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Curtis B.
Curtis B., Technician
Category: Small Engine
Satisfied Customers: 26016
Experience:  have worked on and around most engine models for 35 yrs./Polaris ATV expert
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Regarding a 21 HP Briggs and Stratton motor model #331977:

Customer Question

Regarding a 21 HP Briggs and Stratton motor model #331977: My motor is backfiring. I have changed the spark plug, rebuild the Nikki carb, and changed the gas. The backfiring is almost rhythmic and I am at a loss. Help!
Submitted: 1 year ago.
Category: Small Engine
Expert:  Curtis B. replied 1 year ago.
Have you checked the valve lash? If a valve is not fully closed when the spark occurs, you get a backfire. Valves set .003 to .005 on intake, and .004 to .006 inch on exhaust. Or just set both on .005 inch! That is the place to start!
Customer: replied 1 year ago.
I have not checked this yet. Do you have a link to instructions?Also, I guess I am not sure if it is backfiring or misfiring... Not sure if this can make a difference.
Expert:  Curtis B. replied 1 year ago.
Same difference. There are U tube videos, Here is link to one.-----------------------------------------------------------------------------------------------------https://search.yahoo.com/search;_ylt=AhWMFmDVwwgGpNhDdYH8K6.bvZx4?fr=yfp-t-700-s&toggle=1&fp=1&cop=mss&ei=UTF-8&p=briggs%20intech%20valve%20adjustment
Customer: replied 1 year ago.
THanks, ***** ***** link did not post. I can search myself.One thing I noticed is that if I turn the flywheel by hand, air is pushed back through the carb air intake, which smells of fuel. Is this normal?
Expert:  Curtis B. replied 1 year ago.
It would seem to be normal. Just do a search for Briggs Intec valve adjustment.
Customer: replied 1 year ago.
That didn't help. Please view my video of the issue. https://www.youtube.com/watch?v=wVsgjWZ9vpU
Expert:  Curtis B. replied 1 year ago.
I see and hear the video. Very strange one! What did you find with the valves? Go back and remove the valve cover, and turn the engine by hand ans see if the valve movement is erratic. Briggs is using a camshaft that has pressed on cam lobes, I wonder if one of the cam lobes is slipping, and causing the erratic operation.??
Customer: replied 1 year ago.
The valves seem fine. Could the fly wheel key cause an issue like this? I have not hit anything however, so I am not sure it would be sheared.
Expert:  Curtis B. replied 1 year ago.
Sheared key would cause hard start or no start. I don't see this as a key sheared. But there is something abnormal here. That is why I think the camshaft needs to be checked.
Customer: replied 1 year ago.
Okay, thanks for the replies. I am still under warranty and scheduled a service appointment. That should fix it!
Expert:  Curtis B. replied 1 year ago.
!! If under warranty I would certainly do that! Thanks for the reply. But I sure would like to know what the outcome is!