The1984 xr350r has a history of running hot and burning up. I think that was blamed on the new head design and the 2 carb system. Possibly the overheating could benefit with richer jetting. I know mine gets hotter faster than any bike I've owned. I haven't run it much and when it is used it has been a buddy's bike, usually run fairly slow. When compared to other bikes running over the same terrain it is noticeably hotter. For that reason I run a full synthetic oil, hoping for more protection with a higher boiling point.
The front disc brake is a very nice feature, best thing about a very good bike.
How-to troubleshoot an air-cooled motorcycle engine overheating problem:
As for overheating, let's start with a systematic troubleshoot. First and obviously, the engine cooling fins must be clean with sufficient air flow. Also, a four-stroke engine does not like to idle for extended periods in still air. This even applies to liquid cooled dirt and dual-sport motorcycle engines. (There are constant internet discussions about overheat, even on models like the liquid cooled XR650R and others.) Two-strokes with premix fuel have less tendency to overheat, though heat can still be an issue.
When we ride extensively in the desert during hotter weather, I'm always conscious of air flow. After a good high speed streak or even picking through basaltic rock flows, my Honda XR350R remains "reasonably" cool. I'll let the engine idle for just a moment to help stabilize temperatures before shutting it off. We do crawl in stand-up-on-the-pegs type challenges, one location in particular is at high altitude. This engine has never reached critical temperatures. Your use of synthetic oil can help, too.
Running cool at high altitudes is in part what you suggest. Proper jetting for sea level to 4,500 feet, then running the engine at 7,000 feet, makes the fuel mixture richer. I've never fouled a spark plug or experienced blubbering or unstable operation with these richer mixtures. In my experience, jetting is not that sensitive on these four-strokes, they are way more flexible than many give credit. Main jetted for best performance at 2,500-5,000 feet (our typical high desert country), this same engine has run flawlessly from 2,300 feet on the floor at Johnson Valley, CA to nearly 7,000 feet—without a re-jet. Of course, if the engine were operated consistently at 6,000 feet or higher, I would re-jet.
Keep the jetting within a reasonable range. Read the spark plug color after a hard, open throttle run and prompt shutdown! That's the main jet's realm. If anything, jet slightly rich. In current jetting, my XR350R can run at sea level without burning a hole in the piston. Again, if I were to run the length of Baja, I would jet for sea level to 4,000 feet. Never jet so rich that fuel can wash oil off the cylinder walls nor so lean that the valves and piston are at risk! Learn to read the spark plug.
Often overlooked is the importance of proper valve clearance. Valves adjusted too tight will cause engine overheating and also lower compression. Unseated valves lead to valve face burnout and seat damage, too. Adjust valves to specification, and also adjust the decompression lever and kick start cables. This is a big part of maintenance on the XR350R, the XR500R and similar Honda dirt bike models. The auto-decompression camshaft design eliminates the kick starter cable on later XRs. However, the manual (handlebar) decompression lever adjustment remains very important on every one of these models.
On the XR models with dual carburetors, folks need to get over the idea that these engines never run right. Actually, this myth is helpful to those of us who value the dual-carburetor era, Pro-Link models—you can buy them at bargain prices! A common mistake is for the carburetors to be sync'd improperly. There is a very clear adjustment here, which I can share if necessary. Be aware that these carburetors do not open simultaneously! The linkage is actually "progressive", with a lag stage as the primary carburetor provides a smooth idle and light tip-in air/fuel flow. Then the secondary carburetor (which has no idle mix screw) comes into play. Think of this like an automotive engine with a progressive four-barrel carburetor or multiple carburetors. Adjust the cables and the staged throttle linkage to specification. These engines will start readily and run fantastically if the jetting, float level and needle settings are correct!
General footnote: Current ethanol fuel wreaks havoc on motorcycle carburetor passageways. Do not leave the bike parked for long periods with fuel in the carburetor bowls. Shut the petcock, and with the bike upright, burn fuel until the engine stalls. Fuel standing in the bowl will clog jets, it did mine. The immediate symptom is an unstable idle and poor low throttle response. (I ended up rebuilding both carburetors after letting the cycle sit for too long with "winterized" and "Ethanol" mix fuel in the tank and bowls!) Use a fuel stabilizer additive, this can help for shorter storage. Long term, drain the fuel tank and the carburetor(s). Without stabilizer, fuel can become stale and worthless in months, depending upon the climate.
Air/fuel flow and ratio are a critical part of air-cooled engine performance. Keep the air filter clean and oiled properly, whether stock type or aftermarket. A clogged cleaner element will make the engine run rich or stall, much like leaving the choke on. Also, when considering a lean fuel mixture, always take air leaks into account. Air leaks on the manifold side of the carburetor(s), between the head and carburetors, can lean out the fuel mixture. Check for leaks at flanges and junctions. Using a can of spray carburetor cleaner or WD-40, spray a fine mist at flanges with the engine idling. This can quickly turn up an air leak as the engine speed flares up or changes. Avoid spraying at high heat areas that can ignite the spray!
Once you've worked through the intake side, make sure there are no exhaust restrictions. I have a "tunable" SuperTrapp exhaust end that is set up with the right number of discs for proper backpressure. I periodically remove the disks and gently bead blast the carbon away to keep tuning accurate. Improper exhaust backpressure or clogging can be a major source of overheat on a motorcycle engine.
After fuel mixture/jetting, carburetor sync'ing, air leaks and exhaust restrictions and tune, there is the ignition spark and timing. Of course, spark must be adequate, and the spark plug should be the correct heat range, especially on an air-cooled engine. A "hotter" plug can be serious trouble if not in place for a good reason. Hotter spark than the stock Honda ignition (in good condition) is rarely necessary.
Note: The compression ratio is higher, so you also need to run better octane fuel, especially at lower elevations. This may not be true for high altitude. I can run 87-octane in the XR350R at 4,000-8,000 feet because the atmosphere effectively lowers the compression ratio. I am not as flexible with the Honda XR650R at 10:1 compression and a bore size within 0.060" of a 327 or 350 cubic inch Chevy V-8! If you're at a lower elevation with high compression, spring for 91-92 octane fuel. This helps prevent detonation/ping, overheat and stress to the engine. For an XR500R, XR600R or XR650R, I would run 91-92 octane all the time.
The last item on our checklist is spark timing. Either retarded or over-advanced, spark timing error can kill a motorcycle engine. Check the spark timing from idle to full-advance at speed. Make sure the timing advance is set properly. You can test with a conventional timing light or a light with built-in advance, this is not rocket science. I can help cast light on the procedure and expected results. With an electronic ignition, spark timing either works or it does not. If a motorcycle is old enough to have a mechanical spark advance mechanism, like our BSAs of yore, a sticking or defective spark advance mechanism can cause overheating as well as performance problems.
Of course, there are overheat causes unrelated to the engine, like too much friction in the gear train or binding brakes (a sticky front or rear brake caliper, brake shoes dragging or warped/defective rotors and drums). Wheel bearing resistance, low tire pressure or chain drag can also overheat an engine. Check for resistance with the motorcycle wheels and tires lifted safely off the ground or floor.
This is a place to start, and I am happy to continue this discussion. Glad that FullChoke triggered this topic, that's what these forums are all about!