tecumseh 17.5 ohv bacfire

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  #1  
Old 09-16-10, 08:38 AM
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tecumseh 17.5 ohv bacfire

I just replaced the cylinder head with a used one on my tecumseh 17.5 OHV because the other one caused the engine to burn lots of oil (smoked). The compression is good and the cylinder walls still have the cross hatch from the last honing. I only changed the head and installed a new head gasket set the valves @ .006 clearance. I did not touch the carb or flywheel, and the only seals I broke were the intake gasket at the head, the head gasket, exhust gasket and the valve cover gasket. I used the same spark plug. The motor ran fine before I changed the head the only issue was smoking out the exhaust. Now i'm wondering if it could be a intake gasket leak the valves set too loose .006 instead of .004 or does the head need to have the valves ground so they seat better. I want to keep the cost down so do not want the valve guides replaced if not needed because it does not burn oil now - looking for advice, thanks.
 
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Old 09-16-10, 10:22 AM
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How much of a backfire is it? An occasional pop popping, or more of a (BANG!)

If it's just popping, I would try the intake at .004, exhaust should be ok at .005/.006.

Could also try spraying some carb cleaner or such at the intake gasket with the engine running and see if RPM or tone changes.

I would guess, (perhaps incorrectly ) that the original smoking problem was more a head gasket than head, since it is not doing it now....but as long as it aint doin it now AND you still have the proper oil level
 
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Old 09-16-10, 11:28 AM
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The original head gasket was good but the valve guides were shot I could wiggle the valves with the springs still on them, compression was good and still is (same). The backfire is not too load but does pop pretty good enough to kill it at lower throttle. Holding the throttle at the lower end of Choke helps. The backfire happens every few seconds but not exactly the same time each time. I will try the valve adjustment and spray the starting fluid at the gasket (never would have thought to try that) when I get home as those are easy things to try. After that I'm guessing that i'll need to pull the head and get it worked on?
 
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Old 09-16-10, 06:39 PM
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You might post the make and model of your tractor and engine in paticular. If you have one of those carbs with the flip bail on the bowl, I would guess that could well be the issue.
It has been years since I have seen one but they were 20 bills then for the carb.
I think there is an o-ring on one of the stems that goes bad....others here know much better, but posting your info will still be helpfull.
 
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Old 09-20-10, 03:04 PM
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It's a tecumseh 1999 17.5 OHV don't have the model # handy right now. The local lawnmower guy told me the carb was the problem so I took it all apart and soaked it in Gunk made sure all the holes were clean by forcing water through them, verified the electric fuel shut off was working, put it back together it does run a little better but still pops and spits out of the carb randomly maybe 5-6 times a minute sometimes it's enough to kill the motor. The rear flap (governer?) pulses back and forth "searching" constantly. If I hold the throttle almost wide open and partly in choke it runs ok with very little or no searching. Do you think it's still carburator related or is one of the valves not shutting all the way due to carbon that worked loose and is on the valve mating surface?
 
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Old 09-20-10, 08:25 PM
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Sure sounds like the carb to me. Does yours have a plastic bowl?
 
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Old 09-21-10, 08:02 AM
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Plastic bowl yes with electric shut off valve. I took it off and verifed the o-ring was clean and in good shape the plastic plunger rod has three holes in it and all were very clean, the hole down the middle of the plasit pluger rod clean, I forced water through the gas line inlet and it flowed through ok. I was unable to soak this as the label on the Gunk can said not to put plastic in the product. I noticed that in the center of the bowl next to where the plasic plunger rod is there is a round copper or brass part but it did not appear to have a hole in it not sure what that does. Are you suggesting that I just get a new bowl assembly and try that first? I looked up parts on Partstree.com my engine is part# ohv175 204803A carb bowl repair part # 730639 $31.75 or bowl assembly 640039A $17.72
 
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Old 09-21-10, 09:01 AM
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I don't recall seeing a spring under the center gas plunger rod (main nozzle tube) part # 632545 spring maybe that could be part of the problem I'll have to pull it back off and check.
 
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Old 09-22-10, 02:06 AM
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Remove the fuel solenoid, then farther in the hole that it was in, there is a jet. Remove it, make sure it is clean, then behind that jet is a tiny o-ring. This is usually the problem. It is under pressure from the jet pushing on it and it swells shut. Pluck it out with a pick.
 
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