I have a 1997 250 Polaris trail blazer. back fires and blow off the exhaust.

ck these sites...

https://www.google.com/search?q=2004+polaris+250+trailblazer+no+spark&newwindow=1&sxsrf=APwXEddfFqPFyOq6sx1QyeZupt5MDMKV4g%3A1685999668141&source=hp&ei=NFB-ZPfzBdKJptQP7KS_UA&iflsig=AOEireoAAAAAZH5eRDnFu7tx71C5t-tAJh_4l8ZGIDny&ved=0ahUKEwj33buahq3_AhXShIkEHWzSDwoQ4dUDCAo&uact=5&oq=2004+polaris+250+trailblazer+no+spark&gs_lcp=Cgdnd3Mtd2l6EAMyBggAEBYQHjoFCAAQgAQ6CAgAEBYQHhAPOggIABCKBRCGA1AAWMcyYM9CaABwAHgAgAGjAYgBkguSAQQwLjEwmAEAoAECoAEB&sclient=gws-wiz

Raze the track off the ground , there is a adjustment bolt on the chain case losing the check nut and turn the bolt by hand until tight, then turn the track and recheck, you might have to take the exhaust off to get at it, If i was you i would take the chain case cover off clean and every thing and put new oil in, most people don't even check the oil and then blow the chain, the oil thins out in the summer and drains out a littly each year until theres none.

replace the crankshaft seals
they are letting air in on the piston upstroke and sucking the mixture in through the carby

double check the timing and timing chain tensioner. maybe a leak on the intake manifold?

Make sure the kill switch is in the run position.

AN engine need three things to run:
Compression
Timed spark
Fuel
now.....
1*. Check the spark plug to make sure it is firing.. remove the spark plug.. make sure you have metal to metal contact and then turn the engine over.. (this might be a two man job).. the spark plug should have a nice big blue white spark,,(some electronic's will have a weak orange and still fire fine!)
2. Make sure you have gas in the tank and (going into the carb/FI)... (have to ask-actually have had 3 bikes in my career come in complaining -won't start...!! Add gas and they ran.. WOW!.. never charged any of them....the embarrassment on their face was payment enough!)
3*. While the spark plug is out... place your finger over the spark plug hole firmly .. keep the spark plug cap away from your hand..turn the engine over and see if it will blow your finger off..(it should blow it off the hole easily).

If any one of these three are not nearly perfect.. you have an issue!!

Check these and repost with the results...

Hi Sandy:
Hmmmmm.
Something must have happened.
Back to basics.
Pull plugs, check for hot blue spark.
Plugs wet? Fouled?
If dry and spark then no fuel.
Drain/flush carb.
Change fuel filter.
Think of it as entertainment.
Cheers,
Happy New Year.

Could be a carb issue, did you set your timing correctly?

This is a timing issue. The crank and cam are out of time. It might be excessive wear,

take carburetor apart and do a good deep clean. also check valve clearance.

There are several places you can go on the web that have free atv repair/owners manuals. The owners manuals usually have a wiring schematic on or towards the last page also trouble shooting diagrams with instruction in them.

Type in your search bar: atv polaris (year) model:trailblazer/500cc.
If you like email: [email protected] with specs and I will email it to you.

Hope this was helpful
Try2B8me