Tecumseh backfire

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  #1  
Old 05-15-10, 02:23 PM
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Tecumseh backfire

[B]I've a Tecumseh OHV 17 that's driving me nuts. I hate these motors and have steered clear of them for a lot of years...but now I have one. What I've done so far: The carb/fuel line/fuel filter all brand new, pulled the head and cleaned out the carbon, lapped both valves very well, neither valve spring was broken, both still "felt" very strong (no way to test), both valves have "slight" clearance in their guides and move very freely, lash @.005, stems aren't bent whatsoever nor have any roughness, pulled the flywheel/checked the keystock, checked the coil/module to flywheel clearance, used old plug then new, plug wire is easily within resistance specs. Now the problem. It starts fairly easy and runs good but ONLY with the choke almost full on. While running it OCCASSIONALY pops through the carb. IMO an occassional pop sounds like a sticking intake valve whereas a timing issue should cause more constant popping IF it even started. Right/wrong? Even IF a valve were sticking occassionally, that souldn't cause the choke to be almost full on to run. Right/wrong? I'm whopped with no ideas left of what the cause could be. What am I missing? This has reminded me of why, 40 years ago, I swore I'd NEVER touch a Tecumseh again. HELP!!!
 
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  #2  
Old 05-15-10, 02:27 PM
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Is it backfiring through the intake or the exhaust? I just saw you had it does it through the carb. It could be a sticking intake valve. That can do a lot of stuff because it kills the intake vacuum. When you do that the vacuum isn't enough to draw normally through the main jet and the choke circuit serves as a substitute. That circuit can hog a lot of fuel with just a little draw.

It could also be something on the cam. You can check movement of the valves to see if that's a possibility. A mixture problem is often a popping through the exhaust and normally a lean mixture. That can give you preignition.
 
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Old 05-15-10, 06:42 PM
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I guess you have the one with the darned plastic bowl on th ebottom of the carb, right? If so, take the bowl off, remove the solenoid or plug from the bottom of the bowl, remove the spring and jet from inside that hole, and down in the very bottom of that hole is an o-ring that seals against the jet. This o-ring swells shut and restricts the fuel to the venturi and causes your problem.
 
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Old 05-15-10, 09:25 PM
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Thanks to you both for the ideas. The rocker arms have good travel. Whether it's enough or not, dunno, but they do have plenty.
The carb "O" ring: Could be, but after installing this carb brand new, the popping continued as though nothing had been done. In doing a parts lookup on a website I noticed they offered a "cam repair kit" but couldn't tell me what it consists of or what on the cam this repairs. I suppose the next step is to take it apart and check the cam. For what I don't know, but I'm at my wits end trying to fix this. I'm tempted to part the thing out but whatever I decide to do, guaranteed this IS the last Tecumseh I'll EVER deal with. Kohler, Briggs, whatever, but no more of these. For my money they're a GREAT boat anchor, nothing more.
 
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Old 05-15-10, 11:15 PM
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I missed the part about the carb being new. I would think that would rule it out, but the fact that it takes full choke to run well means it is running too lean and that points back to the carb or the related gaskets.
 
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