Having an issue and thought I'd get ask here just in case someone thinks of something I missed.  Right now I'm trying to figure out why my 05 yz125 doesn't have anywhere close to the output that my friends 09 yz125 has.  The top end and upper midrange is pretty much identical but his has much more bottom end and lower midrange output that mine has.  I need to figure out why this is.

The comparison:

- His bike has 2 hours on it and is brand new basically.  Mine has quite a few but has 1.4 hours on a brand new Wiseco with oem gaskets.
- We both use the stock pipe and both have a PC spark arrestor silencer.
- We both used the same jetting - 37.5p / Clip 4 / 410m - 70F outside during ride.
- Brand new air filters in both bikes.

I never realized how much mine was down on output until riding his.  It pulls of the bottom so much better and has much better throttle response than mine does that carries output all the way to the midrange.  They feel similar after they hit mid power band.  I tend to have to mess with my pilot jet and air screw quite a bit to get good response off the bottom.  I can generally achieve it but it's very finicky.  He didn't even touch his at all during the ride and it ran great from start to finish even though the temp changed +5F.

Thinks I've confirmed over the last couple of days:

- New top end
- Timing correct
- PV working correctly
- Carb clean and jets clean
- 1.4h on silencer packing
- Put 690cc of ATF and got 660cc back out after 1.4h ride.  Not sure how much normally sticks in the engine though.
- Sprayed around base gasket, head gasket and intake boot without change in idle

Things that have an impact:

Checked the reeds and one side had a gap on 2 out of the 3 petals.  Bought new ones and they do the same thing.  Once the reed stopper is tightened down it lifts the petals up a little off the mating surface.  Probably need to try a used reed block and see if it fixes the problem.  Tried both stock and both aftermarket reeds as well as switched the stoppers around and it pulls them up every time.  A new one is like $150 or so.

I don't think the reeds are causing the problem I'm having.  They may be contributing to it but definitely not causing the output lose that I'm feeling over the 09 yz125.

Still need to check float level again just to verify.

My jetting also doesn't very much from the norm to make me think there is a "bad" air leak somewhere.

Edited by kan3, 19 June 2012 - 10:10 AM.

Biggest differences IMO would make compresion ratio, timing and porting.
That apart what gearing you guys use, identical or he's using 12 and you 13 up front?

Sorry, forgot to put that in there

We both use 13/48 gearing

Have you done a pressure test? A couple of small leaks around seals or reeds may sap some throttle response. Maybe worth checking before you chaes your tail too much.

#1 ...yours is an 05, vs an 09 !
..yamaha (multi big $$$ company)
just spent 4 years on the dyno, trying to
squeeze as much hp out of a 125cc as they can,
with different pipe designs, carbs, intakes, reeds
porting, head volume, etc,.
...yours can easily be modified to make the same, or more
...but need to find the right balance of mods / parts.
Anybody who's ever ridden a '95 vs a '96 yz125
knows yamaha found some magic somewhere,
and kept it goin for a lot of years.

#2 ...check to see if there are any leaks anywhere
in the exhaust / pipe to cyl / pipe to silencer /
...the magic of a 2 stroke is the pipe, and any leaks
will just cause mega hp loss ..you can spend mega hrs, porting
matching, jetting, etc, etc, ...but unless it has the right pipe,
...almost a waste o time.
Permatex ultra copper silicone
works great on exhaust leaks. The motor needs to be
sealed from the airbox, to the end of the silencer...any leaks
= hp loss. Also check how much carbon is in the pipe, and let it soak
in carb cleaner to clean it out. Too much carbon in a pipe will change
the powerband / the heat in the pipe, and act as an insulator.
...the pipe has to be bare metal inside, for it to work properly
to gain max hp (for example, we repacked a silencer, gained 2hp on the dyno
...changed a leaking exhaust to cylinder seal, ...gained 1 hp)

#3 jetting is "critical" across the entire rpm range for making max power
...lets say your off a bit on bottom end, and midrange
...the engine is struggling to make peak hp, and even if your main jet
is the right size, insted of making (36hp , as an example), the peak
hp might be 32 hp, and feel like a slug getting there
...carbs are where some races are won, or lost

Edited by yamaha7442, 19 June 2012 - 06:29 PM.

The only significant change in the 125 between 05 and 09 was in 06.
The engine is different. The head is different. Many parts are the same though. If he is a good friend, just swap parts (carb, reeds, head, pipe, whatever) until you find what makes the difference, then replace that. As others have said, first do a compression test, and a crankcase leak test.