HP Briggs and Stratton L head Back Firing through Carby

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  #1  
Old 11-05-16, 05:28 PM
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HP Briggs and Stratton L head Back Firing through Carby

HI, I have a 12HP Briggs and Stratton L head motor Mod No 281707, have replaced the fly wheel key, adjusted the valves, inlet is a bit over .009 should be between .005-.006 exhaust is at .010 timing seams correct, fitted carby with new kit, and the motor still backfires through carby, is there any other things I should be checking? this is driving me mental.
 
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  #2  
Old 11-05-16, 06:27 PM
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Intake valve is not fully seating. Might be sticking. Valve job recommended.

RR
 
  #3  
Old 11-06-16, 01:48 PM
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HI, will give that a go this ride on mower has the head facing the control end of the machine so I will have to take the motor out of the machine, bit of a pain
 
  #4  
Old 11-06-16, 01:57 PM
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Did you go ahead and reset the valve lashes to the .005?
 
  #5  
Old 11-06-16, 02:51 PM
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How did you adjust the valves and just how did you measure the clearance...?
 
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Old 11-06-16, 06:19 PM
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Right people have the head off and cleaned up with the piston up and both valves closed still get the same measurements neither valve appears to be sticking open I don't think I will be able to hand lap the inlet valve to obtain .005 will the engine still run at .009 the head bolts did not appear to be tight not 185lbs ins could that have been the problem? Questions should I buy a new valve or reassemble the motor and see what happens?
 
  #7  
Old 11-06-16, 08:32 PM
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Does it look like the vale is seating all the way around? With the valve open, can you see a shiny ring around where it seats to know that it is seating? Can you spin the valve when it is closed? How loose were the bolts, and is the head gasket blown?
 
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Old 11-06-16, 10:02 PM
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I have opened the exhaust valve and can see the seating lip, can turn it by hand when open, closed it will not turn I have the inlet valve out it also has a bright seating lip, I did not try to turn it before I took it out, the head bolts I could remove with a hand tool, open ended spanner, the head gasket does not appear to be blown,
 
  #9  
Old 11-06-16, 10:51 PM
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Hi, I measured with feeler gauge with both valves closed through the PVC breather after removing the carburettor, as you will see from my other reply to cheese I have now removed the head and will see where I go from here.
 
  #10  
Old 11-07-16, 10:26 AM
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Sounds you wind up with late valve timing. Were you at TDC with the piston when you checked the lashes? That being the case, with both outside the specs on the loose side that would say valve train wear and probably at the cam.

You might see if the lashes would tighten up any by rotating the crank a few degrees either direction. That would point to either lobe wear or something else. You may have to change the cam.
 
  #11  
Old 11-07-16, 10:59 AM
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Actually the valve clearances are on the close side. This is a L-head, not OHV, the clearances are different. As the engine wears, the clearances will actually get closer on these, not wider.

I don't see where the spark plug was mentioned. This would be a good time for a new one, no matter how good it looks. Especially if it's a NGK.
 
  #12  
Old 11-07-16, 11:27 AM
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Being an L-head was the reason for my question. Doesn't seem right for the clearances to be out that much in that direction..???? From wear the clearance should be less than spec if out....????
 
  #13  
Old 11-07-16, 12:21 PM
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If the cam lobes wear the lashes get tighter? How is that engine set up to do that? I can see where the valve seats wearing would tighten the lashes.
 
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Old 11-07-16, 03:46 PM
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The only valve adjustment I know of on these is to grind the stem of the valve to obtain correct clearance.

marbobj: I agree and it bewildered me for the longest time and I did once figure it out, however it has been so long since I have been into a flat head, not to mention the aging, that I don't have the answer.

According to this chart and how accurately it was measured, you are not far if out of spec.

280000 L-Head Aluminum/Cast Sleeve Single Cylinder .005 / .007 .009 / .011
 
  #15  
Old 11-07-16, 04:26 PM
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The valve and the seat wear more than the cam lobes and this results in what shade-tree mechanics refer to as "stretched valves" and you have to grind the valve stem to obtain proper clearances. The valves don't stretch, they just seat lower than they used to. Yours are not out of spec enough to cause any problems. Something else is the problem. Spark plug? Keep it simple.
 
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Old 11-07-16, 04:42 PM
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Thanks Cheese. That's the way a K341 is.
 
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Old 11-07-16, 05:26 PM
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And if they were out of spec, they are in the wrong direction to cause the problem you are having.
 
  #18  
Old 11-08-16, 12:01 AM
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Yep, all of the K-series kohlers were that way and most small 4 stroke engines made from the early 90s and older. The longer they run, the tighter the valves get. I've seen some that got so tight that they barely would start because the compression was so low and they would puff out the carb a little, wetting the filter with gas, all because of tight valve clearances.
 
  #19  
Old 11-08-16, 08:13 AM
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not to mention the aging
We never admit it and they can't prove it.
 
  #20  
Old 11-26-16, 04:49 PM
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hi Guys if the inlet valve is at .009 and the exhaust at .009 should the motor still run?
 
  #21  
Old 11-26-16, 04:59 PM
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That's a lot of clearance, but it should still run fine. Did you take them out and grind them or something?
 
  #22  
Old 11-27-16, 11:56 PM
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Hi Guys,
I have done all the right things I think, replaced the flywheel key, cleaned the carby and fitted a new kit, had the head off and checked the valves which look OK, fitted new gasket refitted the head, the plug is a champion, so am at a loss as to why it is still backfiring through the carby any thoughts, by the way I am Australia which does not help some tomes as we work on a metric system
 
  #23  
Old 11-28-16, 11:44 AM
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I assume it's still not a new spark plug?
 
  #24  
Old 11-28-16, 12:56 PM
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Hi Cheese,
The spark plug is a new champion copper plus 861/J19LM bought from a lawn mower shop that deal in B and S parts have you any other ideas.
Regards,
 
  #25  
Old 11-29-16, 12:21 AM
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Ok, thanks. I wanted to be sure before going further.

What about some history here... did it run fine before and suddenly this? Did someone take the engine apart to repair something?
 
  #26  
Old 11-30-16, 04:17 PM
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Hi, Engine was running when I purchased ride on, Grand Children were using when it stopped, it has not run since I have disconnected all safety switches and install an on off switch and a push button to start all work fine I have just checked the spark at the plug and have plenty of spark, fuel is fine and I have tried to start with aro-start did not help I have not stripped the motor down, just removed the head to check the valve operation which looks OK, just tried to start it again still backfiring through carby, what else could cause the timing to be out could the flywheel have moved without shearing the keyway, I have just removed the flywheel bolt and rechecked keyway which also has not moved, I did replace it a few weeks ago I am at a loss as to what would continue to cause backfiring.
 
  #27  
Old 11-30-16, 05:31 PM
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Does it have good compression?

If so, I guess I'd have to say the coil is the most likely culprit. Try taking the kill wire off of the coil and see if it will start. If so, you will have to choke it or pop the plug wire off to kill it.
 
  #28  
Old 02-15-17, 09:37 PM
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Si it possible for this type of motor to be trying to start in reverse which way should the flywheel rotate clock wise or counter clock wise does the polarity of the starter motor control rotation?
Tweed2
 
  #29  
Old 02-16-17, 06:38 PM
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No. Polarity of the battery connection can change the rotation of the starter but not the engine, because the starter drive will not engage the engine in reverse rotation.
 
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