I have recently been fouling pugs in my 250r like a **** and I finally found out what jets I should be running. Will stock Honda jets fit in my aftermarket carb? How hard is it to change these jets and are there an specific things that I need to watch for like springs flying out when I remove something? I need to get this done soon so that I can get a good feel for the bike for the upcoming season. Thanks in advance

 

Piece of Cake!!!!
You must have a Keihin carb, to change the main jet you don't even have to remove the carb. Just loosen the screws that hold the carb tight, twist it to the side so the bottom od the carb is facing you, take off the big 14 or so MM screw in the middle of the bowl, and the main jet is directly below it us a 1/4 driver to remove it. To get at the pilot you have to remove the bowl.

 

You're going to have some more info on the carb you have since it's aftermarket. Chances are slim-to-none that stock jets will fit your carb now. You must have something on your current carb that explains how to do these things or at least where they are.

 

Look at the carb and see if you can see a name on it.

 

Ok I have rejetted and now the bike is bogging down and stauling in low rpm's and when i dont have good traction. Here's what I changed:

I had a 138 main jet changed to 165
changed pilot jet from 52 to 48
moved clip on needle from bottom to half way

Before I rejetted I had no low end power it seems better now, but now the top end is slow compared to before. What do I need to change now

 

What do I do next. Should I change the main again or adjust the clip on the needle. C'mon guys I am really struggling here so any help would be appreciated. Chris

 

I'd like to help but. . . I still need to know what kind of carb you're using. Jet sizes are different for aftermarket carbs than for the stocker.
All you said about your carb was that it was "aftermarket."

 

OK it is a kehin 36mm. I worked on it today and changed the main back to the 138 and moved the needle clip to the lowest setting. I also turned the gas screw out only one turn. I didnt have much of a chance to ride it but it seems better. let me know if you see anything wrong.

 

OK, I'm going to try to help.
I think you have a stock carb, probably a Keihn PJ05A. Correct? It's a 34mm, not a 36 and it's a good carb.
First, do you have an aftermarket pipe/silencer and airfilter?
The stock main jet size for an 87R is a .152. If you have a stock pipe, etc., start at .152. Airbox lid on, all stock.
If you have an aftermarket airfilter and pipe/silencer you will need to go up to at least a .158, maybe a .160.
Air screw. Turn in all the way and then back it out 1 7/8 turns.
Put your needle in the 3rd position, right in the middle.
After you do this, do a plug check. Get it warmed up and ride it for a few minutes and then pin the throttle in 3rd gear for several seconds. Then pull in the clutch and shut the engine down and coast to a stop. Pull the plug out. You want a nice tan color (sorta like a paper bag). If it's black and oily (it should've been crapping out on top-end) then you are too rich and need to come down one size on the main jet. If it has any white whatsoever it is way too lean (and probably ran great) and you need to go up on main jet size at least one and maybe two. Don't run it too lean for very long, you'll smoke it! I speak from experience.
This isn't the easiest thing to do but this is what you have to do to dial it in.
Better to have it a little on the fat side than the lean, running fat you won't burn it up.
Main jet sizes are as follows,
.150, .152, .155, .158, .160, .162, .165, .168, etc.
My 86R has a K&N, no airbox lid, FMF pipe/silencer, Boyesen reeds, stock carb,
needle in the middle, .158 main. When I go duning, I increase the main to a .160. When I go high elevation I drop to a .155 or a .152 if it still blubbers on topend.
I know this is a pain in the butt but it has to be done.
Keep in mind that elevation changes, air temperature and humidity can affect the jetting.

 

first off, stock 'honda' jets will fit in any keihin PJ or PWK series carb.

and to add to trapperb's post above, as a general rule of thumb, the pilot jet affects idle and off idle throttle response.

the needled mainly affects mid range

the mainjet mainly affects wide open throttle (not upper RPM's, wide open carb position)

do not change things that are ok and do not need changing