Hello everyone. My son and I managed to blow up both his '06 YZ125's this past weekend and I would like to get some input on what you all think may be the cause. I think it is a lean jetting problem. Both bikes are stock except for an FMF Fatty pipe & shorty silencer. I use 92-93 octane pump gas mixed with Maxima 927 at 32:1. The carb jetting is stock (410 MJ, 40PJ, stock needle 3rd clip) on both bikes. The air temp was around 55-60 degrees and he was riding on a sand track when both bikes lost compression and quit running. On the first bike I tore down the piston was broken away on the intake side and the ring was collapsed. The cylinder has broken away all around the top of the bore and there is also damage to the head. Any input would be greatly appreciated. Thanks!
Help! I blew up two '06 Yamaha YZ125's this past weekend
- Thread starter Motorob
- Start date
You did mention you mixed the oil at 32 : 1 , not being clever but can you post the amount of oil you put into what amount of petrol , just maybe your calculations are off .
Standard jetting is normally too rich so that wouldnt have been the problem .
Did you ride the bikes at WOT for long periods of time without backing off ?
Standard jetting is normally too rich so that wouldnt have been the problem .
Did you ride the bikes at WOT for long periods of time without backing off ?
- Jul 16, 2004
- 1,207
- 0
if it was a lean condition I believe it would burn up on the exhaust side of the piston. I may be wrong, can never remember exactly. How hard was he riding the bike? If he kept it in a gear for a long period of time without backing of the throttle that can cause damage. I assume a hole wasnt burned through the center of the piston?
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Perhaps the YZ's aren't jetted all that rich from the factory...? 50 degrees and riding hard in sand calls for a "don't be too lean" mixture. But a broken piston can also mean excessive piston-slap (worn out top-end)
- Thread starter
- #9
Thanks for the replies. I'm trying to post pics of the piston and cylinder, but it keeps popping up "Spamkiller...please remove" every time I try to update it. I'm not sure what I'm doing wrong. Any suggestions?
One of these bikes has aproximately 8 hours on a new top-end and the other one is new and has less than 3 total hours on it. I tore the second bike down last night and the piston crown is burned down to the ring on the exhaust side. I have been working on bikes for over twenty years and these have me stumped.
KDX CRAZY= I mix at 32:1 (4 ounces of oil per gallon of gas)
One of these bikes has aproximately 8 hours on a new top-end and the other one is new and has less than 3 total hours on it. I tore the second bike down last night and the piston crown is burned down to the ring on the exhaust side. I have been working on bikes for over twenty years and these have me stumped.
KDX CRAZY= I mix at 32:1 (4 ounces of oil per gallon of gas)
Of the two engines it sounds like two different problems. Getting back to basics; did the 1st bike get warmed up before loading, i.e. to prevent cold seizure? In the second bike; is the coolant level correct and is the pump working, i.e. to prevent overheat? Since the jetting is stock, I would say the gas&oil mix is not a concern. I run the same mix in a wide variety of Hondas & Yamahas. Never had a problem like that with low hours. WOT, sand, heavy rider, could be causitive. The airfilter clean and not oiled too heavily? Correct sparkplug heat range? Chain not too tight? Go back to the basics. Let us know what you find. You must be at Lejune, maybe CPoint...I may be going back to that area soon.
sand
sand sucks, 2 strokes hate sand ,airfilters hate sand, it can suck sand in the damnest places, the smallest amount will roast an engine,32;1 is rich, arguement goes that richer burns hotter, too lean melts motors. 927 is arguebly the best castor oil but when you use a ratio rite have you ever noticed how much oil residue is left and how hard is it to get off even with gas and it don't like cold weather.
sand sucks, 2 strokes hate sand ,airfilters hate sand, it can suck sand in the damnest places, the smallest amount will roast an engine,32;1 is rich, arguement goes that richer burns hotter, too lean melts motors. 927 is arguebly the best castor oil but when you use a ratio rite have you ever noticed how much oil residue is left and how hard is it to get off even with gas and it don't like cold weather.
whenfoxforks-ruled said:
What on earth are you talking about?
richer oil
Hey I had a Montessa also, I,ve heard and it makes sense to me that in the atomical world of 2-stroke engines more oil in the mixture results into a hotter and dirtier burning temp. Mind you less oil burns cleaner and cooler to the point that friction takes over , but this guys engine that I seen now is lean because of an air leak.
