Tuning tips on a kx125 93

[kx250.1698]

Hi guys, While i am waiting for my sprocket shaft that i bought from a gr8 member (DoldGuy) of this site i want some tips how to tune my kx125 to have max torque and power and need a few Edee's.

Oke to start of with my bike have a Air age SILENCER .

Do a front header of the pipe make a big difference when racing pipe?
Do v force reeds make a big difference?

Any tips please??

[DoldGuy]

KX250.1968,
The best set up for the 93 KX125 is a #6 Slide (5 is stock) , Bore the Carb to a 38mm, use a Boyseen RAD Valve (made for the 93, it's bigger than the 94 & up) not the V-Force, FMF pipe & silencer (they were called Gold Series back then) , and probably the most important part is to have the transfer ports matched in size (one side is larger than the other) but do not change ANY of the ports angle. These motors were known to "Blow Up" if you did not run race gas in them, and they don't run alot of compression, my experience has been that the transfer ports (the large side) created a Lean Area on one side of the piston causing these problems & after you correct the ports the problem will be eliminated. Also make sure your piston & plating of the cylinder is within specifications (Compression should be 165 on a fresh motor) . The last thing is to Advance the timing. If needed I will dig up my old racing notes to find the exact jetting used if you bore the carb. If you do these mods I think you will be surprised how "fast" this 16 year old really is. Hope this answers your question & gives you a goal to work towards. Keep us posted how it works out.

[kx250.1698]

KX250.1968,
The best set up for the 93 KX125 is a #6 Slide (5 is stock) , Bore the Carb to a 38mm, use a Boyseen RAD Valve (made for the 93, it's bigger than the 94 & up) not the V-Force, FMF pipe & silencer (they were called Gold Series back then) , and probably the most important part is to have the transfer ports matched in size (one side is larger than the other) but do not change ANY of the ports angle. These motors were known to "Blow Up" if you did not run race gas in them, and they don't run alot of compression, my experience has been that the transfer ports (the large side) created a Lean Area on one side of the piston causing these problems & after you correct the ports the problem will be eliminated. Also make sure your piston & plating of the cylinder is within specifications (Compression should be 165 on a fresh motor) . The last thing is to Advance the timing. If needed I will dig up my old racing notes to find the exact jetting used if you bore the carb. If you do these mods I think you will be surprised how "fast" this 16 year old really is. Hope this answers your question & gives you a goal to work towards. Keep us posted how it works out.

Thanks Doldguy, A guy told me to buy v force reeds but thanks to your help i will order me a set of boysens,haven't checked yet compression but  will do so,I think when i am going to stripe the motor i will go and buy me new bearing ext. If the piston is still looking like new should i just replace rings or must i replace whole Piston?Is it better then to replace the whole piston?

A another thing the problem of the kick starting.Haven't looked at i well jet because of was waiting to get a sprocketshaft and all the parts that i need to fix the bike as well as i can but any way i also think it is compression that makes the bike don't want to start(the bike have week spark  and i have changed the coils  makes a little bit difference but not really)  Do you think i should rewire the stator coils or what is your opinion about the bike don't want to kick start?  (Do compression have any thing to do with spark?

Thanks 

[Jopiz]

When you test your spark and you can see it, it doesnt mean it fires under high pressure. The resistance may grow so big that the voltage isnt big enough to produce a spark. For a general rule you need 1000 Volt to make a spark jump 1mm in atmospheric pressure. In high pressure the voltage needs to be a lot bigger. So pressure (compression) has everything to do with the spark. The spark should be fat and blue.

How much difference does boring the carb to 38 or buying a new one? Pain to jet or pure bolt-on power when jetted right?
Eric Gorr recommends 34mm carb for low to mid powerband and 38 to high powerband. Does 38 work on nearly any 125cc engine better if higher peak power is wanted? I've noticed modern bikes such as KTMs use 38 carb. I have also  a 125 from early 90's. 1991 kx 125. Im intrested if 38mm would give me better performance also.

-J

[dangerous dean]

hi does the piston/ports modification work also for the 95 model? im just rebuilding mine and to start i have a pc platinum expansion with boyesen reads,what else can i do to help it?many thanks !!!

[kx250.1698]

Hi, when my 125's engine was took apart i saw that the bike already had v force reeds in.Is it bad or can i go with it?


Thanks 

[KXcam22]

V force are good. Very good. Cam.