QuestionHow do I adjust the valve clearance of my 1991 Honda Baja 250R? Thanks..
AnswerHi Alvin,
I can help you tech problems, but you'll have to obtain the Service Manual to get the procedures for tasks such as adjusting valve clearances. I make references to the manual, so you'll need it anyway.
The manual has procedures, specifications, and illustrations for every maintenance and repair topic for the 250R. Use Allexperts for assistance with diagnosis, or if you do not understand something in the manual. The manual will pay for itself with the first repair.
I reply with test and repair information files which are pre-typed, and by corresponding personnaly with what I suspect is the source of problem symptoms. See a sample file below.
Use "www.bikebandit.com" to see microfiche parts on line.
Visit "www.atvfrontier.com" ATV forums on many topic
Respectfully,
Mark Shively
Sample Files:
Valve Clearances: Differences between Excessive Clearance and Lack of Clearance
Two things can happen to a valve train as it wears. Either the seat can wear, causing the valve clearance to decrease, or the cam lobe or bucket can wear, causing clearance to increase.
In the former case, the clearance will eventually reach zero if left unattended, and then the valve will actually be held open slightly when it should be closed. This allows hot gases to swirl around the valve at all times, and prevents the heat transfer to the head that would occur if the valve was properly seated. Eventually the steel will burn, causing permanent and relatively expensive damage.
In the latter case, performance will be reduced slightly (not enough that you'll notice, since it happens gradually) but no permanent damage will be done. You MAY hear a slight ticking noise coming from the head, or you may not.
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Adjusting Valves
It seems four-stroke bikes have found a home in the garages of three distinctly different types of owners. First category includes riders who truly prefer the characteristic torquey, traction-hunting, sure-footed power delivery. No doubt, thumpers rule in really slick conditions and often dominate Veteran and Senior classes. Another camp is filled with riders who have had a four stroke thrust upon them for political reasons and/or other compromises--dual sport bikes, kids beginner mounts and the like. Finally, a third group of owners are those who aren't much interested in bike maintenance chores, preferring the care-free longevity typical to four stroke motors.
We're hard pressed to find fault with anyone who prefers riding their bike to wrenching on it. Even thumpers, however, require occasional preventative maintenance to ward off compounding problems, and one such task near the top of the "must do" list is also one of the most often neglected. Valve adjustment, or actually, adjustment of the valve/valve rocker clearances should be accomplished every two or three thousand miles throughout the life of most sleds, and a couple of hundred miles after break-in of new or recently rebuilt motors. Failure to do so invites poor performance, hard starting and even the potential for eventual engine damage.
Valve adjustments are always completed with the engine cold. This means either before the engine's been run or after a two or three hour cool down period. On many models, you'll find that this time can be well spent removing obstructive body work like radiator shrouds (where applicable) fuel tank, seat, etc., to improve clearance around the valve covers and rocker adjusters. Don't expect miracles, however, as even in this state of undress, access is still dicey on some models (like our KTM RXC), confounding the adjustment process.
With all that out of the way, set the piston to top dead center (TDC) to start, using the kickstarter (carefully), or rotating the flywheel by hand. TDC is located by aligning a mark on the flywheel with a mark on the engine cases. On some models this will require removal of the ignition cover, whereas others have a small access port. Its important to ensure that the motor is at TDC on the compression stroke. Since four stroke motors only produce power every two revolutions of the motor (see the graphic on the side), it's possible for the piston to be at TDC, with the motor on the exhaust stroke, thus the exhaust valve(s) remaining open.
An experienced mechanic can usually tell if it's the compression or exhaust stroke simply by observing the rocker arm position and/or feeling for the valve clearance. Obviously if the exhaust valve rocker is depressed (thus indicating the valve open) then this is not the compression stroke. However, if the valves are badly out of adjustment and over tight (no clearance) this might not be so obvious. To be sure, remove the valve covers or caps to expose the rocker arms. Next, rotate the motor for several revolutions while observing the valve train action. Choose the TDC with both intake and exhaust rockers in the closed position, and realign the timing marks found on the flywheel and case cover.
With the rocker arms exposed and the motor at TDC (compression stroke), valve clearance can now be checked. The clearance itself is the gap between the rocker arm adjuster screw and the valve stem. Clearance is adjusted by loosening the locknut that secures the valve adjuster screw and subsequently turning the adjuster screw in or out, to change that "gap". Measure the clearance using a feeler gage. This task can be accomplished using standard automotive store variety feeler gages, however you'll find that they are cumbersome due to their size and bend (or lack thereof). Really trick dedicated valve adjustment tools, such as those available from Motion Pro, White Brothers, Four Strokes Only, et. al, greatly simplify the task. These adjusters fit into the cramped access holes much better and often allow the adjuster screw to be turned and locknut tightened, all in one easy action.
Valve clearance varies from model to model, and sometimes between the intake and exhaust valves as well. For our Trail Rider XRL250, the intake valve rocker clearance is 0.03 to 0.07 mm (0.001 to 0.003 in), while the exhaust valve rocker clearance is 0.06 to 0.10mm (0.002 to 0.004 in). Our KTM on the other hand has a single adjustment clearances, 0.10 mm (0.004 in) for both intake and exhaust valves. The only way to be sure of the proper valve clearance is to consult your shop manual or call a dealer friend.
There are two schools of thought regarding the adjustment procedure itself. Some mechanics like to snug the adjuster bolt down using the feeler gage as a spacer, while others make the adjustment, snug the adjuster bolt down, then re-measure the clearance. Either way is fine, however, be advised that valve clearance adjustment is inherently a trial and error process. The adjustment often changes when the lock nut is secured, especially if you're a little careless when snugging down the lock nut. Thus, the final step should always be to recheck the final valve clearance, after the lock nut is secured. If and when the adjustment has been found to change (it will) simply loosen the lock nut and readjust.
At this juncture, an aside on valve adjustment philosophy is in order. It should be noted from the start that the results achieved are dependent upon the care and patience exercised during the job. Be aware, that tight valve clearances (smaller gap) lead to burned valves and associated engine damage that costs big money. Loose valve clearances, on the other hand, could cause excess valve train noise, premature wear of the rocker adjuster screw or the valve stem itself-- minor stuff. As a result, the rule of thumb is that if you're not going to be perfect, looser is less harmful than tighter.
Once you're satisfied with the valve clearances, the adjuster lock nut should be tightened to the proper torque. Check your shop manual for the torque applicable to your motor, as tightening torques again vary model to model. Recommended torque for our Honda was 23 Nm (17 ft-lbs) while the Katoom specified 20 Nm. All that's left at this point is to fasten the valve covers back down. If you're really meticulous, rotate the engine through several revolutions and then go back for one final clearance check. After that, button up the valve covers, replace whatever body work that was removed, and set your calendar for your next adjustment.
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