Why does my 2002 eiger 400 run like S&$@ as soon as I put it in. Gear?

check fuel pump --filter--cdi https://www.atvrepairmanual.com/suzuki-eiger-400-repair-manual/

spark plug replace. fuel system clogged up carb filters clean?

Hey Fred, I AM SOOOOO JEALOUS!!! My teenage years Saturday nights were spent in a 64 Fairlane...(4 speed - 390 - WOW!!)..
Any way.. your carb needs some TLC, a valve clearance setting-get a qualified pro to do this!!), new spark plugs (wires maybe) .. a new air filter.... mainly the carb needs a QUALIFIED PROFESSIONAL to clean and set it once you have put in the plugs/air filter.. This will eliminate possible rough idle issues and give the mechanic peace of mind when he is adjusting the carb.
Good luck...

Could be a lot of thing's , you should have it checked at a qualified repair shop .

Hmm...........
1985..........
Probably carb? Automatic choke not working? Remove air filter and spray carb with cleaner, Make sure all parts move freely. With engine cold, check automatic choke position. Start and check function as it warms up.

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is that a semi-auto or manual clutch?

check and clean the carburator... doble check the fuel line in the carb....

If 2 stroke check crank seals, check plug cap for proper resistance,check for good fuel flow to carb.Spark should jump at least a 1/4" gap,NOT JUST the .030 or plug gap.

u have no spark in #2 cyl check for spark with plug if its week regap it