Jetting spode

Crisp BRAAAP!
Edited Date/Time 1/26/2012 10:46pm
I bought a 2006 YZ 125 this year and love it. But yesterday I encountered some serious jetting issues. The kid that I bought the bike from ran premix at 40:1 and puts in an octane boost. I used the tank that was in it, but when I went to refill, I put premium (what he said they ran) without the octane boost (I assumed that was hooey) and ran it at 32:1. I had also put Stabil into it because I had assumed that my season was over already. It ran fine for about 3 laps (or what I call a moto) and I put it on the stand. I assume it was still sucking the old gas primarily at that point. It pulled hard and didn't blubber or anything.

The next time I went out, it seemed to clean out normally in the pits, and it ran fine getting to the track. Once on the track, it loaded up quickly to the point where I wasn't sure if I could even get it off the track and just died. It was bad enough that I wasn't even sure a plug would fix it. It did, cleaned out and seemed fine again. But, once again, taking it back to the track gave me only about a lap before it started to hesitate and it tanked quickly again.

I haven't gone into the carb to see how it's jetted: the kids that I bought it from had extra jets, so they might have done something silly in there. Having said that, it had run fine previously. It's always been on the muddy side and did eat plugs on a somewhat regular basis - more than your average 125. But it's never quit on the track like that. Usually once I am on the pipe, it's good.

So I guess I have three questions: 1) Does Stabil fatten up the gas and cause fouling? 2) Is the octane booster really needed in premium on a stock bike with a stock pipe (FMF silencer). 3) This winter I'll dissect the carb: how should it be jetted? I am riding at Englishtown in NJ (roughly sea level). The thumper talk thread on this seemed to be a 400 main (maybe a 410) with a 30ish pilot. Any other help/knowledge would be greatly appreciated! Thanks for sharing.

Oh - and feel free to haze me for being a spode. I've earned it at this point. Nothing worse than having to push the bike back to the car. Twice. I am sure the 2 stroke haters were smiling.
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scooter5002
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scooter5002 11/28/2011 7:16am
11/28/2011 7:16am
What was the talent level of the previous owner and what is yours? Was he a blazing fast,always on the pipe rider, who used to the rear brake only to slow for corners, entering with the throttle still pegged? And you, more of a novice/useless old vet rider like me?
mx5471
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mx5471 11/28/2011 8:27am
11/28/2011 8:27am
We have an 06 yz125 that's been sitting a while waiting a motor rebuild and I don't remember what the carb setup is,but the carb is bored and the motor is modded. If you have extra jets,I would begin by setting up to the stock specs and take it from there. Stock pilot is #40. Stock main is #410. Stock needle is 6bfy42-74. Clip position is #3, (counting from the top). Pilot air screw is 2 and a quarter turns out.
mxtech1
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mxtech1 11/28/2011 9:01am
11/28/2011 9:01am
You should not be running stabil through a high performance engine. You are right the stabil causes the fuel to richen. And besides that you increased your oil ratio to 32:1. Dropping from 40:1 to 32:1 with no other changes requires a jetting change. I would recommend draining your fuel tank to get rid of that stabil, that stuff does more damage in MX bikes than it does good. I would also suggest returning the oil ratio to 40:1. That is a very safe level to mix your oil at and you won't have any reliability issues due to lack of oil. It will run alot crisper than 32:1. I typically will only run 32:1 during the first tank on a break-in ride after motor work has been done.

Next time you need to winterize the bike, forget the stabil. All you need to do is remove the bowl plug from the carb and let the fuel drain and air out for a few minutes. For the fuel in the tank, just pull the fuel line off at the carb and drain the gas into a gas can and put it in your truck or something. During spring time, fill with fresh pump gas and you will have a guranteed startup with fresh fuel in the tank and the carb.
mxtech1
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mxtech1 11/28/2011 9:07am
11/28/2011 9:07am
93 octane pump fuel is fine, you don't need any type of octane booster. Obvisouly, race fuel is the best choice but not usually an economical option. Try to run 93 though, some stations still have it so check around. It's getting harder to find because alot of the stations are now blending ehtanol into almost every fuel grade - and you want to stay away from ethanol blends.

for jetting, follow the advice and check and see what the jets are currently. If it doesn't run good when you correct your fuel and mixture issues, buy jets to put it back to factory position. The stock jets let the engine run pretty crisp and clean. You may want to drop the main jet 1 or 2 sizes to clean it up just a touch. If you have slow-idling problems increase the pilot size by 1. This might be needed for the aftermarket pipe. Also make sure you are running the recommended spark plugs.

