tecumseh 8 HP engine backfire surge

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  #1  
Old 08-15-14, 01:34 PM
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tecumseh 8 HP engine backfire surge

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Size:  44.8 KBHi, I have a Tecumseh 8hp model#HM80-155651S on a Troybilt rototiller #12194. It's about 20 years old but works fine. It was purchased about 2 months ago and ran roughly. It starts on the first pull with full choke and full throttle. Once the throttle is turned down to medium it surges and backfires. If the choke is off it will do this most. If the choke is in the middle it will do this least and when the choke is open I believe it will die. So, in trying to get the surge and backfire issues resolved the following steps were taken: New OEM fuel line from tank to carb. Carb cleaner was added to the gas. Gas was dumped and tank was cleaned and only gas this time. New carburetor p/n 640152a. Adjusted governor according to Donyboy's youtube video (excellent) Cleaned carb again today but still the same problems. I may have inadvertently bent the governor link to the throttle control bracket but that seems unlikely. All the linkage is oiled with #20 machine oil. The filters are new including the breather. The gasket screws are tight but the WD40 test hasn't been done yet. Also a new spark plug was dropped in. Any suggestions you guys may have would really be appreciated. You'll find some pictures that may help. Thanks for all your knowledge. Firegraymon
 
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Last edited by firegraymon; 08-15-14 at 03:07 PM.
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  #2  
Old 08-15-14, 06:07 PM
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The carburetor needs to be cleaned. Most likely one of the tiny holes in the bowl screw is blocked.
 
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Old 08-15-14, 07:39 PM
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Thanks Cheese, I'll give it a good cleaning this weekend. By the way, here's a picture of the carb. Name:  imagehandler.jpg
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Size:  26.9 KB. It's one of those California carbs
 
  #4  
Old 08-15-14, 07:46 PM
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The bowl nut, as cheese suggested, is probably the problem, there are several styles however the tiny hole near the top is always the problem, refer to the pic to see it, I use the wire from a twist tie to clean it. Have a good one. Geo

[ATTACH=CONFIG]36555[/ATTACH]
 
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Old 08-15-14, 08:34 PM
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Geo. You have shined a ray of light on what has become a storm of disappointment. Thank you.
 
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Old 08-18-14, 10:13 AM
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In addition to all the previous steps, the bowl nut along with the rest of the carb was thoroughly cleaned this weekend. It no longer backfires when the choke is off at low throttle. And the surging is deep but constant. Thanks Cheese and Gee, it runs much better but still backfires at other choke positions. As previously mentioned, the governor to throttle assembly arm was slightly bowed so that was also straightened. While straightening the arm I noticed the speed control spring on the throttle control assembly seemed to have a lot of movement. You can see how it sets in a couple of the pictures. Also, the WD-40 test on the gaskets was done and they all checked out fine. None of these measures stopped the surging, backfiring issue except the bowl nut. Hope you can help.
 

Last edited by firegraymon; 08-18-14 at 11:20 AM. Reason: mor info
  #7  
Old 08-18-14, 08:36 PM
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Something in the carb is still clogged. Part #20 is very commonly the problem. It has a tiny hole in the end of it that gets clogged.
 
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Old 08-19-14, 10:34 AM
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Thanks Cheese, I'll add a fuel filter. Probably should have done that to start with
 
  #9  
Old 08-24-14, 07:46 AM
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Thanks for all your help guys. Cleaned the plug and still doing the same thing. There's something obvious that i'm missing. It's going to the lawnmower mechanic Tuesday. I'll let you all know what he found
 
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Old 08-24-14, 01:56 PM
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While it is running spray/dribble some car/brake parts cleaner behind the carb and along the intake mating surfaces if the engine tempo changes you have an air leak. I had a 8hp briggs that drove me nuts, ended up being a crack on the under side of the intake. Have a good one. Geo
 
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Old 08-25-14, 01:04 PM
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Thanks Geo, would the surging still remain the only issue if the muffler is removed? That would allow better access around the intake and mounting surfaces to thouroughly test it.
 
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Old 08-25-14, 01:21 PM
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Shouldn't make any difference. Have a good one. Geo
 
  #13  
Old 08-26-14, 11:11 AM
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Thanks Geo.
It stopped surging once carb cleaner(all that was available at the time) was sprayed on the carb/manifold mating surface. That looks like that will be the final chapter. You and Cheese solved what was way out of my grasp and I want to thank you for that and the knowledge you share with us all.
 
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Old 08-26-14, 12:40 PM
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Success is a wonderful sound. Have a good one. Geo
 
  #15  
Old 08-30-14, 12:15 PM
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Still having problems with this carb 640152A on the Tecumseh HM80. Sprayed both carb and brake cleaners on the carb/manifold mating surfaces and the problem isn't there. The surging actually levels off when the throttle shaft or the choke shaft are sprayed. Is this normal? The cleaner is just dripped on like Geo had suggested and that makes all the difference. If too much is sprayed on the engine will actually die. Any help will be greatly appreciated. Also, checked the whole manifold and carburetor for any sign of cracks under a magnifying glass and that all appears to be ok.
 
  #16  
Old 08-30-14, 12:26 PM
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That would just indicate a worn throttle/choke shaft. Did you clean the hole in the end of part #20, it is very tiny. Have a good one. Geo
 
  #17  
Old 08-30-14, 12:56 PM
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Here is a link to positively identify the carb and another to clean them. Have a good one. Geo
http://progreengrass.com/reference/t...etormanual.pdf

Small Engine, Lawn Mower, Snowthrower Troubleshooting, Repairs and Safety
 
  #18  
Old 09-02-14, 11:09 AM
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Thanks for the info Geo. The strange part is this is a brand new carb and for both throttle and choke shafts to be worn seems peculiar. If that's not supposed to happen I'll go ahead and order one from TPO and get an OEM. It'll be 3x's as much but if that's the problem, it would be well worth it. The idle screw was cleaned again but still surges. Everything you and Cheese are recommending seems to point to a carb issue. The one currently on it is aftermarket and it sounds like Tecumseh is very finicky.
 
  #19  
Old 09-02-14, 05:08 PM
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In your photos above, look at that little black plastic cap on the side of the carb. Pop that off and underneath is a brass jet that comes out with a flathead screwdriver. Take it out and clean the tiny hole in the end of it. It is very tiny and takes a little wire to clean it, like a bristle out of a wire brush.
 
  #20  
Old 09-09-14, 11:39 AM
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Thanks to everyone that helped with this rototiller. After all your advice I decided to spend the $100 or so to get an OEM carb. Threw it on today with some trepidation and it works perfect! No surging, no backfiring, no leaks! DIY.com is a great site and will recommend it to all my friends. Thanks again, especially to Cheese and Gee.
 
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