98 400 oil leak

Just purchased a 98 400,previous owner claims he just installed a new top end,problem is after engine comes up to temp, oil is seeping from the base gasket on the flywheel side.Question is, are oil leaks common with this motor or do I need to tear the motor back down because of a poor rebuild job?Question #2,on the flywheel side, on the lower half of the case ,under the shifter, there is a anodized blue cover,is this factory,and if not what is it for?Question#3,on the float bowl of the carb there is a red adjuster,I am assuming this is adjustment for the accelerator pump?and whether it is stock or aftermarket.This is my first fourstroke dirtbike so I am a little unsure on maintenance ,valves, timing chain etc.,so any advice would be helpful.Thanks.

Well, you have to decide how bad the oil leak is. If it just weeps a little, maybe you can live with it for a while. Is the small bolt at the cylinder base tight? Warning though: It's just a 6mm bolt, so it doesn't take much torque.

Maybe the surface wasn't perfectly clean, gasket deformed....who knows? Gaskets leak, even new ones sometimes. Doesn't mean the rebuild was poor quality. These bikes aren't known to leak oil any more than anything else.

2. The anodized cover is an aftermarket part to replace the neutral switch. Using the cover gets rid of the switch (which doesn't really have much of a function on the YZ) and the wiring.

3. The red adjuster on the carb is an aftermarket mixture screw. Sticks down at the front of the bowl? It adjusts the idle circuit of the carb. Out is richer; in is leaner.

Thanks for the info,I think I will ride the bike a little while before tearing it down (I dont want to get off of it,this thing is awesome)!

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