99 CR250r Jetting at 100+ Fahrenheit


I'm riding at sea level in temps around 40c (104 Fahrenheit) and trying to get the bike set up nicely after rebuild. Pre-rebuild the temperature here was lower and I was running 178 main 42 Pilot and 1370 Needle.I have rebuilt the bike over the past couple of months (including new piston and rings) and on first test the bike was really rich and smoking and spooging like crazy.

I've spent a lot of time on the jetting and now I'm at 158 main, 38 pilot, 1371 needle at top clip.

Still rich up to 1/8 throttle so will try a 35 pilot.

Has anyone got experience of this bike at these temps? Certainly my jetting seems very lean compared to all the charts available but performance doesn't lie I guess...


Are you losing tranny fluid at all?

I don't have any experience with a 99 CR 250 but do have the 99' carb on our 01'. It seems odd that you've had to go so much leaner, how did it run with the richer jetting prior to your rebuild?

Assuming that it is mechanically sound at this time, you might consider going leaner with the slide. The 99' CR 250 carb came stock with a #5.5 slide which seems a bit rich when compared to other 250's. In 00' Honda used a leaner #7 slide in the CR 250. The slide cut-away has an effect from @ 0 to +1/4 throttle, which sounds like the area your looking at. With the leaner slide you may actually want to go back richer with some of your other settings. When it comes to slides you have a couple choices.. 1) purchase a new slide with a leaner cut-away, they are available in many cuts but are a little spendy, starting @ $55 plus shipping around here. Or 2) have your current slide modified (cut) to a leaner version for a lot less. You need to realize though that once it's cut you can't go back. FWIW I tried the 99' carb with the stock #5.5 slide but have changed it in favor of the leaner #7.


The bike ran fine before the rebuild. It's running ok now but I'm just surprised that I needed to change the jetting so much to achieve it and that the jetting I have ended up with is so much leaner than any of the charts.

I will monitor the transmission oil - I have my suspicions with this. I replaced the entire right case during the rebuild with a new unit direct from honda. This came assembled complete with bearings and seals but I guess it was prepared 12 years ago so the seals are probably past their shelf-life. I will replace the right crank seal and see if it makes any difference.

Do you know the part number for the #7 slide?


Edited by mtrehy

couple pointers on a bad seal. alot of white smoke and spooge and the smell is different than normal premix. pretty easy to tell.

ive ran my bikes in 100+ weather. you will need to lean it out alittle but thats to be expected. never had to mess with the slide though. using 6.0 in a pwk.

I have the same year/carb as you... I've had no problems jetted at 180/42, needle on second clip riding around sea level to 1000ft. I may smoke a little at start up, but goes away after it warms up. I mix 40:1, your mix may have something to do with your smoke/spooge problem.

I've ran in high temps, but not quite as high as 104. I'm assuming you no longer live in the UK at 104 degrees??

I mix 40:1, your mix may have something to do with your smoke/spooge problem.

Not if the jetting is dialed in.

Was burning gearbox oil. It wasn't the crank seal but the gasket between the 2 cases - the new gasket I got from honda was significantly wider in the portion that seperates the gearbox from the crankcase. Anyway, all good now.

Played with the jetting from scratch and ended up at 190, 42, 1370 (stock setting except I'm on top needle clip). On tarmac it feels a bit rich at 1/8 throttle but on the track it is not noticable at all. I have a #7 slide coming so will try that but I'm not worried either way. Also will try a 195 main as it felt a tiny bit lean in top gear wot (again not an area I see on the track anyway).

A 190 main jet? Like you said, performance doesn't lie, but that sure sounds fat. I ran my '97 CR250R in 100 heat at 300 feet above sea level with a 168 for pump gas, 175 with oxygenated fuel.

The performance and the plug reading are good. If anything I'll try a bit richer (195) as occasionally will run it in the desert in very soft sand. At the track this morning it was perfect all round though.

Everything is fresh - new top end, silencer repacked etc. etc.

just rode it today in 100F, 172 main, 45 pilot, it was a little rich but ran well

I asked a related question in the Jetting forum, but haven't gotten any feedback yet. Same bike but with a PC pipe. Temps also hot. Here's the link to PC's recommendation: http://www.procircuit.com/jettingspecs.htm#1999 What do you guys think of what they say to use compared to stock?

I got round to sticking the #7 slide in and it's definitely cleaned up the first 1/8 throttle. Thanks for that tip.

I'd say the jetting is near perfect now at 190, 42, stock needle and a #7 slide. It runs great throughout.

I'm fairly new to the mx track so still lugging it a bit so I get the tiniest evidence of spooge - I guess this may disapear if/when I ride it harder.

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