426 help

[fastredrider44]

Hi, I just found this site, and love it. Last week, I rebuilt my 2001 400ex, top-end only. I bought the fourwheeler a little over a year ago. It had a pro circuit slip-on on it, and that was about it for go fast parts. When I bought it, it had a jet kit intstalled, but ran way too lean. It had a 148 main, which I swapped out to a 155 main. It ran much better, and had much more low-end. It started to smoke right away though. I rode it that way for a year. I decided it was time to rebuild it, and did it with a wiseco piston to make it a 426. Ive been ridin it quite a bit this week, breaking it in, but I'm starting to ride it a bit harder now and am kind of dissappointed. When I'm bout to climb a hill and lay into it, the motor cuts out for less than a second before making me hang on. Also, when I am boggin it a little bit, it tends to make a slight knock before taking off. It does the same knock when I'm idling, and snap the throttle. Any Ideas?

2001 400EX
22" holeshot XC fronts,
20" iRazrs
T-9 Polished rear wheels
450R swing arm
450R front shocks
DG Nerfs and Pegs
14/38 Gears
Uni air-filter
Pro Circuit 304 slip on
426 kit 11:1 compression

[zrpilot]

A couple of questions first:
1) I'm assuming it was professionally bored. What gaskets (base and head) did you use?
2) What piston are you running? Wiseco, JE, or Ross
3) Are you running the stock cam?
4) Did you run higher octane gas? How old was it?
5) When you say jet kit, do you mean Keihen or Dynojet jets and needle?
6) What is the needle set at?
7) What pilot jet are you running?'
8) How many turns out on the pilot needle?
8) Are you running with air box lid on or off?



I'm guessing that your idle/pilot circuit may be lean. Also if your running the stock cam it would definitely DEMAND higher octane because your dynamic compression ratio would be high....higher then with a Hom cams stage one or two...

[fastredrider44]

professionally bored... Yes. give me a little credit.
wiseco piston and gasket kit
stock cam
fresh 93 octane... Would it hurt to mix in 110 octane half and half?

Keihin jets
needle i belive is on the middle
not sure about pilot size or pilot screw. haven't messed with it at all yet.
running airbox lid, but thinking about puttin those 1 inch uni inserts in the front of the box.

[zrpilot]

Originally posted by fastredrider44
professionally bored... Yes. give me a little credit.


Dude, I'm trying to help, you have look at everything..

I think your getting some detonation. If so you have some options:
1) Jet slightly rich and see if the problem goes away
2) Raise the octane (mixing would be OK)
3) Reduce the dynamic compression (one way to do this and maybe gain more HP would be by switching to a Hot Cams)

Being as you probably need to make some jetting changes, that is where I'd start. Up the main to a 160 (or higher if you are going to open up the air box) Then switch the pilot to a 42, finally adjust the pilot air screw to achieve a good idle.

[fastredrider44]

I know your tryin to help. I just thought that was a no-brainer on the boring. I'll try a little bit of that and see what gives.