I need more power out of my 99 yz400

13 replies to this topic
  • speedfreak391

Posted August 01, 2008 - 09:19 AM


OK I know its not a new 450 but it still runs strong. I I got stupid the other day and rode my friend 08 KTM 450sx. Then I realize how much power the old 400 is lacking compaired to the new 450's. No I don't, and won't be buying a new 450. I need to know what I can do to get somemore top end out of it. I see most of the cam's all say they work best with the 426 and up cc. My motor is for the most part stock with a wiesco 12.5:1 piston in it. Any Help wouold be appreciated. :ride:

  • Justin89

Posted August 01, 2008 - 10:39 AM


Do you have an exhaust system on it?...if not, start with that:thumbsup:

  • DRZ04

Posted August 01, 2008 - 08:27 PM


Athena makes a big bore kit for it. I hav eone on my DRZ400 & no problems after ~4500 miles. You get everything you need for the change & get to keep your stock cylinder & piston. You could also go for a set of Hot Cams.

  • zuch95701

Posted August 01, 2008 - 08:29 PM


also i belive ice cube makes a big bore for your bike. The 400s are good bike they are first gen yzfs but they ran strong.

  • cm6501

Posted August 02, 2008 - 11:07 AM


Put a 420 cylinder on it. They mount right up and the power gain is huge.

  • FlashGordon

Posted August 02, 2008 - 09:49 PM


The 400 has plenty of power if it's jetted right. Get a JD jetting kit, and use a 175 main minimum. Do the AP mod, and go with an aftermarket airfilter and pipe. Go up 2 sizes on the pilot jet to make it easy to start.

  • BigBlueTractor

Posted August 03, 2008 - 05:29 AM


I've got a '98 YZ400F that eats 450s. A little bit of port work makes it a monster. The White Brothers can helps too.

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  • BASSic

Posted August 03, 2008 - 08:01 PM


I've got a '98 YZ400F that eats 450s. A little bit of port work makes it a monster. The White Brothers can helps too.

I was going to say, I thought the 400/426 could still hang with any of the newer 450s, at least as far as power goes. That's been my experience with the people I ride with. (In fact I think my 426 feels faster than my friend's KX450F)

Weight and handling are different issues though. Suspension mods can improve handling, but there's no cheap way to make them lighter.

  • 1998YZ400F

Posted August 04, 2008 - 03:47 PM


just the basics, pipe, jetting, some carb mods like zip ty, maybe a bigger rear sprocket. like others have said, these 400's can hang with new 450's pretty well.

  • Birdy426

Posted August 04, 2008 - 09:19 PM


Maybe just needs a refresh...piston, rings, valves and such? What's the gearing? If it's geared tall for top end, it can make it feel underpowered...

Hi compression piston, big bore kit, JD jet kit, some hot cams or web cams, and a pipe, and that sumbitch will run circles around the new 450s.

  • FlashGordon

Posted August 04, 2008 - 09:52 PM


Took me a long time to tweek my 99 YZ400F until I had it running almost perfect. Have a 2006 YZ450F now, and got it pretty close based on what I learned from the 400 and what others posted on Thumper Talk.

Here's what I had if it helps:
Lived at Sea level, but mostly road at 3000-5000ft, trails only; no MX

Pilot 48
Main: 178
Leak Jet: soldered shut
Pilot screw 2 turns out. (Get the MSR or Tusk pilot screw)
JD Jetting kit red needle, 3rd clip (best improvement for the buck)
Did BK Mod to the Accel Pump with a drilled and tapped screw setting squirt to .5 sec.
Home made Powernow on intake side of carb, where you cut a slot, and insert an aluminum plate.
Uni Airfilter
Wiesco Piston after it was 5 years old. Replaced cam chain, tensioner and guides, which were all worn.
Had both a Big Gun full SS exhaust, and stock with a PMB spark arrestor. After installing the Silencer and SA in the Big Gun, I didn't see much difference from the stock pipe, unless the silencer was out.

Ran an 8oz flywheel weight, and 13/53 gearing, which was perfect for the tight trails we have in the NW, but it had lousy top end on dirt roads.

You absolutely have to spend time getting your jetting right, to get the maximum performance out of any bike. Half the people I ride with don't want to mess with jetting, and complain about poor performance or hard starting. I know 2 guys with WR450's who refuse to mess with them. The exhaust has an opening the size of your thumb, and it feels like riding an XR250. My son's 2003 YZ250F, with a Dr.D and jetted perfect, just eats them alive. You have to let them breathe and drink to get them to do what they were designed for.

When you snap the throttle, you want that thing to launch off a root or whoop 6-10 feet, or fall on it's face? It's all in the jetting and pipe. It is 10x more fun to ride a bike that is responsive,

  • speedfreak391

Posted August 05, 2008 - 05:41 PM


OK, I guess I should clarify. It is a stock bore with a wiseco hi comp piston, white bros. big tube header and the yamaha gyt-r carbon titainium can. I think the gearing is 15/50. I do have a WR trans in in from road racing it more top end. Oh and I am running a twin air filter. I am thinking big bore, and cams, port work too.

  • 1998YZ400F

Posted August 05, 2008 - 08:03 PM


will you still be riding it at high top speeds? you could go smaller on the counter sprocket and bigger rear sprocket to help put the torque to the road, but youll obviously lose some top speed in doing so.

  • BigBlueTractor

Posted August 06, 2008 - 09:06 PM


Stock cams and a stock bore, stock compression Wiseco piston (12.5 - 1) and some port work is all it takes to smoke 450s. Next time you have your head pipe off take a flashlight and look into the exhaust port. The big step from oval to round just before the pipe flange is most of the power loss in the stock casting. There's also a bit of flash just under the valve seats on all five bowls that cause turbulance. If your comfortable with a grinder you can do it yourself or... PM me if you're not sure.

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