Carb jetting mystery...

[axellmusic19]

I have rebuilt the engine with a mild bore and port and a high comp. head and stock exhaust. Over the summer I ran a #210 main and it ran great and had the air screw out about 1-1.5 turns. I then changed the jetting progressively once the weather changed, and had gotten nothing but lack of power. I went to a #220 then #230 and skipped #240 and right to a #250 size but still I manage to get great heaps of torque and power until half throttle and it sputters after that. I tried moving the powervalve adjustment up in rpms by 90 degrees, however that just lead to a barely noticeable movement in torque band. What is happening? It starts great with barely any help on cold mornings with the #250 as expected but it doesn't seem to be helping my power from 50%-100%. Any ideas? Thanks.

[LTandRaptorider]

I can't remember the jet sizes in the stock carb... But when tuning remember this-

Pilot jet affects up to 1/4 throttle
Needle affects up to 3/4 throttle. Two things that affect the needle are, clip position and taper.
Main jet affects 3/4- WOT.

The air screw should be 1.5 to 2.5 turns out. More than that, you need a smaller pilot. Less turns, bigger pilot. Sounds like you are ok on the pilot.

If the engine is sputtering and no power, sounds like you have it too rich. Start with moving the needle clip up a notch to lower the needle, see if there is any improvement. Get the mid throttle right, then check wide open. On a warm engine, hit WOT then kill the engine. Check plug color, looking for light brown. Black=too rich on main, white=too lean on main, go bigger immediately! I would say it's a little rich on both needle and main.

Hope this helps! Just getting back into the 2-stroke after finally finishing my LT250R rebuild. Not sure now which I like better, my 250 or the LT-R450! :D

[axellmusic19]

Well thank you very much for your advice! It helped a lot except I experienced a major problem. I moved the needle up to the 2nd notch and the main down to 230 and the air screw to 2 turns out. It ran great except for the half throttle on up and still sputtered. I guess I was mistaking the sound for being too rich when really it was too lean, or so I think so far. I will move it to the fourth notch and leave the main and everything else. The thing that became a major problem was that when I went to idle after riding, it shot the revs up to the max and wouldn't shut off even with 0 throttle and the ignition switch off. I had to drop the clutch to kill it. I almost blew up :ermm: I took the switch apart and cleaned it and now it works, replaced all cables, and will be checking to see if the clip fell off the needle, causing it to drop in to the main and leaning it out so much as to rev so high. :confused: Any advice or similar experiences to the sputtering or random redline for no reason?

[axellmusic19]

PS: I checked plug color right after and it was a crispy light brown :huh

[LTandRaptorider]

On the high rev... You didn't turn the gas petcock off did you and forget? Not being smart, I"m in the habit of always turning it off at shut down, then sometimes forget to turn it back on when firing it back up. I've made the mad dash several times back to the quad when it revs to the moon! So that, or almost out of gas. Otherwise check the carb, lean condition def causing that. Since you just had the carb apart, most likely something there. Maybe the needle clip didn't seat all the way and fell off.

[axellmusic19]

The fuel was left on and the trans was left in neutral. My guess thus far is the clip dropped the needle and since the engine was warm enough, it acted like a diesel and used the heat off the head to burn the tiny particles of fuel allowed in and therefore not needing a spark from the ignition, which I switched off. Also, the head is high compression which increases my confidence in this being the case (hopefully it's correct and easy to fix). I'll take it apart soon and reveal the madness. :rolleyes:

[axellmusic19]

WTF! The needle was in the correct spot and everything! The idle screw was out 4 turns however. So after readjusting everything you think it'd run right? Hell no! It barely could idle and the second I gave it gas to help it stay running, It revved to the moon! What is going on with this @#$^%&* thing? The spark plug wire will be replaced soon and the ignition box seems okay but I have no way of checking. Any help would be awesome cause I am sick of it destroying itself.

[fearlessfred]

sounds like maybe the crank main seals are leaking

[89trx250r]

my trx250r did once and like you said it wouldnt shut off even after i pulled the switch it turned out to be a ****ty throttle cable and old gas in the carb from when i bought it. basically cleaned the carb real good by hand cleaned the jets checked the float etc make sure your slide isnt getting stuck if youve ever gotten carb cleaner in there it strips the chrome from the slide and hangs up sometimes... and make sure youve got good ground point on the frame and this last part is hugly important make sure you dont have any holes or air leaks in your air boot from the filter to the carb and the carb to the reeds etc it sounds like if youve got a cripsy brown its lean it should be golden youll know when its perfect youll be like nice.

[axellmusic19]

Thank you very much for the ideas! I checked the boot first and sure enough there was a hairline tear around the clamp. I am going to replace it and see if the problem persists. I never thought about the slide getting stuck so that will be something I'll try to clean and fix in the mean time.

[axellmusic19]

So just for why nots, I cut the boot in front of the crack then refastened the carb in the remaining boot, closer to the reeds. This disconnected it from the airbox, which I removed and now and running just a giant K&N filter and the carb with the shortened boot. The throttle response, power, and torque all have jumped significantly even with the carb set on the rich side. Hopefully I will have finished pictures soon and have further results. I'm even working on a duct system that would route a pipe from the frame intake to the carb in case I would be riding near water.