Things to know for your Yz 125

Hi guys! I have done many things to my 2006 Yz 125 over the 2 years and I found little things that work and things that dont work. So I am going to tell you what I do and what works. This post is what I did to my 06 Yz 125 but Im sure 05-10s can benefit this too.

Pipe

The first mod you should do to get better power is get a pipe. I have the Pro Circuit works with the R-304 and it works well. I used the stock pipe before but the Pc gives more Mid-Top end power.

Reed Valves

Another mod that many others do is get a reed valve. Many talk about the V-force and Boyesen. I got the V-force first and it is not worth it. I didnt notice any power gained. Also my reeds on that V-force dont close, and started to chip at about 30 hrs. Not satisfied at all so dont get it. Later I got a Boyesen RAD valve and it was worth it! It made better power and its a metal frame instead of plastic. Because it was Metal, the inside of the valve was rough so not to long ago I used a dremel too and took off all the rough castings. Then I used a drill that I wraped the drill bit with scotch brite and used WD-40 as a lube. This makes metal so smooth. It made better airflow and gave it more power. Boyesen wins this.

Oil/jetting

Pre-mix, and jetting is key to have a strong 125! I use 32:1 Maxima SuperM because 32:1 gives it enough oil to run at high Rpms on my bike. I tried 40:1 and it seems ok but more oil = more life so go with that. The first thing 05-10 Yz 125 riders should do to their jetting is get a 420 main. I ride at tracks at 2000-4000 ft and it runs great! The bike is fast and has given me no issues. I tried the 35, and 37.5 pilot with a 410 and it was too lean. Same thing with the 420 with those small pilots. After messing with the jetting alot, the richer I went, the more power she made. Right now In my 2006 Yz 125, I have a 420 main, 40 pilot, Needle #6BFY42-74 at position 4 with the air screw around 2 - 2 1/2 turns out. Always check your plug. With this jetting and riding it with mid-full throttle, plug was golden brown wich is perfect and I am always back with a smile on my face!

Fuel(IMPORTANT)

Many people just run 91 in there bikes. I have too but 91 will give you issues. I have blown about 3 Clyinders in 1 year because of detonation. I dont abuse the bike. I ride it like it should. You know when you have detonation when you hear a ping sound while you put a load on you motor. Also when pull your head off and see chipping on top of the Clyinder, or you look inside your exhaust port and your piston is scored and chipped is a sign of something going wrong like detonation. The first time this happened to me destroyed my clyinder, piston, and crank berrings. The first time it was a slow process because I noticed it when my bottom end made a squeak sound around 25 hours or so. That was a $1000 rebuld. 18 hours later with a new clyinder and bottom end it did that same thing!!! Luckily this time my bottom end was ok. Because It was damamged I wanted to get it ported by Eric Gorr. I dont know about Gorr guys. I hate to say this but he did a bad job on my clyinder. The first time I sent it to him(1 week of waiting), I got it back ported nice, the head cut for 91, but the nikasil had a chip below the intake ports. So I sent it back, and 1 week later I get it back but its not mine!! I had a B with ink stamp on mine and the one I got back had the same porting but has some rough casting in the ports and there was a ingraved C76. Looks like Yamaha did that Ingraved C76 to say its a C clyinder. The total of that was $225 for a exchange. So my $400 clyinder my dad got me was traded... Turns out he sends them to Millennium Technologies. They have this exchange thing so you send them your damaged one and you get one that has been replated that is not yours. That was not cool and he didnt tell me about this untill I figured it out. He kept telling me "No that Clyinder is yours"... (Not at all!!) So I ran it with 91 that day with low-mid porting and the bike didnt make it 2 hours. 1.6 hours of detonation and this top end looked worse than it did the first time I had this issue. So I sent it back agian (1week) and I think he knew he screwed up. I got Eric Gorrs 125 Clyinder and head with the porting free of charge. Slaped a Wiseco Pro-lite and oem gaskets with oem o rings. I went to my local station that sells Sunoco 100 octane and I mixed one Gal with 1 gal of 91. Went to the desert for Thanksgiving after giving it a correct break in. Going full throttle across the desert with hill climbs and riding it like it should gave me no issues. When I got home, I took the pipe off looked inside and the clyinder walls, and piston looked perfect!! So I learned that do not be cheap! It is wise to run a 50/50 blend with 100 octane. I know its $8+ a gal but its better then rebuilding your top end every 2 hours... Sorry this is so long guys!

