Posts
3945
Joined
4/1/2008
Location
Visalia, CA, USA
Fantasy
Edited Date/Time
5/28/2015 6:30pm
My son has had little opportunity to ride the YZ125 he got for his birthday in November, but it's always had a very, very slight bog when snapping the throttle open. About 3 weeks ago we pulled it out, threw some new 100LL / Maxima @ 32:1 in there, and took it for a spin. The temp had gone up a bit since he first rode it, and the bog got worse as the day wore on. Took the YZ to the track a week later in 90 degree weather, and the bog is much worse. When rolling on the throttle slowly, it will pull fine all the way through the top end. No issues snapping the throttle open on the stand, it's when it's rolling along in gear that it's an issue. Snapping the throttle open produces a huge bwaaaaaaaaaaah, and then after a couple seconds of bogging it starts pulling and cleans up as the rpms climb. Once the rpm's get high enough, it pulls great.
The prior owner had ridden it with this setup before, so there's not much changed. He did neglect to put oil in the fuel the last time, so I rebuilt everything. Fresh crank, bearings, seals, and .5 oversize piston (it's sleeved- yuck). Motor is ported/case matched, with a PC pipe/silencer. Clean air filter, reeds look great. Maybe 2 tanks of fuel through it since the rebuild.
Thinking it was rich on the 430 main, I kept working my way down to no avail. Overall I tried everything from a 390 main, up to a 460. Although the extreme small/big mains certainly had a different feel once the bike started pulling, the bog was still there. Raising the clip to the highest spot on the blue JD needle made no difference.
Tested the coil, stator, and CDI with a multi-meter. The coil was off a smidge, so I ordered a new one (cap and wire come with it). Installed it, headed out to the track and it's bwaaaaaaaaah again!
On a whim, I dropped the needle clip to the lowest setting, and the bog got better. Still there, but noticeably better. &%$#@!? It's getting hotter out, and the bike wants more fuel? This makes no sense.
Thinking that maybe I was fighting 2 battles at once (jetting and ignition), I picked up a stator. No improvement. Just to see what would happen, I raised the clip again. Bog city all over again. Dammit, I hate spending money on nonsense!
Yesterday I did these 2 videos in 70 degree weather. Video 1 is with the blue JD needle in the richest setting, video 2 is the leanest (clip at the top) setting where the bog got worse.
One thing that stood out, and I now remember being true before today was that as the bike got fully warmed up, the bog came showed up. When it's cold (well, warm cylinder- I warm a motor up before getting on the throttle), there is little to no bog.
Video one:
http://youtu.be/VAzQLGBzmJY
Video two:
http://youtu.be/6x8LRCSwZlg
Thanks for the suggestions.
The prior owner had ridden it with this setup before, so there's not much changed. He did neglect to put oil in the fuel the last time, so I rebuilt everything. Fresh crank, bearings, seals, and .5 oversize piston (it's sleeved- yuck). Motor is ported/case matched, with a PC pipe/silencer. Clean air filter, reeds look great. Maybe 2 tanks of fuel through it since the rebuild.
Thinking it was rich on the 430 main, I kept working my way down to no avail. Overall I tried everything from a 390 main, up to a 460. Although the extreme small/big mains certainly had a different feel once the bike started pulling, the bog was still there. Raising the clip to the highest spot on the blue JD needle made no difference.
Tested the coil, stator, and CDI with a multi-meter. The coil was off a smidge, so I ordered a new one (cap and wire come with it). Installed it, headed out to the track and it's bwaaaaaaaaah again!
On a whim, I dropped the needle clip to the lowest setting, and the bog got better. Still there, but noticeably better. &%$#@!? It's getting hotter out, and the bike wants more fuel? This makes no sense.Thinking that maybe I was fighting 2 battles at once (jetting and ignition), I picked up a stator. No improvement. Just to see what would happen, I raised the clip again. Bog city all over again. Dammit, I hate spending money on nonsense!
