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03 yz125 40:1?

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Just picked up a 03 yz125 and the guy said he ran 40:1 in it. I got it home and took it for a quick spin around the neighborhood, afterward spooge was running down the can, and it was a very short ride. It's my understanding that 40:1 is LEANER than 32:1, is this correct? He also said that on 32:1 it fouls flugs like crazy. Looks like I get to dive into some jetting yet again.. everyone's favorite past time.

Also, the bike came with a FMF SST and FMF Fatty, will one jetting setup work for both?

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40:1 is a leaner oil ratio than 32:1, but jetting wise it will run richer (air/fuel ratio) than 32:1 premix since less oil is displacing fuel. If it is not fouling plugs with 40:1 premix, then it shouldn't foul with 32:1.

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I guess I should have clarified when i say leaner i meant jetting wise. So as far a jetting is concerned, 32:1 is leaner than 40:1, and may be the propper premix to use. would 40:1 over 32:1 cause extra spooge? There's more oil in 32:1 so I'm temped to think there could actually be more?

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I guess I should have clarified when i say leaner i meant jetting wise. So as far a jetting is concerned, 32:1 is leaner than 40:1, and may be the propper premix to use. would 40:1 over 32:1 cause extra spooge? There's more oil in 32:1 so I'm temped to think there could actually be more?

Generally, if you have a major spooge problem at 40:1, you will probably still have one with 32:1. It may help it a little, but I have a feeling you will still need to jet a little leaner to fix your problem. Personally I would be running at least 32:1 in a 125, but that is a whole other debate.

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repack the muffler, and see if that helps...... and work on the jetting some more

i run 40:1 i didn't have any spooge till i 144ed it and didn't get it jetted perfect like before

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There is no debate ,,, A 125 should run 32:1.

Agreed!!

I prefer the fatty over the SST. Actually I don't like the SST at all. Best thing to do at this point is pull the jets and see what you have. On a stock '03 I've found a 32.5 pilot and a 400 main should get you in the ballpark on the jetting however different brands of premix can also effect the jetting. You will need to repack the silencer as I'm sure it's soaked.

Another thing to look into would be overall mechanical condition. A worn top end, worn or damaged reeds, and even a dirty air filter can give symptoms of rich jetting. Most used bikes always come with a "fresh top end" but unless they provide receipts I would be skeptical.

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Well I pulled out the carb and gave it a cleaning. It hadn't been pulled in quite a while. The vent tubes were routed odd and stuck in thier position, took a little work to get them better routed. I also pulled the jets to see what they were. The main is 420, and the pilot is a 40. I didnt look at the needle.

Right now the bike has the SST on it, the Fatty has a nice dent that i need to get popped out before I put it on the bike. I'm going to jet for the SST and hope the Fatty is close. Either way right now its pig rich.

The engine feels good, and has strong compression you can feel through the kick starter. I just relized I forgot to take a look at the plug while I had it torn apart.

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yea, repack the silencer. While i was running my bike with a bad crank seal, a crap full of oil gets sponged with the old packng,replaced crank seal and threw a diff silencer and ran crisp!! no spooge, threw on the other silencer and a crap full of oil is all over my swing arm etc. but i did go down a step in the main jet after i replaced crank seal. Have fun jetting:smirk:

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Well i threw my fatty in the freezer to get a dent out. Tomorrow I am going to the local powersports dealer and getting some jets and some repacking. I'm going to get 32.5-37.5 pilot jets (have 40) and I'm going to get 350-410 main jets (have 420). What are the chances the needle needs to be swapped? I'm going to play with the clip, hopefully the needle thats in there will be good.

The midrange is where the bike feels strange and lacking.

I also don't really want to put nice new packing in just to soak it with spooge. Should I really be concerned about it?

Edited by Bumperlt

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I'm going to get 32.5-37.5 pilot jets (have 40) and I'm going to get 350-410 main jets (have 420).

You shouldn't need anything smaller than a 390 for the main. What position was the needle clip at?

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I don't know I didnt look while I was tinkering.. I'll take a look tomorrow.

