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· Registered
3 Posts
Discussion Starter · #1 ·
Here's what I have
- 2003 Rancher TRX350TEA 2x4
- 9000+ miles
- Operating at 7200 feet above sea level

Here's are the problems
- Runs pretty good (no backfiring, pretty good power, no hesitation, or bogging) BUT
- Plug fouled with dry black soot (even just at idle for a short time)
- Blows a little black smoke on revving.
- Compression stinks (60-70 out of 97), no significant change on wet test
- Rough syncopated idle. Sounds like a demented drummer

Here's what I have done
- Carb completely cleaned, rebuilt, and all parts measured and in spec.
- Changed Main Jet to 122 from 125, Pilot Jet to 40 from 42, stock start jet, needle jet and needle.
- Needle adjusted to leanest setting.
- Adjusted pilot screw to achieve max rpm at 1300-1500
- No hesitation on acceleration. Returns to idle ok, though sometimes too low for a few seconds.
- I have tried fiddling with different pilot jets from 42 down to 35, no joy.
- Choke a little worn on brass ferrule, but appears to work properly

- Top end rebuild with honed cylinder, new rings, tappets, lapped valves just completed.
- No valve leakance.
- All top end things that I could measure were in spec.
- Could not measure cylinder bore as indicated in manual, but new rings spec'ed perfectly
- No leaks at valves after lapping (solvent test).
- Valves adjusted correctly.
- Tried pressurizing the cylinder at TDC (holding it there with the recoil starter) and can hear a very faint hiss at crankcase breather, which I would expect since SOME air will always pass the rings. Don't have leakdown test apparatus.

- Clean air filter, correct plug and gap
- Spark jumps the gap ok, but does not look "fat and blue" more like "normal size and very light blue", though I have nothing to compare it against. The connector inside the boot seems tight.
- Battery is weak, but I have no better luck when testing with 10amp boost charge
- Alternator appears ok (13+ VDC with a crappy battery)
- Timing seems ok, but my timing light misses flashes (though not clearly in synch with roughness in engine)
- No smoke or gas smell at crankcase vent.

- Since the carb and top-end rebuild, the engine seems a bit quieter, and blows MUCH less black smoke, but no essential change in problems
I will put in a new battery soon, but I don't expect that to fix anything except ease of starting.

I don't really know what to try next.

I could get a peak voltage tester and look at the coil or pulse generator but I would guess that this will not tell me much, since the machine essentially works correctly. It would be interesting to know if the PG is missing pulses but I have no idea how to test for this.

The only compression thing that I can think of is valve timing, but I am reluctant to rip into the camshaft without knowing what I am looking for. I suppose the piston or cylinder could be out of spec, but nothing else down there was, and the new piston rings fitted to perfection and the wet test does not help the compression.

Can anyone out there point me at what might be the next best thing to look at?


· Premium Member
5,683 Posts
Hey Pete
Wow youve done a lot of work on this rig–i have an 02 with 7200 miles. No engine work and it runs well at 3200 asl. You’re quite a bit higher.

Check the service manual for high altitude operation it should list the right jet to start. It may be possible to get a smaller main jet.

How long has it been since you changed the starter (choke) valve?

You could try a different heat range plug but only do this after you’re done adjusting the carb

· Premium Member
5,683 Posts
Reread your post after coffee
I looked up the manual and looks like you tried everything except replacing the SE valve. Yeah I would do that—The oring is supposed to have a conical shape not a smooth shape. Consider putting the carb back to recommend hi alt jet settings when you replace the SE valve. Then try the 122 jet if you still don’t like it.


· Registered
3 Posts
Discussion Starter · #4 ·
Thanks guys! I overlooked the SE valve. It's cheap and easy. I will re-evaluate the Main Jet when I replace it. The ATV was blowin' chunks of black smoke at WOT with the 125 high altitude jet. Possibly due to leaking SE valve!

Ordered a new battery, and new SE valve today.

Any thoughts on low compression? Like what range is reasonable?

Anyone have a video/audio of their TRX idling? I've nothing to compare mine with, so maybe the rough idle on a TRX350 is normal?

· Registered
3 Posts
Discussion Starter · #6 ·
Well damn! Amazin' what a screwed up SE valve can do to you. Turns out the valve seal was worn out and the cable damaged slightly at the knob. Still worked, but there was always a little choke on.

Replaced valve and returned pilot jet to original 42s. Everything good, engine totally smooth, no soot on plug, MUCH less black smoke at the pipe on acceleration.

Thanks for the help guys!

PS. If anyone knows what might be a reasonable compression range for a 9K miler, please let me know!
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