# 10 horse verticle Briggs backfiring



## Jakebob (Jun 15, 2006)

I have a 10 horsepower Briggs that starts fine , but misses and backfires. I have cleaned the carburetor ( float type ), set the points and coil gaps, changed the oil, and installed a new plug. This is an old Jacobsen lawn tractor. When I got it the engine would not start. I found that the soft key had been sheared so I replaced it. It fired right up but after a few minutes and a couple of starts it would not start again. The key was sheared once again. I thought this occured due to me not tourqueing the flywheel down sufficiently. I then cleaned the carb and so forth and it started fine but soon began to backfire again. I haven't pulled the flywheel again yet and I might not get to it until next week, but doesn't this sound like a timing problem? I think the key might have given a little. I hope some of you experts can give me some diagnosis and advice on this.

Thanks

I leave tomorrow for Florida, ( driving from Ohio ) so if I don't respond right away don't give up on me


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## scrench (Dec 8, 2004)

sounds like the flywheel might be cracked but be sure the crankshaft and flywheel are spotless clean no oil at all clean and dry !!!! if the points are off and it misfires it will pop the key every time make sure the points are good and the condensor is good i hate points i run them in my old shovelhead and they drive me nutts from time to time


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## Munkey (Jun 12, 2006)

The crankshaft could also be bent torsionally, depends what happened to it.


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## Jakebob (Jun 15, 2006)

*Good advice*

Thanks for the information , when I get home I'll start with new points and condenser and cleaning the crank and flywheel. I'm not sure how to determine if the crankshaft is damaged, I hope not, and can a crack in the flywheel be determined visually? Thanks again Scrench and Munkey. I'm glad I happened upon this group.


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## tom tilson (May 1, 2006)

I know this is highly irregular. Sometimes when folks shear a key and need to finish the job right away, they will substitute a piece of steel keystock, they probably never go back and put the aluminum key in and as a result can end up with a mag crack, as already suggested, but a crank keyway that is waalowed out. Remove the flywhl and closely exam the mag, which cracks straight out from the shaft hole, then exam the fly wheel key slot and the crank slot for any wear at all. If the key is replaced later with the aluminum one, then it will wallow aroynd unti it ti gets a deep set or shears and stops all together. If this has happened, you will have to determine if th engine is worth a crank and mag wheel. The one that I found was on my son's and since the unit was12 yrs old, I fitted a steel key and put more than normal torque on the retainer nnut. Two years later she was still chugging.


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## Jakebob (Jun 15, 2006)

*keyway information*

Thanks Tom for your ideas on my problem. I recall that the keyway looked a little ragged when I changed the key. I'll be back home on Wednesday and I'll let you know what I find.

Thanks again


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## certifiedtech (Jun 30, 2006)

Does it back fire through the carb, or the muffler? Either way, it sounds more like a valve problem. If it back fires through carb, it`s the intake valve, through the muffler it`s the exhaust valve. The valve may be burnt and not seating properly.


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