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Thanks Cyborg, yes we thought of that also, and checked. the key is there and in one piece. Could it be perhaps due to my nephew on;y tightening his nuts to the torgue of his impact driver and no more? i.e. not using a torgue wrench?

Hey Cyborg and Snake, just to let you two know, today I went down with nephew to have a look once more at his timing and even though it appeared to me that the timing was accurate, we went ahead and moved the camshaft sprocket 180 deg. out of sure desperation to get this motor to fire. With no luck. same as before, the engine turns over, but does not start.  Thanks anyway for your input, and if there's anything else you can thank of please advise. thanks

I believe the error I was making was that There will always be two different cycles that the piston can be in when we're at TDC. One of them is the correct position (for setting the timing) the other is not. So we must be sure, of the two cycles, which one we're at.

Thank you snaker for your reply, that helps a little, knowing the ratios between the crankshaft,camshaft,and piston. The way I read your message, the CDI is in fact stupid, and does fire on the other revolution during the expulsion of burnt fuel? It's just that it doesn't matter, since the exhaust valve is open/or closing, and of course there is no new fuel or compression to create another explosion? Or does the CDI box know somehow that this is not the right time to fire?

Ok so nephew buys a taotao 150cc with like 36,000km on odometer. has for about 3 months, then engine fails to start. We swapped out carb. with a known working carb. no go. we spray starter fluid into rear of carb. no go. We swap out coil and CD-BOX both with known working ones, no go. I may add before we swapped out any parts we checked the spark by removing the plug and laying against valve cover and turned engine over and we get big bright spark from original equipment. I then removed his carb, coil, cdi box and one by one added them to my working taotao. (Better to add his parts to my running bike, then to add my running parts to his bike) all three parts check out. So we think compression problem. we place a test gauge into spark plug hole and get a reading of 60PSI, now at first you may think there's the problem. but first understand this, we bought the tester from harbor freights, and it's a cheap tester. we soon discovered that the gauge must not be calibrated properly, because we tested my moped that is running and starts, with it, and it reads 30PSI. so assuming 30PSI is not accurate, but enough to start my bike, surely 60 PSI reading should be good enough. at this point we're thinking timing could have jumped. we pull the valve cover off, and according to our knowledge, the timing appears correct. Allow me to explain what we know about the timing. We find TDC according to the flywheel, when at the proper position, both valves should be closed, and the cam-shaft sprocket should have the big hole at the top, and two smaller holes at each side of the sprocket just over the edge of the heads surface. Is that proper? At this position we checked the valve lash which was proper with a feeler gauge. at .004. now I also visually inspected and can tell neither rocker arm is putting any pressure on neither of the two valves and there's a very slight wiggle i can get from holding the rocker arms and wiggling them. to me this passes inspection of the timing. this is the only area so far, that I'm not completely satisfied with, because since this is a four stroke engine, there is one other time the piston can appear to be at TDC but yet NOT be on the power stoke. Am I correct with this theory or am I wrong? Once the combustion is complete, the piston then pushes the exhaust out and once again is at TDC, so how are to know which stroke we are at with the 4 stroke system? It appears that the flywheel will show once again TDC even on the second stroke when the piston is only expelling the burnt fuel/air mixture. So either my nephew who was on the flywheel side of the motor, was wrong in telling me we're at TDC when we were not actually there, or I'm having difficulty trying to wrap my head around the idea of 4 strokes. As I see it, 4 strokes, two different positions where the piston could be, (either all the way up or all the way down) and out of 4 strokes there should only be 1 power stroke correct? But the way I seen it is, after the power stoke, the piston makes a complete 360 deg turn of the flywheel and we're back to another power stroke? I thought the very next time the piston is at the top of the head, it should only be pushing the exhaust gasses out, then the very next down stroke should be bringing new fuel and air into the cylinder then the up stroke compression, until it reaches the top, and only then is it once again at TDC, the spark plug fires, all valves closed, and the piston gets pushed down once more completing it's power stroke. But when we were at TDC I rotated the variator but only until the piston reached the top of the cylinder again, assuming this position is NOT the power stoke, I expected my nephew to tell me according to the flywheel we are not at TDC, but instead he tells me we are. Thinking about the physics here, in order for the piston to once again be at the top, i would assume the flywheel must have rotated completely 360 deg again, but if so, then, this is not the beginning of the power stroke. So is it that the CDI box is designed to ignore every other rotation and not fire the plug at this point? I should have taken a little more time in noticing  where the valve positions where in relation to the second TDC when this was going on, but the news that we were once again at TDC threw me off and caused me to sidetrack. but thinking about it now, I'm definitely missing something here. I doubt very seriously that the CDI sends another release of charge command to the coil at the second TDC stroke, since it would be useless anyways. So how does the CDI know this is the only the 2nd stroke of a 4 stroke system. Better yet, how do we know which stroke we are at? Again perhaps my nephew was wrong at calling TDC on that second stroke, but instead of us pulling the valve cover off and performing the test again, I thought I would ask the pros maybe you guys can clear this up for me.

 Back to the engine failure, after checking all these things and nothing is proving bad to us. My nephew decides to just purchase a complete engine from some on-line outfit to just get his bike back on the street. It's his main source of transportation. As is my moped. So he orders one. it comes in, and right before we ready to place his into his frame, we notice that he ordered a short block, yet his old engine is (not sure to say long block) 1 inch longer in the transmission to rear wheel area. we thought it may still be enough space there for us to work this thru, but no, the rear wheel does not have enough clearance to fit, and even if it did, the right side swing arm will not fit, neither will the rear shock fit in the place it should. So we either return it, get his money back and then place another order and wait for it to arrive, which he has no patience for.....or....hey I got an idea, why don't we just keep the bottom block of the old engine, but replace the cylinder, head, and piston from the new engine? If it was a compression issue, or bad valves or rings issue, we would do away with that, we wouldn't have to explain to the seller our mistake in ordering the wrong part, and best of all he would be on the road by tomorrow. Also I forget to mention, not only did they ship us a new engine, but also a new carb, coil, and cdi box. so with all these new parts, surely the engine will fire right up right? Nope. we're still at the same problem we were at in the beginning. with all new parts, nothing. the engine turns over, but no start. anyone have any ideas short of just buying a new bike?

 Here's where we are at, the bike has now a new carb, gas is flowing from tank to carb when a vacuum is applied to pee-cock valve. Spark plug is firing with a good bright spark at spark plug, compression should be at maximum although we have not put the compression gauge onto it. We're assuming with the new cylinder, piston, rings, and new head that that should not be the problem, and hasn't pointed in that direction when we did test for compression. I would also assume that by having a good spark, we can also rule out the kill switch being an issue correct? So I'm at a loss at this point and having a hell of time accepting defeat here. someone please help!!!