The noise is between a rattle and a spining noise.It does not rattle when it is idleing but it does when the choke is on or if you hold the throttle open a little bit. I thought maybe it was the timing chain tensioner but I replaced that and its not the problem. The valve clearance is good, and i put new rings in it.
Could be your Flywheel,The yz426 has a riveted flywheel,over time these rivets wear from the flywheel spinning around,causing a "rattling sound"or it may be your timing chain is loose or has been stretched.
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I suggest you have your car checked as soon as possible - while a rattling exhaust isn't desperately urgent it could be the cat breaking up which might later restrict the flow of gases. Most importantly excessively loose timing belts or chains tend to make most noise when the engine is idling. If the noise is being caused by this delay could be very expensive.
Sounds to me like the throttle plate is too far open at idle. Look for something hanging up and holding the throttle more open that it should be-like the choke like the auto choke or the fast idle cam screw set to far in. Look up the fast idle speed adjust procedure in the books and check out how yours is set up. Also, if the carb was all the way apart the throttle plate may be off center on the throttle plate shaft causing extra air to bypass the throttle plate. Good luck.
I take it that you have a carby in which case adjust the choke cover so that the cam engages the throttle lever and holds the throttle open until the bi-spring in the choke cover moves enough to move the cam out of the away allowing normal idle. If you have efi (Not from your choke word) then run fault codes and see which sensor is faulty. There should be an idle speed around 1200 RPM idle in efi for around 3 minutes and then the idle should drop back.
it could be a wrist pin, and im not sure if those amc's had hydraulic lifters but sometimes with old lifters when the oil pressure is lower at an idle the lifters arnt filling up all the way so they develope a little valve lash, and when the rpms are raised the oil pressure raises and fills the lifters. you could try chasing the noise down with an automotive stethoscope.
Is the engine running at high idle or are you having to hold the throttle open to keep it running? If it stays at high idle on it's own be sure that when you push the choke back to the "off" position, check your linkages and be sure that the choke cam is returning to the low idle position, also be certain that your throttle grip operates without obstruction when returning to idle position. If you are having to hold the throttle open to keep the engine running you are probably going to find carburetor issues. Hope this help to get you looking in the right direction.
To change the choke cable, you must take the carb off the engine. Remove the air filter and backing plate. Take the nut off the backside of the choke cable and let the cable come out of the bracket that holds it. Make sure the petcock is turned off and disconnect the fuel hose at the carb. Loosen the throttle and idle cable at the handlebar throttle assembly. Pull the carb out of the intake manifold seal. Work the throttle and idle cables out of their positions in the carb. Bring the carb with the choke cable out and away from the engine. Take the choke cable out of the carb with a 14mm open end wrench. Take the plunger and spring off the old cable and swap it to the new cable and reinstall the choke cable. Put the throttle and idle cables back onto the carb, put a new intake manifold seal on the intake manifold and push the carburetor back into the new seal. Put the choke cable in it's bracket and secure the nut. Be careful and do not over tighten the nut as the plastic cable housing will break. Reconnect the fuel line, the vacuum line, and anything else that was taken off. Replace the air filter backing plate and the air cleaner. Readjust the throttle and idle cables making sure that the throttle operates freely and returns when let go on it's own. Never put any kind of lubricant on the choke cable. There is a knurled wheel that you can use to tighten the cable so that it stays where you put it.