01 yz125 jetting help

[Meshmez]

ok.. so i know that there is no "setup sheet" for jetting, and it depends on conditions and this and that.. but can someone please give me a good starting point?

i have the stock expansion chamber, and a FMF Q silencer on it.

i normally run my premix 40:1 or 36:1

jetting is currently stock

LOTS of junk coming from the silencer.

what im having problems with is low end (yes i know its a 125 2stroke.. ) going up hills the bike is VERY prone to stalling and bogging. thinking of getting a flywheel weight which i think is supposed to help..

anyway does anyone have a good idea of what i might need to do with the jetting?

im thinking it needs to be run leaner right?

[yzboy18]

try 32:1, thats what i ran my stock 01 at and it ran pretty good.

[SpDyKen]

Ok, do you want to work on the jetting, or make other changes to make it more trail worthy?

Read up on rejetting; Lots of threads out there, search for them. You'll start with leaning the pilot jet (only.) This also gets you the low end your looking for. Yes modern 125's can have good low end.

I've been where you are with an '03. Yes, get the flywheel weight. Absolutely NO downside. I did it first, but I wish I'd started rejetting sooner; major gains in low end result! Get the most heavy weight Steahly sells. Add a 2 or 3 tooth larger sprocket, unless you ride a lot of fast, wide open riding (high top speed.) Then only 1-2 t.

Your engine will last longer mixing at 32:1, no loss in power.

[Dudly C]

You are definitely rich on your pilot circuit. Try adjusting your air screw out first, if you end up at more than two turns out from slightly seated then reduce you pilot jet one size and retest. After you get the pilot circuit right and you have your bottom end back (no bog) work on the main circuit. You may also have to adjust your needle clip position if you are still rich in the midrange.

[dtkiko]

currently my '03 125 is 1 step leaned out on both pilot and main jets. i'm running at premix ratio of 32 : 1. prevailing temperature ranged 90 to 100 degF. leaning out would definitely improve your low-end power. other mods at the present are VF 3 and FMF rev and shorty. shorty silencer will also improve your low-end; with a reduce pipe length comes also a reduced back pressure from the pipe, thus providing you instant throttle response. A/F screw 2 turns counterclockwise (always stock setting). rear sprocket up 2 teeth, giving me 13/50 combi.

on my next home leave, will install the JD jet kit (thank God, it finally arrived home), and the 8-oz steahly flyweight. IMO, the biggest flyweight that would fit for a 125 could only be 9-oz; better check it out before ordering one.

like you i'm also into trails...come no mods for a 125 in trails, it all boils down on how good you are in using that clutch

[Meshmez]

thanks for all the help/suggestions guys!

i have to run the Q pipe because of the sound and spark arrestor laws around here. i have a FMF SST, and a couple small silencers, but i dont really use them for the above reasons. so i figured i should get my jetting set for the setup i use most.

i forgot about mentioning gearing. i have changed it some but i forget right now what i actually went to. i know i went for more torque/less top end so its probably right about where you guys are saying to have it.

what weight flyweight should i go with? dtkiko talked about it a little but what is everyone elses opinion? is there a disadvantage to going too heavy? or is it just going to make it rev slightly slower and not really matter?

hopefully this week i will have time to start messing with it. i have been meaning to but i have been so busy with work and school!

i think i also have a problem with my float/valve cuz sometimes gas will just leak out the bowl overflow vent. im sure that isnt helping with my rich condition either... haha

thanks again guys. you have reinforced what i was thinking i needed to do, and given me a direction to go in.

[Dudly C]

Whoops. The first thing I should have said to do was check the float valve. Get yourself a new valve and seat install those and set your float height where the float lever is level with the bowl gasket surface. After you get this done start with your jetting procedure. You may not be as far off as you thought once your float level is correct. Good Luck.