KX420 port timing specs

[John Orchard]

Anyone got a KX420 barrel off their engine?  I was wondering if I could get the port timings to compare to my KDX400.

[Lozza]

Pipe is more important than the port timing.

[GMC]

I compared some RM & PE barrels a few years back and the only thing I could pick was the exhaust port on the RM was around 2mm higher.

[John Orchard]

I've read (in original magazine tests) that the KDX400 is very strong in the mid range but falls over once it gets into the higher rpm.  I was thinking of raising the exhaust port a touch but you guys would know more about pipes than I could ever learn.

I also wonder if the original muffler was too restrictive on the stock KDX, maybe I should just try a muffler with a straight-thru core first?

Geoff you amaze me with where you can dig-up information  ;-)

[Lozza]

In a 125cc superkart we run really moderate port timings, still has peak power at 12,800 and revs to 14,000. Check the ignition timing as well.

[John Orchard]

Why do you think the 400 is best?  Tests on the 400 state that it's a bit low in peak-power and hits a wall in the top-end.  I'm not young enough, strong enough or stupid enough (although some might argue on that last point lol) to want an all top-end open classer but getting the hole shot is important to me  ;-)

I just wondered what stood-out between the 400 & 420 barrels other than bore size.

[Speed]

Anyone got a KX420 barrel off their engine?  I was wondering if I could get the port timings to compare to my KDX400.

G'day,
Maybe I can help.  I have some KX420, KDX400, KDX420 AND KDX450 Cylinders here
What is it you're looking for?

If it was me, I'd be using the KDX400 cylinder as it should have the boost ports in the intake leading to the transfer ports.
Assuming you're going to increase the bore size.

[Speed]

Why do you think the 400 is best?  Tests on the 400 state that it's a bit low in peak-power and hits a wall in the top-end.  I'm not young enough, strong enough or stupid enough (although some might argue on that last point lol) to want an all top-end open classer but getting the hole shot is important to me  ;-)

I just wondered what stood-out between the 400 & 420 barrels other than bore size.

The 400 has the boost ports already in the intake port.  I have done very many of these cylinder 400,420 and the 450..
There is much more to it that just the port timing.  If you want to PM me I'll drop you my number and I'm more than happy to chat to you about them.  To hard here.  I can't type for sh%$...

[Lozza]

Why do you think the 400 is best?  Tests on the 400 state that it's a bit low in peak-power and hits a wall in the top-end.  I'm not young enough, strong enough or stupid enough (although some might argue on that last point lol) to want an all top-end open classer but getting the hole shot is important to me  ;-)

I just wondered what stood-out between the 400 & 420 barrels other than bore size.

The pipe was built to make a lower peak power, there are ways to stop the brickwalling. With the KX they lifted the exhaust port and maybe a slightly different pipe.
Boyensen tunnels don't really add to the cylinders abilty to shift intake charge (the transfers are closed when the reeds are open and vice versa) but allows a lot of intake to 'park' for when they do open.
KDX ignitions are not the best even late models ran a fixed ignition curve which will limit top end.

[Speed]

Why do you think the 400 is best?  Tests on the 400 state that it's a bit low in peak-power and hits a wall in the top-end.  I'm not young enough, strong enough or stupid enough (although some might argue on that last point lol) to want an all top-end open classer but getting the hole shot is important to me  ;-)

I just wondered what stood-out between the 400 & 420 barrels other than bore size.

The pipe was built to make a lower peak power, there are ways to stop the brickwalling. With the KX they lifted the exhaust port and maybe a slightly different pipe.
Boyensen tunnels don't really add to the cylinders abilty to shift intake charge (the transfers are closed when the reeds are open and vice versa) but allows a lot of intake to 'park' for when they do open.
KDX ignitions are not the best even late models ran a fixed ignition curve which will limit top end.

That's why I suggested John give me a call..  And he did, we had a great chat.  The KDX and or the KX engine can be made to run great with power from almost zero too and to the moon.  But many things need to be tweaked.  One thing I can say is they absolutely run better with the tunnels in the cylinder or two holes lower down on the piston and a arc as well.  This doesn't make much difference in all out power but takes away some of the narrowness and kick....  I've lost count of how many of these cylinders I have done over the years in many different configurations.  Mostly it's about what suits the rides style.

[John Orchard]

Thanks guy's, I am loving all the help I'm getting with this project, I really appreciate it.  It's making me more excited about completing the bike  :-)