Other things

With the porting, I noticed that the bike pulled. So Porting does do something.

Just because your bike is stock doesnt mean it will be ok to run 91 pump gas. I had the detonation happening when the bike was all stock with just a pipe and reeds.

With Pistons I tried Oem and Wiseco. Oem and Wiseco are great pistons. They are priced similar, and perform well. I have a wiseco in my 125 now and I am happy with it. As long as your jetting is correct and you maintain your bike, any piston should give you great power. I did notice that the Wiseco made my bike a little louder. It made it sound like a real 125!

So there you have it. Hopefully these mods help you guys in the future. One more thing. With clutches cranks or anything else on your motor, go with oem. Oem clutches work. Aftermarket didnt work for me.

Hope this helps you guys!!

-Robbie

Nice write up!

I agree 100% about the vforce reeds. I went back to stock and noticed no difference in power. I also just started mixing 91 pump with 100 octane. I currently have a 420 main, 37.5 pilot and stock needle on 2 clip from top. My cylinder head has those little chip marks too but I'm not sure if it's from detonation or from the vforce reeds it ate???

Nice write up!

I agree 100% about the vforce reeds. I went back to stock and noticed no difference in power. I also just started mixing 91 pump with 100 octane. I currently have a 420 main, 37.5 pilot and stock needle on 2 clip from top. My cylinder head has those little chip marks too but I'm not sure if it's from detonation or from the vforce reeds it ate???

2nd Clip from top??? If its 2nd from top, you needle is low wich is a lean mixture. You should have a middle position aka Pos 3 or even Position 4 (go up one clip from the bottom). Trust me the richer I went, the more power it made. The leaner I went the less power it made. Try a 40 pilot and pos 3 and 4 on your needle. That works well for me.

2nd Clip from top??? If its 2nd from top, you needle is low wich is a lean mixture. You should have a middle position aka Pos 3 or even Position 4 (go up one clip from the bottom). Trust me the richer I went, the more power it made. The leaner I went the less power it made. Try a 40 pilot and pos 3 and 4 on your needle. That works well for me.

Also thoes chips on you clyinder is probably detonation. You should get that repaired or else it can cause more damage.

http://s218.photobucket.com/albums/cc110/KX85Bandit99/?action=view&current=P4042174.jpg - this is my first clyinder that detonated. Notice thoes chip marks. If your plating is scored, get it repaired and replated.

The "chips" or pitting marks on the dome of your head are probably from where the bike had blown up before and the previous owner just bolted it back on after a rebuild.

IMHO nothing wrong with exchanged cylinders - and while the chipped plating is not great - it is also a risk that is run when only porting an existing cylinder (without replating). Sometimes the plating flakes/chips no matter what you do...typically an indication of poor bonding between plating and cylinder - which I see 1 in 10 OEM cylinders...

So long as the chip is smoothed...it has no influence on power or longevity...

Anyways - I do not agree with polishing the boyseen - it's a waste of time!

But I have found the stock yz right on the lean verge on the needle. Going richer here but tad leaner on pilot from stock works great.

Main jet all depends on fuel - altitude, and temps...hard to say run x jet! I have run down as far as a 380 - and up at 440 just depending on weather and track...and fuel.

Being off one or two jets on race fuel is not to big a deal for reliability....off two jets in rich direction kills peak and over rev - off two jets lean and it will blow up on pump almost right away. On race fuel when too lean it "chirps" letting you know its time to jet richer!

When I dynoed a rad valve vs the V-3 on my 05 YZ 144. The V-3 showed a 1HP gain while the rad valve was only a .5 gain.

