Air-Cooled Dirt Motorcycle Engines and Overheating

[Moses Ludel]

At the forums Garage Photo Gallery, member "FullChoke" (Greg) responded to my photo of the magazine's 1984 Honda XR350R motorcycle.  We have identical '84 XR350R motorcycles.  Greg's cycle has engine heat-up problems, and this raises the issue of how to keep any air-cooled dirt or dual-sport motorcycle engine running cool enough.  I'll begin with sharing our exchange at the Garage (below), followed by pointers on how to keep an air-cooled dirt or dual-sport engine from overheating.

 

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My comments at the Garage Photo Gallery:

 

Moses Ludel
23 September 2013 - 02:33 PM

The Honda XR350R cycle has been in our stable since the late '90s. Rick Sorensen, an A&E aircraft professional, had set up this dirt motorcycle for desert enduro riding. Rick's attention to detail, tuning and appropriate upgrades has made this one of our favorite dirt motorcycles to date.

When new, the 1984 XR350R came with many advanced features, including Pro-Link rear suspension, a disc front brake and the four-valve, twin-intake and exhaust thumper engine design. The performance, dependability and flexibility of this engine has been a constant source of satisfaction. Under the most challenging conditions, including crawls through milder "rock gardens", the cycle and four-valve engine have delivered tractor-like stability.

In other online forums, there is much talk about the "failure prone" and "problematic" dual carburetors on the XR350R and XR500R engines. In my experience, nothing could be further from the truth. I have rebuilt these carburetors and set them up to factory specifications, adjusting linkage and cables accordingly. The dual 26mm Keihin carburetors are stable and flexible enough to get by with the same jetting from Johnson Valley (King of the Hammers) to timberline at Nevada's high mountain ranges.

Note: I jet for our base at 4500 feet elevation. The carburetors tolerate short rides to 8,000 feet and drops to 2,500 feet. That's very flexible, and though I would re-jet for extended riding at either of these high or low altitude points, these carburetors will "function" over that range. Simply put, I'm not fiddling constantly with the carburetors.

The transition to the secondary carburetor is seamless, and by making sure there are no vacuum leaks, the tuning stays rock steady. Sure, EFI delivers more power and refined tuning, but these twin Keihin 26mm carburetors do work well and can be adjusted, or even re-jetted with care and patience, when you're 120 miles from a paved road.

As for handling, I always ride the motorcycle that's under me. Sure, a CRF450 or XR650R could "run circles" around these earlier XRs, but that's not what I'm riding here. The XR350R can provide a great ride when handled like, not surprisingly, a tuned XR350R and not a CRF450. This logic applies to road cycles, too. I never rode our Honda Gold Wing or the BMW K1100LT like I did my two 1969 BSA motorcycles, the lightweight 650 Lightning or the 750 Rocket III. When I hopped on my youngest son's Yamaha YZF600R6 a few years ago, that was a different story as well.

For this XR350R workhorse, I'm considering a dual-sport conversion kit from Baja Designs to enable highway access. We have an '84 XR500R waiting in the wings for restoration, another dual-sport candidate. Each of these bikes would weigh under 300 pounds converted, much nimbler on dirt than a KLR or BMW in any form. Nevada's new OHV permit program makes it just as sensible to opt for a DOT-standards conversion and license ("plate") an XR for dual-sport use. We carry insurance either way.

On that note, expect details shortly on what direction this takes. If the dual-sport conversion route, I'll cover the steps at the magazine in an HD video 'how-to'.

Moses

 

Greg's comments at the Garage Photo Gallery:

 

fullchoke
Yesterday, 09:22 PM

 

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.

Greg

 

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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!

 

Moses

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[fullchoke]

Thanks Moses for the comprehensive post. Leaving stale gas in the carb is so easy to let happen even though we know the bad it will do. I know that I've drained the gas from the tank after forgetting to drain it and letting it sit over the winter, put in fresh fuel and was relieved that it started up and seemingly ran ok. The danger here is that the jets  could be partially plugged in essence acting like a smaller jet and when you have 2 main jets that are partially clogged... well you see the problem. Some engines may be more susceptible to this than others and fall victim to a bad reputation when poor maintenance is the real problem. The early Honda RFVC head design may have been one of these engines. Moses also mentioned how 4 stroke engines don't like to idle for long periods, because of the heat buildup. Possibly the RFVC design tolerates even less of an idle period than the head designs before it. Once the oil gets cooked, the valve train is damaged. The valve seals go 1st and the engine smokes on startup but then clears up, with more wear it can smoke all the time, with more heat, the valve train fails completely. 

 

When I 1st got my XR350R it smoked a cloud on startup. I didn't know it but the end of the dipstick was broke off and I was overfilling the engine with oil. Eventually I found a picture of a good dipstick and reduced the amount of oil. No more smoke. That dipstick is metal and that missing piece has concerned me. I need to find another dipstick, but not from Honda, they are obsolete.

 

Greg 

[Moses Ludel]

I suffered firsthand from leaving fuel in the two carburetors of the XR350R.  The only solution was removal and careful rebuilding of both carburetors.  The "slow"/pilot jet on the primary carburetor was not just clogged, it was ruined.  As most know, it is virtually impossible to clear a solidly clogged brass jet.  Jeweler's drills, welding tip cleaners, a stiff wire, you name it, most attempts like this result in the enlargement or oval shaping of the soft orifice.

 

The cleaning attempt went nowhere, and fortunately, jets crossed over to a Kawasaki application with a much less costly (under $8 including shipping and tax) replacement Keihin jet, purchased through the local Kawasaki source rather than Honda.  The overhaul gaskets came from Honda. 

 

Lesson well taken.  I now shut off the petcock and run the bowls as dry as possible, bike upright, before stopping the engine.  This drops the fuel below the pilot and other passageways.  So far, long periods of setting have not seen the problem return.  I do try to ride the bike to avoid stale fuel, otherwise draining the tank and refilling with fresh gasoline.

 

As for overfilling the crankcase and creating blowby and valve seal and guide seepage, that's an excellent point to share, Greg!  My XR500R has a dry sump system, and overfilling issues are a bit less likely to happen, though they could.  Worth investigating on this machine.  It only ran 30 minutes when I made the blue smoke judgment call.  Was I too hasty in condemning rings and such?  Maybe the valve guide seals are bad or damaged from overfilling the oiling system.  The crankcase did have oil seepage, but that, too, could be from excess pressure caused by overfilling. 

 

At this point, an engine teardown is warranted, the crankcase seepage won't disappear by simply dropping the oil level.  The guide seals are likely ruined from the bike setting up for years now.

 

Moses