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1992 YZ 125 Bogging and fouling plugs


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I just bought a 1992 YZ 125, previous owner says he put a "Top End", like everyone else does when they sell a bike.  He felt strong it was jetting, i ran it for 1/2 a day before it started fouling plugs black, and now it is hard to start. I pulled the carb, cleaned it out, but can't tell what jets I should change, or if that is my issue.should I staet over and go back to stock jet sizes and see what that does?  It has an Mikuni aftermarket carb, and a pro circuit pipe that I think is for a 1993, not 1992.

 

Any suggestions or thoughts?

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Since you probably have no idea what you have in the carburetor, the first thing I would do is take it apart and see what size jets are in it, and find out what size and model the carburetor is. Clean it if course, but cleaning the jets doesn't cure slobbering and flooding. Look at one of the videos on setting the float. Set it and check for leaks around the seat. 

If the size has changed, as it was popular to out 38mm carbs on them, you can find the jetting for a 2001 and compare yours to it. 

But do some homework before you start throwing money at it.

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It most likely is jetting.  If you brought it over to my garage, I'd start with a compression and a two stroke specific leakdown test to confirm the motor is fresh.  Takes 20 minutes tops.  If it isnt, you can chase your tail with jetting. 

If it is tight and fresh, then I'd identify what jets are in it as well as what needle and its clip position along with air screw.   You can try leaning the bike the with the airscrew and raising the needle clip a position or two (lowering the needle) to see if it improves.  Ultimately, I'd suggest buying few jets and doing a proper step by step jetting session.   Jets are dirt cheap and having a few on hand is highly recommended for any two stroke owner.  

Lots of two stroke jetting instructions here and elsewhere on the net.  What works for one person doesn't always work for others.  Its as much as about how the bike is being ridden as it is where, temperature, elevation, humidity, and mods to the bike.   Lots of folks willing to help.   

Edited by Captain.Olives
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Monitor the gearbox oil level / amount.

If you find it regularly low despite no visible external leaks, the engine might ingesting oil thru a worn RH crank seal making it act rich/foul plugs.

Edited by mlatour
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One other tip-whatever you do, DON'T get discouraged!!!  I was exactly where you were a month ago, and with help from people on here we got it dialed in. Good news is you will be as fast as a pit crew chief when it comes to changing jets to get it dialed in 🙂  Jetting seems super intimidating at first (at least to me), but then when the clouds lift u feel pretty good hearing it run well!  Listen to the guys above-they definitely know their stuff!

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On 6/17/2020 at 12:20 AM, Captain.Olives said:

It most likely is jetting.  If you brought it over to my garage, I'd start with a compression and a two stroke specific leakdown test to confirm the motor is fresh.  Takes 20 minutes tops.  If it isnt, you can chase your tail with jetting. 

If it is tight and fresh, then I'd identify what jets are in it as well as what needle and its clip position along with air screw.   You can try leaning the bike the with the airscrew and raising the needle clip a position or two (lowering the needle) to see if it improves.  Ultimately, I'd suggest buying few jets and doing a proper step by step jetting session.   Jets are dirt cheap and having a few on hand is highly recommended for any two stroke owner.  

Lots of two stroke jetting instructions here and elsewhere on the net.  What works for one person doesn't always work for others.  Its as much as about how the bike is being ridden as it is where, temperature, elevation, humidity, and mods to the bike.   Lots of folks willing to help.   

Thank for the response!

I have 4 main jets stock 330, which was in it, then 320, 340, 350. The clip is currently set 1 down from top, or 3rd up from bottom, book says 3rd position is factory setting, which way is 1? (Just making sure, a little new to this). The needle jet is factory as well.  

Should I go up in number on the main jet or down 1st?  Will also do the compression test next.

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17 hours ago, getherejune said:

One other tip-whatever you do, DON'T get discouraged!!!  I was exactly where you were a month ago, and with help from people on here we got it dialed in. Good news is you will be as fast as a pit crew chief when it comes to changing jets to get it dialed in 🙂  Jetting seems super intimidating at first (at least to me), but then when the clouds lift u feel pretty good hearing it run well!  Listen to the guys above-they definitely know their stuff!

glad I read your quote, not gonna lie, was feeling pretty beat up!!

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On 6/17/2020 at 10:20 AM, mlatour said:

Monitor the gearbox oil level / amount.

If you find it regularly low despite no visible external leaks, the engine might ingesting oil thru a worn RH crank seal making it act rich/foul plugs.

Thanks!  Will watch that too!

