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Yamaha yz125 athena factory top end kit no low end power

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I recently bought an 09 yz125 with full pro circuit exhaust, vf3 reeds, and the athena factory top end kit. It had 140 compression and didn't have much low end power but plenty top end power so I re-ringed the piston and it had 150-155 compression which I've been told is fine. But still no low end power. My friends 06 yz125 had good low end, what's wrong with my bike? Ill appreciate any help

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Try getting a heavier flywheel, that and the pro circuit pipe is probably a race pipe, FMF's SST Pipe is for trail riding and isn't a true "fatty" pipe, believe it or not on 2t's the expansion chamber (pipe) can really impact on top and bottom end power

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Thanks for the Fast comment! And I tried an fmf factory pipe in place of the pro circuit factory pipe and it didn't help. And I do motocross an I have a ktm 150sx with full fmf and it absolutely rips down low and mid and top. My Yamaha is just up top and I can't just pop the clutch cuz it will just pull back down. But if I feather the clutch It goes really good. Jetting seems to be good too

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Had a year of horrible results trying to keep an athena 144 running. Just found out what to do. Here in Florida I had to drop the 410 to a 390 and the slow jet to a 35. Bike finally has pull with no clutch.

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My yz is a 125 still and it has a 440 main, stock needle and pilot and stock needle position. The plug looks great and its got full pro circuit and vf3s but maybe the main jet needs to be dropped some. Thanks for the input! P.S. I've tried needle position and no luck

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My yz is a 125 still and it has a 440 main, stock needle and pilot and stock needle position. The plug looks great and its got full pro circuit and vf3s but maybe the main jet needs to be dropped some. Thanks for the input! P.S. I've tried needle position and no luck

Tried the needle to, that fried a head gasket.

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Lol that's not good. I've heard these athena kits have lots of head gasket issues.. And ok ima order a bunch of jets and see if I can get it to run right. I have a lot of jets but they are numbered differently..

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I think when it comes to the elusive bottom end of a 125, it comes down to two things, what kind of working order your complete motor is in and getting that carb dialed, which is trickier than it seems with the 125 "bottom end".

 

This is all coming from trying for a year to get a bottom end out of the notorious turd 2001 cr125. I have over time made this thing a little ripper, but what was most important is abuse of the clutch. The bike actually does have a decent bottom end now after dumping WAY too much cash into it, but I still find better results and traction by using the clutch.

 

What I'm getting at here, is do whats necessary (probably just a matter of dialing in your pilot jet and needle clip position) to get the best throttle response, and then work hard at riding that thing with 1 or 2 fingers on the clutch at all times. Yes I know, basic motocross technique, but I found I didn't have to "try" to keep my finger there when I realized how useful it is. Not only does it provide instant power and traction, I have great control over the bike in the air. Any goon can panic rev, but when I'm really pulling hard out of a corner with a fat jump straight ahead, it feels good to know I can stay on the throttle and when I'm about to fall off the bike mid air just pull in the clutch and smash the brake lever. I can do this in my sleep now, and I remember practicing at first how difficult it was to do all the motions in the air.

 

So basically, your best friend on that bike is going to be the clutch, don't make it a nice two way street friendship, abuse the shit out of it until its burned up then buy new plates. You can spend $1000 in sub par add ons and hours constantly jetting and re-jetting your bike for conditions, but i've found on the 125 a little bit of clutch goes a LONG WAY.

Edited by dan2581

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Id check the reeds, make sure the power valve is working, then go to work on the jetting.

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The Athena cylinders "port timing" has always been suspect. You might be able to get it closer by trying different thickness base gaskets. What you look for is to have the top of piston even with the bottom of transfer port "at BDC". Also adjusting the squish by tightening it up .032 to .030 is prime and maybe having the squish band widened a little. This creates more "MSV" Maximum Squish Velocity which will give it more bottom and mid power. Brent at TDC 2 Stroke Performance can cut your head and if need be adjust the port timing if the gasket trick doesn't work. Also sounds like a low compression ratio, if your not already just use straight 90 to 93 octane fuel it will run better.

 

www.tdc2strokeperformance.com

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I recently bought an 09 yz125 with full pro circuit exhaust, vf3 reeds, and the athena factory top end kit. It had 140 compression and didn't have much low end power but plenty top end power so I re-ringed the piston and it had 150-155 compression which I've been told is fine. But still no low end power. My friends 06 yz125 had good low end, what's wrong with my bike? Ill appreciate any help

I have an '08 YZ125 with an Athena 144 kit and I can tell you that out of the box athena cylinders have very poor internal port cast. Lots of irregularities and casting flash. This all compromises flow. I sent my 144 cylinder out to Pinnsonault Racing to have a "low/mid" port job. Worth every penny. I run a 7 oz FWW, and V-Force reeds, stock exhaust and my 144 is a fantastic and broad engine. I think your other factor of poor low end is your running a Pro Circus pipe which tend to be more "mid/top" type pipes. If you can put the stock pipe back on (or find an FMF Fatty), and bump your rear sprocket up to a 50T . This will be a huge help in getting some additional bottom end power.

