Craftsman Mower No Start - Backfire Thru Muffler

Discussion in 'Small Engines' started by Friday, Aug 11, 2016.

  1. Friday

    Friday Hero Member

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    Older Craftsman 15.5HP rider. B&S 31C707-0230E1 engine. Some history: For some time now, the mower has not spun up correctly when starting, like it has a weak battery connection or bad starter. After mowing for about an hour the other day, I stopped for about 10 minutes. When I went to start the mower, it would not start, and it was backfiring through the exhaust. While it has had the poor spin up problem for a while, it has always started. I read up on this condition and decided it needed the valves adjusted. Got that done. The spin up problem has gone away, but the mower still will not start and still backfires. I checked and I have spark at the plug. Checked the flywheel key, it looked ok, but I put in a new one anyway. Still no start. No fire. Spins up fine. Still backfires through the exhaust.
     
  2. billr

    billr wrench Staff Member

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    Check the valve operation visually as you slowly turn the engine. Many of those B&S engines have an "easy spin" system that re-opens the exhaust valve on the compression stroke, to make starting easier. This is accomplished by a fairly crude mechanism on the cam, and activated by centrifugal weights. That could be bad and keeping the exhaust valve open a bit. Note that because of this feature, a compression test will be meaningless. To see a good IPB (Illustrated Parts Breakdown) and parts prices for that engine, I recommend taking a peek at <smallenginepartswharehouse.com>
     
  3. Friday

    Friday Hero Member

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    Bill: Is it possible the exhaust valve is stuck? It doesn't look like the exhaust valve moves as much as the intake when turning the engine by hand. Can I just remove the rocker arm and spring to check if the valve is stuck? (new to small engine repair).
     
  4. billr

    billr wrench Staff Member

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    Yes, you should be able to move the valve even without removing the rocker. Those springs are pretty weak compared to what we are used to seeing on autos, you should be able to move the valve with just finger-pressure.

    Usually intake/exhaust valves have the same lift, so if the exhaust is "lazy", then that is another hint that the "compression-release" thingy is stuck and holding the valve a bit open all the time; thus reducing the observed lift.
     
  5. Friday

    Friday Hero Member

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    Bill: I'll check out the valve. Checking the compression release arm will involve tearing down the motor to get at the camshaft. I decided to try something else. Please read all of this: I have two riding mowers. Mower A was purchased new at Sears in 1997. 15.5 HP Craftsman 42" cut, Hydrostat. Model 917.270622. B&S engine 28U707-1174-E1. Several years ago, the mower deck cracked at a weak point and was unusable. A new deck was over $400, so I got on Craigslist and found a similar mower for $300 and bought it, Mower B. 15.5 HP Craftsman 42" cut, 5-speed manual transmission. Model 917-272352. B&S engine 31C707-0230E1. Mower B is the one I am having trouble with. Mower A has been sitting in my shed for the past 3-4 years, with no mower deck on it. Recently I had to sell my pickup, so I got Mower A out and was going to use it to pull a garden trailer to move brush on my property. Charged the battery. Put in some gas. No start. Squirted gas into carb and it started, ran for about 10 seconds, then died. I figured the carb was gummed up/needed rebuilt. Being low on funds, I put that project aside. Then came the problem with Mower B last week. No start, possible camshaft problem. Today I decided to merge the two problems. I removed the known working Nikki carb from Mower B and put it on Mower A, removing the Walbro carb. Mower A then fired right up, but died immediately. If I squirt gas into the carb, it starts and runs until the gas runs out, then it won't start. I realize there may be design differences in the two carbs, but all the fittings are the same, the choke assy is the same, etc. What am I not seeing? Oh, I pulled the vavle cover off Mower A and checked the clearances, and they are right on the money. Tempted to push Mower A next to Mower B and throw a match in there, but I cannot afford a new/different mower right now.
     
  6. billr

    billr wrench Staff Member

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    Well, I read it, now I need to get a piece of scratch-paper and make a chart of the "facts" to try understanding it all.

    I'm not sure if I was clear, you can check to see if the cam is holding the valve open by just taking the valve-cover off. No need to get deep inside the engine to check that.
     
  7. Friday

    Friday Hero Member

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    Bil: Yes, I understand what you are saying and I am planning to check that exhaust valve tomorrow. Spent the last two days helping daughter move.
     
  8. billr

    billr wrench Staff Member

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    Yeah, I'm 500 miles from home, helping a son rebuild a shower.
     
  9. Friday

    Friday Hero Member

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    That sux. I hate plumbing.
     

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