yes there definetly was a performance increase but jetting it is a little tricky as you can see here I am 4-5 years later still straightening it out. My upgrades can be seen in my signature, but now I am .040 over bore not .020


I had the head shaved for higher compression as was recommended to me when having the cylinder ported.

 

are you ALWAYS using 92 or greater octane fuel? if not then there is the answer to your pinging,,,,,,,,,,,,,,,,,,,,,,,,,,,

 

i am not a 250 guy, but i think i can help a little here.
How much was the head shaved?
what is your compression in psi and or un corrected ratio?

the 250 is a piston port right
so on the bottom end it will need leaner tuning components then the 400 reed motor.
look on the bottom of the carb slide, you should see a number.
the stock 400 carb will say 1.5. look on the stock 250 carb it will likely have a bigger number. after a port job this needs to be changed to fix up the 1/8 to 1/4 throttle range.

you know how the slide has an angle cut on one side. the greater that angle, the leaner the slide. the number is how many 1/16"'s of an inch you will measure if you were to place the slide on a flat surface. measuring how high that cutaway is
this can be modified with just a file and some good measuring.
tuning this is like magic to the driveability of a ported machine or one that had carb swap.
also if you suspect you are way on the lean side of the magic air/fuel ratio. don't slowly jet up, that can have bad results. you could actually melt a piston after having jetted up. instead overshoot where you think you going to be and start dropping back down till the engine is happy.

if anyone is wondering when my piped and ported 400 still had the 34mm. my carb slide was up to a 3.5. a mild port job on a 400 would need a 2.0-2.5.

 

yeah,i actually got the 34mm carb with a #250 main.ran like an angry chainsaw.i am running 92 octane and could use some more.i'm pullin 145psi.the plug was alittle darker than it should be at 600 ft up. so when i luckily acquired a carb kit it had a #230 main and perfect color with 0 change in performance.so in essence i did just that Mrtwostroke.but it idled perfect only after i cleaned and replaced the pilot.and the needle clip is two from the bottom.

 

you should still check that slide. there is improvment to be made. what pilot size are you running?

 

I dont understand why a higher performance machine should run leaner than a stock machine?
the head was shaved .020 I believe and the compression I didnt check because it was just rebuilt. I dont have time to do all this until probably next week so I will post then with some results. Keep subscribed!

 

I will post more in the coming days, but here is something to think about.

A high performance two-stroke will need leaner tuning components in the small throttle openings. The engine's powerband has been shifted so the engine makes lots of power on top but less down low. requiring a larger main jet but maybe a smaller pilot and different slide cutaway. don't forget the needle might need to be fatter up high and with more taper increasing fuel in the midrange
to quote mikuni's manual

"change of slide cutaway is made, when the low speed fuel system is out of balance with the main fuel system"

keep in mind here there is over lap everywhere when talking about a tuning components job at different throttle openings. There is a great chart to describe it but i don't have time to scan it right now. i will later

straight from my 2000 scrambler 400 factory service manual
pilot system's main function is to meter fuel at idle and low speed driving.
though its main function is to supply fuel at low speed, it does feed fuel cont. over the entire operating range.
fuel for the pilot jet is drawn from the float bowl, mixed with air reg. by the air screw and delivered to the engine through the pilot oulet.
when the air screw is closed, the mix. is made richer as the amount of air is reduced. when opened the mix is leaner. adjusted from closed to three turns out.

throttle valve cutaway effect is most noticeable at 1/4 throttle. but has an effect from 1/8-3/8. a steep angle would indicate a lean mixture because there is less resistance to air flow. a flat angle the opposite beacause of greater resistance to air flow.

more on that stuff later. one more thing and a few links.
the 400 is a reed motor. the 250 a piston port.
when the piston is travelling down the charge in the crank case is being forced through the transfer ports to the top side of the piston. in a reed motor this charge can't leave the engine because the reeds are being forced closed.
In a piston port engine part of this charge is pushed back through into the intake boot and carb. this often called an intake backwash. it then pulls more fuel out of the float bowl richening the mixture further. at high engine speeds this doesn't have any real effects. but at low rpm it does. giving you one reason why reed motors make more power, it allows the reed motor to have greater intake port duration.

2 Stroke Tuners Handbook by Gordon Jennings
http://www.kreidler.nl/artikelen/per...raham-bell.pdf
http://www.macdizzy.com/

 

here is a good vm carb tuning manual. start with page 14 for proper selection of pilot jet. rich lean conditions on page 18 are good starting points. The last page shows tuning components effective range. there is a different one I have that lumps the needle jet and jet needle together for simplicity, I will scan it sometime.