Originally Posted by Zecatfish
The weakest links on the XT350 engine,
1. Timing chains are not long life items. They stretch bad. I had to replace mine at about 10k miles.
2. Timing chain tensioner.
3. Intake boots.
I've you an complete YZ490 chassis beside the shop,
I'm actually thinking about putting my XT350 engine in it,
Would solve all the weak links of the XT350 chassis.
The timing chain tensioner can be easily converted to a manual setup. Push out the roll pins that hold the tensioner together, bin the spring and locking mechanism but keep the final "actuator rod". I used a long 8mm Allen bolt / socket head to replace the spring and locking parts and ground a curved depression in the end of the socket head to match the end of the actuator rod. The threaded end of the Allen bolt passes through a new thread cut in the original tensioner housing and protrudes ~ 30 - 40mm out from the housing for adjustment.
The adjustment process now is to release the locknuts on the visible threaded section of the Allen bolt and just screw in the Allen bolt until light finger-tight
only. The best confirmation of correct cam chain tension is to remove the cam cover and use a long probe to lift the chain away from the flexible tensioner / guide. If there's 5mm - 6mm of movement of the camchain away from the tensioner guide surface at mid span then it's fine.
The intake "boots" / joiners have a built-in multitude of poor engineering. Restricted internal diameters, delamination of rubber from angled manifold surfaces, rubbish materials in the rubber itself. I binned em and fabricated new pipe-based manifolds with clamped, large diameter fuel hose as the new flexible joiners.