Not long ago I bought a used 2000 TRX400EX that I was hopeful my wife would like to use with us at the dunes. It ran great on the test ride I got to make. The machine is not stock, it has a Full DG Exhaust (pipe and headers), K&N Filter (which suuuuucckkks! I can feel grit inside the intake boot that is getting past the filter), air box mod etc. The machine does smoke a bit on startup when cold, so I know a ring job minimum is in its future.
The machine has a 38 Pilot Jet and a 188 Main jet (seems awfully big). I have a feeling whoever did the initial mods and put alot of $$$ into this thing originally, and might have even thrown on a big bore kit. No way to know without pulling the head and measuring.
But... it starts right up, runs fine on trails and has alot of power, it will run away from the other stock '02 TRX400EX we have. That said, we have some sand dunes here (Little Sahara, Utah) with its prominent feature being Sand Mountain, a 1,000ft high sand dune that is a blast to climb from various places.
But, here's where the trouble and my question comes from. I can make a run at the mountain, the machine takes off like a bat out of hell, pulls hard though the gears: first, second etc... but half way up or so, when I start to loose RPM and speed, just about the time I need to downshift, the machine gets hella-boggy, like its overloaded running rich, and I have to pull in the clutch, turn back down the hill and rev it a bit to get it to run solid again. It makes dropping to the next lower gear on the climb impossible as I've lost all the engine power. The other stock 400 would just slowly walk past me right on up to the top. This machine seems fine on trails, or conditions where I'm not full wide open for extended periods of time like the long wide open pulls up Sand Mountain.
So, I'm trying to figure out if this is a rich condition and I need to play with the Main jet, as well as the needle... OR, if this is due to rings / oil fouling the plug... or compression issues meaning a top end is in the works. I tried to get my compression tester to thread into the spark plug hole but for the life of me I couldn't get it to thread in (lack of clearance). I know I really need to read the compression to really identify how bad the cylinder is (without yanking the head anyway). I replaced the spark plug yesterday, the old one was fairly dark.
Bottom line, I'm trying to figure out if I want to keep this machine long term, rebuild the top end over the winter... or sell it to someone else now, who wants to deal with the issues (with full disclosure of its current state of course).
Here's a short video of it running with my son trying to jump it. Sounds great here:
Everything else mechanically is great... I'm not into this very much at all in terms of $$$, but I don't want to do a top end job and still have this sputter / bog on the long full power pulls.