Okay so i finally finished my 400ex build. I have a 426 Big bore (Wiseco Piston), HD studs, cylinder was honed after stud install, webcam stage 2 cam, all new gaskets, White Borthers slip on, open air box. I started it up for first start, cranks over quick (no choke in 35 degree weather).. it runs mint, idles as it should, good fuel mixture, no hesitation at all on throttle response, good compression. The problem is, it is smoking VEEEERRRRYY bad and blowing ALOT of oil out of my exhaust. My exhaust, swingarm, pretty much whole back end is covered in oil. What would cause this?

 

Did you replace the valve guide seals,could be seeping into the combustion chamber,also it may take a little while for the rings to seat against the cylinder walls. Also I know it might be stupid to ask,but did you make sure you had the numbers on the compression rings facing up? If not will run but shoot out oil! I did that on and old 67 305 Honda when I was 18 years old! Ran good ,but shot oil and smoke out the snuffer pipes against the garage wall. OPT

 

That would be #1 on my list.

One things for sure, something is screwed up bad and if that much oil is going through it and if you have a pretty high compresson piston you can detonate the hell out of it.

You need to tear it down again and see what's up.