04' cr250 jetting...

Hi guys new to this forum, it looks like the best place to come for some much needed help.



My uncle and i both own CR250s, i have an 03 he has an 04 but, my uncle has been having major issues with his jetting now, here's a list of problems he faces:

bog at low end, so much so that he nose dives of most jumps becuase of the bog.

every ride tons and tons of black oil flows out of his pipe all the way down his swingarm.

his bike chows fuel, we rode the track the other day ( we are pretty much the same sort of skill level on the track) and his bike used a full 8 liters of petrol where my bike only used 3 :o

when he starts the bike up it smokes like crazy his right side crank seal has been replaced like 3 times in 3 or 4 months now.


what we found:

when we opened up the carb (stock mikuni) it was running a 40 pilot jet and 350 main with the stock needle and the carb slide has been modified in a way, there was an upside down "U" ground out of the slide on both the left and right sides of the slide, maybe to let more air in :? my uncle did not ask for this to be done though.


His bike is completely stock except for an fmf fatty header pipe, he runs his bike 50:1(to try lean the bike out), motul 800 only, we live down here in johannesburg south africa the average elevation here is 1753 meters and temperatures range between 20 - 30 degrees Celsius. Any help would be greatly appreciated as my uncle is feeling very bleak about his bike, never riding always fixing and anything better than the current jetting spec would be a whole lot better as we dont have the money to buy a PWK carb, you guys seem to be the ones to come to, as the mechanics over here are mostly dodgy :D thanx alot guys



Based on your story, I can see why your uncles bike is running poorly. It is running extremely rich for MULTIPLE reasons.


Here is the list of problems I see immediately.....


Elevation 1753 meters (5259 feet)

Temperature 20-30*C (70-85*F)


^---- high elevation and warmer temperatures cause a richer mixture.


Running 50:1 gas/oil ratio is making the bike run RICH. You are putting more fuel into the engine.  Run 32:1 in your gas, this will help lean the bike out.  50:1 is a richer ratio than 32:1 (remember, Rich means more gas and has nothing to do with how much oil you are adding)  The extra gas is what causes black "oil" out of the exhaust. Its not oil, its actually unburnt fuel and exhaust gases.


A 40 pilot is too large!  You need to go down around a 30 pilot.


If the slide has been cut or modified, throw it away and get a factory original condition slide. There is no need to be guessing on what someone else did to that slide. The bike will run great with an unmodified slide.


The 350 main jet sounds like it could be right based on that elevation but I cannot tell you for sure.


Here is what you need to do:


1) Stop running 50:1 fuel mixture.

2) get a 2004 Honda CR250 factory service manual to see what the factory jetting recommendations are for your elevation and temperature.

3) Install those jets that are recommended (they will still be a little rich but way better than what the bike has now)

4) Throw that modified slide off a mountain and get an original slide.

5) See how the bike runs.


It will run MUCH better and from there, you may only have to do minor adjusting to get it running great.

wow very interesting stuff :) learn somthing new everyday thanx so much, he does have the service manual from honda i think it said somthing like a 390 main and a 30 - 35 pilot but we were worried we werent factoring the fatty header pipe into the equation by running stock jetting and all, ill be sure to show this to him tomorrow at work, his needle is also position 2 from the top, he also needed advice on whether getting a new slide will help or not  we also only have access to 93 and 95 unleaded gas he uses the 95, thank you so much for the help (((: one more thing should he clean out his header and re - pack his silencer once again  :lol: after the new jetting has been installed?

My 05 use to guzzle fuel like that like I'd go out with a full tank and after it warmed up and it ran clean i could only ride for about 30 mins in the open trails until it ran out of gas had gopro on for time. Well I bought a rebuild kit for the carb and its fixed but where the needle seats or whatever (not sure of the name)the hole was twice the size of the new one so it was getting way to much fuel.

Yea I'm thinking that slide is the culprit.....messing up the airflow over the needle jet tube and making it suck raw fuel straight into the motor.........

Who does that sort of stupid shit??? Grinding shapes into a perfectly good slide??

Or the float needle is worn out and flooding the engine or dripping onto the dirt while riding which would explain double the fuel being used compared with the good CR.

Have you tried swapping carbies?

Do you have a pic of that slide?

I have my stock 03 tmx that I might have a stock slide you can have for free I will be in my shop this weekend I can check it out if you would like

I have my stock 03 tmx that I might have a stock slide you can have for free I will be in my shop this weekend I can check it out if you would like

I have my stock 03 tmx that I might have a stock slide you can have for free I will be in my shop this weekend I can check it out if you would like

hi guys,


yea i don't know who would do it, i work with my uncles, they were having like a discussion like the mechanic he sent it to just swapped his carb out for the one that's been modified, you never know down here in south africa way to dodgy probably break it so you will come running back, he spent 2000 rand on replacing all the seals and gaskets in the motor because of the mad amount of unburnt fuel pissing out his pipe after every ride the mechanic was stumped but im also sure its the slide, he has the mikuni TMX10a carb with the TPS so my 03 carb wouldn't work in his bike? if i can get him to stop being lazy and pull his carb off ill get a picture up for you guys :D


and hyphooker thank you so much for the offer but your all the way in washington O: 

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