Jump to content
Sign in to follow this  

YZ 125 mods

Recommended Posts

I am looking to buy a 2005 or new YZ125. What are the best mods to look for on a used bike.

I saw a one with a big bore kit 144 do these run good? reliable? Do they mod the carb to compensate?

What pipes are good on these and where do they increase the power?

What gear ratio are you guys running for MX?

Who does a good port job on these? How big improvement? Big bore or port job?

Share this post


Link to post
Share on other sites

Add a pro circuit pipe w/ a 304 silencer, vforce 3 reed cage, vp race fuel and spend some time jetting it. You'll be more than happy with these mods without breaking your wallet

Share this post


Link to post
Share on other sites

The pro circuit pipe moves the power into the mid and upper rpm's. Vfoce reeds are supposed to add power all over. Compared to other 125's the Yamaha has great bottom end power in stock form.

My 05 125 had both the pro circuit pipe and vforce reeds when I bought it and I ended up switching both back to stock. The pipe didn't suite my riding style and the reeds made jetting a little more difficult and then eventually chipped. I did not notice any difference in power that I could feel with stock reeds vs the vforce. That's just my experience.

My favorite gear ratio for mx is 13/50.

*Peak horsepower means nothing if it's not in the usable rpm range.

Share this post


Link to post
Share on other sites

IMHO dont waste your money on a pipe. I tested the PC the FMF and the stocker on the dyno and for what I consider the best overall power, the stocker was best.

VF3 are decent improvement - I do recommend them.

I posted a link to a write up on the 144 kit from athena - as delivered it's about 7 percent better peak - and barely better off bottom than a stock bike.

With extensive mod work it can be 25 percent more powerful than stock peak - and 35+ percent better early in the powerband with no hips or dips in power.

Same applies to an OEM cylinder punched out to a 144.

Best bang for your buck is without a doubt and OEM cylinder modified to a 144. For around 650 bucks if you are willing to trim your own cases - SERIOUS power improvements can be had using stock pipe and reeds.

Buy a pipe, silencer, reeds and a top end - and your almost at the same number..and you will not gain much.

Share this post


Link to post
Share on other sites

I'm pretty happy with my stock bore, VF3, DEP shorty, stock pipe and Phathead Racing head with f14 insert to run 105+ octane.

Share this post


Link to post
Share on other sites

Is there any dyno charts on stock 125 vs 144 vs 250f just curious to see the differences

Share this post


Link to post
Share on other sites

best thing i did to mine is go down a tooth on the counter sprocket. i makes the gear ratios closer (bike gets more bottom end). some people think it requires too much shifting, but its not bad when you get used to it. sure you're gonna lose top speed, but for me it's ok cuz i tap out 5th gear (mine is an 04) only at the fastest local track. for comparison, before i used to barely get into 5th at all. now it's way easier to carry speed out of corners and do jumps out of corners. if you're gonna do this i also recommend going up on the main jet to avoid top end meltdown because you're gonna hold it WFO more than usual

Share this post


Link to post
Share on other sites

I posted this earlier - disregard the "numbers" the scale is off.

DynoComparisons.jpg

Green or most powerfuel - 10rmz450

Red/second most powerful, full mod 10crf250r (cams, porting, piston, pipe, 4.4 race fuel, mapping)

250f with pipe and 4.4 race fuel and mapping is the similar curve as red just less power..

Lowest power on chart is full mod 125, stock on this reads around 23.5

And blue is full mod 144.

Use a scaling factor if you wish to get numbers that "are similar" to what you would read in a magazine - the 10-11 RMZ450 is about a 51-52 horse bike

51/34.5 = 1.47. Multiply the rest for "results" that seem "normal"

Share this post


Link to post
Share on other sites

I agree with most of the info posted above as it is very accurate as long as you know the history of the bike but I always hesitate buying a used bike thats been heavily modded, Im not talking a pipe and aftermarket reeds. Im talking big bores, port and head work, case/ crank work, etc. etc. which can all be sketchy unless done by someone who knows what they are doing and trust me when I tell ya Ive seen some real hillbilly hack jobs that were considered mods when they really probably hindered the performance of the bike. The first question you should ask yourself is how much experience do you have on the dirt and what type of riding will you be doing? Buying a heavily modded bike usually makes me think one of two things, it was raced HARD and maintained but none the less it was raced hard... and if it wasnt it was some teenagers with mommy and daddies wallet at his disposal and it was abused and put away wet. If Im going to buy a used bike I look for bikes that are as close to stock as possible, and in very good mechanical shap. Not necessarily how many mods it has done, considering Ill undo half the mods to fit my style anyways.

