thru the carb. These are my assumptions; please correct me if my thinking is wrong.


I do understand your comment about an obstruction for the exhaust port. I'll take off the muffler and take a look for anything obvious.
 
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Old 04-17-09, 10:58 AM
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Explaining further...

The valves (both of them need a clearance = gap) between the rocker that pushes them and the end of the valve stem to allow the valve to fully close. Should you have no clearance on the intake valve it would not be closing fully and you would be getting a compression backwash through the valve and into the carburetor = possible backfire when the engine fires.

Regarding the exhaust..

If you had an exhaust obstruction or an exhaust valve not opening, the expanding gases of the combustion wouldn't be able to escape through the exhaust system, but would backwash trough the intake valve when it opens.
 
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Old 04-18-09, 12:57 PM
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Thanks for the explination. I understand better were the gap is located.

So here is what I've tried next:

Take the muffler off and no obstructions

With no muffler or carb attached to the engine I check for compression and there is none (0psi). I then check my compression hose/gauge on an edger and it works just fine.

So what does this mean if there is no compression. How is this possible? When I had the head off I could see the piston and valves moving. I did damage the head gasket so this might be a possibility but I would have assumed some pressure to build but there was none. I tried it a couple of time and pulled plenty hard on the cord.

Does this point to anything obvious? I can still feel a slight suction and exhale from both intake and exhaust each when the engine is turned slowly by hand.
 
  #8  
Old 04-18-09, 02:38 PM
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* Problem: Engine backfires.
Possible causes:

1. Carburetor set too lean.
2. Magneto is not timed correctly.
3. Valves are sticking.

Lawn mower will not start This is very detailed, enjoy!

Notes on the Troubleshooting and Repair of small Gasoline Engines and Rotary Lawn Mowers Be safe, GBAR
 
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Old 04-18-09, 09:01 PM
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For it to have no compression, a valve is not closing all the way or the piston is not moving. Assuming the head gasket is in half-decent shape.
 
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Old 04-18-09, 09:31 PM
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Cheese,

Thanks for the reply.

I took a few pictures and moves, links below.

The pictures are of the engine at TDC and every 90 degs as I spin it by hand. Does it makes sense?


http://www.youtube.com/profile_videos?user=ahenchmen

Craftsman non-working

Some of the links may be bad or refer to old pics/movies.... sorry.. I'll try to clean it up.
 
  #11  
Old 04-18-09, 09:52 PM
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Looks like it is all operating normally, but I can't tell if the valves are closing all the way...especially the intake valve. Inspect each valve seating surface, and the seat with the valve open. If the contacting areas are shiny, they are closing and seating. If the contacting areas are dull and black, they are not seating.
 
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Old 04-19-09, 11:56 AM
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Thanks to all for all the guidance.

It appears to me that the seating surfaces of the valves are making contact with the body, but they are not shiny. When the valves are raised they can be spun around easily. When they are down they can not be spun around, indicating to me they are making contact. The edges of the valve when they make contact do not appear to be damaged, pitted or any carbon build up.

I think pretty soon this project will come to an end since my signficant other wants to park back in the garage.

As always, it's been fun and educational.

If anybody has some more suggestions please let me know.
 
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