YZ250 jetting help?

[DVS ME]

Hi my top end went on my yz now its back together again i want to jet it to try and stop it happening again its got a fmf fatty power core2 v force reeds wossner forged piston new rod and all bearings seals etc.
Im just worried about the main jet its got the std 178 jet the manual says for europe spec use 180 ive bought new jets 180 182 185.
Any input is welcome

Thanks

[Sensei]

Run the standard plug then do a plug check to see its colour & then work from there ... the last thing you don't want to do is seize it so run on the richer side of the colour tuning

[gwynfryn]

If it seized because it was running lean due to incorrect main jet size then yes you need to jet it correctly. Read up on plug chops and remember you want to look at the base of the plug carefully, cut the thread off if you have to. Google yz250 jetting and go with a conservative main to start with . Correct it for elevation eg Auckland = sea level. Look at needle position as well.
Correct jetting is essential for 2 strokes and factory settings are often too rich if anything.
Spend the time to get it right and you will be rewarded.

[CRF119]

Also remember the oil ratio and oil will greatly affect performance and jetting. So chose a oil and ratio and stick with it. When trying to see what others are running for jetting around the world make sure they are also running the same oil to fuel ratio and same oil. If they are not then there is not much point in copying them.

[DVS ME]

Thanks for the replys ive started with the 180 seems ok going to try the 185 next.
Was running motul at 40:1 then got two liters of agip syn oil used about 1.5 ltrs then blew up not blaming the oil but never had a problem with motul with my old 99 yz250 ran it for two years no problems.
First ride after the rebuild ran it at 35:1 with motul and 98 the manual say 30:1 so might give that a go also.
But............. The chain link came off and it smashed the engine case so piss off ive spent so many hours and so much money on the rebuild all for and hours worth of riding anyway got some cases coming off ebay and wil swap them over in the next few weeks.

[B0000M]

by changing from 40:1 to 35:1 you have made your air/fuel mixture leaner. this is because more of the volume of fuel/oil flowing through your jets is oil.

if your bike was already on the lean side then this may be enough to take it over the top.

i ride a cr250, i use 40:1 with castrol TTS, why - its easy to obtain anywhere, its cheapish, and its given me about 18 years of no troubles.