Out on the track the V-3 was noticeably smoother than the Rad. My personal opinion is the V-3 is excellent and what i run. Not just because it performed the best on the dyno, but because it felt best to me on the track.

Reed valves obviously don't add a ton of HP, so often what is judged as power is the hit, the V-3 is smoother so that can fool ya.

I ran 91 octane pump gas mixed with Yamalube R at 32:1 here in Indiana in the spring/summer and never had one problem. Bike ran amazingly strong. Other experienced riders thought the bike ran like something much larger than a 125cc motorcycle. I left mine all stock, even had a dented-in stock pipe on it.

I ran 91 octane pump gas mixed with Yamalube R at 32:1 here in Indiana in the spring/summer and never had one problem. Bike ran amazingly strong. Other experienced riders thought the bike ran like something much larger than a 125cc motorcycle. I left mine all stock, even had a dented-in stock pipe on it.

How do you ride it? 91 was Ok for me when I first got the bike. When got better over the years and faster is when I had issues.

When I dynoed a rad valve vs the V-3 on my 05 YZ 144. The V-3 showed a 1HP gain while the rad valve was only a .5 gain.

Out on the track the V-3 was noticeably smoother than the Rad. My personal opinion is the V-3 is excellent and what i run. Not just because it performed the best on the dyno, but because it felt best to me on the track.

Reed valves obviously don't add a ton of HP, so often what is judged as power is the hit, the V-3 is smoother so that can fool ya.

What confuses me is that many people say the V force and Rad work. I guess you got a good V-force lol. I must of got a lemon. All I know is that when that Rad was in, the bike was much faster.

The "chips" or pitting marks on the dome of your head are probably from where the bike had blown up before and the previous owner just bolted it back on after a rebuild.

Not at all. When I first rebuit the top end nothing was wrong. When I took it apart when things were going on is when I noticed that chipping. Even Eric Gorr and many other people I know who run 125s in races say its detonation.

IMHO nothing wrong with exchanged cylinders - and while the chipped plating is not great - it is also a risk that is run when only porting an existing cylinder (without replating). Sometimes the plating flakes/chips no matter what you do...typically an indication of poor bonding between plating and cylinder - which I see 1 in 10 OEM cylinders...

So long as the chip is smoothed...it has no influence on power or longevity...

Anyways - I do not agree with polishing the boyseen - it's a waste of time!

But I have found the stock yz right on the lean verge on the needle. Going richer here but tad leaner on pilot from stock works great.

Main jet all depends on fuel - altitude, and temps...hard to say run x jet! I have run down as far as a 380 - and up at 440 just depending on weather and track...and fuel.

Being off one or two jets on race fuel is not to big a deal for reliability....off two jets in rich direction kills peak and over rev - off two jets lean and it will blow up on pump almost right away. On race fuel when too lean it "chirps" letting you know its time to jet richer!

Ok exchanging Clyinders is ok If you want to. Its NOT ok when the guy who sends it back saying no its yours when its not. He never told me Im exchanging yours for this one. He made up stuff. Also the exchanged clyinder I got back had fresh plating with a head mod by Eg for 91 octane. It did not do anything. 1.6 hours and done. This is proof that this is detonation. And I was running rich jetting.

Also polishing the Boyesen Rad was fun! Was not a waist of time because it did make a difference in power. More air flow then before.

Ok I made this post because I wanted to share my knowledge on my 125. This post is to give users an Idea of what will help make your 125 run better like mine did, and to let you know what issues that can happen, and a solution to that issue.

Nice write up!

I agree 100% about the vforce reeds. I went back to stock and noticed no difference in power. I also just started mixing 91 pump with 100 octane. I currently have a 420 main, 37.5 pilot and stock needle on 2 clip from top. My cylinder head has those little chip marks too but I'm not sure if it's from detonation or from the vforce reeds it ate???

Reeds would disintegrate in the combustion chamber before screwing up a cylinder.

