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Briggs 8 Hp


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#16 Bruce Dorsi OFFLINE  

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Posted July 18, 2012 - 09:34 AM

Hey everybody. I have this Troy built tiller with a 1970's Briggs and Stratton 8HP engine; that will not run. It has gas,spark and compression. It backfires out of the carb. The valves are fine. The carb. was cleaned and rebuilt. I was thinking maybe the carb is adjusted wrong??? Was wondering what you guys thought?


A little history will be helpful. ....What was done to the engine recently? ....When was it last run? ....Was the engine apart?

If you have adjusted the carb to its initial settings (1-1/2 turns from closed on high & low speed needles), it should be close enough for the engine to start. ...Final adjustments are made while the engine is running.

Backfiring thru the carb indicates incorrect ignition timing (wrong coil gap, or partially sheared flywheel key), incorrect valve timing (sticking intake valve, insufficient intake valve clearance, incorrect valve timing, or intake valve not seating properly). ....What makes you say, "the valves are fine?"

In order to backfire thru the carb, the ignition is firing while the intake valve is partially open.
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#17 LTD OFFLINE  

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Posted July 18, 2012 - 10:40 AM

A little history will be helpful. ....What was done to the engine recently? ....When was it last run? ....Was the engine apart?

If you have adjusted the carb to its initial settings (1-1/2 turns from closed on high & low speed needles), it should be close enough for the engine to start. ...Final adjustments are made while the engine is running.

Backfiring thru the carb indicates incorrect ignition timing (wrong coil gap, or partially sheared flywheel key), incorrect valve timing (sticking intake valve, insufficient intake valve clearance, incorrect valve timing, or intake valve not seating properly). ....What makes you say, "the valves are fine?"

In order to backfire thru the carb, the ignition is firing while the intake valve is partially open.

Heres everything I did to it. New flywheel key,New magnatron, spark plug,rebuilt the carb, ground the valves and lapped the valve seats. I have no idea when it ran last it wasen't apart.

#18 LTD OFFLINE  

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Posted July 18, 2012 - 10:43 AM

Is it possible that the magnetron is on upside down? That would drastically change the timing.

Its not possible to install it upside down because it has to contact the magents on the flywheel.

#19 LTD OFFLINE  

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Posted July 18, 2012 - 08:25 PM

Update. I got it running. Turned out it sheared the new flywheel key after I put it in. Put another one in and it ran. Thanks for all the replies and suggestions.
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#20 MH81 ONLINE  

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Posted July 18, 2012 - 08:35 PM

Wow, two keys? I wonder what would cause that?

At least, if it ever does it again, you know where to start looking

#21 HDWildBill OFFLINE  

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Posted July 19, 2012 - 09:24 AM

I agree with Alan, two key's maybe something else is going on. I think if it happens again I would start looking further to find out why it keep shearing key's.
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#22 DougT OFFLINE  

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Posted July 19, 2012 - 09:30 AM

Wow, two keys? I wonder what would cause that?


Grease on the taper, flywheel nut not tight enough, or something like burrs on the taper that would make it not seat. The taper has to hold the flywheel and crank together. The key is just there to line it up.
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#23 Bruce Dorsi OFFLINE  

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Posted July 19, 2012 - 03:09 PM

Grease on the taper, flywheel nut not tight enough, or something like burrs on the taper that would make it not seat. The taper has to hold the flywheel and crank together. The key is just there to line it up.


I agree !! .....Also, the cupped washer needs to be under the flywheel nut or starter clutch. ....The concave side of the washer goes against the flywheel. .....If the washer is cracked, it needs to be replaced, as it will not hold tension on the flywheel.




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