Tecumseh Backfire Getting Worse

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  #1  
Old 02-15-07, 12:05 PM
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Tecumseh Backfire Getting Worse

I have a 13 year old Noma snowblower with a 12 HP Tecumseh OHV engine (model number OHSK120). It starts up without any problems. For the past few years when I encountered really heavy, wet snow it would backfire until I eased up feeding snow into the auger. For the backfire to occur the machine would need to have been running for some time, say 20 or 30 minutes. I checked the main jet on the carb and it's set at one-and-a-half turns out. The person from whom I bought the snowblower claims he had the unit professionally prepared for off-season storage. Since I have owned it, I run it dry at the end of the season and use the fuel dump valve on the bottom of the carburetor bowl to drain the gasoline out. So it doesn't seem likely there's a varnish build-up problem. The bolts that hold the carb to the engine intake manifold are tight. I haven't checked the flywheel key but have no reason to suspect it's sheared off. I did check the compression and if I recall correctly it was about 85 psi.

When I recently ran the unit for 20 or 30 minutes it suddenly backfired and died when it was not under load. I had just finished snowblowing (good timing of failure!) so I just put it away. This is the first time it backfired when not under load. So it seems like the problem is getting worse. It started right up the following day and ran fine for about 20 minutes. I started working on some heavier snow and again it backfired and died again.

I have also noticed an oil film over parts of the engine and snowblower. I checked the oil breather assembly and it seems fine. I'm wondering if this is related to (or caused by) my backfire problem?

What else should I be checking for?
 
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  #2  
Old 02-15-07, 12:36 PM
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My Snapper did this with the Tecumseh Snow King 5.5 hp motor. It was getting worse and would need to have the choke on towards the end to keep it running. It was a sheared crank pin that had the timing way off.

Unfortunately this was only the beginning of my problems with this motor.
 
  #3  
Old 02-15-07, 02:42 PM
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As mentioned, it could be a partially sheared flywheel key although I've found this quite rare and if the timing was off, it would backfire every time it fired.

The first thing I would look at is the valves. They're built into the head on these OHV's. Be sure to have a new head gasket on hand and pull the head. Valves could be carboned up, valve stems sticking after warming up, not seating properly after warming up, etc.

My experience with OHV's backfiring has been traced to the valves almost every time.
 
  #4  
Old 04-01-07, 05:06 AM
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back fireing ?

in my 22 years of experience working on small engins i have found most back fireing to be caused by a lean mixture , possibly a partial blockage of the main jet this can also cause hard starting. an easy way to check this is to turn the choke partially on, if this fixes it youve found the problem.
 
  #5  
Old 04-12-07, 01:41 AM
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valves

the usual valve clearance on O.H.V. engines is .005 on in and ex a tight clearance on inlet valve will cause backfire when the engine has heated up
 
  #6  
Old 04-12-07, 08:10 PM
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I think I'd start by replacing the spark plug. If it still exhibits problems, I'd pull the carb bowl and check for trash/water in the carb.

It could be a valve problem too, and without listening to it first hand, it's hard to say if it's a valve problem or not. Usually backfire caused by valve clearance is a puffing pop repeatedly, if it's really bad. Valves don't get too tight on an OHV engine though, so I doubt that's your problem unless someone has made attempts to adjust them prior to this problem.

A sticking valve backfire is usually a loud bang one time and the engine shuts down unless the valve comes unstuck quickly enough...and then it may continue to bang unexpectedly at random with no pattern. A loose valve seat will usually cause a loud bang at random, and at some time will usually kill the engine with a series of loud bangs until it dies. A lean fuel mixture usually causes the most backfiring during the introduction of a load to the engine. As the engine "catches up" to the load, it can continue to pop and spit.
 
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Old 04-12-07, 08:39 PM
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Originally Posted by cheese View Post
I think I'd start by replacing the spark plug. If it still exhibits problems, I'd pull the carb bowl and check for trash/water in the carb.

It could be a valve problem too, and without listening to it first hand, it's hard to say if it's a valve problem or not. Usually backfire caused by valve clearance is a puffing pop repeatedly, if it's really bad. Valves don't get too tight on an OHV engine though, so I doubt that's your problem unless someone has made attempts to adjust them prior to this problem.

A sticking valve backfire is usually a loud bang one time and the engine shuts down unless the valve comes unstuck quickly enough...and then it may continue to bang unexpectedly at random with no pattern. A loose valve seat will usually cause a loud bang at random, and at some time will usually kill the engine with a series of loud bangs until it dies. A lean fuel mixture usually causes the most backfiring during the introduction of a load to the engine. As the engine "catches up" to the load, it can continue to pop and spit.
Good stuff cheese thx for the info. Won't find that in any manual
 
  #8  
Old 03-18-13, 05:08 PM
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Tecumseh ohsk120 snowblower engine

Bikerbill did you ever find out what caused your tecumseh 12 hp backfire and quit, mine is doing the same thing. I changed the coil and carb. Still stops after running for about 25 to 30 min's . I just wondering if you ever figured your's out
Thanks TIM my email is******* Please email me if you got your's working.
 

Last edited by Shadeladie; 03-18-13 at 05:18 PM. Reason: Email not allowed.
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