Could really use some help

[fixkick]

sorry ,
I am not talking about backfiring , back to the air clear box. (is it?)
nor the other backfiring, called muffler explosions,

ONLY misfiring.
Misfiring means the spark plug failed to fire and burn the air fuel mixture , each time it happens.
that is mostly bad spark, no spark no boom.

it's heard , an felt. both, and is violent.

this is a gas engine, and spark must be good, compression good, an fuel rates good.
if all that is good misfire does not happen, unless fuel is super rich. then it does too rich misfiring, spark plug tips turn black too, as proof. (and can smell it)
it is very hard to explain misfire to some one that never seen or heard this,before.

ever have a lawn mower, string trimmer, chain saw, or 2 stroke dirt bike misfire, all do with regularity.

makes a sound bam, bam , bam, pifft, pift, bam, bam bam bam, pffit, bam. over and over, that is misfire, each Pfifft is unburned fuel to the CAT and a RED hot CAT, if CAT still alive as most are NOT this old.
then you smell the unburned gas stench, power loss, and huge gas bills, even 15mpg is easy to get , doing this.

Read up on misfire , internal combustion engines.