350x engine mods advice

[LA 3W]

I'm looking to build up a 350x that needs the motor gone thru anyway, not currently running The bike will only be ridden a few times annually but one of the rides will be at the 3 Wheel national MX race in Florida. Trying to decide if I go the Hi-comp route with cam (possibly head work) or if I go a little simpler with a Big Bore (414cc?) and a cam but pretty much leave the head alone? I do already have a 400ex carb for it. I'm trying not to totally break the bank as I have an 86 200x and possibly a 250sx to build for that race also..... Thanks in advance for any help or insight.

[ATC200X4716]

I've had several 350x's over the years and always liked one I had with a 12-1 piston and a better cam. It was very crisp, and best suited with race gas or at least Sunoco 94. The high compression gave instant throttle response, which the 350X kind of had anyway it only got better. Get it bored to fit the piston properly and a good valve job. I bore and hone my own cylinders and have found so often many unscrupulous shops over-hone the bores so you end up with an already worn out piston fit before you even start the bike up. I've been accused of machining the cylinder head to raise the compression when in fact all I did was fit the piston clearance properly, within .0002" of proper size. Proper size and roundness does make a difference and not everyone has the proper gauges to measure it. Never go to someone honing with a 3 arm hone in a hand drill. I use a sunnen hone which gives you the roundness. The stock pipe worked pretty good, adding an aftermarket just made an awful lot of noise. I've never ridden a 350X bigbore, maybe someone will chime in who has.

[LA 3W]

Thanks for the response. Did you rejet the stock carb or use a 400ex carb or something else? Thanks!

[shortline10]

The 400ex carburetor is a decent upgrade and starts the bike easier IMO . I’ve installed a bunch on 350x’s . If the motor is still healthy the 400ex stock jetting is fine . The intake boot and air box boot need to be stretched a little and the snorkel will need to be heated up and reshaped with a heat gun some for clearance .

[ATC200X4716]

On my 350X's I always used stock jetting on stock 350X carbs. I'll add I that owned 350X's before 400EX's even existed. The 350X being a 4 valve motor breathed very well, against say a warrior which was a 2 valve motor and kind of a turd. I probably could have gotten more out of the 350X with jetting changes, but I also used 100 octane fuel back then. The build was a 12-1 wiseco piston, Web cam I believe, and aftermarket valve springs. It ran fantastic. Don't underestimate quality machine work and good fuel. 12-1 and 87 octane is a bad idea. I did add a super trapp pipe but it didn't really add much of anything other than a lot of noise.

[LA 3W]

Thanks for the info. I already buy race gas for my Quad so high comp and running race fuel is not an issue. Update: I did go ahead and buy the 414cc big bore with a cam and valve springs. Giving the cylinder to my local engine guy to sleeve and bore this week. Interested to see how it turns out, especially since I have a stock 350x to compare it to.

[350for350]

I'll bet you start looking to see if there's an anchor on the stocker when you ride them back to back.