Posted: 08/20/2011 8:47:05 PM
Hey all. I bought my son his first dirtbike yesterday. I bought a 2003 TTR125 for 950 bucks. It is in mint condition, except when giving it in the higher gears it sputters and doesnt really want to get to full speed to quickly. Also if you are say in 4th gear cruizing and you hammer the throttle it wants to try stalling out unless you return the throttle back to min throttle. I was thinking of rebuilding the carb, but till then was wondering if any of you might have any useful tips on this problem? Also if anyone has a good guide on setting the carb at a starting point for the idle and air screws, and a guide on rebuilding the carb that would maybe help as well. I threw about 6 oz of seafoam cleaner/conditioner in a full tank of gas to see if this might help. I am open to all your suggestions. Thanks in advance.
P.S. I just was looking at the carb and it is a Mikuni UCAL carb. Anyone know much about these?
Reply by Admin on 08/21/2011 10:16:57 AM
If the gas doesn't look and smell old and stale, I would suggest taking the carb apart and cleaning it. Blow out all the passages, jets, etc. Dirt in the carb making the bike run rough is a very common thing. If you search the entire forums for running rough or idling rough, you'll see many topics on it. Being that you're experiencing issues at higher speed, I would guess the main jet is partially plugged.
4Strokes.com Technical: Carburetor Overview
Reply by bluearc74 on 08/22/2011 09:50:46 AM
Thank You. I will definately be taking the carb apart tomorrow.
Reply by Admin on 08/22/2011 12:08:01 PM
Assuming you've already checked to make sure the air filter was clean and not over-oiled or over-treated. Sometimes I will remove the air filter and hold a clean rag or something over the inlet while revving the bike to try and clear out any debris in the carb.
Reply by bluearc74 on 08/22/2011 12:53:42 PM
Yepper. Tried that. Thanks though. I am actually still trying to find good start points on where to set the pilot air screw and idle screw at when this is rebuilt. My honda 124 fourtrax says in the manual to turn the pilot air screw till it seats and then back 1 3/8 of a turn, and 1 turn from seat on the idle screw? It would be nice to know incase the current setting to the pilot air screw is not right.
Reply by Admin on 08/22/2011 1:17:52 PM
1-1/2 to 2 turns out is pretty common for most carbs. If it runs better with fewer or more turns, then jetting is off or obstructed. If you plan on keeping the bike, you might want to get a service manual. Here's an owner's manual download that might help:
I was just looking at the last page of the Mikuni VM manual (it is a VM20SS, right?) and it could also be the Air Jet, which I'm assuming is just one of the holes on the inlet side of the carb.
Reply by bluearc74 on 08/22/2011 4:19:02 PM
Thanks alot once again. I did have a downloaded manual, but couldnt find much on carb settings. I would like to find a Clymers manual like the one I have for my 1986 honda fourtrax. It is a life saver.
Reply by Admin on 08/22/2011 6:20:13 PM
Unfortunately it doesn't look like Clymer has one for the '03 TTR. There are Haynes, factory/OEM, and others on eBay though. Maybe try Amazon as well.
Reply by bluearc74 on 08/23/2011 7:19:54 PM
Well I took apart my carb and it seemed pretty clean and in good shape to me. I will put it on tonight and see if it made a difference.
So I put the carb back on. Adjusted the pilot air screw to about 1 1/2 and the idle scre to about 1/2 turn and it didnt want to idle very good once warmed up. Adjusted the pilot screw to 2 1/2 and the idle screw almost all the way in and it idled awesome and ran great.After cruising it for 5 minutes or so I thought I would check the plug for indication on if it is running lean and sure enough the plug was white.Thought maybe it might of been from before and put a new plug in and took it for a zip again. Checked the plug and still white. My next step was to check the Jet Needle to see what clip position it is in. Apparantly you can move it to one of three spots depending on if you want it to run lean or richer. I could adjust the pilot screw out some more but I don't know how much a guy can or should be adjusting this screw? Anyone with some ideas would be great. Thanks
Reply by Admin on 08/24/2011 09:06:50 AM
Originally you indicated the bike had a problem in the higher range.. the idle screw has no affect on this. It's simply to keep the bike running when the throttle is at rest. The pilot screw is also for the low speed circuit. If you're sure the carb is clean, the main jet is not obstructed, and there are no leaks in the intake boot, then I would bump up one size on the main jet and see what that does.
Reply by bluearc74 on 08/30/2011 12:29:14 PM
Well I took the carb back off and replaced the main jet with a 110 and the pilot jet with 17.5 and moved the throttle needle position to the bottom one and the bike ran great and no whitish grey on the spark plug now. Thanks for all your help on this. My boy is happier than a pig in shit now!! Cheers
Reply by Admin on 08/30/2011 12:47:50 PM
Glad you got 'er all tuned up! Hope your boy has many hours of enjoyment with his new ride!
Reply by swalter3559 on 10/10/2012 09:16:47 AM
Bluearc74, after changing the jets which I have also did, about how many turns out did you go with the pilot jet?
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