Two Stroke Motocross

Two Stroke Motocross Forum => Technical => Topic started by: bunghole731 on March 07, 2012, 06:28:35 AM

Title: YZ125 jetting questions.
Post by: bunghole731 on March 07, 2012, 06:28:35 AM

Hey everyone, Im new to the forum but i have been lurking here for a while, great stuff.

Anyways, a few months back I ditched my  07 yz250f 4 stroke and picked up an 05 yz125. Relatively new to these 2 stroke beasts but i couldn't be happier with my decision.  The only thing is I noticed alot of stuttering and cutoff in higher rpms. It just doesnt sing like it should. It idles perfect, for about 8 seconds and the mid range power is fine though. I am in the process of doing the top end and could use some insight onto where to go from here jetting wise. When i opened the powervalve top it was packed full of gunk unburned oil. Im almost positive its running too rich. The plug is black and wet and the piston doesnt look to great, and i think it was a contributing factor to the poor operation in higher rpms.

I am looking for some advice on where to go from here, runing 40:1 maxima K2, jetting is stock, 410 main, 40 pilot, 4th position clip. Along with a new top end, the silencer is going to be repacked, reeds cleaned and checked, powervalve cleaned so i have a fresh base to jet off of.

also, i notcied some pretty bad blow by on the piston, the ring was out of spec aswell.

So what do you guys think? Rich? what to change first? any help would be greatly appreciated!
















And heres the bike just for some eyecandy

Title: YZ125 jetting questions.
Post by: TMKIWI on March 07, 2012, 07:56:44 AM

Welcome.
You are doing the right thing by rebuilding the top end. (There is no point trying to jet a worn engine.)
It might pay to check your gearbox oil level as well. The piston looks very wet.
You may have a leaking crank seal.
Also check your needle and seat are not leaking and your float level is correct.
It should be straight forward to jet if everything is good.

Good luck.

Title: YZ125 jetting questions.
Post by: SachsGS on March 07, 2012, 04:37:32 PM

First fresh top end,clean powervalves,repack silencer and make sure airfilter is clean and not over saturated.Then air screw 1 and 1/2 turns from seated,return needle clip to middle (third) position and start going down on the mainjet one size at a time doing plug checks as you go.Not super familiar with the latest YZ125s but have a hunch your bike will be happy in the 380 range.

Title: YZ125 jetting questions.
Post by: Recovered on March 07, 2012, 10:15:09 PM

air screw for your stock jetting should be 2 1/4 turns out from seated try going down 1 size leaner on your main and re adjust your air screw

Title: YZ125 jetting questions.
Post by: bunghole731 on March 08, 2012, 01:55:17 AM

Got a 390 and a 400 main to try out. still have to finish up on the silencer and top end. Also i bought some drain cleaner to attempt to clean out the head pipe, going to give that a try because if its anything like my powervalve was it has to be robbing some power.

Thanks for the advice!

Title: YZ125 jetting questions.
Post by: cnrcpla on March 08, 2012, 02:14:44 AM

Quote

Also i bought some drain cleaner to attempt to clean out the head pipe, going to give that a try because if its anything like my powervalve was it has to be robbing some power.

  Let me know if the drain cleaner works, I would like to try that on my header pipe too 

Title: YZ125 jetting questions.
Post by: bunghole731 on March 08, 2012, 03:30:01 AM

For sure, ill take a bunch of pics.

Title: YZ125 jetting questions.
Post by: Recovered on March 08, 2012, 08:54:20 PM

non chlorinated break parts cleaner also works for cleaning carbon out of the pipe

Title: YZ125 jetting questions.
Post by: bunghole731 on March 08, 2012, 10:38:14 PM

so i just got done cleaning the pipe. I used sodium hydroxide drain cleaner crystals, and it didnt do anything. literally came out a slight hue of brown, no carbon taken off nothing, and the stuff burns your skin and takes the paint off your pipe. Waste of time. I ended up putting a piece of cable on a drill and spun it/ rinsed like 5 times. That worked really well. So my advice, just either burn the carbon out or use a piece of cable and a high power drill.

Im trying 400 main, and 3rd position clip for the jetting. When i checked the air screw it was like 4 turns out so i adjusted that back to 2 1/4 and i set the float height.

going to do the heat cycles and plug testing tomorrow i think. still have to repack the silencer, change tranny fluid and refresh coolant.

