Converting a 250 to a 300

[Niall-griffiths]

Hi all for the winter I want to turn my 2002 250 sm into a 300 if possible,

can anyone tell me what I need ? A cylinder,cylinder head and piston is this it and is it just a straight swap or is there rod differences??Or do I need a 300 exhaust? At the moment I have the standard messico front pipe and silencer,

also what should I do regards to jetting? At the moment I have a 178 main and 40 or 42 pilot where should I go from here?

Is there any other mods or parts I would need? Ignitions different plug ect ?

 

[Fred1956]

Hi all for the winter I want to turn my 2002 250 sm into a 300 if possible,

can anyone tell me what I need ? A cylinder,cylinder head and piston is this it and is it just a straight swap or is there rod differences??Or do I need a 300 exhaust? At the moment I have the standard messico front pipe and silencer,

also what should I do regards to jetting? At the moment I have a 178 main and 40 or 42 pilot where should I go from here?

Is there any other mods or parts I would need? Ignitions different plug ect ?

That should do it, 250 & 300 have the same exhaust as far as I know..obviously you'll need gaskets & head o-rings. Jetting you have won't be far off, close enough to start with

 

[desertgasser300]

Hi all for the winter I want to turn my 2002 250 sm into a 300 if possible,

can anyone tell me what I need ? A cylinder,cylinder head and piston is this it and is it just a straight swap or is there rod differences??Or do I need a 300 exhaust? At the moment I have the standard messico front pipe and silencer,

also what should I do regards to jetting? At the moment I have a 178 main and 40 or 42 pilot where should I go from here?

Is there any other mods or parts I would need? Ignitions different plug ect ?

Needed:
Piston and rings
300 cylinder and head
Jet according, the 300's like to run leaner then the 250's. Prepare for more torque and more vibes.
Pretty straight forward swap. Rod is the same. Some like the 2k3 ignition with the 300's. But it boils down to what you like.

 

[sweeper]

Looking at buying an 03 ec 300.I want to swap it out to a 250.I'd have the 300 stuff you'd be looking for if it comes together.

 

[Niall-griffiths]

I'm just after abit more power, do u think I would notice much difference?i will get the head skimmed accordingly to raise compression and port and polish the cylinder and carb. Would this make a big difference ?

sweeper let know I wouldent need the piston as I will be putting a new one in,are you in the us or Europe mate ?

 

[sweeper]

Oregon, U.S.A.

 

[Niall-griffiths]

Does anyone know if a 05 300 cylinder will fit onto a 02? Also will the 250 cylinder head fit onto the 300 barrel or not ?as the mount on the 300 has broke

 

[gassergrant]

jetting in my 03 300 is
mj 178
pj 38
slide 6
needle n3eg clip 3

runs well but not done many miles with this set up can report back after Sunday on that.

going head to head against my mates 250 I could not notice any real difference I also have a full fmf system and carbon reeds

 

[Fred1956]

Does anyone know if a 05 300 cylinder will fit onto a 02? Also will the 250 cylinder head fit onto the 300 barrel or not ?as the mount on the 300 has broke

Cylinder heads are NOT interchangeable

 

[nambo-trev]

You need a new cylinder and a reed spacer

 

[pscook]

Does anyone know if a 05 300 cylinder will fit onto a 02? Also will the 250 cylinder head fit onto the 300 barrel or not ?as the mount on the 300 has broke

As above you need a 300 head to match the 300 cylinder. However, I have read that the newer 300's don't even have the lug, or at least aren't attached to the frame. It might be worthwhile to run without the lug bolted to the frame, supposed to reduce transmitted vibrations.

What condition is the 250 top end? Looking to sell it?

 

[Niall-griffiths]

The condition of the 250 head cylinder is very good, put a genuine piston kit in 20 hours ago also I might sell it but am unsure yet it really depends what I could fetch for it to make it worthwhile,

gasser let me know how u get on please mate !

 

[Niall-griffiths]

Could I get some options on getting the cylinder head and crancase ported do people think it will gain much in terms of power and smoother revving?

 

[pscook]

Could I get some options on getting the cylinder head and crancase ported do people think it will gain much in terms of power and smoother revving?

I believe the general consensus is to clean up the intake and transfer ports, then raise or lower the cylinder for exhaust port timing, and finally have the head cut for squish and compression. I don't remember reading any threads here about raising or lowering the port timing through port modification, it's typically raising or lowering the cylinder to expose or shroud the intake and/or exhaust port by adding or removing base gaskets (cheap and easy to undo).

I would be interested in hearing if someone widened the exhaust port, or moved the intake or transfer ports, or did any other porting and what gains (or losses) they experienced.

 

[Niall-griffiths]

Thanks for the reply, I have been told it would cost ?150 to port and polish my intake ,transfer and exhaust ports from a 2 stroke specialist who also makes his own performance pipes so I might send my cylinder to him and report back the results,I couldn't find any threads on anyone widening the ports either