Help! I blew up two '06 Yamaha YZ125's this past weekend

[BradC62]

Jetting

32to1 is not rich
Not only about the main jet but also the pilot jet along with other parts of the carb your wrong . many yz125 bikes on the circuit are running small main jets 410 stock and turning there air mixture screws 1 to 1 1/2 turn out thats bull . I have sean many bikes being walked off the track , ring locked on the piston .how about a 490 main 37.5 pilot and some carb mod's to thrash 250f's outdoors ! We have no problems. You can not do that on a stock bike , un' less you know what you are doing. this is not a144, this is stock ! 926 oil is good but yamaha r is also, those are the best. If and when you take apart motors you will see a difference between oils, Some lube, some don't . There is always pro's and con's with almost anything , the bike's come to lean from the factory , along with fork seals that leak !!! :cool:

 

[pidro_el_pirata]

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?

Just post them on a host site (like http://photobucket.com) and then post the links. You can set up a free account that will host PLENTY of pictures for you to link to, it's way easy too. I recommend you check that out.

 

[Motorob]

BradC62,

Thanks for the info. I've had quite a few people tell me that they are running a 470-500MJ on these bikes. I plan on getting me an assortment of MJ's and testing them. For the winter I have switched to Yamalube 2R @ 32:1. Thanks again!

 

[BradC62]

Motorob,
Great choice on the Yamaha 2R @ 32to1. My friends and myself have been using it for 30 years, no problems. Clutch Saver has been very good to us also. The main jet & pilot jet is a whole nother issue with these bikes ( YZ 125 ' 05' 06' 07' ). They are to lean from the factory. Out door summer on a stock bike 430 or 450 , indoor 460 or maybe 470 . But there is also the pilot jet, & needle! My son & I are runing a 490 to 500 main out door summer (hot) and a 500 to 560 indoor (cold) the pilot jet is 37.5 it has never changed ( stock is 40 ). There is a very nice light coco brown on the plug. In order to run these larger mains you have to to some carb mods. What I have come across is the poor mans fuel injection. This bike flys.We have been doing this for 4 years racing even with the smaller bikes , by burning more fuel and oil efficintly it lubes every thing in it's path. The motor run's like a clock. We have done this set-up on a few bikes in the last 5 month's, the riders can not believe the difference. Alot of the bikes new vibrat, the cranks are not balanced that well from the factory, along with the fork seals that leak after only a short while, and also the stopper lever that disenagrates and wipes out the tran's. Keep your eyes open , and yes you are on the right path get that 410 main out!
THANK's for the Comp.

 

[bruno670]

What color was the plug? One thing to keep in mind that the Lean/Rich is GAS to AIR not gas to oil. The more oil you have the leaner it will run.

 

[BradC62]

Bruno670
The plug look's perfect. you want to look for a quarter size carbon deposit on the center of the piston. Like I said before there is alway's pro & con. I have seen many YZ's blow up, our local dealer has sold many piston's, ring's, and rod kits, along with bearings! The choice is your's, just trying to help out. We have had great luck . THE carb holds many annser's. 32to1 is not a lot of oil , 25 year's ago 20to1 was the magic number but that was year's ago. Bike's have come a long way since then, and also the oil's . I have seen people running 40to1 & 50to1 walking their bike's off the track, OoP's what did they do wrong , to small of a main or was the gas / oil mixture to lean . Good luck with your YZ ? . Maybe i will see you indoor at local track's or in the spring at baja mx in michigan or motoplex in ohio. There are a few bike's runninig the same set-up. should I look for 670 ?

 

[steve125]

I use 92-93 octane pump gas

Theres your problem. If you had been running a race fuel like VP C-12 this would have never happened. And the stock jetting would be fine.

The FMF pipe adds power but along with that comes detonation. The late model YZ's are borderline on Det. stock, add an aftermarket pipe and its worse. Running pump fuel and large jets is a band-aid fix at best and will lower HP.

 

[BradC62]

race fuel

Mr. Steve125
Race fuel will not solve the problem. THE BIKE RUNS TO LEAN FROM THE GET GO! You can run race fuel , the ideal octane is 100. PR2 racing does 144's along with other people, they are using stock main's (410 or 420's) the motor's blow up. We are not! The bike's we are running on the curuit are smoking fast, with no motor failure's after we have installed the new stomper lever.We have been working on the jetting along with the rider's for 10 month's. Old school does not apply, keep the bike stock at first you can always go to a 144 get the jetting RIGHT. Contact
RACEJMC(at)AOL.COM

 

[steve125]

Brad have you ever tuned with race fuel before? Sorry Brad theres more holes in what you are saying that I can count. You must be using some real junk for fuel to even have to go near a 500 main jet.