They say timing is everything. When it comes to small gas engines, timing isn’t everything, but it can impact everything. If an engine’s timing gear is off by just one tooth, it will cause the engine to backfire and lose power — if it runs at all.
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Certainly an engine’s timing is something that rarely — if ever — crosses your mind. Despite this, the list of parts that affect timing and, in turn, engine function and usability, could take a bit of time to list. There are timing marks, a timing chain or timing belt, timing chain guide and a tensioner. There’s also ignition timing, the ignition module and the ignition timing control circuit.
As engines become more advanced, however, many of these settings are fixed in the best-run position, making your job easier. Nonetheless, it’s important to understand what timing is all about, as well as how to set it correctly when rebuilding an engine. So, let’s get started — the clock is ticking.
Camshaft timing determines when the valves will open and close with regard to the piston’s position within the cylinder. A small gas engine’s cam timing will be controlled by one of two basic methods. In the direct drive method, the cam gear and crankshaft gear rotate together. The crankshaft gear is half the size of the camshaft gear, and the timing marks on each are matched up when the engine is assembled. The second method is prevalent in overhead cam designs and uses a belt or chain to drive the camshaft, which does not touch the crankshaft gear. While a belt’s material will deteriorate and require occasional replacing, a chain is made of hardened steel links and should never need replacement.
No matter the method of timing, the theory is the same: The camshaft and crankshaft must be in the same relative position to one another, and the crankshaft must rotate at twice the speed of the camshaft. Matching the timing marks on each gear sets this position.
If the timing is not set correctly, the timing will be advanced, with the spark coming too soon, or retarded, with the spark coming too late. When the timing is advanced, the piston will still be traveling upward when the spark occurs, which will push the piston back downward too quickly. Starting an engine this way will basically create opposing forces within the engine, with the combustion process turning things one way and the starter cranking it in the opposite direction. If the spark comes too late, the piston will already be traveling back downward and the spark won’t carry the necessary force to push the piston down the cylinder, which will weaken combustion. This will burn excessive fuel besides causing the engine to run hard, if at all.
Most engine manufacturers will recommend placing the piston at top dead center when setting the timing. From there, it’s simply a matter of lining up the timing marks on the camshaft gear and crankshaft gear if it’s a direct drive engine. If it’s a belt- or chain-driven engine, set the chain on the gear sprockets with the timing mark on the crankshaft matched with the mark plate on the timing chain, and the timing mark on the crankshaft aligned with the mark plate on the opposite end of the chain.
Overhead cam engines that use a belt or chain often include links to help set the timing correctly. The links indicate where the timing marks should be set. While the links are convenient — especially for anyone who hasn’t done much timing before — they aren’t necessary to get things set correctly. As long as the piston is at top dead center and the camshaft marks are aligned with the cylinder plane when the chain is placed on the gears, the engine will be timed correctly.
A spring-loaded automatic tensioner keeps the timing chain at the correct tension. This tensioner is built into overhead cam engines to keep the tension tight, and it needs no adjustment. Belt-driven engines do not use tensioners, however, since belts are replaced once they become loose. Either way, tension need not be a concern when setting timing.
Some engines also use a balancer shaft, which smoothes the minor vibrations within an engine. While engine harmonics are different than timing, the balancer shaft includes timing marks, as well. When rebuilding an engine, mount the balancer on the crankcase so that the timing mark on the balancer gear matches the mark on the crankshaft gear.
As with camshaft timing, there should be little need to adjust ignition timing with modern engines, which are set to the ideal position by the manufacturer. Transistor-controlled ignitions, for example, are called “breakerless” and the parts are all fixed.
A capacitor-discharge ignition — also a breakerless ignition — has no requirement for timing, either. Rather, a gap must be set between the flywheel and trigger module. To do this, align the projection on the flywheel with the trigger module, and then loosen the adjustment screw on the trigger module. Insert a feeler gauge that corresponds to the manufacturer-recommended size — usually around 0.010 inch — and retighten the screw until tight against the gauge.
While necessary for an engine’s function, cam timing can be relatively simple to set. Additionally, ignition timing is almost effortless — if not completely unnecessary — in newer engines. It’s just a matter of understanding the concept of timing and some basic steps. Once that’s accomplished, you’ll be able to have perfect timing every time.