This is going to be hard to diagnose without a better description of what is happening. This might sound silly but are you using the clutch? Is it adjusted correctly? If the clutch is working as its designed to do you need to disengage the clutch, squeeze the clutch lever, when you slow or apply brakes; just as you would in a manual transmission car. I can't begin to help you with the noise you hear. The clutch in most of these engines (you didn't tell us what kind of engine you have) that is the manual clutchs, can be noisy but if you're slowing with the clutch engaged then it could provide you with some interesting sounds both from the clutch and the chain and sprockets. Get back to us with a little more information and we'll see if we can help you out. Your problem could be one of several things such as sprocket/chain/wheel alignment, or some other things that you'll need to check. Tell us more first.
Tom
Okay, now lets see where we are. Did the coaster brake make any noise before you installed the engine? Is the coaster brake lever attached to the chain stay and not rotating when you back-pedal? When you had the rear wheel off to install the sprocket did you rotate the axle cone nuts which might have changed the bearing pre-load? Try removing the chain and see if there is any noticable binding/noise in the rear hub while rotating the wheel. If the rear bearings/brake are binding you'll need to adjust them. If you're not sure about the procedures seek help from a local bike shop. Watch what they do and ask questions. There is also lots written here regarding bearings and coaster brakes. Use the search feature and do some reading. Do you feel the noise you hear is from the clutch or the rear hub? If you find nothing wrong with the rear hub/brake the problem might be in the clutch but personally I've never encountered the symptoms you described.Heres what I found so far the bike uses a coaster brake which sems to stop the bike too well and its bumpy a litle bit when braking the chain does have slack and the wheel turns ok when raiseed off the ground its just when i brake therea a jerk accompanied by a grinding noise and i do disengage the clutch when braking and everything sems to be alligned ok the chain is not binding up but the problems only occur when braking and its a 2 stroke engine
Well I have isolated the problem its not the clutch at all its the coaster brake itself nothing goes wrong until i apply the brakes. when applied lightly its as if you just slammed the brakes real hard and the wheel seizes im worried its going to break the chain or something any fix to this?