Honda Rincon ATVs have a reputation for abnormal knocking/rattling noises from the upper engine.  Resolution ranges from “do nothing it's normal" to complicated theories involving the relationship between ignition timing and engine speed.

I spent hours on the internet reading about alleged causes without finding any answers about why it happens or what actually causes the noise, prior to diving into the 2003 Honda Rincon with an ill-sounding rattle parked in my garage. 

 About a year ago, this particular Rincon began to knock/rattle when started cold (less than 50 degrees) and ceased as the engine reached operating temperature.  This isn't a subtle noise that could be missed; a sixteen year old girl heard it from a hundred feet away and said "that sounds really bad".  The noise would occasionally stop (with a cold engine) if the engine was briefly revved.  Listening to the engine with a stethoscope revealed the right front of the cylinder head/cam cover area as the origin. 

Proper engine oil pressure was confirmed and the timing chain tensioner was removed and inspected for travel and operation. It was not maxed out and it held position when compressed.  Admittedly, I was skeptical of a loose timing chain from the start because the noise produced sounded nothing like a loose timing chain and more like a broken valve spring, a loose valve seat, or lack of oil to the cylinder head.  To be certain, oil flow was also confirmed to the cylinder head.

Removal of the cam cover revealed no cause.  The rocker arms and related components measured well within the specified range.  Other than running the engine, the noise could not be reproduced.  The only clues found were a sharp feel to the edge of the exhaust cam follower (when compared to the intake follower) and a very small chip in the finish of the same follower. 

With the cylinder head in place, not much of the camshaft can be seen, but the irregular follower wear prompted additional scrutiny and questions.  The decompression mechanism is activated by the front most edge of the exhaust cam follower and would make an eerily similar noise when tapped on.  The frequency of the sound was correct as was the location, but what was the cause?

Understandably, the owner decided against additional exploratory surgery at that time and a year had passed before the noise became constant rather than intermittent.  Hot or cold, it sounded like something was ready to detonate.  I concluded the next logical step to be cylinder head removal for closer inspection of the decompression mechanism. 

Numerous individuals on internet forums stated that replacing the timing chain tensioner resolved the noise temporarily, with some replacing the tensioner 3 times in 1200 miles to keep the engine quiet.  Few said the noise had been resolved after the timing chain, tensioner, and camshaft were replaced-a labor intensive repair that struck me as throwing parts at the problem in hopes of one of those parts being the solution.  I decided to take another look at the timing components prior to removing the camshaft, but could find no faults.   Because I do not know the extended position of the tensioner when new, I could not accurately determine the travel remaining, but estimated it to be 30 to 50%.

Inspection of the camshaft showed the lobe height to be within specifications, though the exhaust side had signs of what I’ll call unusual scarring, with one score being enough to catch a fingernail-nothing that could cause our noise, but something of concern and another piece of the puzzle.  A similar wear pattern could be seen on the exhaust lobe follower. 

According to the Honda service manual, the decompression mechanism worked as designed, though I questioned of the smoothness of its operation and suspected it may not have been releasing properly, possibly hanging up and disengaging harshly, causing the mechanism to rattle.  Because of the exhaust lobe scarring, I felt the camshaft and followers should be replaced regardless, which would also allow me to compare the feel and operation of the decompression device. 

When the new cam arrived, I studied the mechanism carefully and could not find any differences in operation, feel, or measurement and strongly suspected the noise would still be present once reassembled. 

While fondling and spinning the replacement camshaft by hand in some native attempt at mediation-to become one with the camshaft-I had an epiphany.  As the camshaft sped up and slowed down, the decompression device would change direction enough to make noise.  This reminded me of a slow motion video I had once seen of a camshaft in operation, where the camshaft appeared to actually stop at points through its rotation, caused by the friction of valve actuation and the coinciding speed changes of the camshaft.  In reality, even with a properly tensioned cam chain, the camshaft is not spinning at a constant speed.  Also, with low engine speeds, the camshaft speed fluctuates more than at higher engine speeds where momentum begins to smooth out the fluctuation (this can be easily demonstrated by pulling an engine over slowly by hand versus a quick pull). 

What if the cam chain wasn’t “loose” by normal standards, but loose enough to cause the decompression mechanism to flutter and rattle?  I could hardly wait to get it reassembled and test my theory, but how could I adjust the timing chain tension with the engine running?

