My old engine had developed a few issues so it was time for a replacement. I have been collecting parts for a while and I took all the best parts to come up with a replacement. The only parts I reused were the cam, valve cover, and cylinder after a fresh bore.

The engine started up first kick and I put about 250 miles on the bike last weekend. The engine ran great, but I noticed one thing that was not right. At idle it makes an occasional loud tick. The engine also misfires once when the sound occurs. I happens a bit more when you let off the throttle as the revs decrease.

Last night I got a few minutes to look at it and the noise appears to be coming from the head for sure. I'm hoping to get a bit more time to look over the next couple of days and I wondering what to look for. I'm thinking that it must be the decomp system engaging when it is not supposed to be. Then the loud tick noise would be the cam follower coming off the decomp cam and snapping closed. That would also explain the misfire which happens at the same moment. I have another cam to try, but I would like to try and figure out what it is and not just blindly change parts.

Any ideas?

Check the autodecomp on the exhaust cam very closely. Search for the several recent threads on the autodecomp. The small pin may have come out and the large pin is wallowing around in there occasionally cracking open the exhaust valve. Just an idea but easy enough to check. Mine had done this and I just happened to notice when doing a valve clearance adjustment.

Sorry - I really have no idea on these XR's. I thought I was in the DRZ forum - have no idea how I landed here.

Ryan Nelson said:

All I can think of in the head that could cause a misfire and loud ticking is something in the valvetrain. The auto compression release seems like a likely suspect. I don't have one, so I am just speculating.

The issue was the old well used cam. I changed from old cam "A" to not quite as old cam "B." The noise is now gone. It must have been an issue with the starting decomp system. It's the little wobbly thing next to the cam lobe. It has a pin and a little spring that engages it when the engine is stopped. When the engine is running it is supposed to disengage from the centrifugal forces. Well it was engaging when it was not supposed to.

The cam is slightly worn, but other than this decomp problem is still in spec. I may take it apart and see if it can be fixed. I guess it's possible that a bit of crud got in there while the engine was open. I sure don't remember it happening with the old engine.