Hey I had a Montessa also, I,ve heard and it makes sense to me that in the atomical world of 2-stroke engines more oil in the mixture results into a hotter and dirtier burning temp. Mind you less oil burns cleaner and cooler to the point that friction takes over , but this guys engine that I seen now is lean because of an air leak.
whenfoxforks-ruled said:
That myth is older than Malcom Smith's original boots. :)
The amount of oil in the mixture doesn't influence combustion temperatures to any significant degree.
Without seeing the whole thing first hand we are all just guessing as to why these engines broke but I'll add my guess just to muddy things further. :whoa:
Fuel requirements (jetting, octane, distillation curve) are very dependent on how much work you are asking the engine to do, in other words the load being placed upon it. With smaller two-stroke engines this becomes even more significant.
A good way to illustrate it would be to go up a hill in a specific gear and throttle opening that causes spark knock, and then ride back down in the same gear and throttle opening. It will tend not to knock on the downslope due to the reduced load on the engine.
Small two-strokes running in sand present you with the rare situation where you can use nearly all of the power available and do it for extended periods of time. Lots of load and lots of heat, for long periods of time.
If you add in how tough it is for radiators to work effectively on a sand track you get and engine that runs much hotter, is being loaded near it's maximum, and requires more fuel and better cooling (especially on the piston crown) to balance the temps and load.
All of this is a perfect environment for knock (aka detonation) which is influenced by temperature, pressure and time. Raise the temps, the pressure or the time under pressure (load) and you increase the risk of abnormal combustion and eventually detonation.
Pick the right fuel and it will tolerate high temps and pressures better and have a longer delay time before it crosses over into ugliness. Adding in more fuel with richer jetting might cool things enough to fix things but experience has shown that if you load the engine hard enough jetting alone won't compensate for the wrong fuel.
I hope this helps some.
- Thread starter
- #20
Thank you for all the replies. I was able to "salvage" one cylinder and head assembly. I put in a new piston kit and ran it today with no problems. Before running it I drained the gas tank, re-jetted the carb (430MJ) and it is running awesome. The old cylinder might just get made into a 144.
I hope you get things all sorted out. I would like to suggest you consider this though...
Your engine is seeing the higher end of the load spectrum and as a result heat plays a big role.
Also as a result of the higher loading,
the bearings and other wear surfaces are also providing a demand
for protection at the higher end of the spectrum.
A 32:1 ratio is normally sufficient to use with most all of the top shelf lubricants and fuels,
and for the majority of riding, even with the higher revving small bore 2t engines. But when the demand is higher,
so must the providing of protection.
I think that the harder ridden engines do much better when they are given an ample supply of an oil that is sufficient for
higher heat situations and the protection the bearings and surfaces demand.
I'd think you much better off with more like 20:1 being the lower end of the scale.
I would also consider using a different oil. I know that is panoras box of statements here....
but consider a couple of things.
Jr. loves the smell and so do you. So do I...we all like it.
Smell does squat, other than let a person know he's running bean oil, albeit castor one of the best lubricants.
I think that a good synthetic ester or combination of esters provides the very best of protection at the level of
protection a hard ridden engine needs.
If you are a Maxima fan, I would give their synthetic ester a try.
Klotz is hard to beat. They combine synthetic esters with some bean oil.
(they get it both technically and marketwise)
If money is an issue with increasing oil ratio, inexpensive Mobil racing 2t can offset it.