If I were you Id think about buying a bike in good mechanical shape and save my money for fuel/ premix, oil, filters, fresh tires, top ends, and having the suspension set up for your ability and style. Thats where the average weekend rider will gain the best performance thier bike can provide and most importantly what they can afford. The more mods done to the bike the more maint. it requires and the more money you'll be dumping into it, so unless you have alot of cash at your disposal for a bike that will eat up the top ends and tires and nt to mention if you have to run race fuel b/c of mods your dumping money into a bike that you can not afford to maintain properly.

I dont know too many people that can actually ride a 125 to its full potential, and if your ability is beyond that of what a 125 can provide maybe your best option would be to go to a 250 2 stroke or 4 stroke considering that most local mx tracks dont allow 144's to race the 125 class so theres really no point. If you are one of those people that can ride a 125 to its full potential then bump to a 144 and go out and smoke the 125 thumpers, I have on friend who races a 250f but from time to time likes to get the 144 out and go have some fun beating up on the guys riding bigger more powerful bikes.

Sorry for the long post but thats my .02

Share this post


Link to post
Share on other sites

I agree with you on the mods to some extent just trying to get some input what works good a yz125. I don't know think I want a used bike that has a big bore kit or eveything trick on it. Like you said the OEM company spends million making their bikes work for the average person. However there are mods that can make a stock bike better. I am not sure that 125 will be enough power so I was trying to get an idea what the good riders are doing for more power. I currently ride a Crf450 so the power difference will be huge but should make me better. Additionally, If I like the bike I may spend the many on a big bore kit or porting. I am looking for a bike that is well maintained first and second has desired mods. If the mods are not there I buy the later.

Share this post


Link to post
Share on other sites
I posted this earlier - disregard the "numbers" the scale is off.

Green or most powerfuel - 10rmz450

Red/second most powerful, full mod 10crf250r (cams, porting, piston, pipe, 4.4 race fuel, mapping)

250f with pipe and 4.4 race fuel and mapping is the similar curve as red just less power..

Lowest power on chart is full mod 125, stock on this reads around 23.5

And blue is full mod 144.

Use a scaling factor if you wish to get numbers that "are similar" to what you would read in a magazine - the 10-11 RMZ450 is about a 51-52 horse bike

51/34.5 = 1.47. Multiply the rest for "results" that seem "normal"

How kind of bottom end do you think you can get out of a 125 if you ported for low end?

Share this post


Link to post
Share on other sites
How kind of bottom end do you think you can get out of a 125 if you ported for low end?

For a 125 - the yamaha's are about as good as it gets off bottom. More compression helps a little - but frankly the powervalve does exactly what low end porting would do. Shy of a major pipe change - you can not achieve serious low end gains from porting.

So - if you want a lot more bottom - run a longer overall length pipe...

Share this post


Link to post
Share on other sites
For a 125 - the yamaha's are about as good as it gets off bottom. More compression helps a little - but frankly the powervalve does exactly what low end porting would do. Shy of a major pipe change - you can not achieve serious low end gains from porting.

So - if you want a lot more bottom - run a longer overall length pipe...

Wow, I'm silly. You were talking 144 and I was thinking 144 but wrote 125. :smirk:

So to ask again correctly, what kind of improvement can you see in a 144/pipe setup ported for low end. I'm looking at either a 144 or a 250 2t upgrade for woods riding. Would really like to keep the weight of the 125 but really need some more low end power. Have friends with variety of 2t bikes but have no reference for the 144 setups.

Share this post


Link to post
Share on other sites

My brother has a Gorr 144 ported for low end. The low end is ridiculous for a 125. It is flat on top though. For tight woods it is awesome. I cant say how different a mid range ported bike is.

Share this post


Link to post
Share on other sites

Do you guys have any issues with the reliability of the Eric Gorr 144 engine? I heard the Athena ones are prone to the head gaskets leaking.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×