Big bore kit 426 Up Date

2 replies to this topic
  • marv02

Posted January 26, 2013 - 02:53 PM


After put the motor back together with the 3mm over size big bore kit, [color=#000000]High Velocity Port[/color] my head, WR transmission in my 2000 YZ426 make out to be 450.5 CC.

2004 YZ carb FMF power bomb head pipe and Q4 muffler.

After geting it up and running playing with the jetting still not 100% on the jettiing yet but close. I going to try a 170 Main and a 50 pilot jet next the 168 main working but I feel 170 may be better.

The bike runs good motor is very electric motor type power I think if I still was using the YZ tranmission instead of the WR it would hit a lot harder, it wil lift the front tire on tap just roll on the gas up the front wheel comes off the ground.

The WR trans is what I was looking for works great for the open desert where I ride and live at before it seemed with the YZ trans you bang though the gears looking for a 6 gear that not there the WR trans you get up in to 5th and I happy for Now LOL.

The 3mm oversize piston works good but ordering a custom head gasket $38.00 a pop is a draw back but he bike runs on Pump gas with no issues.

I feel the big bore and the porting makes up for the diffrence between the WR Trans and the YZ trans spacing if it was a stock bore non ported the bike would feel on the lazy side due to the WR trans the big bore kit pulls topend power and speed much easier before the motor STOCK felt like it was laboring a lot now just cruises along.

But I do notice at lower speeds you need to be more in the right gear (Drop down 1 gear) when you roll on the gas it chugs easier if you'r in a higher gear compaired to the YZ trans.
As in before if you'r in a higher gear with the YZ trans it was not much of a big deal; the motor did not care.

I running a 13/50 sprockets now I am going to try running a 12 front sprocket see if wakes up the bike up some.

Edited by marv02, February 02, 2013 - 06:07 PM.

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  • marv02

Posted February 02, 2013 - 04:08 PM


I found time today to play with the jetting some more it getting there.
170 Main I want to try a 172 main the 175 is to rich.

52 Pilot jet seems to be the trick for now.

The Needle second from the top.

When the bike was stock it would never hit the rev limiter now it hits it with out breaking a sweat.

I rode the bike behind the house working on the jetting seems to wake the bike up some more I never out 2nd gear or could run it hard so I will not really know until I get to take out and run it hard.

Edited by marv02, February 02, 2013 - 06:10 PM.

  • marv02

Posted February 09, 2013 - 02:47 PM


Got a chance to take the 426 out today the jetting is spot on runs great all the lazyness is gone, pulls strong though the power band and every gear.

Before I was thinking it was the WR trans that made fell lazy and make sure it was in a lower gear to make the bike pull hard from cruising part thorttle then hitting it wide open not any more it was the jetting.

I am happy in 5th pined it has good topend but you always could use some more LOL out here on the open desert fire roads.

Edited by marv02, February 09, 2013 - 02:48 PM.

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