Edited by Polar_Bus

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Thanks for the advise guys. And I've checked my reeds and they were good and also I've checked the powervalves and they work but they are not smooth when they work. Is that how they r supposed to be? And yes I almost never take my finger off my clutch. And I do abuse my clutches but when the engine runs like this it sux. My ktm is amazing. The Yamaha won't even let me pop the clutch because its got no power when it free revs it seems, but if I feather the clutch it goes good. And ok I'll check the squish and stuff too.

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Thanks for the advise guys. And I've checked my reeds and they were good and also I've checked the powervalves and they work but they are not smooth when they work. Is that how they r supposed to be? And yes I almost never take my finger off my clutch. And I do abuse my clutches but when the engine runs like this it sux. My ktm is amazing. The Yamaha won't even let me pop the clutch because its got no power when it free revs it seems, but if I feather the clutch it goes good. And ok I'll check the squish and stuff too.

 

I would strongly suspect a power valve problem. I have personally experimented with the PVs on the yz125 by mechanically holding them either in the full open or full closed position and I can tell you that the effect on top end and low end respectively in drastic.

 

The PVs must be fully closed at low RPM or you will have very poor low end power and an abrupt hit when transitioning into midrange. If one or both of your PVs is hanging up and not closing this could very well be the problem. The operation of the valves should be silky smooth. You mentioned they are not, so something is wrong. How exactly are you testing the operation?

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Ya I took off the pipe to take off the powervalve cover and put the pipe back on and started the bike and at idle it was closed but kinda wanted to bump up a tiny bit once in a little while and once I hit the throttle at all it would be all over the place and at higher rpms it was fully open but it didnt need very much rpms to open. At least thats how I remember it. My ktm's powevalve has nice and smooth operation. and like you said, when it hits the powerband its like a big yz85. no low end or mid and just top end. it like its got no powervalves. But nothing is broke that I can see. I checked that stuff when i pulled the cylinder off. Could something be wrong in the engine case? Thanks

 

And I think the flaps in the cylinder are good but not 100% on that. But I think I checked them when the cylinder was off

Edited by kaidpederson

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Ya I took off the pipe to take off the powervalve cover and put the pipe back on and started the bike and at idle it was closed but kinda wanted to bump up a tiny bit once in a little while and once I hit the throttle at all it would be all over the place and at higher rpms it was fully open but it didnt need very much rpms to open. At least thats how I remember it. My ktm's powevalve has nice and smooth operation. and like you said, when it hits the powerband its like a big yz85. no low end or mid and just top end. it like its got no powervalves. But nothing is broke that I can see. I checked that stuff when i pulled the cylinder off. Could something be wrong in the engine case? Thanks

 

And I think the flaps in the cylinder are good but not 100% on that. But I think I checked them when the cylinder was off

Interesting. So if I understand correctly, you observed that as soon as you give the bike the slightest amount of throttle the PVs are opening and bouncing (open/closed)? That sounds suspicious. It sounds like what might be expected if there was no spring in the governor system. Try this. take the pipe back off and unbolt the PV linkage arm from the PV shaft. Then see how much effort it takes you to pull the linkage up out of the case by hand. It should be hard but possible, and when you let go it should very quickly snap back down. Here is a diagram of the governor assembly. The shaft goes up due to centrifical force of the 4 spinning balls overcomming the force of the spring. The spring hold everything in check and stable. Maybe somebody messed with your spring?

 

https://www.thumpertalk.com/shop/oem.php?partcategory=345398&manufacturer=7&category=5&year=2009&model=6511

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I don't remember how hard it was to pull the rod up but there is resistance when I pulled the rod up when I had the cylinder off. But I'll check again. Ill check allot of stuff you guys suggested. Any other ideas?

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I took the bike for a ride today and the low end power was about were it should be. Today was much cooler then the day that the power was bad. So it's probably jetted just a little rich right? Thanks for all the help guys!

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What??? I don't know how you could confuse a minor jetting issue small enough to be compensated by a 1 week temperature fluctuation with the problem you described as seeming like it had no powervalves and no bottom end. You made it sound like a serious lack, not some little jetting squabble, mini tweak-tweak thinggy. Anyway, if you're happy, then I guess it's fixed.

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Ok this is an old post but here's an update. When I said that I took the bike out on a cooler day it was better but not that good. It just seemed to be good because it was better than what it was usally. Then next time I took it out it ran bad again. I swapped the 48t rear sprocket that was on the bike out for a 50t rear sprocket. That helped dramatically! I didn't have to feather the clutch anymore to keep it from bogging down when I poped the clutch anymore. I wasn't impressed with the low end power but I was ok with it. Now I was just replacing the right side crank case because of slight damage and fount out that my powervalves had a broken peice! I was pretty happy when I saw this. So the bike did end up having a power valve issue afterall. Here is the parts part # 3JE-11922-01-00. I had zero powervalve funcnsion so it was just like a really big bore yz85. I do not know how it runs yet because it not together yet and plus it winter but we are going to area cross next Saturday so I'll be able to test it then. Thanks everyone who posted!

Edited by kaidpederson
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Is the powervalve the stock one, or the Athena 144 valve?  Both my Athena valves broke, so I had my stock ones machined for the 144 Athena Cylinder. 

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