I have a 99 explorer 400 and It has no power what so ever. It has a lot of problems going uphill. I have adjusted the air screw, but it doen't really help. It just started up it never did this before. If it helps any it is being run at about 4000 feet ubove sea level. If anyone could help I would really appreciate it.

 

check for a dirty air filter...

 

 

I checked the air filter and it's clean.

 

Check the plug,airfilter and do a compression check. You might even need to clean the carb out or rejet. But start with the simple things first.