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Thoughts on maxrpms Yz 139 kit

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This winter I'm looking to send my cylinder out to get a big bore. I have it narrowed down to the maxrpms yz139 or the eric gorr 144. If anyone has had one what were your thoughts on it. I'm leaning towards the 139 because I've heard the 144 is a little flat on the top end and I've read on the maxrpms website how much they disliked the 144 on the yz. They are both about the same on reliablity right?

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Why not Derek @ HP bikes?

 

 What Monk said we ran one of his 134s this year and it worked flawless ran like a top on 50/50 avgas and pump gas I may consider selling the complete top end which has 45 hrs total if you have a good stock top end to exchange. Pulls stock gearing at 3400 ft like nothing with a 145-150 pound rider on it.

Edited by gedakbx

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What Monk said we ran one of his 134s this year and it worked flawless ran like a top on 50/50 avgas and pump gas I may consider selling the complete top end which has 45 hrs total if you have a good stock top end to exchange. Pulls stock gearing at 3400 ft like nothing with a 145-150 pound rider on it.

the cylinder isnt perfect theres a few marks but I dont think they catch the finger nail so their alright

Why not Derek @ HP bikes?

I havent heard as much about him ill look into him

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Personally I would stay away from the EG kits. I didn't have a good experience with him on my kx144. You get what you pay for

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Personally I would stay away from the EG kits. I didn't have a good experience with him on my kx144. You get what you pay for

what problems did you have?

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Had my eg yz144 on my 06 for 10 years now not had any problems with it ever. Runs sweet I would check you can still find the 57mm pistons easily for the yz. The 144 is the more popular big bore kit don't think you can get the ti nitrate coated ring for 139 57mm pistons either.

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Had my eg yz144 on my 06 for 10 years now not had any problems with it ever. Runs sweet I would check you can still find the 57mm pistons easily for the yz. The 144 is the more popular big bore kit don't think you can get the ti nitrate coated ring for 139 57mm pistons either.

the availability is what I was worried about. How do you like the power of it? Do piston and rings last as long as a normal 125?

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Power depends on what you ask for in the porting and head mods. I went with the mid/top porting cause I mostly ride mx with it and generally rev the piss out of it. I also went with the high compression race gas only head mod to help retain some of the bottom end and give it a nice hit in the mid. It still has more bottom then a stock 125 even with the 14t counter sprocket which is a must do mod.

Reliability with the wiseco piston with a standard ring 25 hrs before the ring was shot and the piston started getting grooves worn into it from the exhaust bridge.

With the wiseco ti nitrate coated ring closer to 60 hrs before the ring wears out. Keep in mind that my high compression head setup requires 114 trick race gas and it does put more wear on the piston ring.

Slight trade off for better power with a pump gas setup the wear won't be as hard on it.

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Power depends on what you ask for in the porting and head mods. I went with the mid/top porting cause I mostly ride mx with it and generally rev the piss out of it. I also went with the high compression race gas only head mod to help retain some of the bottom end and give it a nice hit in the mid. It still has more bottom then a stock 125 even with the 14t counter sprocket which is a must do mod.

Reliability with the wiseco piston with a standard ring 25 hrs before the ring was shot and the piston started getting grooves worn into it from the exhaust bridge.

With the wiseco ti nitrate coated ring closer to 60 hrs before the ring wears out. Keep in mind that my high compression head setup requires 114 trick race gas and it does put more wear on the piston ring.

Slight trade off for better power with a pump gas setup the wear won't be as hard on it.

thanks for the response. Is there just the two options fot pump and race because I ran av gas mixed 50/50 so its not quite race gas but more then pump. I race harescramble so I wanted a little more bottom end but I want it to be able to do track decent also

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thanks for the response. Is there just the two options fot pump and race because I ran av gas mixed 50/50 so its not quite race gas but more then pump. I race harescramble so I wanted a little more bottom end but I want it to be able to do track decent also

When Derrick sent ours back his instuctions stated 50/50 c12/ 91 pump or 50/50 100ll avgas/ 91 premium ran the same on both we ran 100ll

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You could maybe ask but for harescrambles and Desert racing I would go with the lower compression pump gas head setup.

You'll notice that the 250x and even the new 125x (sold only in Japan currently). They come spec'do with lower compression heads because it let's them rev out better.

Now if you could get your hands on a 125x cdi box from Yamaha Japan that would be the ultimate off road mod.

Yamaha part number B1B-85540-00 per Yamaha Japan parts search it's roughly $156 usd.

Webike Japan a awesome Japanese based parts seller does oem parts requests for Japanese models. I've used them before sweet service.

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You could maybe ask but for harescrambles and Desert racing I would go with the lower compression pump gas head setup.

You'll notice that the 250x and even the new 125x (sold only in Japan currently). They come spec'do with lower compression heads because it let's them rev out better.

Now if you could get your hands on a 125x cdi box from Yamaha Japan that would be the ultimate off road mod.

Yamaha part number B1B-85540-00 per Yamaha Japan parts search it's roughly $156 usd.

Webike Japan a awesome Japanese based parts seller does oem parts requests for Japanese models. I've used them before sweet service.

no deserts around here lol. I thought higher compression gave better bottom end?

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It typically does but the 125X has different porting and head mods and like the 250x power valve opening with a stiffer spring and power valve blade grinding to match different cylinder with different porting specs and cylinder head with lower compression then a yz the 125x like the 250x, the CDI has a different power curve that pulls sooner off the bottom end smoother through the middle and further up top with not as much hit giving it a wider power band.  The 125x was just released this month finally in Japan as a 2017 model if you want this bike here better start hitting up your dealers and demanding it.

 

A video of the bike kind of a lame promo video mostly talks about the changes but at least you can see it's real and it in action.

 

http://www.bing.com/videos/search?q=2017+yz125x&&view=detail&mid=074B0AA1F94FFD12DC04074B0AA1F94FFD12DC04&FORM=VRDGAR

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It typically does but the 125X has different porting and head mods and like the 250x power valve opening with a stiffer spring and power valve blade grinding to match different cylinder with different porting specs and cylinder head with lower compression then a yz the 125x like the 250x, the CDI has a different power curve that pulls sooner off the bottom end smoother through the middle and further up top with not as much hit giving it a wider power band. The 125x was just released this month finally in Japan as a 2017 model if you want this bike here better start hitting up your dealers and demanding it.

A video of the bike kind of a lame promo video mostly talks about the changes but at least you can see it's real and it in action.

http://www.bing.com/videos/search?q=2017+yz125x&&view=detail&mid=074B0AA1F94FFD12DC04074B0AA1F94FFD12DC04&FORM=VRDGAR

that's interesting this is a budget build though so there's no money for new cdi's at this time lol. Would it have a wide ratio trans? Would a 125 even pull a wr trans?

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Race HS? Leave it a 125 and run the piss out of it. A 144 kit isn't going to be some drastic motor improvement. Throw a oversized rotor on the front and come into the corners a little hotter, you'll see more benefit...

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The plating on my 285 that was done by Max Power is still holding up many years later. It's a beast. Running 110 leaded fuel and Motul 800 @ 45:1

 

                             Craigus

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I ended up ordering the 139 kit the other day. Had been planning to go with eg 144 but went to go order it and saw he wasn't excepting new work until May because he was so busy, and I need my bike by May so I went with max power. Does anyone here have one? What's a good base line for jets to start out with at sea level?

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