I need some help with my 86 trx250R. I am new to the world of race quads. I bought this thing used and I have just put a new topend in it. It was marked 66mm on the jug and the manufacturer said it looked like the first rebuild. Everything was installed correctly but I am fouling plugs and running rich. (too much fuel due to plug being black and wet). I have a keihin carb that says PJ but no "mm" indication. I checked the jets and the main is a 152 and the slow is a 48. FLoat level by way of measuring in the manual looks to be about right..just over 1/2". Running valvoline 2 cycle oil and supreme pump gas at 32/1 ratio. I have a FMF pipe and silencer. I have lots of black spray which indicates rich mix to me. THe engine bogs and spits and spudders until I'm wide open then it seems to line out but still isn't quite right. It just seems like I am getting way too much fuel. My airscrew is 1 7/8 turns out. My throttle adj **** is only about 1 turn out. I have noticed that these two ajustments don't do much of anything..especially the throttle ****. I have been throught the carb and all is clean and flowing freely. I turned my reeds over and they seat with no gap and look to have no signs of wear and tear. I have good spark as well. Please help me get this thing running right or stear me in the right direction. I'm about 3400 feet in elevation and running the BR8 plugs. Thanks Guys!

 

I don't know much about 2 strokers but let me just say that your post is one of the best I've ever seen with all the information someone who knows 2 strokes needs to help you. So many times we see posts that are void of information. "Guys, I have this motor that won't run. Can anyone help me?" Well, ok, I'm exaggerating, but you know the kind. Anyway, on behalf of whoever helps you, I say thanks for lots of info.

 

Jets are cheap, I would buy some ranging from 142 to 150, and jet down until it cleans up, then you may have to play with the needle settings to get the mid and top end right. Sounds like that 152 is just dumping way too much fuel in there.
I just did a Google search, your carb settings of 152 main and 48 slow jet are stock for sea level, you should go down one size for every 1700-2000 feet of elevation, so you need to go down 1, maybe 2 sizes to start, maybe even drop one size on the slow(pilot) jet if i bogs on the bottom end. Is all your riding at 3400 feet? you may want to look into a product called Dial-a-Jet, it allows you to run leaner jetting, then set an auxillary fuel jet for richer/leaner conditions ( elevation, temperature,etc) that way you are not rejetting for every change in temp or altitude, which affects 2 strokes a lot more than 4 strokes. For a cheap start, get a couple jets, a 148 and a 150, and see if that cleans up your running. then you can tweak the air screw or change the slow jet to clean up the bottom end. Do a Google search for " jetting for altitude", I found a lot of good articles and tips just doing that. Good luck, let us know how it turns out.

 

Thanks for the reply's and good information men. I was leaning toward going to a smaller jet but wanted to hear your thoughts. I just don't want to lean the engine out too much but one or two sizes smaller shouldn't hurt I would think. Yes I am running at 3400' 90% of the time. Anyways, I'll get some jets asap and try them out. Thanks again for the advice.

 

You said it bogs until full throttle, that would mean you need to move the clip on the needle up a notch at a time until the bog disappears.
The main jet provides fuel flow at WOT and the needle affect mid range throttle. Buy moving the clip up, you are placing the needle further down into the main jet, thus reducing the amount of fuel delivery mid-throttle. It can be a bit time consuming because you have to move the clip, run the bike to check throttle response and then possiblt move the clip again.
Only go on notch at a time, it doesn't take long to burn a piston buy running too lean, even at mid range throttle.