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new top end - issues

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Hi everyone,

Been a reader of this forum for quite some time, but just joined up now to get some input on an issue im having.

Bought a 2002 yz 125 in excellent condition off of a mechanic buddy of mine (30 or so hours on bike since new) last week. Took it out for a rip and it performed perfectly.

The next day, we changed out the top end (new piston/ring etc..) as i wanted to get the bike back to as close to "new" as i could.

Couple days later, threw in some fuel, pulled up the choke, gave it 2 kicks and away she went at a typical choked high idle. The first thing i noticed was a light pinging sound which appeared to be coming from the engine, but this is not verified yet. Second, as the bike warmed up and the idle regulated to a lower rpm, it died. Kicked it over again, it started up right away again, turn the choke off and again it died. Third time, gave it a tiny bit of throttle (16th of a turn or so) and the engine maintained an idle, but if the throttle was increased to around a 1/4 turn the bike would totally bog out (lack of snap and power completely). Sounded like a 4 stroke for a second there. I havent done anything with the bike yet, but the first thing (of course) will be taking a look at the sparkplug for any tell tale signs and from there who knows. I'm thinking it could be a fuel/air mixture issue with the increased compression of a new piston? Maybe not. im not sure. was looking for some input from anyone who might have crossed this bridge before or any tips or leads to start on.

Other things to know - I know everybody has their "perfect" fuel/oil premix ratio. 32:1 being the yamaha "yamalube" reccommendation. For this bike since day 1, its been on amsoil synthetic at 50:1 (20$ a quart) which has worked flawlessly. The old piston still had factory cross sections on it when taken out, which to me says its been lubricated nicely. The new (OEM) piston was coated with oil when installed, so don't fret about gas/oil ratios or lack of lubrication. I just know someone will have a seizure, when anyone mentions 50:1 around forums.

Thanks!

Mitch

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sounds like it would be a top end problem but your signs sound like a jetting issue? specifically your pilot jet! 50:1 (not knocking it) is too lean for my taste so i cant give you any suggestions on jet sizes but you said it ran fine the day before so id start by making sure the bike is getting fuel! check your boot to make sure its seated correctly and your air filter is clean! the only other suggestion i have is to make sure your head is torqued to proper spec and your ring is aligned correctly with a proper end gap and check your reeds to make sure there are no cracks or gaps! good luck with you diagnosis and keep us informed!

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Thanks ftball90. We are tackling this tonight with high hopes of hitting the track tomorrow. The torque on the head was one thing that came up to be checked as we just wrench tightened it to feel. Air filter is tip top and ring is on properly. won't fit in the cylinder any other way, as i found out! haha

Yeah, 50:1 isn't my choice, but thats how it's always been run and when i had it out, it really ripped. Around here we like to say "if it ain't broke, don't fix it" so im leaving it at that ratio. Im an older guy and i was impressed by the little smoker when i had it out. im REALLY thinking it has to be a minor problem (im an optimist).

Thanks for the good advice. ill check the boot and reeds as well.

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Thanks ftball90. We are tackling this tonight with high hopes of hitting the track tomorrow. The torque on the head was one thing that came up to be checked as we just wrench tightened it to feel. Air filter is tip top and ring is on properly. won't fit in the cylinder any other way, as i found out! haha

Yeah, 50:1 isn't my choice, but thats how it's always been run and when i had it out, it really ripped. Around here we like to say "if it ain't broke, don't fix it" so im leaving it at that ratio. Im an older guy and i was impressed by the little smoker when i had it out. im REALLY thinking it has to be a minor problem (im an optimist).

Thanks for the good advice. ill check the boot and reeds as well.

my guess is its a minor prob too

but if your ring end gap is out of spec it can create a lot of problems... even if its in the correct way! and i agree if it has been ran 50:1 and your not having problems with it i wouldnt change it, i didnt pay attention to where you were from but if the weathers getting colder the air gets more dense which will cause the mixture to lean out even more, and at a 50:1 ratio id be extremely cautious! i didnt think about this the first time but your mixture could be too lean causing it to bog/die! and you know as well as i do a lean mixture will cause serious top end problems!

i would hate to see you have a blown top end because of your bike leaning out too much! personally i always rejet my bike in regaurds to the current weather (richen it up when its colder and lean it out when it gets warmer)

just another suggestion, you might look into a jd jet kit it will come with everything you need for around $75 and i think its well worth the money! then you can change your ratio to something your more comfortable with and jet it accordingly! and have the peace of mind to not worry about your bike being too lean.

good luck:ride:

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The trouble shooting process should start with a comp check. A auto zone $40 one is fine , and will tell you whats doing in there.

Then onto other potential probl areas like jetting.

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amsoil does reccomend 50:1 for dominator. thats what i run, not at 50:1 though, i run 40:1. you shouldnt have any problems with 50:1, im assuming amsoil knows what they are talking about.