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Agreed!!

I prefer the fatty over the SST. Actually I don't like the SST at all. Best thing to do at this point is pull the jets and see what you have. On a stock '03 I've found a 32.5 pilot and a 400 main should get you in the ballpark on the jetting however different brands of premix can also effect the jetting. You will need to repack the silencer as I'm sure it's soaked.

Another thing to look into would be overall mechanical condition. A worn top end, worn or damaged reeds, and even a dirty air filter can give symptoms of rich jetting. Most used bikes always come with a "fresh top end" but unless they provide receipts I would be skeptical.

*****************

Yep a 32.5 x 400m should get you up and running. also could try a 35x390m.

These sugestions are for 32:1

Never ever believe a used bike you just bought has a fresh top end.

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+1 on the repack. I recently bought a used 125 and it was splooge city even once I sorted the jetting. It was just left over from the previous owner that didn't know how to twist the throttle. He just putted around on it.

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*****************

Yep a 32.5 x 400m should get you up and running. also could try a 35x390m.

These sugestions are for 32:1

Never ever believe a used bike you just bought has a fresh top end.

Im going to get a 32.5, 35, 390, and 400. And a repack kit. And some new plugs. And a new clutch lever. Tomorrow.

The guy never said it has a fresh top end. I would replace the piston but I don't have the extra 100 right now. I'm going to pull the jug off tomorrow and take a look though. I can feel strong compression through the kickstarter and I think thats a good indicator. If it looks like its in bad shape it'll stay in the garage for a few weeks til I've got the cash. I'm hoping it will last through summer, and then I am going to buy the wiseco top/bottom kit and redo the whole thing. It vibrates alot more and just doesnt feel as smooth as the 96 rm125 i traded it for (that i had just rebuilt).

Generally its not a good idea to work on jetting without new packing and a fresh top end, I know this, but to be honest I want to hone my jetting skills and I aint got shit else to do tomorrow.

+1 on the repack. I recently bought a used 125 and it was splooge city even once I sorted the jetting. It was just left over from the previous owner that didn't know how to twist the throttle. He just putted around on it.

It's my understanding that while 125's love to be on the pipe, if its jetted right, it will run decent no matter where the slide is and won't foul plugs.

Although if I have my info right, when run at slower speeds, the plug doesnt have a chance to get hot enough to burn off the spooge, therefor spooge can still be present.

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I'm going to pull the jug off tomorrow and take a look though.

Don't pull the jug just yet, pull the pipe first and take a look through the exhaust port. If you don't see any signs of blowby you should be fine for a little while anyway plus you will save the cost of the gaskets.

It's my understanding that while 125's love to be on the pipe, if its jetted right, it will run decent no matter where the slide is and won't foul plugs.

Yes, for the most part that is correct. There are some premix oils that have an extremely high flash point. These oils can cause fouling even with proper jetting if you putt around on the bike long enough.

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Run the ratio the oil manufacturer recommends. One jetting does not fit all everywhere and one bike can differ from another. Jet "your bike" accordingly to your altitude, temperature and humidity.

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I repacked the silencer, put a new air filter on, remixed the fuel to 32:1 (added 1.6oz 2gal at a time), and I think its improved.

Silencer- There was no allen screws on my turbinecore2. I had to use my ezouts to get them out. They may have been screws at one point, but they aren't anymore. I just pounded them back in after i was done. Also, only a little over half the packing came out, the packing around the SA stayed. I thought about taking out the what appeared to be rivets to get to it, but I didn't. I just repacked what I could.

Jetting- I put in a 390 and 35. Seems to be working well. Only time will tell. My rm125 seems like it was faster and had a harder hit, maybe because it was bored to 56mm (making it a 134). Is the yz125 known for lack of a hit?

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most people think its dumb... but if you really want you can run 50:1 and it will run crisp... well that is if your jetting settings are already rich... if i really wanted i might even be able to run 70:1 but im not guna take that risk : )

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Never ever believe a used bike you just bought has a fresh top end.

Unless there's receipts and photos like mine. 👍

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