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Definately try a spark plug first. What does the old spark plug look like? ( white, oil fouled, melted, metallic?). Usually a fuel issue will make the inoperable cylinder very hot. Might have a loose valve guide in the head, or a bent valve.
Back fire is an indication that the bike is firing incorrectly.
Your timing chain is probably stretched, due to the year.
inproper valve timing due to stretched timing chain will make the bike backfire.
I have other thoughts like stuck valves or inproper sparkplug wires on wrong plug,
i always start with easier and less expensive method.
1st check your sparkplug wires to make sure they are on the right plug.
2nd pull sparkplugs and do compression and leakdown test.
3rd pull componets to see if valves are opening and closing, (valve clearance check)
4th if valves are good, then timing chain is probably stretched.
5th if timing chain and timing marks are good, and valves are good, then proceed to
testing the signal generator to make sure your getting a signal to the cdi,
6th if you have a signal to cdi, the cdi tells which coil to fire. either # 1&4 or # 2&3.
if signal generator is working, but coils aren't firing right then its the cdi.
obviously your coils work because its firing, just back firing.
Thats all it could be from easiest to hardest.
Good luck and have a great weekend.
You may have issues with;
Fuel flow, check filter for blockage, line for obstructions and floatbowl needle and seat for dirt.
Air/fuel mix. Clean carby with a aerosol carby cleaner and check operation of slide needles (rise/fall in unison)
Sparkplug; Reset gaps or replace if wary.
Top End, is camchain tensioner working, any undue valve train noise, has it been serviced at all.
Electrical. Check wiring connections and plugcaps, coil earthed and pluglead free of other wiring.
1. First thing is when was the last time you had your valves adjusted
2. On cold start up twist about 15 times on your throttle, as you do that the accelerator pump in carb will shot gas in cylinder, I have a 2003 yx426 & 2005 yz450 have to do to both of them when cold out to get started
try to perform these :
1. Check the sparkplug if it is good or not,.if it will
ignite when you are going to start.if it doesnt ignite,replace with new one.but when after replacing it is still does not ignite,then its time to check the primary coil.You can check it by looking the wire coming from the said coil through the CDI..(mostly it is a black color with a little stripe of any color.) if you already found the said wire,start the motor and ground the said wire to the body with on & off motion.If while doing this you can notice a spark coming from it,then your primary coil is good.,Thus,its time to check your CDI or need to replace it.
But,if there is a fire in the sparkplug when you start the motorcycle but it doesnt work or fire at all,then you must clean the carburator.
Onthe other hand,if your motorcycle is a dynamo/contact point operated,you must check the "timing" into right settings,or give an importance to the condenser if it has a leak and may cause short during the motor will start..