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sounds like jetting to me. However, that 50:1! I nearly had a seizure! :bonk:

:excuseme:

NOT if your useing a good oil like Amsoil... We've ran everything from KX65 to 125's to 250's to my 300exc on 50:1 mix with no problems.... Living up north I belive your problem is jetting. It's gotton cooler and the air is heaver....

i'd start with a bigger polit jet and go from there....And..$20 qt for Amsoil???

I need to be selling you, your oil...I pay $9.59

BTW...how do you nearly have a seizure???? Mine have always been good until it just locked up.... Even when the clip jump out and trashed the cylinder...

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yeh $20 a quart is outrageous. i pay about $9 a quart too. which is still outrageous, but its good stuff. i pay about $12 a gallon for the cheap stuff for my jetski

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NOT if your useing a good oil like Amsoil... We've ran everything from KX65 to 125's to 250's to my 300exc on 50:1 mix with no problems.... Living up north I belive your problem is jetting. It's gotton cooler and the air is heaver....

i'd start with a bigger polit jet and go from there....And..$20 qt for Amsoil???

I need to be selling you, your oil...I pay $9.59

BTW...how do you nearly have a seizure???? Mine have always been good until it just locked up.... Even when the clip jump out and trashed the cylinder...

Sorry, was a joke. No engine seizure. OP made it sound like some here may freak out about 50:1 so I went ahead and "did" that.

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yeah let us know what the problem is. I just put a new ring on my bike and it runs fine bunt just dont sound right.

I have a question since i know nothing about jetting, if the bike ran totally fine for this guy then he put a new top end in, why is the jetting all the sudden off if it was fine before?

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yeah let us know what the problem is. I just put a new ring on my bike and it runs fine bunt just dont sound right.

I have a question since i know nothing about jetting, if the bike ran totally fine for this guy then he put a new top end in, why is the jetting all the sudden off if it was fine before?

As the weather gets colder the air gets heaver which means the bike need more air.

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As the weather gets colder the air gets heaver which means the bike need more air.

The bike would really need less air in cooler temps, or more fuel. You would jet more rich for low temp conditions.

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And wouldn't replacing a worn out topend possibly restore lost compression (my guess on where your pinging is coming from) and become a much more efficient air pump with more power (after jetted correctly)?

A fresh piston/rings renews a motor's ability to provide power through standard compression....a worn out topend is obviously a poor air pump and lacks compression. It makes sense that it was time for you to replace the topend, and the need to re-jet validates that necessity (in my book)....

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Hi everyone. First off, a very sincere thanks to you all for being very helpful and giving great tips!

We cleaned out the carb, did a few very slight idle adjustments, broke in the new piston proper and away she went screaming! I was being hesitant with doing too much with carb adjustments, as im still new to working on these things, but my mechanic friend helped out.

Also, checked the plug and it was BLACK! imagine that! running rich at 50:1! My buddy's white garage door now had a HUGE black mark on it from clearing out the carbon and oil. That will teach him to mess with bikes after a few drinks! One quart works out to around 10 gallons of fuel mix or a bit more. 20$ i spent at the fancy downtown dealership is brutal for a quart, but i got in touch with my local amsoil rep, and he's going to sell to me direct @ $110.00/case of 12. So now i get it under 10$/quart. :banghead:

anyway, i took it out to a national track in my home city yesterday, revved her to the moon, checked the plug again, and it was that sweet brown/tan color that everyone loves to see! Synthetic seems to be working well for me.

Noticed that driving like a teenager at a track can make an older fella like me feel REAL sore the next day, but noone can get the smile off my face, so well worth it!

Cheers guys!

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Great Thread, Always nice to hear a happy ending, and great to have that smile after a day riding. Hope my plug stays that nice brown too.

Cheers,

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I was wondering about break-in while reading this as you never said anything about it!! using the oil to lube the piston must have fouled the plug??

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I was wondering about break-in while reading this as you never said anything about it!! using the oil to lube the piston must have fouled the plug??

y'know, i initailly thought i fouled the plug and grabbed a few more. after i pulled it and saw that it was black as hell, i checked for spark and there was a healthy one against the case. so she was still good. still using the same plug now with 2 spares in my kit. i think you are correct in the extra oil on the piston contributed to the bike bogging initially. That and the crud in the carb.

Like i said before, im very new to working on these 2 strokers even though i realize their simplicity and was very hesitant in pushing the revs after the top end was done without experienced help. When the bogging started and the idle died, i felt that going further than my knowledge was just asking for an expensive repair. Thus the forum information request here and having my mechanic friend to help me out before i went too far and regretted it.

edit: ALSO, after we had it out to the track, i was lucky enough to have my buddy around to try her out for a few laps, and he figured that a bit of jetting adjustment is in order for peak performance. So Butta, you are correct in your assumption according to him.

Edited by mitcho

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