Also polishing the Boyesen Rad was fun! Was not a waist of time because it did make a difference in power. More air flow then before.

IMO

I think you find that having a smooth surface is worse than having a rough

surface. You want that surface rough to help the mixture stay mixed. When the path is too smooth the air/fuel mixture will start to separate causing a loss of power.

IMO

I think you find that having a smooth surface is worse than having a rough

surface. You want that surface rough to help the mixture stay mixed. When the path is too smooth the air/fuel mixture will start to separate causing a loss of power.

Agreed, Boyesen does not polish the Rad on purpose. Same with 99% of all other engine builders. It was found out long ago that a slightly rough but not bumpy surface increases flow. The rough surface creates a slower port surface boundary area that the faster moving air "above it" can travel faster do to less friction. The rougher surface also encourages that the air fuel stays mixed and encourages fuel droplets and their formation at lower port velocities/engine speeds are kept to a minimum.

Edited by 455
Ok exchanging Clyinders is ok If you want to. Its NOT ok when the guy who sends it back saying no its yours when its not. He never told me Im exchanging yours for this one. He made up stuff. Also the exchanged clyinder I got back had fresh plating with a head mod by Eg for 91 octane. It did not do anything. 1.6 hours and done. This is proof that this is detonation. And I was running rich jetting.

Also polishing the Boyesen Rad was fun! Was not a waist of time because it did make a difference in power. More air flow then before.

Not saying the cylinder and head you got WORKED well - obviously it blew up!

I AM Saying an exchanged cylinder is never a bad thing...often you get one that is better than the one you sent - in fact almost always - it's freshly plated to a better spec than stock - with perfect bore tolerances!

Anyways - when you can prove to me the polished boyseen flows more air on a flow bench or any other device that measures flow - I will start polishing them in a heartbeat!

Here is an analogy everyone seems to overlook when looking at ports and surface finish.

If you walk through a door way - do you walk brushing up against the sides, or through the middle?

Air is the same - with the majority of flow passing through the easiest part of the port to flow through - aka not brushing up against the walls. I have said it before - but google flow boundary layer - you might learn a lot you didn't know before!

Not saying the cylinder and head you got WORKED well - obviously it blew up!

I AM Saying an exchanged cylinder is never a bad thing...often you get one that is better than the one you sent - in fact almost always - it's freshly plated to a better spec than stock - with perfect bore tolerances!

Anyways - when you can prove to me the polished boyseen flows more air on a flow bench or any other device that measures flow - I will start polishing them in a heartbeat!

Here is an analogy everyone seems to overlook when looking at ports and surface finish.

If you walk through a door way - do you walk brushing up against the sides, or through the middle?

Air is the same - with the majority of flow passing through the easiest part of the port to flow through - aka not brushing up against the walls. I have said it before - but google flow boundary layer - you might learn a lot you didn't know before!

I can not prove that smoothing the reed block will have better airflow but it makes sence that it will because there is less friction so more air will flow then before. But I can say when I first got the porting done with the reed block stock, it didnt seem to have as great of throttle response. When I got the porting work done for the second time, I polished my reed block. Then it had better throttle response with no hesitation. its crisper than before. It was faster too. I was always told this. The more air and fuel you can get into the combustion chamber and the faster you can get it out, the more power you will make. Remember that. Im not trying to prove anything. I am just sharing what I did that made the bike run better. And from what I did it made more power. I had a fun time trying all this stuff that I wanted to share it to others.

Edited by 125 racer 981

My bone stock bike ran at sea level on pump 91 mixed at 32:1 with a 420 main and a wiseco piston for well over 35 hrs with nothing but oil and airfilter changes at an upper Florida B class mx pace. The final blow was the inside of the ring groove cracking and leaking compression. Everyone has said to only run race fuel, but I was wanting to do a big bore so I didn't much care...

The piston, cylinder and head show no signs of detonation and the piston wash looks good... maybe a tad on the lean side.

Thanks for Posting!! This will Def come in handy!!

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