have a Sears Snow Blower with a Briggs engine 21a414 0540-E1 , it back fires then the throttle is reduced , some sites say this is normal.
Checked valve lash intake is 4 th , exhaust is 5 th, armature gap is 15 th.
Sears had replaced the coil and the radius of the flywheel doesn't match the curve of the armature legs. Could this be the problem?
Sears said this is common and no concern.

 

Does it do it when you do it slowly? If not then that's the answer.

 

I does it after it warms up , also it idles poorly , what do you think about the coil legs not being the right radius?
I've read that it could be the muffler design causing after fire.
Also the plastic cover over the carb area , has a metal shield inside it and in no time it gets really hot , could this be leaning out the carb?

 

does it back fire through the carb or just through the muffler?

 

it back fires thru muffler only

 

That can be caused by retarded timing or a lack of resistance in the exhaust system. The latter would be usually the muffler design. Used to, when kids would soup up car engines you would often find the same thing with high performance mufflers. Some times they would put a spark plug in the exhaust so the unspent fuel would ignite and give an extra "bang".

When you have a snow blower and a muffler change the problem is a little worse because of the lack of an air filter.

The armature gap and type of module used isn't likely to be the cause, although you usually match the module configuration to the flywheel curvature.

 

I called Briggs and they said the valve lash and coil gap was ok , and the coil was a new design.
They said try a RC12YC Champion plug , and check the valve guides ,
some times they come loose and move at random intervals and stop the valves from opening/closing properly.
Doing this check next.