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Bought my son his 1st 2 stroke and have some questions.


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Got a great deal on an 2005 YZ 125 with about 40 original hrs on it. I've never owned any 2 strokes. The bike is stock except for a gold series FMF fatty pipe and exhaust. I did not get a manual with the bike so that's why some of the lame questions, need to buy one.

What is the correct premix ratio for this bike?

What is the best method for mixing the oil with gas. I don't want to have a separate gas jug just for his bike if I can avoid it.

What 2 stroke premix oil is recommended?

My son is a tall 12 yr old and is just coming off a 2008 TTR 125. He rode a 2001 KX100 2 stroke and loved it. He has not yet ridden the bike.

I want to tame the bike down a bit for him. I've heard about a fly wheel weight witch I think I will go with to start.

What else could could be done to tame it down a bit more while he transitions into this bike. Don't want him hurt.

Is there any throttle limiter?

Would the stock pipe and header help tame it down more?

What is the stock suspension weight range for rider for this bike?

Anything else I should know about 2 strokes please feel free.

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I run 40:1 with a quality synthetic 2 stroke oil, I prefer silkolene. Yes you will have to have a second fuel jug for this bike. My prefered method of mixing is using a ratio rite measuring cup https://www.rockymountainatvmc.com/p/933/3568/Ratio-Rite-Measuring-Cup?term=ratio%20rite  and mixing in your 2 stroke dedicated fuel jug and shaking around a bit and rinsing the ration rite with fuel and dumping it back into the jug. ok thats the best im going to do at 5 am.

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You have made an excellent choice.  A while it is definitely a faster bike than TTR 125, it is not necessarily more dangerous, especially if your son has developed good riding skills, such as using the front brake appropriately.  The TTR is overweight, under-suspended, and has lousy brakes.  My son used to race 2 strokes like the KX 100, and 4 strokes like the TTR, and even though he turned much faster laps on the two strokes, I always felt he was much safer on the smokers, simply because they were so much better with regard to braking, suspension, and weight/handling.  

1. Get yourself a manual right away.  There is technical information in the manual you will need.

2. 32:1 is the premix ratio you want with pump premium.  No need for any super expensive oil.  Yamalube 2R or Lucas is fine. Mix ONLY what you need to ride, and never store gas more than a couple of weeks.  When you buy premium in small amounts (like 2 gallons), always run a gallon or two in your car BEFORE you measure the 2 gallons for premix in your fuel jug, just to make sure you are getting all premium.

3.  Cleaning the air filter is going to be your most important maintenance duty.  This bike will suck FAR more air than the TTR, and the filter will get dirty quickly.  You can make life much easier on yourself by using PC racing filter skins (just google it).

4. I wouldn't worry about taming the bike, as kids adapt quickly, and the YZ probably has the most easy to use powerband of all the major 125s.  You may well have some jetting issues (fouling plugs) depending on how your son rides and how the bike is currently jetted. DO NOT let some yahoo talk you into running 50:1 premix or some other crazy ratio.  If the bike is fouling plugs, and you are not comfortable with jetting, get yourself some help from someone with experience.  It will be well worth the time and money.  Once you see how it is done, you can do it yourself fairly easily, but it's not something that can be described very well on an  internet forum.

5.  I would probably leave the suspension stock, and use the manual to set the sag and learn about how the clickers work.  I've spent a lot of money over the years on springs to match my son's weight as he grew when he was racing, but if your son is just play/trail riding, it's not worth it.  He might be a tad light now, but he will soon gain the weight to match the spring rate!

6.  Enjoy the bike!  

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Get yourself a red, 2 gallon, gas jug from WalMart. Use a permanent marker and write "Pre-Mix" on it, then use this for the dedicated YZ gas mixing jug. If you're fortunate enough (as we are here in S.W. MO) to have ethanol free premium available, you don't have to worry so much about the leftover pre-mix setting around for a few weeks. 

rpt50 has laid out some good advise on several things, including the pumping a gallon or two into your car/truck before filling your gas can, and avoiding trying to correct improper jetting with fuel mix ratios. Jetting in a 2 stroke is a very important element to making them run right, and also one of the most over thought. Get yourself a service manual, and do a Google search on properly jetting a 2 stroke MX bike, several very good tutorials out there on the subject.

You two are going to have a good time, YZ125s are great fun and the best advanced learning tools made for the new motocross rider (and woods rider too for that matter).

