Hi Everyone,



I'm hoping you can help out a guy with a KFX 400. My family has owned a KFX 400 since 2006, and it used to run absolutely amazing. Never had an issue until I started bringing the bike to the dunes.



8 years ago this problem started, then I moved away to college, and now I own the bike again after it just sat in a garage for 5 years. We could never figure it out the issue all that time ago, and now I have the time and $ to actually fix the bike, but everyone I take it to is at a loss for words..



The KFX 400 is basically unusable out in sand dunes or hilly terrain. In 3rd gear ONLY, when the bike is in sand or going up an incline, it will completely lose power and sputter for about a second or two. This will happen anywhere from low to high RPM, WITH THE EXCEPTION of when I am pinned on the rev limiter. It happens most frequently in low to medium RPM. When the bike stalls / loses power / whatever you want to call it, I can rapidly pump the throttle and usually the power will come back and the throttle response returns, but most of the time I end up down shifting into 2nd to finish the climb up a hill because of the speed I've already lost.



It sat and did not run for 5 years while I was in college, and I picked it up from my father a few months ago. The KFX is in great shape physically, never had any serious mechanical work performed, and bone stock except for a Biggun exhaust. I just had everything tuned up last month (carb pulled out and cleaned, new spark plug, new air filter, new coolant, oil change, valves adjusted back to spec) still experiencing the same issue I've had with the bike for years out in Glamis or any other dunes. I've taken this quad to more than 5 shops and no one has ever been able to diagnose or fix this issue...



The quad will run perfect with dirt tires and rip up and down dirt roads all day long. The minute it's in sand and or going up a steep hill, it's guaranteed to have the sudden power dump / loss in 3rd gear everytime.... all day long....



My mechanic, said if it was the CDI this issue would occur in other gears and terrains. But it only happens under a load, and in 3rd gear.... We have no idea where to turn on this thing... Trying to not walkaway from it if I don't have to...



I appreciate the advice and help!

 

CDIs are mounted with vibration isolating rubber. That rubber can harden and cause intermittent issues with the CDI that are hard to diagnose.

Maybe the gear box causes a certain vibration in third that resonates through the frame and makes the CDI crap out until it stops... Try a fresh mount.

 

Thanks for the response! I'll check this out and let you know what happens.