The timing chain tension device utilizes a worm gear and spring to adjust tension.  Inserting a small flat blade screwdriver into the rear of the tensioner and turning it clockwise retracts the device against the spring.  Letting go of the screwdriver from this position releases the spring and causes it to rapidly unwind, extending the tensioner shaft.  The beauty of a worm drive, by design, is that it cannot be forced to retract unless rotation is applied-at least that’s how it’s supposed to work.  In my opinion, the greatest weakness to this design is the spring.  As the timing chain and guides wear, the spring unwinds further and applies less force.  At the same time, there becomes increased slack in the chain, which increases the leverage against the tensioner.  In otherwords, the automatic tensioner progressively becomes weaker as the force against it is increased.   

By running the engine with a screwdriver inserted into the tensioner, I’d be able to increase the timing chain tension by turning the screw counterclockwise.  If I was right, the noise would cease as the chain was tightened. 

When the engine was started, I was surprised to discover my screwdriver rapidly turning in either direction-the tensioner was retracting with the uneven forces of the valvetrain.  As suspected, the noise ceased as the chain was tightened and held in position.  Because I could adjust tension as the engine was running, it was interesting to hear the noise as the chain is slightly loosened-a faint rattle could be heard that sounded like excess valve clearance (the valve clearance was set at the middle of the specified range).   So much for the theory that valves are noisy on these engines by nature. 

I disassembled the automatic tensioner to inspect for damage or wear that would prevent it from holding tension, though no defect could be found.  Because of this, I ruled out replacement of the tensioner as so many others have tried.  As the timing components wear on this engine, even though the automatic tensioner has travel remaining, I believe it can no longer retain proper chain tension.  This would explain why others could not resolve the issue for long by replacing the tensioner.  In order to utilize the factory automatic tensioner, the timing chain and guides would need to be replaced. 

When modifications are done to a motorcycle engine, such as stiffer valve springs, the automatic timing chain tensioner is often tossed in favor of a manual adjuster.  A manual adjuster allows for more precise adjustment and cannot suddenly back off tension, causing a potentially catastrophic failure.  I’ve installed several of these to various motorcycles over the years, none of which exhibited this type of noise when the chain was loose, though none of these engines used decompression mechanisms either. 

In the relatively young ATV world, manual adjusters are practically unheard of.  The bulk of these tensioners are built by American Performance Engineering, more commonly known as A.P.E. (www.aperaceparts.com).   The ATV forums I read indicate that most ATVers are not familiar or comfortable with the concept of manual tensioners-with fear of over tightening the chain being a major factor. 

As it turns out, numerous Honda motorcycles use the same automatic tensioner and APE has a replacement to fit these applications, though it is not listed on their website as a replacement for Rincon ATVs.  A company called CRU (Cycles R Us) does produce a replacement that appears to be an A.P.E. clone; even the part numbers are similar.  Naturally, I ordered and installed one and the noise is gone.  Rather than start on the topic of how to install and adjust one properly, I suggest having someone with previous experience do the install.  They can easily be improperly installed and/or adjusted, causing all sorts of trouble you don’t want.  The included instructions are very generic and do not provide enough information for a first time installer.   

So why do some Rincon ATVs have this issue while others don’t?  And why did the exhaust cam lobe exhibit unusual wear? 

I believe both questions have the same answer.

Engines that utilize the engine oil as automatic transmission fluid will place more demand on the oil and cause the oil to breakdown more rapidly-this is a simple fact of engineering.  There’s also the longitudinal engine design that dictates a unique valve train configuration, a design that I believe requires more operating friction than convention designs, especially on cylinder heads utilizing four valves.  Keeping that in mind, consider the following:

·       This is a unique engine that places additional stress on the engine oil.  High quality oil that meets Honda’s specifications should be used.  I recommend the Honda recommended synthetic.  This does not extend the oil change interval and the oil should be changed more frequently than “normal” ATV engines. 

·       As the oil breaks down more rapidly, it will also be consumed more rapidly.  Running any engine low on oil is not good; running this engine low on oil is much worse. 

·       Idling and low speed operation causes accelerated wear to the timing chain and related components.  This is why engine manufacturers recommend more frequent timing belt replacement when used in this manner.  Starters are cheaper to replace than timing components. 

The owner of this Honda Rincon rarely goes trail riding and the bulk of its use is around the homestead; excess idling, low speed operation, oversized tires, and heavy pulling.  Valvoline 10w40 oil was used-a decent oil, but wrong for this application.  Judging by the color of the oil, it needed to be changed more often too.  This certainly explains the “why”. 

How long-term of a fix this is has yet to be determined.  Because the factory tensioner was not maxed out, I believe this is a viable solution that will provide many years of trouble-free service.  This Rincon has approximately 3,000 miles on it now; I believe it will go another 3,000 miles as is, especially with high quality oil and more frequent service.

CRUT900 is the CRU part number; HT900 is the APE part number.  Both are sold at various retailers for $50 to $70.00.