More top end speed wanted for suzuki ltz400 year

I would check your spark plug to see if fouled, if so clean with wire brush and carburetor cleaner or replace plug. You think it is possible the fuel mix got higher than 40 to 1? If mix is too high it can cause the bike to run lean.Usually if mix is off you will have some blueish grey smoke coming out of exhaust.

Well, yes and no. A person can do almost anything he wants, but is the cost worth the Gain? You'll need to have a connecting rod custom made as the crank journal is different. Deck clearance will be an issue too.
You can however use the 750 head which will bolt up and has better flow. A slim planing, and they respond well to a meager port and polish. On the intake side keep a satin finish and just clean it up, exhaust a little more aggressive, (shinier).

Your ATV does not have lifters. Valve clearance estimate (FROM EXPERIENCE) is close to .004" intake and .0055"-.006" exhaust. This should be a little loose, much more preferable than tight.

GET THEM PROFESSIONALLY SET!

the fan doesn't run unless they get really hot take the front plastics off and check the coolant in the radiator instead of the plastic jug

can put a larger sprocket on back or smaller sprocket on front. will gain low end acceleration but may loose top end speed

You do not need to alter anything except the chain, increasing the tooth on the front sprocket and decreasing the tooth on the rear will give you more top end, remember more teeth longer chain required, less teeth shorter chain!!

To increase the top end speed, just put on a smaller rear sprocket. Alternate to that, is to mount a larger front sprocket. You can do either or both depending on how much top end speed is wanted and how strong the engine is to carry the load. The length of the chain will need to be changed.

take to dealer, big, big job

2002 gsxr 1000r started drove up on trailer went to town came back went to unload it and would not start was running great turns over good lots of fuel good battery fuel pump is working no f1 code ???????