why do new motorcycles need to get their valves shimed alot? can someone explain how the whole valve adjusting thing works? why do some engine types require more attention? i have owned thumpers my whole life and never really thought about it. i was looking at this like cheap beater bike (hyosung gt250 v twin) and i read the valves need to be checked like 3 times in a short span, wtf??
Valve Adjustment Motorcycle
Valves and the valve seat wear as they are used. To a lesser extend, the adjusting system of the engine valves also wears during use. Wear can cause the valves to open too far or not open enough, depending on where the wear is. This in turn can cause the engine to not run efficiently or if severe enough, it can cause damage to the engine or cause then engine not to run at all. Most car engines have self adjusting lifters, which keep the valves in the proper place. Most motorcycles don't have self adjusting lifters (HD is one that does on many of its engines). This is why the valves need to be adjusted manually. The first adjustment on most motorcycles is within a thousand or so miles of new and the next one depends on the manufacturer. Some manufacturers recommend valve adjustment checks as often as 6000 miles, others recommend 12,000 to 25,000 miles checks. I've never looked at a hyosung or its manual, but the manufacturer will have the recommended check schedule in the owner's manual of each bike.
Motorcycle Valve Adjustment
Your prospective Hyosung may require some more frequent adjustments because the initial wear on its machined surfaces would still be polishing each other off.... surfaces like your hardened valve seats against the sealing surfaces of the valves... Or, the hardened surfaces of the cam lobes against the rocker arms. If your Hyosung's engine has been properly engineered, then its components have been properly case hardened. After their initial microscopic machined imperfections have been polished smooth, they shouldn't have to be checked as often. The Hyosung 250 was designed in Hamamatsu Japan by the same people who design Suzukis. It should turn out to be a great motorcycle. Perhaps frequent valve checks are because it's high revving and air cooled.?.? I owned an air cooled KZ550 Kawasaki, once. It required valve checks every 3000 miles. On the other hand, my wife has a 2007 Corolla. It has a low revving, Double Over Head Cam, 4 valve per cylinder LIQUID cooled engine. It requires valve adjustments every 60,000 miles.
Valves have to have some clearance when the engine is cold so that as the stems expand the valves will still close all the way. Most of the wear in the valve train occurs at the valve and seat, as the seat tends to recede and the valve pulls further into the head.(SOHC and OHV designs may also wear in the rocker arms and bushings, which tends to open the valve clearances, usually not as quickly as valve seat wear, however.) As an inevitable result of this wear at the seat/valve juncture, the gap between the valve stem (or bucket or shim) and the cam lobe (or rocker arm) tends to close down. If the gap is too small, the valve may not close all the way when the engine is warm. If the valve does not have enough contact with the seat, it cannot transfer heat away, and may erode. There can also be a loss of compression if the valve cannot close. This wear happens most rapidly with a new engine and the valve clearances tend to close up relatively quickly. As the parts bed in, clearances tend to stabilize. That is why the first one or two valve adjustments are most important.
