Yz 125 exhaust pipe

My yz 125 is currently getting the shocks rechromed but i was also thinking about the exhaust. I was riding my friends 125 the other day and he has a stock pipe. I noticed i could wheelie out of powerband which is great for logs tight corners etc. However mine has no where near the bottom end which makes me think its the pipe or jetting. My bike is jetted a little rich but my dad wont let me make it any leaner as he doesnt want me blowing it up. My current pipe is pro circuit platnum with shorty.

You could lean the pilot circuit a bit to make it run a little crisper without risk of damage

but keep your usual main jet as a 'rich' safety margin for WOT moments.


If your bike responds well to having the air screw turned open 1/2-1 turns more (more air)

that indicates that a one size leaner pilot jet is required.


As you've experienced, the stock YZ125 pipe (2005-15) is a very good 'all-around' performer,

and better suited to low speed/low rpms than a Pro-Circuit.

Edited by mlatour

Ok thanks. I twisted the mixture screw today and it didnt seam to do much should i lean the pilot off anyway?

The air screw fine-tunes the pilot jet circuit, allowing more or less air to mix with the fuel

but it has a limited range of adjustment, say between 1 turn out to 3 turns out from fully bottomed.


If outside of the screw's optimal adjustment range, a leaner/richer pilot jet is required.


Notice in my above post I wrote: 1/2-1 turns more,

meaning opening it more than the usual setting, says from the standard 2-1/4 turns to about 3 turns out.


If by 3 turns out to the engine does seems to run better but you still require some air screw tuning, it's sign that your pilot jet is too rich.

Okay...I see you like to make things complicated for everyone to follow as you have 2 threads going on the same subject:



You mention here that: 'your dad doesn't want you to jet any leaner' but yet you premix at 50:1 in a 125 !!!

(discussed in the other thread)


50:1 is actualy richer than 32:1 , by richening the air/fuel ratio,  less oil = more fuel,

no wonder your pilot jet circuit is too rich and your bike does poorly at lower rpms.

It's richer yet you have much less of a safety margin for lubrification at 50:1 when your riding allows for wide-open throttle.


Do yourself a favor and start over from scratch :

-use the stock jetting and carburator settings as per the service manual

-premix at the recommended 32:1

-then fine tune the jetting for your riding conditions*


*which would most likely be a leaner pilot jet for slow trails,

and a richer main jet for your aftermarket Pro-Circuit pipe.

Edited by mlatour

Ok ill start over.

Ill put the stock jetting in and run a tank of 32:1. If that does run/ work better ill put in a new spark plug. Ill check the spark plug as i may need to change it for the exhaust. Im sorry to confuse everyone when im opened up to different topics at the time.


If this is an 05 or newer yz125 the pro circuit pipe will never feel as good as stock with stock silencer.

Other pipes may ad power on dyno but they never feel as lively( can't think of a better word) as the stock system.I have tried fmf fatty/ fmf factory/ and ptocircuit not a sst and stock always felt better everywhere.

Edited by qwert321

Ok thanks i think ill look into a stock pipe

I have a near mint stock 06 pipe not using it has been sitting not getting used. It doesn't work well with my 144 setup.

Stock Silencer important also

Back in 05 or 06 there was a lot of talk about it.

True about the silencer, if you are going to use a stock pipe, use the stock silencer as well.


I've experimented with a shorty on a stock pipe, even when re-jetted slightly to best tune that combo,

I noticed a very small gain in throttle response but at the price of less overrev ability.

Also a decrease in torque, an already rare comodity on a 125 (required even more downshifting).


One ride and I re-installed my stock length silencer back on.

Edited by mlatour

Honestly these bike work really good with everything stock.

On top of all the aftermarket pipes sitting in a box I also have 2 vforce and 1 boyesen rad valve sitting around.

I know ( have read here on thumpertalk) the vforce is suppose to add a hp and the fmf fatty is suppose to add another hp( Harris performance post) but it just does not add upto a better powerband.

You can buy the parts and try but I believe you will have aftermarket parts sitting in a box just like I do.

Yeah i understand thanks for the offer smokinjoe but i have mate who wants to do a trade my pipe is slightly dinted and im gonna give him a little cash i loved riding my friends as he has stock pipe and its so torquey compared to mine!

The last thing I changed on my 06 when I had it 144'd was the pipe silencer. I had to the stock pipe was hurting the top end with the setup. I swapped it out for the factory fatty and it was much better for the 144.

I have a Eric gorr mid top porting high compression race gas only head. The increase in compression allows it to make better use of the larger cones. I got the top end back it had been flat with the stock pipe Still pulled the same off the bottom through the middle.

Can't say if it do anything for a stock motor had all ready 144'd the bike. It was the first mod I did to it and was the only intended motor mod, the 144 kit sort of made me have to get a pipe in the end.

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