What mods YZ125?

36 to 38 would be imperceptible...technically make it richer. If you need to change jetting, one step at a time with small changes no matter where that need may be.

Jetting on that bike stock is way, way off.  We went with a JD jetting kit and it cleaned it up nicely.  We also had some porting done with Tom Morgan and the bike performs really well.

I think pipe and head (in that order) are the must changes. After you've done those the rest is just fine tuning. An aftermarket ECU can make it rev a little further. Porting will add some, but pipe and head will take you 90% of the way. A silencer is often more sound than power on the dyno. Jetting you will have to sort for your setup/conditions/riding either way you do it. 

Race fuel... I run VPR in mine. I'd start there rather than a head or ECU on pump gas

Has anybody tried the apex head?

I was at Millienium Technologies Tuesday. There constructing a big addition on the building.  I walked all ovr the building. They hv a lot of new engines they designed.  I talked with the lead guy. Super knowledgeable. He designed his own cr125 pv shaft also.he explained the Honda exhaust port designs. Honda had a design that wasn’t as wide but didn’t make as much power. The wide with a bridge makes more power but the bridge is weak & cracks.  Quite the sharp team there. I went into the office also & introduced myself. Hi I,m Profeshenal 125. I asked what does it take to get on the wall here ? I saw one pic of Ty Davis on a yz250 doing off rd. You can schedule a tour of the facility. I asked going for max power what bike make more s cr125 or s yz. I was shocked to here the cr does . In low threw the midrange. But the yz is very torquey. I would throw a shorty on your bike . Pic is just a part of the building.

The Yamaha head has a very big squish and a big radius to the dome. I like neither of them for the 125. I tried to copy the APEX XC dome that is very popular on the 250 for the 125 with a small squish and approx oem compression and that completely lacked "hit" :-). Sort of guessed it would give up to much on the 125 but had to try it....
I'm sure the Apex heads designed for the YZ works fine like most aftermarket heads.

You probably were talking with Eric gorr 

More like Gorr was talking with Profeshional.

You can solve jetting issues/loss of power at elevation with one mod and you can greatly reduce clutch work too. Swap in a STIC Super X carb for small bore magic. It looks like a Keihin PWK, but it creates a strong unstoppable low end and at slow speed it will loft the wheel for logs with just a blip. Mid and extended top is greatly increased. Imagine 3rd gear at 1800 rpm on an incline and you can grab a handful and it wont stall. 

I think it was a thread by Tractor a few years ago on the original STIC insert metering block. He stated that his kids and their friends all favored a block mod only 125SX over a second 125SX with a high dollar race engine package. 

There are no other aftermarket carbs, ignition, exhaust or head mods that create additional power at the level of the STIC products. Stamp it.

Between the SA and elevation I would stick with bumping the CR and jetting/pipe.

I have an apex head with a mx and sx dome sitting on the shelf. Sold the bike and never put it on. Set for 91 octane, 22+ yz 125 only.

I’m assuming you have this for sale? I don’t believe they are 22-current specific tho? Shoot me an email if available. 
r.chaulk94@gmail.com

I have the Apex head running the Supercross Dome with V Force reeds, and the RV Spec Pipe and Silencer from PC.  Running the same jetting as Keefer used for his sons bike.  Can't remember exact numbers but they are on his page.  Love it.  But i'm also at sea level.  Good Luck

I have a 20 yz 125 with v force , lectron and FMF shorty I recently picked up . It runs nice and crisp but was thinking it needs more over rev . It also could be I forgot how slow these are but damn fun bikes . Also should the spacer be in with the v force ? 

If you want to have your bike rip more like a late model carb KTM, but also end up with a lot more bottom thru mid and ultra responsive throttle. The STIC Super X2 carb by STICSuperTorque.com will work a treat. 

You're basically never out of the powerband. A buddies 125X w/7oz FWW, 3rd gear @ 1700 rpm and ham fisting it...no bog.  To manage the mayhem under a 1/4 throttle on my 200SX, I went with the slowest turn G2 throttle cam...for MX.  

I've been running the Apex head with the XC/SX dome set up for 93 pump gas for a little under a year now and it made a very noticeable power change. It gave the bike a lot more luggable power in the lower RPMs but I didn't notice any change in the top end power.

A 125X buddy prefers the MX insert over the XC/SX insert for trails, as it has the free rev'g broad powerband.

Finally got around to testing the apex head. Very noticeable upgrade as you said. I’m very happy with the SX head bike pulls very hard. Probably the best bang for your buck.