Yesterday I did these 2 videos in 70 degree weather. Video 1 is with the blue JD needle in the richest setting, video 2 is the leanest (clip at the top) setting where the bog got worse.
One thing that stood out, and I now remember being true before today was that as the bike got fully warmed up, the bog came showed up. When it's cold (well, warm cylinder- I warm a motor up before getting on the throttle), there is little to no bog.
Video one:
http://youtu.be/VAzQLGBzmJY
Video two:
http://youtu.be/6x8LRCSwZlg
Thanks for the suggestions.
Bog off the bottom is your Pilot Jet - circuit. Too lean will cause the "Bog" when snapping the throttle. Check your Air screw setting. Count the number of turns it takes to bottom the screw. Then back it out 1-1/2 turns as a starting point. If it bogs on the stand when checking the response, turn the screw in 1/2 turn (reducing the amount of air flowing through the circuit) this will richen the mixture. If it get better but still there, try another 1/2 turn in. If better then you may want to go to a larger Pilot Jet (35 to a 37 or 40 for example).
Basically the engine is not getting enough fuel (through the pilot circuit) to match the air flow, thus causing the bog....
Set you Clip back on the second groove of the Needle.
Good luck, let us know how it does.
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like arkmx says, if you snap the throttle open and it lean bogs/cuts out, its the pilot circuit. either the pilot jet is too small or the pilot circuit is clogged.
dont mess with the main or needle to try and fix it, go back to stock and get the pilot circuit squared away first.
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Stock jetting is very good on these and anywhere close to stock will not run that shitty.
Also, the extreme bluing of the pipe suggests major leanness.
You aren't running a V-Force by chance? If you are, get rid of that POS. I had one on my 2006 YZ125 and I could not get that thing to seal up tight to save my life. Always had gas ooze under the reed block on top of the cases.
When my V-Force was sucking air it acted just like that. Would be fine on the stand and would be fine early in a moto. But as the bike got good and hot it would start sucking air and get the major throttle snap bog.
Either way, seal the reed block and manifold up with a nice even coat of Moto-Seal or Permatex copper.. have to use something that is impervious to gas.
Other possible air leak locations... cylinder base gasket, crank seals. Even the right crank seal can cause that on this bike. Most people think of a bad right side seal as sucking tranny oil, but on the 2005+YZ125 the engine configuration puts the crank really high above the oil level, and the tranny is vented to the atmosphere so it can suck air.
Also check right side power valve shaft seal. Can suck air here too.
And buy a new float and needle/seat set for it. Checking the float level is a PITA on these, easier to just buy a new one and be sure.
http://youtu.be/6-NgipCJYTM
As you can see in the video, the PV works fine, and it's a stock reed cage.
I have a new float needle in there, although it will sometimes leak out the overflow while sitting on the bike stand, not running. However, a high fuel level would make it run rich throughout the rpm range, not lean. That also lends credence to FGR's idea.
1.) Clean the power valves-if they are gummed up and stay stuck closed it could create a major bog going to wide open throttle, probably not the issue in this case because the bike seems to be totally dying where if the powervalve was stuck it would steadily bog
2.) Who did the cylinder sleeve? Someone reputable? I have seen shops that supposedly install them incorrectly bore the cylinder and create tight spots making the piston to cylinder clearance to small in certain areas. I've seen as much as .010" difference in the port areas
3.) Although I kind of disagree with it being an airleak, double check your left side (stator side) crank seal for damage
4.) The one thing you mentioned that you messed with that made it better was the needle. Check for a bent needle. Try going back to stock needle in stock clip position just so you can count that out. The blue needle is the one for sea level in that kit right?
Did you do a compression test? What was it. Did you do a leak test?
If fuel leaks out of your bike just sitting there you need a new o-ring around the float seat.
thats slavens video i always link people to since im not gonna go out an make my own. this is a video he made to show people what a lean pilot circuit does.
