Valve seals or oil from exhaust and more questions

[WoodTRD]

I recently noticed my 400ex smokes a little when you let off the gas. I'm assuming the valve seals are starting to leak a little, but it doesn't smoke on startup, just when it gets warmed up good and I let all the way off the throttle from a decent rpm. I did install a white bros e series slip on from a bike that had been smoking bad about 8 hours ago so it could be just getting hot enough to burn oil residue from the other bike off the inside of the pipe, what do y'all think? I checked the compression just to be sure and it's perfect at 120psi, so pretty sure it's not rings.

The journals on the head that's in my bike were a little worn last time I had it apart, so I have a spare head at the machine shop getting new exhaust studs right now and am planning on new valve seals going in it, lapping the valves, and throwing a stg 1 hotcam in it with a xr400 headgasket when I swap it over. With 120lbs of compression I don't think I'm going to worry about
doing rings right now, it runs perfect other than the bit of off throttle smoke. I'm pretty much at the end of my winter budget and plan on doing a 11 or 12:1 416 next winter.

Im at about 150 ft above sea level, my setup has a stock piston, white bros slip-on, sparks 6 degree key, uni with filterskin, no lid, 155 main, stock pilot, and will be getting a fresher head, what I believe to be a stage 1 hc (more on that when it gets here), and xr400 headgasket. Would there be much of a noticeable benefit from a light diy port and polish on the exhaust side and smoothing out the flaws on the intake side along with smoothing out the header welds while I have it off?

I have the xr headgasket already, haven't ordered the valve cover gasket, cam tensioner gasket, or valve seals yet though, any reason to go with Honda or cometic on these or would cheaper ones work just as well? Thanks

[VTredneckgames]

If it happens while you are at WOT, then it is the seals.

Now, the way you described it, when you are at high rpms and you let off, it starts smoking, that is an indicator that the rings are going out. When you let off from high rpms it creates a huge vacuum in the motor, this vacuum pulls oil from the crankcase into the combustion chamber.

You need rings. Do them while you can. Even if you have to put off the cam. Do the rings. It is only going to get worse. Before long it will smoke more and more.

And you had better have the shop measure it to make sure it doesnt need boring.

[2001400exrida]

yeah it sounds like your rings. My seals were blown and it would only smoke on startup, then it wouldn't smoke at all. I'd go ahead and get a new piston and rings, unless the one you have is somewhat new. Then u might be able to get away with just rings.

[250rforlife]

its your rings. like they said replace them right now. as for the gasket kit. get a oem honda gasket kit a. its gasket kit a or b. but wich everone it is it comes with so many more gaskets then cometic. it gives your valve seals, header gasket, everything you need to replace your topend to the top quality. and its oem so u can trust that.

[WoodTRD]

Even with 120lbs of compression cold? I'm really thinking it's either in my valve seals or just burning oil that's accumulated in the exhaust from the other bike that was smoking bad...but then again I could very well be wrong. I already have the cam on the way and my spare head at the machine shop getting new exhaust studs. If the consensus is I need new rings now though I may go ahead and hone the cylinder and throw new rings in it while I have it apart though...but at 120lbs of comp cold I'm thinking they're still ok, and I'm just wanting to make sure they'll last a summer. Next winter I'm planning on going to a 416 and higher compression anyways

[WoodTRD]

I'll look into getting a Honda gasket kit, that's the way I was leaning. All I'm lacking though is the valve cover and cam tensioner gasket, I've picked up the xr headgasket and copper crush gaskets already. Anyone know if it'd be worth it to remove the base gasket or run a thinner one along with the xr headgasket?

I'd love to go ahead and bore it and throw a higher comp piston in with a CRF timing chain and hd studs, but it's not really in the cards right now.

Thanks for the replies