I just rebuilt the upper half of the engine on my ATV. I replaced the Cam Chain, along with a new OS piston, and had the shaft board out. I out everything back together, and after cleaning out the carb, and applying a carb rebuild kit, the bike started right up and was running great. I let it idle for 20 minutes then drove it around a bit. After I got done I parked it. The next morning went out to start it, and it would not start. I figured something might have gotten lodged in the carb so I took that apart cleaned it again, and put it on, nothing. Then I took a look at the Cam Chain and it had jump almost 180. I put it back in time, but the bike still will not start. I hope with a new plug that this might fix the issue, but I think that I am only being hopeful. What would cause a brand new chain to jump like this. I also replaced the upper cam sprocket, and but in a new Chain tensioner. Any thoughts would be great as I am all out of ideas at this point.

 

If you turn the crank through one revolution you are back to where you should be surely? If it has jumped you probably didn't get the "fixed" chain guide in right at the bottom when you put the head back. Did you leave the alternator case off so you could see it slip into it's socket?

 

Thank you for the reply, Yes after resetting the timing I have rotated the crank several times, by hand, and with the starter and every thing has stayed in alignment. When I was putting everything back together, yes I had the Crankcase cover off when I out it all back together and made sure the chain guide slipped back into the slot. As far as the fixed chain guard, I'm assuming you mean the rear one, that one I never removed during my rebuild.

 

What I was getting at is when you move the crank one rev the cam moves through 180 so the T mark will line up at 180 out. Also you can't get the cylinder off without removing the plastic "fixed" chain guide. The "moving" guide is the one pushed by the chain tensioner.

 

merryman,

Yes you are correct, I thought the cam moved 360 for each revolution of the flywheel. Last night I put on a new Carb, and I tried to get it my moto 4 started and it did not work. Then I Moved the cam 180, but still no luck. I guess I will have to keeping playing with the timing hoping that I can get it set. I'm just glad that it turns out it was my own mistake, and not something else that caused the cam to be 180 off. Is there any way other then to get the bile running to know if you have the Cam set correct, or if it is 180 off, it sure would be nice not having to keep switching while trying to diagnose what is going on. Also I noticed that being the cam sprocket there is the Dowel pin that the sprocket goes onto, is it normal for this to be able to move back and forth? There is nothing in my book, and I don't know if this needs to be all the way left or right, or if it is broken or something.

 

I just had a thought on this whole 180 cam issue.

If I understand all of the components there is no 180.

The Flywheel moves the piston up and down, there is a magneto on the fly wheel that causes the spark plug to spark every time the Piston is at the top, regardless if what the cam is doing. In other words The Piston at the top is either compressing the gas and air, and the spark-plug fires making combustion. Then the next time the piston moves up, it is expelling the exhaust, but the magento on the flywheel does not know this and it fires the spark-plug again because it is a wasted Spark system. Unless someone knows that this is not the case.

http://en.wikipedia.org/wiki/Wasted_spark

 

Yes, that was my point, you can't get the timing 180 degrees out. You can check the timing easily, the mark on the sprocket should be at the top, I think it lines up with a mark in the head casting on the Yam, when the T mark on the flywheel lines up with the middle of the hole. The "key" between the cam and the sprocket is rarely a tight fit but should not be sloppy. If it is, your timing could be a bit out with the marks correct but provided a valve hasn't hit the piston I would have thought it would run. That is why I suggested checking the compression, which would show up a bent valve.

 

The compression feels good, I know when I first got it back together and it was running, I was around 130. I have not checked since, however when turning the flywheel there is positive resistance as the piston moves up to the top position. and then you can feel that resistance release and you can hear what sounds like a small swoosh sound coming from the head. (I'm assuming this is the exhaust valve opening allowing the pressured air and gas to leave the cylinder. There is Gas getting in the cylinder I checked this last night by holding a lighter to the open Spark Plug hole, and getting a flame shooting out. The one thing that I can think of at this point is that the Timing is still off even though I have the marks lined up due to the sloppiness in the bearing that the Dowel Pin goes into the cam sprocket for alignment. Literally I could turn the flywheel so the cam is 10 degrees either way off the mark, then take the sprocket off move the chain on the sprocket that same 10 degrees, then moves that Dowel Pin to alignment and then put the sprocket back on and the Cam would show that it is alignment, and I did not actually move anything in the head. I'm worried about doing this, because I don't want to cause the piston to hit the valves (Did not know this was possible). But I don't see any other way of trying to figure out the timing then doing as I disrobed over and over until I get it to fire.

 

You can set valve timing independent of the mark on the sprocket. You need a timing disc, available from motorcycle engine tuning shops. Remove the plug and pull start, fix the centre of the disc firmly to the end of the crank somehow, make a pointer from a piece of wire fixed to the engine cases and set the pointer to 0 on the disc with the T mark on the flywheel set and valves closed. Then turn the crank in the direction of rotation until the exhaust valve just loses clearance, (starts to "nip" your thinnest feeler gauge) this should be at the angle it says "exhaust valve opens at" in the shop manual. Too early? Move the cam away from the direction of rotation a bit, re-tighten and try again. Too late? Move the cam the opposite way, all very tedious. Once set, check the "valve closes at" angle, and the "inlet valve opens at" angle to make sure all is correct.

 

merryman,

Thank you for your advice on this, and be patient with me as I am sure you can tell I am a novice when it comes to working on small engines. I was all set to pull my cam shaft out last night when I got home, thinking that something might be wrong with it. When I got the game gear off, and looked closer I realized the movement in the Dowel pin that sits in the gear is because the dowel pin is moving the shaft its self. I get about 90 degrees of movement back and forth as the tear drop portion of the cam shaft hits against the lifters (I believe this is how it works). So I am satisfied that this is all functioning the way it should. With that said, I had a neighbor over who happens to know much more about small engines then myself. He checked everything that I had checked from verifying the timing based on the marks, and checking that fuel is getting into the cylinder, and checked the spark coming from the plug. We even sent some carb cleaner directly into the carb and could see it getting pulled into the Cylinder. So the conclusion that he has is that since even with all of this it is not firing even once the whole time we did all these checks that the spark while present, is not strong enough to ignite. So I ordered a new ignition coil, and will be putting that on in the next few days crossing my fingers that it was just bad timing and that after doing the ending rebuild and getting it running, it went out making me think that something in the rebuild went wrong.