Mods and jetting differences!

[1998RM250]

After looking on fmf's website on jetting specs
for different bikes, I was wondering why the huge differences in jets? For example my 2001 RM 250 it says a 158 main jet, and for 2002+ it has a 170 on some bikes when those years are all pretty much the same motor. That seems to be a pretty big difference, and i noticed even some of the older RM 250's running a 152 main jet on fmf's site which seems really lean! I was wondering what gives with the jetting, I'm trying to figure this stuff out. :bang:

 

[rmc_olderthandirt]

The motor may be roughly the same but the carburetor could be different.

While the number on the jet is a reference to its size there are several different ways that the jets are measured so without knowing a lot of technical details it is hard to compare a size 158 jet in one carburetor to a 170 on a different carburetor. In addition, the size of the jet will vary with the size of the venturi, which may vary with model year of the bike.

Rod

 

[whenfoxforks-ruled]

Fuel and air, in a specific volume, compressed, exploded and expelled. For the specific volume, requires a specific flow of fuel and air. Even the texture of the ports carrying the mixture have drastic differences in the final result. And the final expelled gases, even the packing in the silencer makes a difference! So you have the airbox, carb, engine, pipe and silencer that can all be slightly different, and thats the same year make and model. One year to next? They go and change the electronics a tad? If the jetting is different, something else is also. Vintage Bob

 

[RM_guy]

Porting changes probaly have the big affect from year to year on the same model.

 

[FatBiker]

Oops, I got the wrong impression! I thought this was gonna be a thread where the mods. were gonna "throw down" about jetting differences?!!?? My bad. :joke:

 

[flyinzuki]

Some carbs like my 00's have an electronic power jet.
Or it could just be a different nozzle, needle, or slide combo.
Even if the carb body maintained the same dimensions.

 

[1998RM250]

so what would happen if I ran a 2004+ piston in my 01 RM 250 when physically the cylinder is the same size stroke etc?

 

[_JOE_]

It could possibly hit the crank or head.

 

[helio lucas]

the ring peg could break in a port. even the port timing could be radicaly changed.
or maybe just nothing...