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Discussion Starter · #1 · (Edited by Moderator)
Hi Guys:

My son spotted this mower in the trailer of a guy who picks up scrap material at his shop. After pulling it over to make sure it wasn't seized up he bought if for $10 and gave it to me. The mower is is pretty good shape but looks like it sat outside for many months, wheels and deck are very good, gas tank was about 1/4 full of water. I fix these up and sell them donating the money to a local cat rescue; this helps the cats a bit and keeps the mower out of the landfill. This mower has me stumped.

So far I have:
  • Confirmed presence of spark
  • Remove muffler, piston looks okay, no major scoring and exhaust ports were surprisingly clean. There was a lot of carbon on the top plate in the muffler and I scraped it out
  • Replace top and bottom seal
  • Disassemble the Walboro carburetor, remove throttle shaft, clean carb with spray, run a thin wire up through main jet and gave it a bath for two hours in an ultrasonic cleaner with water and Dawn dish soap, blew out with compressed air.
  • Clean out and dry the gas tank. Make sure cap vent is open.
  • New CJ14 plug gapped to .036

I put it all back together and it started. Now the problem. When cold it starts easily and engine will run for 3 or 4 minutes then begin to lose RPM. Once this begins RPMs steadily drop until the engine is barely running. Like 1000 rpm. Applying choke slows it even more and it will usually stop. Trying to restart it might start but again just barely runs. Throttle has no impact and choke kills it. If I let it sit and cool it behaves the same way. Even when it starts cold and has good RPM it does not sound right, there is sort of a popping sound from the carb.

Troubleshooting
  • Check compression it's 130 PSI warm and 125 PSI cold.
  • Replace ignition coil with a known good one
  • I don't have any spare Walboro carbs, but I jury rigged a known good plastic carb for F engines and connected a primer and hose. For the plastic carburetor I used a new gasket that does not have the small hole near the top for the little vent / port on the intake manifold.

Engine started and ran, but behaved the same way. After 3 or 4 minutes RPMs drop and mower eventually quits.

I believe I have eliminated the likelihood of an ignition or carburetor problem. The compression appears to be acceptable and the seals have been replaced. As the engine warms up the problem appears. Without knowing any history of the mower and no former owner to talk to, I'm guessing this is why it was scrapped.

I am missing the root cause. This thing it telling me something but I don't know what it is; I gotta be missing something. Now I am turning to you guys, the experts.

Any suggestions / advice would really be appreciated. Please tell me what have I missed.

Thanks for for your time.

Dave
 

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It sounds to me like a gas supply problem. I would suggest taking the fuel line off the carb and put it in a small pot, and verify that it has a steady flow of gas for several minutes.
 

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Sounds like it’s not breathing good. You stated there was lots of carbon on the top plate of the muffler, and you scraped it off, but you didn’t say anything about cooking the sludge/carbon out of the inside of the muffler. Put it on the barbecue grill and cook it on high for 30 mins, let it cool and just hose it out with the garden hose. Those F series mufflers need this done from time to time and they are stainless steel so you won’t hurt it with the high heat. A note for those folks with cast aluminum mufflers, DON’T put those on the grill. They will deform severely.
 

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Simply remove the muffler for short term. I don't think this will hurt anything, and it will tell you soon enough, if that's your problem.
 
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On those F series mufflers if you remove them and tap with a screwdriver it should sound tinny and hollow. If it has a dull sound then it needs to be cleaned. As stated I have cleaned these on a gas grill on high for 45 minutes to an hour. Let cool and shake out the burnt stuff. Flush with water if you like for good measure. Bill
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Discussion Starter · #7 ·
Very good advice. Thanks to everyone.
Gas cap vent looks okay.
I had not considered the muffler. I'll put it on the BBQ this weekend and report whether it did the trick.
If that does not work I'll try an alternate fuel tank / hose.

Thanks again for all the replies.

Dave
 

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Discussion Starter · #8 ·
Success!

I followed Lawnboy77 and Yard Nazi's instructions and baked the muffler on the high setting of the BBQ for an hour. After the muffler cooled and with a little shaking I could hear the loose carbon moving around. I got as much out as possible and a few taps on the muffler from a screwdriver handle even more carbon came out, mostly from the top hole. I got at least as much carbon as Bill's third photo shows. Surprisingly large chunks too. As Bill suggested I also flushed with a garden hose.

Before heating the muffler I weighed it on my Harbor Freight scale. Before heating it weighed 25.35 ounces after heating and carbon removal (but before flushing with the hose) it weighed 23.85 ounces, a difference of 1.5 ounces of weight. That might not seem like very much, but this carbon / soot is quite light so 1.5 ounces is quite a bit of material.

I put the muffler back on the mower and it now runs fine! Thank you guys so much for the help; it is much appreciated.

As this was a scrapped mower with water in the gas tank, I did not even attempt to start it without cleaning out tank, carburetor and replacing top / bottom seals. At least the fuel valve was turned off so the water from the tank did not get into the carb, and the carburetor was clean when I took it apart. There was a bit of leakage around the bottom seal but I have seen much worse. Once the gas tank was cleaned I bet the mower would have run even if I did not change the seals. This kind of leads me to conclude that a plugged up / restricted muffler probably led to the scrapping of an otherwise usable mower.

If anyone has a new muffler that they could weigh and post the results, it might be useful to have a reference weight for a new muffler and compare it to the weight of a restricted one and use the weight an indication when to undertake the BBQ procedure. I have never had this problem before despite working on about twelve F engine Lawn-Boys over the years. You guys came to the rescue; lesson learned.

In the future whenever I remove the muffler to check exhaust ports / piston, I will weigh it before reinstalling and if its approaching the 25 ounce mark, I'll BBQ it.

Thanks again guys!

Dave
 

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Good deal! Just another tip in regards to F series engines with the Walbro carb. Try and keep the engine clean, especially around the carb. If you get too much grime build up around the carb it will keep the choke return spring from completely moving the choke butterfly to the off position when the throttle is moved out of the choke detent and into the " Run" position. This will cause it to run rich and clog up the muffler even quicker. I learned this the hard way. The Walbro and choke setup is really good, but not perfect. A weak choke return spring seems to be it’s weak link. I’ll still take them over the plastic carbs that warp any day.
 
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