Good luck on the big bore.
Your engine is seeing the higher end of the load spectrum and as a result heat plays a big role.
Also as a result of the higher loading,
the bearings and other wear surfaces are also providing a demand
for protection at the higher end of the spectrum.
A 32:1 ratio is normally sufficient to use with most all of the top shelf lubricants and fuels,
and for the majority of riding, even with the higher revving small bore 2t engines. But when the demand is higher,
so must the providing of protection.
I think that the harder ridden engines do much better when they are given an ample supply of an oil that is sufficient for
higher heat situations and the protection the bearings and surfaces demand.
I'd think you much better off with more like 20:1 being the lower end of the scale.
I would also consider using a different oil. I know that is panoras box of statements here....
but consider a couple of things.
Jr. loves the smell and so do you. So do I...we all like it.
Smell does squat, other than let a person know he's running bean oil, albeit castor one of the best lubricants.
I think that a good synthetic ester or combination of esters provides the very best of protection at the level of
protection a hard ridden engine needs.
If you are a Maxima fan, I would give their synthetic ester a try.
Klotz is hard to beat. They combine synthetic esters with some bean oil.
(they get it both technically and marketwise)
If money is an issue with increasing oil ratio, inexpensive Mobil racing 2t can offset it.
Good luck on the big bore.
Last edited:
motorob
Your main jet is too small for a stock 06 yz125 at 50 deg. Around a 430 to a 450 depending on outdoor or indoor. I run 32 to 1 Yamaha R and have been for 30 years. My son has a YZ125 06 no problems other than a stopper lever going bad! ouch! We have been running a 470 to a 490 mian jet in a stock bike all year , it will take on 250 F's. After the stopper lever went, I had to split the cases , very mild port work was done. We are running a 500 to a 560 main jet ! at the end of outdoor and the beginning of indoor racing depending on temp, dry or damp air. The bike is stock . There is a huge potential to unleash from with-in the carb! with a stock bike. Good luck !
Your main jet is too small for a stock 06 yz125 at 50 deg. Around a 430 to a 450 depending on outdoor or indoor. I run 32 to 1 Yamaha R and have been for 30 years. My son has a YZ125 06 no problems other than a stopper lever going bad! ouch! We have been running a 470 to a 490 mian jet in a stock bike all year , it will take on 250 F's. After the stopper lever went, I had to split the cases , very mild port work was done. We are running a 500 to a 560 main jet ! at the end of outdoor and the beginning of indoor racing depending on temp, dry or damp air. The bike is stock . There is a huge potential to unleash from with-in the carb! with a stock bike. Good luck !
Glad to hear you got one back and running again. I like that idea of making the one, with a toasted jug, into a 144.
I'd just like to add one thing to the discussion (I'm sure most everyone knows this). It was mentioned that your fuel/oil mix was too rich (32:1). This only causes plug fouling and a loss of power. Oil burns cooler than gas, so richer oil mix=cooler, imo. Also- I don't believe that 32:1 is too rich... in fact, I agree with jaybird.
With jetting (air/fuel), lean= more power/heat, while rich=less power, less heat.
I'd just like to add one thing to the discussion (I'm sure most everyone knows this). It was mentioned that your fuel/oil mix was too rich (32:1). This only causes plug fouling and a loss of power. Oil burns cooler than gas, so richer oil mix=cooler, imo. Also- I don't believe that 32:1 is too rich... in fact, I agree with jaybird.
With jetting (air/fuel), lean= more power/heat, while rich=less power, less heat.
Most definitly detination look on top edge of cylinder their will be pitting and on edge of head. you can not run a yz 125 on pump gas in the manual it says 95 or higher.did the gas you bought have ethonal in it thats been causing lots of problems. we build and race yz 144s since you have to rebuild go 144 its worth it call me if i can help you out jody backbay motor sports 603 569 3200
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