Edited by OLHILLBILLY
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You have made an excellent choice. A while it is definitely a faster bike than TTR 125, it is not necessarily more dangerous, especially if your son has developed good riding skills, such as using the front brake appropriately. The TTR is overweight, under-suspended, and has lousy brakes. My son used to race 2 strokes like the KX 100, and 4 strokes like the TTR, and even though he turned much faster laps on the two strokes, I always felt he was much safer on the smokers, simply because they were so much better with regard to braking, suspension, and weight/handling.

1. Get yourself a manual right away. There is technical information in the manual you will need.

2. 32:1 is the premix ratio you want with pump premium. No need for any super expensive oil. Yamalube 2R or Lucas is fine. Mix ONLY what you need to ride, and never store gas more than a couple of weeks. When you buy premium in small amounts (like 2 gallons), always run a gallon or two in your car BEFORE you measure the 2 gallons for premix in your fuel jug, just to make sure you are getting all premium.

3. Cleaning the air filter is going to be your most important maintenance duty. This bike will suck FAR more air than the TTR, and the filter will get dirty quickly. You can make life much easier on yourself by using PC racing filter skins (just google it).

4. I wouldn't worry about taming the bike, as kids adapt quickly, and the YZ probably has the most easy to use powerband of all the major 125s. You may well have some jetting issues (fouling plugs) depending on how your son rides and how the bike is currently jetted. DO NOT let some yahoo talk you into running 50:1 premix or some other crazy ratio. If the bike is fouling plugs, and you are not comfortable with jetting, get yourself some help from someone with experience. It will be well worth the time and money. Once you see how it is done, you can do it yourself fairly easily, but it's not something that can be described very well on an internet forum.

5. I would probably leave the suspension stock, and use the manual to set the sag and learn about how the clickers work. I've spent a lot of money over the years on springs to match my son's weight as he grew when he was racing, but if your son is just play/trail riding, it's not worth it. He might be a tad light now, but he will soon gain the weight to match the spring rate!

6. Enjoy the bike!

32:1 isn't the right ratio, neither is 40:1 or I personally use 45:1

Doesn't make any of them right.

You jet the bike, then mix accordingly.

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hard to jet a bike properly when you don't have a clue as to how it will react to different mixtures.

pick your preferred mixture, jet to that mixture.

Yes you are right. I should have put it that way.

Sometimes I forget that I'm giving advice to someone that doesn't know. That's for the correction

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Good input guys.

The bike ran really good when I test drove it even thought it's been sitting for a long time. But unfortunately I have no history on the bike. Since it has a fatty on it I would assume it has had some jetting work done on it? But then again would stock jetting be correct for the FMF pipe? Would it?

Are the jets in the 125 the same as my CRF 450R? Not same size but same number of jets and dose it work the same with the different circuits like the Keihin carb?

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Good input guys.

The bike ran really good when I test drove it even thought it's been sitting for a long time. But unfortunately I have no history on the bike. Since it has a fatty on it I would assume it has had some jetting work done on it? But then again would stock jetting be correct for the FMF pipe? Would it?

Are the jets in the 125 the same as my CRF 450R? Not same size but same number of jets and dose it work the same with the different circuits like the Keihin carb?

No, the stock jetting isn't correct for the Fatty pipe. And most of the time the stock jetting isn't correct for the stock pipe. There are a lot of variables to consider, altitude, temperature, motocross or woods, fuel mixture, etc.. Like I said before, Google "how to jet your 2 stroke bike", and go from there. There is no one set jetting spec for any bike, follow the tutorials directions and get it right for your conditions. It's not as hard as it seems when looking from the outside, and I could give you pretty close jetting parameters for a YZ250, but I have no real experience with a YZ125.

And no again on the CRF450R carb. It may be a Keihin, but it's not at all like the one on the YZ125.

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One thing you can to to take some power out or tame it down is to loosen the throttle cable so he can't have WOT.  Another way is to lock down half of the reeds with a metal plate.  Like taking 1/2 the valves out of a motor or a restricter plate.  Been there, done that.  He will respect gravity and the laws of physics eventually anyway.

 

Stay with a quality oil for pre-mix.  Remember, quality doesn't come from WalMart.  Klotz, BelRay, Maxima, yamalube, just a few.  32-1 is a safe mix.  If you fould a plug, and you will, they are cheap.  Cheaper than a piston and cylinder.

 

Have fun!!  Ride safe!  Better yet, ride with him!

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Adding play into the throttle cable is up to rider preference but usually not a wise thing to do. The motor will be all herky jerky over bumps and generally difficult to control.

