jetting help please! 1991 rm250

[tnutcher]

Hey guys i need some help with jetting. The carb isn't the stock one on this bike. I had a problem with it sticking open and so far has been solved with a new cable. I did some spark plug tests and at WOT i got a paper bag brown color or maybe a shade lighter.
For the main jet I assume this is alright?

I took one at ~1/2 throttle and it came up a darker gray brown color. Is this alright or too rich?
I don't know number sizes of jets but the needle is at #2 and air screw is 1 1/2 out.

I ATTEMPTED an air screw adjustment but while holding the throttle constant twist, all turning the screw did was make the RPM's increase no matter how far it turned out and when I let off the throttle it would be a very high idle. I need some help here.

My friends bike kx250 93 has a really big hit at the low end where mine doesn't. His is stock and mine has an fmf fatty and turbine core2 and boyseen 2stage reeds (not the highest level but the second i think pro or something) I would think mine would have a good hit but it doesn't which makes me think the air screw/PJ is off. Any help would be appriciated this adjustment process left me more confused. :think:

Thanks in advance, sorry about the lenght
TNutcher

 

[mxcrazed]

click on this

 

[tnutcher]

Yeah I've read through many posts and talked to a few guys about it, am i that far off?? :bang: haha
I need help with the PJ setting and the air screw. If I hold the throttle at a certain point like advised to and turn the airscrew out from seated until the highest rpm is acheived, the rpm simply keeps raising the farther I turn it out. Is something messed up maybe with my idle adjustment screw causing this to be screwed up or am i horribly off the path??
TNutcher

 

[whenfoxforks-ruled]

What kind of carb is it? And what size?

 

[AwfulSmokey]

Yeah I've read through many posts and talked to a few guys about it, am i that far off?? :bang: haha
I need help with the PJ setting and the air screw. If I hold the throttle at a certain point like advised to and turn the airscrew out from seated until the highest rpm is acheived, the rpm simply keeps raising the farther I turn it out. Is something messed up maybe with my idle adjustment screw causing this to be screwed up or am i horribly off the path??
TNutcher

I read that same stuff and don't like it that way. Here's what you do. Pick what part of the throttle you want to fix. For example The pilot (slow) jet is most effective between 1/8 through 1/4 throttle. If you think you are rich, you would turn the air screw out say 1/2 turn. Then concentrate on just using 1/8 to 1/4 throttle and see how the response is. If it gets better, keep going out. Reverse that if it's lean.

You should be at about 1 1/2 turns out. When i did my KDX200 it ended up being 1 size down for every 1 turn for the air screw give or take.

All you really need is to use the air screw and remember this:
The air screw is most effective between idle through 1/8 throttle.

The pilot (slow) jet is most effective between 1/8 through 1/4 throttle.

The slide valve is most effective between 1/8 through 1/2 throttle

The jet needle is most effective between 1/4 through 3/4 throttle.

The main jet is most effective between 3/4 through wide-open throttle.

 

[tnutcher]

Well the carb is a Mikuni VM round-slide carburetor and not sure on the size. The only numbers I found on it is "T4 65" and that helped me nowhere I'll try to measure it and find out what the hell it is if anyone else wouldn't know. Why would the previous owner put on a different carburetor especially this one. . . is it a hotter carb or maybe just a swap out?

Thanks for the tips that makes much more sense. I'll give that a try. It sounds like it'd be the pilot jet, air screw and slide or needle. . . It seems to be slower than what i think a 250 would be from 1/8-1/4 or 1/2.
TNutcher

 

[whenfoxforks-ruled]

I would at least measure the bore of the carb. Or be looking on evilbay for the proper carb.

 

[tnutcher]

Ok,
bore is 36mm, Intake side is 50mm and reed side is 34mm.
I take it they go off bore for sizing. is this even the right size for a 250rm? what is a standard 250 carb size? Thanks
TNutcher

 

[tnutcher]

This seems to be the same carb at 36mm.

http://cgi.ebay.com/ebaymotors/ROUN...020QQitemZ300139722509QQrdZ1QQsspagenameZWD1V

Another question is why would anyone swap the carbs for this one?
Thanks
TNutcher

 

[whenfoxforks-ruled]

What size are the, main jet, pilot jet, and needle? While you have the carb off, check the reeds.

 

[tnutcher]

number off the main jet is 340, pilot jet is 3.3, and needle was a 6H3. The slide also had 12.0 stamped in it. I have it all torn down and have sprayed with carb cleaner, i'm gonna let it sit in solvent overnight and blow it out with o2 and throw it back together and see how it runs and make some adjustments. Everything seems pretty clean and free of debris but maybe something was blocking a passage. . .

All the numbers have a sign before them or on the part that looks like a corner where two lines meet with a smaller box on the inside corner, the closest I can figure to draw it is: " b_ "
I figure its just the brand of parts but figured i'd include it haha

anyone have any idea why the PO swapped carbs to this one?
Also the float needle has a ring around the tip where it contacts the seal but it isn't a groove, just a visual wear mark, is that still good or should it be replaced? Thanks
Thanks for the help guys, I appreciate this it's helping me learn a lot.
TNutcher

 

[whenfoxforks-ruled]

First off, replace the float valve. 330 main,35 pilot, and 6ael needle is stock. There is no telling about the carb swap.

 

[tnutcher]

First off, replace the float valve. 330 main,35 pilot, and 6ael needle is stock. There is no telling about the carb swap.

Sorry the pilot is 45. 3.3 was stamped on the float valve seat. I'll try and pick up a float valve tomorrow and throw it together and tweek the needle or air screw and see how it affects.
TNutcher

 

[tnutcher]

got it clened and put back together and found that the air screw at 3 turns out ran really nice on the 45 pilot so i swapped in a 42.5 pilot but ran out of daylight. It would pick the front end up like a 250 should, before i had to clutch to get it off. . . :bang: :bang: Thank you very much for the help everyone.

What are the signs of it being too lean on bottom? Can i do a plug chop for the low end to see or just by feel? Thanks
TNutcher

 

[whenfoxforks-ruled]

You can run the bike at the throttle position, relating to that circuit, pull a plug chop and look at the plug for reference. Wet or dry? Rich or lean? Go a few sizes in each direction. Do not run it for long. The dramatic difference can tell you. As long as the carb does not have a power jet, the pilot is just off idle where it comes in.

 

[tnutcher]

well got it tuned very nice. Before it would bog and rev out like a 125 or 85 but now it has that bring the tire up no problem hit down low like the 250 should. Thanks a lot. I ended up sticking with the 45 pilot and had it out 2.25 turns on the AS, i tried a 42.5 and a 40 pilot jet but got issues of hard starting and too lean so this works great for the bike. I did a plug chop here at its just a shade rich which i like better than too lean and I love the feel of it so i'll stick with it.
Thanks again,
TNutcher