1979 KX 250

[splime738]

Just looking for some help with carb and jetting for my 250 A5, anyone running them what set ups are you using? I am in the UK, so it will be different to OZ or US but just interested to hear what is being run if different to stock with new fuel.

[John Orchard]

Sure, I got two of them, they are fast & sorted...

MJ: 170 reverse type
Needle: 6F21 clip 3
Needle jet: R-0
PJ: 50
Air screw: 1.0 turns

In one carb I have fitted a different model needle jet so it runs a 370 hex jet.

I had a bit of trouble at the start as my carbs were the original on the bikes, they didn't run right until I replaced the needle jets, they had worn and were making it way rich at part throttle, lots of gunge dripping from the pipe joins.

[splime738]

Thanks John,

my problem is I have seized a couple of times, been trying to use jetting from manual but it is obviously running too weak. When you say reverse main jet is that the round type with slot?

[oldyzman]

If you are trying to tune a later model vm mikuni carby with hex main jet the the equivalent size to johns spec 170 is 370.
Cheeers Brett

[splime738]

Thanks Brett, it seized at weekend with a 190 in it, I thought it was going to be quite rich.

[JohnnyO]

Thanks Brett, it seized at weekend with a 190 in it, I thought it was going to be quite rich.

If it seized with a 190 main its not lean, you've got another problem.
Mine runs a 175 main,
40 pilot
6FL28 needle in the 3rd position
R2 needle jet

It's well sorted and fast..

[splime738]

That's pretty close to the original jetting there Jonny O, cheers, will have to re-check for air leaks again

[JohnnyO]

The std jetting is a 172.5 main and 50 pilot jet..

[oldyzman]

Splime, What sort of seizure?
Did it get hot somewhere around the exhaust port, transfer some of the piston to the bore and also weld the rings to the piston?

[JohnnyO]

It didn't seize thru being too lean unless it has a bad air leak.
How's the piston to bore clearance?
Is the ignition timing advanced or compression ratio too high?

[Simo63]

It's well sorted and fast..

Anything is fast with you on it John

[splime738]

Ignition is hopefully ok, it has just had a new old stock cdi, stator and coil fitted (silly money), marks lined up so it shouldn't be advanced or retarded, barrel height and head all standard with Kawasaki head and base gaskets. Barrel just been nicasiled and new piston. It has gone as oldyzman says around the exhaust port, went solid then freed off but has scrapped the piston and cylinder will need to be re plated.

[splime738]

Forgot to say, running standard exhaust too with a Holly silencer

[Simo63]

Ignition is hopefully ok, it has just had a new old stock cdi, stator and coil fitted (silly money), marks lined up so it shouldn't be advanced or retarded, barrel height and head all standard with Kawasaki head and base gaskets. Barrel just been nicasiled and new piston. It has gone as oldyzman says around the exhaust port, went solid then freed off but has scrapped the piston and cylinder will need to be re plated.

you can try to remove any residual alloy with hydrochloric acid if the piston has left any alloy behind.  The nicasil bores are pretty hardy

[TT5 Matt]

id be sealing up the exhaust and intake ports and doing a pressure test on the crankcase to check for any air leaks. also id do what a lold timer told me and that is start off very rich and work down not start lean and sieze or cook everything as getting barrels replated aint cheap nor are new piston/rings.also check fuel flow from your tank and that the carb as the right sized needle and seat fitted