building a snappy 400ex motor?

[400man]

lets hear some ideas/input on building a snappy, rev happy motor that is still reliable and pump gas friendly. I have plans to build me a nice xc race/trail bike in the near future, and i have a all stock 03 motor to work with. i plan on spliting the cases and using a fresh crank. i want a motor that revs quick and snappy similar to a 450. but i want to try to stay on pump gas for long weekend trail rides. any opinions and input is welcome.

[BlasterEaten250]

Well if you want to stay on pump gas 11:1 compression will be your limit as far as pistons go. An 11:1 piston with a stage 1 or 2 Hot Cam, with at least a slip on exhaust while jetting nice and crisp will give you a relatively snappy motor. A 450r Carb will help improve throttle response a lot and give you slightly more power overall, especially top end power. I have found gearing to be a big game changer on the 400ex's. If you don't plan on topping out all the time, try going down a tooth on the front sprocket. It adds a lot of pull to the bike. You could also have a company port your head, but that is when it starts getting pricey.

[400man]

ive already got a few parts I can use from a built motor that blew up earlier this year. I have a fcr carb off a yfz450 that im gonna use, and also a lightened flywheel and 2stage hotcam, and full white bros exhaust system.. the head porting is one thing I have never tried, would it make a lot of difference in how well it revs and runs and everything?

[CJM]

I didnt port my 400 and it had plenty of power imho. I had an 11:1, stage 2 hotcam (dont bother with stage 1 the quad fell on its *** trying to climb hills with a similar build my buddy did using the HC stage 1), 04-05 450r carb, full exhaust and open airbox.

[DragonGunner]

ive already got a few parts I can use from a built motor that blew up earlier this year. I have a fcr carb off a yfz450 that im gonna use, and also a lightened flywheel and 2stage hotcam, and full white bros exhaust system.. the head porting is one thing I have never tried, would it make a lot of difference in how well it revs and runs and everything?

Port job will help from bottom to top end, port job helps all round, its adds about 2 hp IMO to the 400EX, maybe a little more.

[Stickman400]

I didnt port my 400 and it had plenty of power imho. I had an 11:1, stage 2 hotcam (dont bother with stage 1 the quad fell on its *** trying to climb hills with a similar build my buddy did using the HC stage 1), 04-05 450r carb, full exhaust and open airbox.

I have the same thing and it is very snappy and fun to ride. I wouldn't call it a quick revving engine though, just because it's not going to ever rev like a 450. But this setup with a port job, you're lightened flywheel and that FCR carb. in place of the 450R carb would be about the best you're going to get out of a 400 motor. The 11:1 piston, 450 carb and open airbox makes the HUGE difference in these engines, makes it feel like a whole different machine.

[400man]

yep, that's probably the setup im gonna use. one other option would be a stroker crankshaft, since I plan on replacing the crank anyways, but I don't know how much it would affect other things, as far as how quick it revs and long term reliability and such.

[CJM]

IDK about you Stick, but mine revved up pretty dang quick once I got the jetting dialed in. What I feel really helped was the gearing, I went to a 14/36 and had plenty of torque and speed.

[MtnEX]

Well, I am going to reply and subscribe to this one to see where it goes :)

I have had some similar thoughts on what I might do different now after getting used to a 450 vs what most might do to the 400EX.

I have real mixed judgement on this subject. Part of me still likes the 400EX for all it is and doesn't want to try to make it what it is not. But the other part of me also thinks that my opinion of what is still OK may have changed after time on my 450, which is as extremely stall happy as it is extremely snappy.

Now that I have gotten used to running more RPM and adjusted to the stalling crap some, I end up asking myself now.... can you really do enough to a 400EX to make it too stall happy or too snappy or too high strung?????

I have thought about the flywheel lightening, the stage 2 and stage 3 cams, going ahead with a race gas motor so long as octane requirement don't surpass 110 octane, and on and on...

As far as what others said, YES, gear and gearing selection is everything on these things, and so is tune. You really wouldn't believe. But I have tuned a few to know. I did my team mates 2012 bike... stage 1 hotcam, stock otherwise, HMF slip on and EHS lid. I had it firing so fast it would be running before the button was all the way in, and running snappy enough I could feel the stock clutch slipping.

I have also once a long time ago made a valve clearance adjustment that was a tad out of the box that did make a noticeable difference. But I don't remember it well enough to put the specs out here. But it was something dumb like down to the tightest spec on one side and the loosest spec on the other or something nutty like that... I sure remember thinking it was something you shouldn't do unless you never skip on maintenance and check things often.