If all the other timing marks are right then the intake and exhaust valves will be closed before the piston reaches TDC and the compression charge will be highest. Rotate the engine until the piston is at TDC, and confirm BOTH the intake and exhaust valve rocker arms are loose, then set the valve clearances. I suspect you are setting the valve clearances on the Exhaust TDC piston position. If the camshaft timing is advanced (opening the intake valve too early) part of the compression charge is lost because the suction action of the descending piston has not effectively pulled in enough air/fuel charge and when the piston reaches TDC on the compression stroke there is less mass to compress, resulting in low compression. If the camshaft timing is retarded (opening the intake valve too late) part of the compression charge is pushed backwards into the intake, resulting in low compression. Think of TDC as Compression TDC and Exhaust TDC, valve clearances are always set at Compression TDC.īut, if the camshaft sprocket timing is off one tooth or more at Compression TDC, it will impact the compression value. But, because the piston also comes up to TDC during the exhaust stroke, if you attempt to set intake valve clearance at that time it could cause the intake valve to remain slightly open during the compression stroke depending on the camshaft lobe profile. At that time both the intake and exhaust valves are closed.
Also, only on the top of the compression stroke the intake and exhaust valve clearance is set, and you can confirm this by moving both valve rocker arms. You can still confirm piston top dead center (TDC) on the compression stroke when the piston is highest with a probe in the spark plug hole.
The procedure to find the compression stroke is just as described suck/squeeze/bang/blow is the process shown on this video Īgain, assuming you assembled the engine correctly and that the timing marks are set right, a degree wheel will only confirm that is done right, IF you also have a dial indicator installed through the spark plug hole to confirm how much the intake valve has started to open at exactly what position of the camshaft and crankshaft. Assuming the timing marks on the camshaft sprocket and crankshaft are properly set, and that you have assembled the engine properly, you don't need a degree wheel to determine top dead center.
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