I have an 400 explorer 4x4 that will not run unless I install a new sparkplug. After I install a sparkplug it will run for about a week then will quite again. Sometimes I can clean the plug and it will start again. Does anyone have any idea what the problem could be? The compression is reading around 90PSI.Any help is appreciated.

I would check the carb settings - gas / air mixture and make sure the air filter is clean. When you take the plug out- if it's wet or cover in black - then turn your air screw out a 1/4 turn at a time and do a pull check. You want the plug to have a "rust" color on the end. If the air filter is dirty, then you will get less air and foul out your plugs. Your compression is in the lower limit for the bike to run well. If it gets much lower then you will need rings or a piston.
hope this helps
hunt

kyxplorer,
Can you send me pictures of your quad, I am wanting to do something similar. I am buying a 95 xplorer for around $800 and fix it up. One question, how good is the front ends with the chain drive?

Lee

400 explorer won't run
 

Your compression is way too low, you are lucky it is running at all. It will need a "top job".

400 explorer won't run
 

Agreed, your compression should be at 115 minumum.

400 explorer won't run
 

same here TOPEND time.

400 explorer won't run
 

why do you run that quad so hard? poor thing, no wonder her compression is so low, rebuild the top end before you go messing with the carb, if it ran fine before on the stock carb settings leave them alone and fix your compression