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On 6/16/2020 at 11:56 PM, Adam Haegele said:

I just bought a 1992 YZ 125, previous owner says he put a "Top End", like everyone else does when they sell a bike.  He felt strong it was jetting, i ran it for 1/2 a day before it started fouling plugs black, and now it is hard to start. I pulled the carb, cleaned it out, but can't tell what jets I should change, or if that is my issue.should I staet over and go back to stock jet sizes and see what that does?  It has an Mikuni aftermarket carb, and a pro circuit pipe that I think is for a 1993, not 1992.

 

Any suggestions or thoughts?

Bro, i'm going to be a harbinger. I noticed you said it recieved a new top end. I say tear it apart. You will know for sure if its rebuilt or if you were shafted. Check your piston, and cylinder for wear, if so, go from there. Jetting? It depends on your living elevation from msl. I you are sea level, stock jetting would be hunky dory. Check that piston and cylinder...

9dd7d1ec53dd8444b0b6e0a3e4764afb.jpg

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10 hours ago, Adam Haegele said:

Thank for the response!

I have 4 main jets stock 330, which was in it, then 320, 340, 350. The clip is currently set 1 down from top, or 3rd up from bottom, book says 3rd position is factory setting, which way is 1? (Just making sure, a little new to this). The needle jet is factory as well.  

Should I go up in number on the main jet or down 1st?  Will also do the compression test next.

The needle works like this when it comes to what the clip position changes.

 

unnamed.gif

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5 minutes ago, SmokinJoe said:

The needle works like this when it comes to what the clip position changes.

 

unnamed.gif

Perfect!!  Visuals are always great!!  I am at the 2nd to the top.  I did notice the tip of the needle was slightly bent as well, may be an issue, ordered a full bottom kit also!  Thanks for the reply!!

Edited by Adam Haegele
didnt finish
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Just now, Adam Haegele said:

Perfect!!  Visuals are always great!!  I am at the 2nd to the top.  I did notice the tip of the needle was slightly bent as well, may be an issue, ordered a full bottom kit

Its bent? huh? Bro, if you aren't getting your slide and needle functioning correctly, you might have troubles. It might be hanging up on something. Bro, get the correct oem parts in there and get it running! Mkay?

250px-VanDriessen.jpg

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51 minutes ago, M E T A L A C I D said:

Its bent? huh? Bro, if you aren't getting your slide and needle functioning correctly, you might have troubles. It might be hanging up on something. Bro, get the correct oem parts in there and get it running! Mkay?

250px-VanDriessen.jpg

Yeah man, they are on order!!

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I’m not familiar with that era YZ125, But two strokes are two strokes.  It will be extremely helpful if you can identity the carb model and size, some folks may have direct experience with it (as you mention it’s aftermarket.)

Just as of not more important, is identityIng what pilot jet it has in it and what needle.  Just like jets, needles have different sizes and tapers.  

Have you tried leaning it out with the air screw?  

What kind of riding are you doing with it?   When is it fouling?  
 

the pilot, airscrew, needle, and main jet all control fueling at different throttle openings.   
 

After getting your new needle to replace the bent one,  go through the carb and ensure it’s in proper running order including float height.  If you don’t have a few leaner pilot jets, I’d suggest getting some too.  
 

I think you are jumping the gun thinking about the main jet unless you are on the track and running wide open all the time.   If you are non expert just trail riding, more than likely be between pilot, air screw, and needle you should be able to clean it up.  
 

Mark off your throttle tube with tape at 1/8, 1/4, 1/2, and 3/4.  Why? Because that’s going to help you isolate what circuit is too rich and fouling your plug.  Those markings will roughly correlate to the pilot/air screw, needle, and main jet.   

Start with the pilot and airscrew and then work your way up. 
 

Its a process of elimination sorting a new to you bike, especially an old one.   So patience will be key.  
 

don’t assume anything, eliminate them systematically.  
 

Personally, I’d want to verify the bike has good compression with a tester and that the motor is tight by doing a leak down NO matter what any seller told me.  I’d the compression is low, it can foul plugs.  If it’s not holding air, the wet side main seal could be leaking and fouling plugs.  And more....
 

if the bike doesn’t respond to to a leaner pilot, airscrew further out,  and leaner needle clip position, I’d be looking closely at the motor.  

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13 minutes ago, Captain.Olives said:

I’m not familiar with that era YZ125, But two strokes are two strokes.  It will be extremely helpful if you can identity the carb model and size, some folks may have direct experience with it (as you mention it’s aftermarket.)

Just as of not more important, is identityIng what pilot jet it has in it and what needle.  Just like jets, needles have different sizes and tapers.  

Have you tried leaning it out with the air screw?  

What kind of riding are you doing with it?   When is it fouling?  
 

the pilot, airscrew, needle, and main jet all control fueling at different throttle openings.   
 