 

If desired ... You can smooth out that YZ125 a lot with the right pipe (eg. Gnarly) and jetting. Also a quieter muffler will smooth it out, and retarding the spark timing a little can make a big difference.

 

If the jetting is too lean at any throttle opening, then the motor will have some hesitation before a surge of power. If that happens then the key is to find out at which throttle position it's doing that, then richen the required fuel circuit. When a 2 stroke is rich it runs rough, wont rev out, spooge drips out the end of the muffler and you foul plugs.

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So I took my son out today for a test ride on the bike.  He liked it, felt comfortable pretty quickly and the bike fits him perfectly.  Its defiantly the right bike for him but dam this is one hell of a fast bike!!  I think you guys who say no need to mod it to mellow it out are nuts!!.  Justin is about 110 lbs, 12 years old, is a good rider but this bike is a bit over his head as far as responsiveness and overall speed.  Its the unexpected accidental jerk of the throttle that will kick you in the ass, loop the bike out,  and put you on the ground as fast as you can say it.  That's what I am most worried about.   IMHO it needs to be tamed down a bit until he gets really used to it.  This bike feels fast and as responsive as my CRF 450R but I weight 230 and I'm 50 years old and have been riding for about 15 years. 

 

I'm defiantly going to put a fly wheel weight on it.

look a stock header and pipe, and anything else I can do to tame it down a bit more. 

 

Anyone have a stock pipe and header for 05 YZ 125 for a good price let me know, IM me.     

 

Technical issue

When the bike starts up there is a decent amount of white smoke, also noticed that when he was lugging it around as he was getting used to it.    Is this normal?   When I rode it and was really on the pipe the smoking was gone.

 

Also dripping oil where the header pipe connects to the engine.  Is this common?  Is there a lead seal that could be worn or missing here?

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Replace or at least check  shift stopper in the sticky section (transmission) to make sure someone else did.

https://thumpertalk.com/topic/512858-replace-your-shift-stopper/  .

 

Nicko thanks for the tip on this.  IF the bike has a problem with the stock shift stopper what would I look for "feel for" to try and figure out if the one on the bike is stock and should be replaced?  What do you mean the sticky section of the trans?   

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So I took my son out today for a test ride on the bike. He liked it, felt comfortable pretty quickly and the bike fits him perfectly. Its defiantly the right bike for him but dam this is one hell of a fast bike!! I think you guys who say no need to mod it to mellow it out are nuts!!. Justin is about 110 lbs, 12 years old, is a good rider but this bike is a bit over his head as far as responsiveness and overall speed. Its the unexpected accidental jerk of the throttle that will kick you in the ass, loop the bike out, and put you on the ground as fast as you can say it. That's what I am most worried about. IMHO it needs to be tamed down a bit until he gets really used to it. This bike feels fast and as responsive as my CRF 450R but I weight 230 and I'm 50 years old and have been riding for about 15 years.

I'm defiantly going to put a fly wheel weight on it.

look a stock header and pipe, and anything else I can do to tame it down a bit more.

Anyone have a stock pipe and header for 05 YZ 125 for a good price let me know, IM me.

Technical issue

When the bike starts up there is a decent amount of white smoke, also noticed that when he was lugging it around as he was getting used to it. Is this normal? When I rode it and was really on the pipe the smoking was gone.

Also dripping oil where the header pipe connects to the engine. Is this common? Is there a lead seal that could be worn or missing here?

I'm pretty sure white smoke is coolant? Could be the head gasket is bad.. Somebody correct me if I'm wrong
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Nicko thanks for the tip on this.  IF the bike has a problem with the stock shift stopper what would I look for "feel for" to try and figure out if the one on the bike is stock and should be replaced?  What do you mean the sticky section of the trans?

Even if it hasn't broken loose jet (THE BEARINGS IN THE STOPPER) It needs to be replaced with the 2007 updated Stopper (NO BEARINGS ) in it because of the potential damage it can cause.

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Small bores need oil, 32:1 is usually recommended, whatever quality twostroke oil you like use

a good name brand that makes racing oils. Yamaha knows their stuff most of the after market pipes don't get you that much more power or hit than stock, so I wouldn't worry about the pipe. Through in a fly wheel weight, retard the ignition timing, and add a tooth or two to the rear sprocket it will be good to go.

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Found the PDF manual on line.  Mix ratio in manual says 30:1.  I have both a Ratio Right and Quick 2 Mix containers and nether of them have a 30:1 ratio on them.  Only 32:1 and seams like most of you recommend the 32:1 is that what I should go with?

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