0-1/4 throttle isnt going to give you that kind of throttle response as the throttle opening isnt large enough. basically whats happening is you have enough fuel to allow for clean idle operation, but not enough to support a clean transition
depending on the needle, the straight section works down to 1/16 throttle. depending on the straight section length, a needle clip change can have an effect that low in the throttle range, which is why you made progress by richening the needle clip.
"spanky's jetting guide" was a good source of info when i first got into bikes. something very important that is mentioned in his guide is that worn rings mimic a lean condition, so keep that in mind if the top end is worn.
Check the pilot circuit for gunk, dirt etc., make sure he passages are clean (spry with contact cleaner) and clear of gunk. It's possible something got in there. Works one day, then not the next time you try to ride. Murphy's Law....
Here are some images for EXAMPLES to help explain how the circuits work together....
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The whole different settings for Europe always seemed stupid to me. Weather matters, not what continent you are on.
The stock USA jetting works very well anywhere from 60-90 degrees and 0 - 5000 feet elevation. Yes, it can be fined tuned better at the edges of those ranges, but it is pretty damn good all around.
Those stock Europe settings look ridiculously rich. My YZ125 would run like crap if I put that in.
When my 2006 had the stock pipe it ran great with stock USA jetting. Tried playing with it and always went back to stock.
I put on a FMF fatty and shorty muffler and it got a little rich. Dropped the main from 410 to 390 and it totally rips now. I usually ride at 2500-4500 feet. If I rode at sea level or in thick sand I would probably put a 400 in for some safety.
Now that my son's back has healed, I've dove back into fixing the YZ.
I ended up bringing it to a local guy to figure out what the issue was.
What he found was that the machinist didn't bore the sleeve properly, and there was a ton of clearance between the piston and cylinder. Once it warmed up, there was too much clearance and it bogged like it was lean. So I sent off another cylinder to Eric Gorr along with an OEM piston. New Nikasil, ported, head mod for race fuel. Assemble it and ride up and down the street so it gets warmed up- no bog!!
Take it to the DT-1 track on Saturday, my son takes it for half a lap and the bike bogs out and dies. WTF!!
He pushes it back to the truck, I fire it up and make it a whole lap before it starts to bog out. Within a minute of it dying, I can restart it and go ride. I make a few laps at low speed and discover that it's more prevalent at large throttle openings, but sometimes it just wants to die.
There happened to be a guy with a stock 05 YZ125 at the track, and he was willing to let me swap the entire carb. I rode it for 3 laps, then my son got one in before the session was over. No issues. The jetting wasn't 100%, but it didn't want to just keel over and die when the throttle was twisted.
Paul Temmerman was nice enough to let me use the little practice track next to the DT-1 warehouse today. I pulled the entire carb off my 03 KX125 and slapped it into my son's 05 YZ125 and headed over there. After nearly a half tank of fuel, there were no big bog issues. The jetting isn't optimized, but it works. Just to test it out, I re-installed the stock Yamaha carb on the bike- big bog city.
So- it would seem that I have a defective carburetor (!?!?!?). Before I shell out just over $300 for a new carb, does anyone have any theories as to what might have caused the carb to have a stroke (heat stroke, really)?
My son's coming off a Judo related concussion sustained on Sunday, so he won't be back on the bike until next weekend. I have a little time to get it right.
Pit Row
Leak on float bowl gasket?
Sticky or leaky needle/seat?
Incorrect float height?
Check your slide for cracks.
http://youtu.be/JXLEAPPTWFc
I have tried every imaginable jetting and float combo, including putting the entire float assembly off the KX125 carb in the YZ carb to no avail. No air leak on the carb that I can find, no crack on the YZ slide. There is a small bit of wear on the bottom/side of the YZ carb, but it's really minor.
I'll be ordering a new carb this week. Trying to decide between another TMX or the Keihen.
One, there was a leak in the intake manifold that was only able to be detected with a leak-down kit that I made with $20 worth of parts from Lowes. Using some Hondabond in addition to the gasket solved that issue.
Two, the carb was defective. Seriously. Swapping to a carb from another YZ125 had it running great as did the carb from my KX125.
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