Hi quick question about engine timing on scooters?

I pulled a 125cc honda sh engine top end apart to change the rings and zip tied the cam sprocket in place so I could just put it back on how it was before which I knew wasn't out of timing.. however my friend cranked the engine over before I could tell him it was important.. basically..,, idk where the crankshaft is at... I made tdc markings there's also a marking on the engine. So ive got a 50/50 chance of getting the crankshaft in the right timing and the camshaft timing is pretty much self explanatory because there's a massive arrow and if I'm correct 2cranks of the crankshaft =1 full cam rotation= this arrow meaning it doesn't have a false tdc... Just curious as to how I'd find the correct crankshaft tdc so the spark fires during the combustion stroke not inbetween exhaust and induction??

Answers

Anonymous

At TDC on the compression stroke, the tappets should be loose. So just open the valve covers and move the tappets. They should move freely when you put a little force on them. If they are stiff (and you will notice when you turn the crankshaft 180) the engine is on the exhaust stroke (or valve overlap as old school guys call it). Another test is to remove the spark plug and put a finger to cover the hole. Turn the crankshaft and as soon as the engine is on TDC on the compression stroke, you will feel your finger being pushed out of the hole.

[ DI 4 REALITY ]

You are telling lies because you can't change the rings by taking the top end apart. You would need to remove the cylinder to get to the piston and rings

don r

Honda designed those engines to be hard to get the cam back on wrong. While the cylinder is off you can see the position of the piston as well as before you put the head back on and the timing mark on the rotor is easy to point TDC. After all that, you cannot lineup the cam wrong since its retaining bolts will not lineup with the sprocket unless the cam is oriented correctly. Make sure you replace both top and bottom gaskets. Do not over tighten the cam bolt or you will break it- the cam is almost brittle as glass. Careful replacing the cam bearings so you don't strip the holes the screws go in soft aluminum. New gaskets there keep them from leaking oil.

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