Title: YZ125 jetting questions.
Post by: dk on March 10, 2012, 05:03:14 AM

Where are you in Cali?  I just bought a 2005 yz 125 and I had the same issues.  Bike ran good but would sputter and not run smooth at high rpms in the lower gears.  I did the top end and the overrev is back with no weird sputtering or hesitation.  My piston looked just like yours along with the power valve being gunked up.  Stock jetting.

My new problem is a slight bog or hesitation after landings.  I have not checked the float bowl height yet and i belive this is the problem.

Title: YZ125 jetting questions.
Post by: bunghole731 on March 10, 2012, 06:48:41 AM

Im in San diego, and don't mess with the float, unless its obvious its the problem. I did and ended up putting it back to the way i found it. Anyways, right now im running 400 main, 37.5 pilot air screw about 2 turns out, and clip at 3rd.

I have some surging and hesitation at about mid throttle if i jump on it from low rpms. It kind of bogs then picks up, i think moving the clip back to 4th position will solve this and then it will be spot on. Plug tests so far show signs of great combustion, not lean, not rich.  The repacked silencer, new top end and the current jetting make it worlds better. Best of luck!

Title: YZ125 jetting questions.
Post by: Dirt Addict on March 11, 2012, 11:59:22 PM

i just picked up a well used 05 yz125.  the previous owner is a douche.  after i got the bike home i found it had a leak from a crack near the drain plug.  he had it nearly empty of gear oil and a skid plate covering the evidence. the cylinder had the plating chipped away from predetonation . the head wasn't any better. 
anyway, fast forward to this week... i got a new cylinder and head, used quik silver epoxy on the case  and have everything cleaned up.
i'm taking it out tomorrow for it's 1st test run.
jetting :
410 mj
37.5 pj
stock needle 3rd pos
a/s 1 1/2 out
elev 3200',  temps 50-60 f  pump premium 32:1 amsoil syn

Title: YZ125 jetting questions.
Post by: bunghole731 on March 12, 2012, 01:22:55 AM

Ahh man thats no good, but it sounds like you got it figured out.

If i was mixing 32:1 i would go back to 410 on the main, so i think you'll be happy with the jetting you got.

Im at about 2200' above sea level, and temps ranging from 60 to 80 f lately.

Title: YZ125 jetting questions.
Post by: Dirt Addict on March 12, 2012, 03:42:01 AM

I'm taking it to I-5 in the morning. I'm thinking I should be close with the jetting. It's ok if it's a little fat for the break in ride.
i ordered some Torco octane boost. from everything I've read it's the real deal and should get me close to 100 octane. That should take care of any predetonation. I put some new plastics on it and it's ready to go....

Title: YZ125 jetting questions.
Post by: bunghole731 on March 14, 2012, 12:07:57 AM

so this whole time ive been trying to jet this bike and i cannot get it right. Im thinking about putting everything back to stock and running 32:1. Its at 40:1 and I put a 37.5 pilot 2 screws out, tried a 400 main and it felt too rich,sputtering at full throttle, went up in the needle still too rich. Tried 390, too lean got surging and a light plug.

Any advice? im stumped on where to go from here...

Title: YZ125 jetting questions.
Post by: bunghole731 on March 16, 2012, 12:01:09 AM

Ive done everything. i really am starting to think its electrical. I get great power up until just after the powerband and it just bogs. It feels like its not getting spark and it cant burn the fuel. Im going to check all the connections and see what it can do.

Title: YZ125 jetting questions.
Post by: cnrcpla on March 18, 2012, 12:07:44 AM

If you are worried about the electrical, try a simple spark test. If it sparks a nice bright blue, then its not an electrical issue. If its dull yellow or worse, change the plug or look at the stator. Sometimes it can get build up that prevents it from generating a nice spark.

Title: YZ125 jetting questions.
Post by: bunghole731 on March 18, 2012, 12:49:05 AM

i got it figured out. When i first bought the bike i messed up the stator being a dumbass. Anyways i bought an 06 stator and the cord was too short so i had to splice in the 05 harness to make it long enough to reach the cdi box. When i spliced it i used krimp connectors, and one of those had come loose causing my whole problem with jetting. I took it all apart and soldered the connections and it runs like a completely new bike. It just keeps pulling.

Thanks for all the help guys now it runs like a dream, wasn't even jetting after all.

Title: YZ125 jetting questions.
Post by: cnrcpla on March 18, 2012, 12:58:07 AM

Nice to hear  Have fun riding it. 

Title: YZ125 jetting questions.
Post by: TMKIWI on March 18, 2012, 05:38:11 AM

Never use a normal crimp on a dirt bike.
Too much water around that will get into the electrics.
glad you fixed the problem.
Lesson learned.