After getting your new needle to replace the bent one,  go through the carb and ensure it’s in proper running order including float height.  If you don’t have a few leaner pilot jets, I’d suggest getting some too.  
 

I think you are jumping the gun thinking about the main jet unless you are on the track and running wide open all the time.   If you are non expert just trail riding, more than likely be between pilot, air screw, and needle you should be able to clean it up.  
 

Mark off your throttle tube with tape at 1/8, 1/4, 1/2, and 3/4.  Why? Because that’s going to help you isolate what circuit is too rich and fouling your plug.  Those markings will roughly correlate to the pilot/air screw, needle, and main jet.   

Start with the pilot and airscrew and then work your way up. 
 

Its a process of elimination sorting a new to you bike, especially an old one.   So patience will be key.  
 

don’t assume anything, eliminate them systematically.  
 

Personally, I’d want to verify the bike has good compression with a tester and that the motor is tight by doing a leak down NO matter what any seller told me.  I’d the compression is low, it can foul plugs.  If it’s not holding air, the wet side main seal could be leaking and fouling plugs.  And more....
 

if the bike doesn’t respond to to a leaner pilot, airscrew further out,  and leaner needle clip position, I’d be looking closely at the motor.  

Thank you very much for spending the time to go through this for me, it is very much appreciated!!  I've learned more in 2 days than ever!!  Been 20 years since I've had a bike, and my 14 yo son likes riding and working on bikes, so thats why we bought a cheap project bike, so this is great stuff for us to try out. 

Just finished up my buddies carb cleaning on his KLR650, so tomorrow, YZ compression test, and go from there......more to come I am sure!! 

Thanks again!!

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On 6/17/2020 at 12:06 AM, ossagp1 said:

Since you probably have no idea what you have in the carburetor, the first thing I would do is take it apart and see what size jets are in it, and find out what size and model the carburetor is. Clean it if course, but cleaning the jets doesn't cure slobbering and flooding. Look at one of the videos on setting the float. Set it and check for leaks around the seat. 

If the size has changed, as it was popular to out 38mm carbs on them, you can find the jetting for a 2001 and compare yours to it. 

But do some homework before you start throwing money at it.

Actually, I was wrong it is the stock Mikuini carb, and all the jets were stock size as well.

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Glad to see the homework paid off. Loads of good advice above. If you go to any thorough jetting guide you will see the first suggestion is to check the float and set it. If you haven't done that before you start all those other drills you are getting the cart before the horse.

You have to ask yourself why your particular rig needs leaner jetting than stock.  It can be the way you ride it, altitude etc. But there is a reason that the best jetting guides tell you to make sure the fuel delivery system is working right, before you start down the other rabbit holes.

 

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On 6/19/2020 at 12:50 AM, Captain.Olives said:

I’m not familiar with that era YZ125, But two strokes are two strokes.  It will be extremely helpful if you can identity the carb model and size, some folks may have direct experience with it (as you mention it’s aftermarket.)

Just as of not more important, is identityIng what pilot jet it has in it and what needle.  Just like jets, needles have different sizes and tapers.  

Have you tried leaning it out with the air screw?  

What kind of riding are you doing with it?   When is it fouling?  
 

the pilot, airscrew, needle, and main jet all control fueling at different throttle openings.   
 

After getting your new needle to replace the bent one,  go through the carb and ensure it’s in proper running order including float height.  If you don’t have a few leaner pilot jets, I’d suggest getting some too.  
 

I think you are jumping the gun thinking about the main jet unless you are on the track and running wide open all the time.   If you are non expert just trail riding, more than likely be between pilot, air screw, and needle you should be able to clean it up.  
 

Mark off your throttle tube with tape at 1/8, 1/4, 1/2, and 3/4.  Why? Because that’s going to help you isolate what circuit is too rich and fouling your plug.  Those markings will roughly correlate to the pilot/air screw, needle, and main jet.   

Start with the pilot and airscrew and then work your way up. 
 

Its a process of elimination sorting a new to you bike, especially an old one.   So patience will be key.  
 

don’t assume anything, eliminate them systematically.  
 

Personally, I’d want to verify the bike has good compression with a tester and that the motor is tight by doing a leak down NO matter what any seller told me.  I’d the compression is low, it can foul plugs.  If it’s not holding air, the wet side main seal could be leaking and fouling plugs.  And more....
 

if the bike doesn’t respond to to a leaner pilot, airscrew further out,  and leaner needle clip position, I’d be looking closely at the motor.  

Couple questions for you if you don't mind.  where do I get the right float height measurement?  What can you use to measure it?

2nd Q - can you do a compression test with the carb pulled?

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