KDX mods re-post
George Couyant
With the renewed interest in KDX's lately, I thought it may be timely I
repost this little gem from some time ago.
As an update on this bike, it's going strong and the owner is really happy
with it. Stone reliable too.
Cheers
George C.
I've had a few responses regarding my offer of some KDX200 tips so rather
than emailing each one, I'm posting it here....
Though I've never owned one, I have helped a couple of guys tweak them and
have had good results. As I modified one I was fortunate enough to have access
to another identical '88 air cooled KDX 200 and compared the effect of each
modification as I went along. This was terrific because I didn't have to rely
on the old "seat of the pants" acceleration meter.
The aim of the mods was to increase midrange and high end power without
sacrificing the already great bottom end. I didn't want to dabble with
porting (couldn't afford it) and to help the suspension work as it should
(again without spending much money).
Firstly, to help kick some high end power, we bolted on a DG KDX200 pipe.
This was the most expensive thing we did. From memory, it was about
AUST$280. This really gave it a power boost. In third gear roll on drags
against the stock KDX200, it pulled 3-4 bike lengths. The only thing was
that the bottom end became a bit fluffy.
To cure the bottom end, I made a 12mm thick KIPS box spacer (on the left
side of the barrel). The KIPS box is a chamber that opens at low RPM to
boost low end power and improve the throttle response smoothness. The
spacer just increases the volume of the box. This brought much of the
bottom end back and it kept up with the stock bike right down low.
Next it needed a bit more compression so at first we shaved 10 thou off the
cylinder head and it made a tiny bit of difference. We ran it on standard
unleaded gas and got it good and hot. It didn't ping so we unbolted the head
and took another 5 thou off. Never ran it on cheap gas again so I'm not
sure how much further I could have gone.....we just left it as it was.
Next the barrel came off to check the porting. All I did was clean up the
ports and made sure the appropriate ports opened when they should etc.
Actually, I was quite surprised at how close the stock ports were.
While I had access to the piston, I had heard that you should open up the
lubrication hole in the back of the piston, so I did. This is a little
tricky because what you do is get a small file and make the corners more
square. You're not actually increasing the length or width of the hole.
You must be careful not to make the corners too square because it can
lead to piston cracking.
When it was all back together, the jetting got some serious attention.
After days of farting around, we decided on a #130 main jet, standard needle
and needle jet, center needle clip position and one size bigger pilot. Oil
was at 40:1 (good stuff, not your regular lawn mower oil.....)
Oh yea, this is all at close to sea level. If you're high in the mountains,
you can go leaner.
Finally, the stock muffler was changed for an Accord (the large diameter
version for the KDX200. Not the smaller *noisy* motocross based one).
This thing really goes now. It leaves the stock bike for dead. You've got all
the bottom end of the stock motor with a real KX feeling top end.
For the suspension, all we really did was to change the oil in the forks and
shock. We ended up using 5 wt both ends. The guy that rides this is a feather
weight, so if you weigh more or ride *real* fast, go heavier and try more
oil in the forks.
One problem though....the DG pipe uses thinner steel, so it dents easier
(it's also easier to repair...).
Try the mods. They're easy, cheap (except for that bloody pipe....) and
make a heap of difference. You'll love your bike even more....
Cheers all...
George C.
Eric Murray
[...]
>Next it needed a bit more compression so at first we shaved 10 thou off the
>cylinder head and it made a tiny bit of difference. We ran it on standard
>unleaded gas and got it good and hot. It didn't ping so we unbolted the head
>and took another 5 thou off. Never ran it on cheap gas again so I'm not
>sure how much further I could have gone.....we just left it as it was.
How much squish clearance did you leave?
>While I had access to the piston, I had heard that you should open up the
>lubrication hole in the back of the piston, so I did. This is a little
>tricky because what you do is get a small file and make the corners more
>square. You're not actually increasing the length or width of the hole.
>You must be careful not to make the corners too square because it can
>lead to piston cracking.
Where are these holes? I haven't taken apart any two-stroke engines
designed after about 1984, and I've never seen oil holes in the pistons.
Are they a new(er) idea? I've seen small (~2mm) holes in the middle of
the exaust-side piston skirt in pictures of Honda 125 GP engines'
pistons. Are those oil holes? How the heck can there be enough oil
going through those tiny holes to make a difference?
--
Eric Murray er...@microunity.com
This package contains one (1) Usenet news posting. Do not eat.
George Couyant
> In article <1992Nov26.0...@convex.com> cou...@convex.com (George Couyant) writes:
>
> [...]
>
> >Next it needed a bit more compression so at first we shaved 10 thou off the
> >cylinder head and it made a tiny bit of difference. We ran it on standard
> >unleaded gas and got it good and hot. It didn't ping so we unbolted the head
> >and took another 5 thou off. Never ran it on cheap gas again so I'm not
> >sure how much further I could have gone.....we just left it as it was.
>
> How much squish clearance did you leave?
Didn't modify the combustion chamber at all (except for cleaning
up any rough edges..) . Just shaved the head, so whatever that
works out to. The thing hasn't pinged since we did it, so we could
probably take a bit more off. Most of the riding is near sea
level.
>
> >While I had access to the piston, I had heard that you should open up the
> >lubrication hole in the back of the piston, so I did. This is a little
> >tricky because what you do is get a small file and make the corners more
>
> Where are these holes? I haven't taken apart any two-stroke engines
> designed after about 1984, and I've never seen oil holes in the pistons.
> Are they a new(er) idea? I've seen small (~2mm) holes in the middle of
> the exaust-side piston skirt in pictures of Honda 125 GP engines'
> pistons. Are those oil holes? How the heck can there be enough oil
> going through those tiny holes to make a difference?
It is a lubrication hole. It must do something for the
manufacturer to go to the trouble of introducing another step in
the manufacturing process to drill the hole I guess. Only trouble
is that they can only drill a round hole and can't go too big in
size for fear of weakening the piston. By opening up the corners
(but still keeping radiused corners), you're increasing the
surface area of the hole to let more lubricating mixture in there.
KDX's are pretty bullet proof but aren't adverse to the odd slight
sieze every now and then.
Cheers
George C.
--
,--_|\ George Couyant email cou...@convex.com
/ \ Senior Systems Engineer Ph 61 3 823-6216
\_/--\*/ Convex Computer Pty Limited Fax 61 3 823-6254
'
86 TT600,90 KX500,88 KX250,85 CR250,86 CR500, oh yea, and the kids 92 PW50.
Russel Wardman
>>
>> >While I had access to the piston, I had heard that you should open up the
>> >lubrication hole in the back of the piston, so I did. This is a little
>> >tricky because what you do is get a small file and make the corners more
>>
>> Where are these holes? I haven't taken apart any two-stroke engines
>> designed after about 1984, and I've never seen oil holes in the pistons.
>> Are they a new(er) idea? I've seen small (~2mm) holes in the middle of
>> the exaust-side piston skirt in pictures of Honda 125 GP engines'
>> pistons. Are those oil holes? How the heck can there be enough oil
>> going through those tiny holes to make a difference?
The holes line up with the exhaust port bridge and provide it lubrication/
cooling. I hear that some of the guys with modern RM's are drilling these
themselves. The bridge seems to be a common failure point, so any lube/cooling
improvements help. 2mm sounds kinda big, I'd guess .about 1mm.
--
** Russell Wardman ** 78 TT-500 (for sale) **
** 613-723-6500 ** 84 Turbo 750 KAWASAKI (for sale) **
** ** 90 KX 500 KAWASAKI **
** (rwar...@gandalf.ca) ** **