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Crisp BRAAAP! 11/28/2011 9:22am
11/28/2011 9:22am
Scooter: I am a useless old vet, too, but I am on the pipe. I mean, not Decotis on the pipe, but it's open. I don't know how fast the guys I bought it from were but they weren't clueless. I see where you're going with this line of questioning but I don't think this is it. I've run the bike before and the only time I've had a problem is after adding the new gas.

Mx5471: I am a scientist so I totally get what you're saying. I might go back to baseline. I'll have to open up the carb to even find out where I am. I was hoping to cheat, largely because I am a scientist so I have to work on grants, papers, etc. all the time. It's tough for me to find the time to put in a few motos, nevermind test. But I totally hear what you're saying, and I might just have to do it. I guess a lot depends on what the setup is right now (for example, maybe it is in the stock location).

Thanks for the help, guys!
Crisp BRAAAP! 11/28/2011 12:09pm
11/28/2011 12:09pm
mxtech1 wrote:
93 octane pump fuel is fine, you don't need any type of octane booster. Obvisouly, race fuel is the best choice but not usually an economical...
93 octane pump fuel is fine, you don't need any type of octane booster. Obvisouly, race fuel is the best choice but not usually an economical option. Try to run 93 though, some stations still have it so check around. It's getting harder to find because alot of the stations are now blending ehtanol into almost every fuel grade - and you want to stay away from ethanol blends.

for jetting, follow the advice and check and see what the jets are currently. If it doesn't run good when you correct your fuel and mixture issues, buy jets to put it back to factory position. The stock jets let the engine run pretty crisp and clean. You may want to drop the main jet 1 or 2 sizes to clean it up just a touch. If you have slow-idling problems increase the pilot size by 1. This might be needed for the aftermarket pipe. Also make sure you are running the recommended spark plugs.
Thanks so much for the excellent advise! Man I appreciate it. Shit - I was afraid of the Stabil. I was just trying to be a good boy for once. Welp, now I know. I actually don't mind spending the money on race fuel - I am so out of shape that I am not burning a ton of gas. My problem is finding it. I think I can find it at Englishtown at the track.

I run the recommended plugs. I do try to do everything by the book, but I'll back out to 40:1. Thanks again for the thoughtful post. I really appreciate the knowledge!
Falcon
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Falcon 11/28/2011 1:44pm
11/28/2011 1:44pm
Just a little reminder about jetting changes with oil ratio changes:

More oil in your gas makes the bike run LEANER. (I.e., 32:1 is leaner than 40:1.)
Thus, I doubt your oil change caused the overly rich condition. That said, I always ran my 125s at 40:1. I had good luck with Maxima K2 and Bel-Ray H1R.
xeroxed74
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xeroxed74 11/29/2011 9:18am
11/29/2011 9:18am
How old is your air filter? An old or just nasty filter can cause fouling issues.
Crisp BRAAAP! 11/29/2011 9:55am
11/29/2011 9:55am
xeroxed74 wrote:
How old is your air filter? An old or just nasty filter can cause fouling issues.
It's clean and good - not the issue, but thanks for that thought. It's a good point.
kippy450
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kippy450 12/2/2011 9:36pm
12/2/2011 9:36pm
mxtech1 wrote:
You should not be running stabil through a high performance engine. You are right the stabil causes the fuel to richen. And besides that you increased...
You should not be running stabil through a high performance engine. You are right the stabil causes the fuel to richen. And besides that you increased your oil ratio to 32:1. Dropping from 40:1 to 32:1 with no other changes requires a jetting change. I would recommend draining your fuel tank to get rid of that stabil, that stuff does more damage in MX bikes than it does good. I would also suggest returning the oil ratio to 40:1. That is a very safe level to mix your oil at and you won't have any reliability issues due to lack of oil. It will run alot crisper than 32:1. I typically will only run 32:1 during the first tank on a break-in ride after motor work has been done.

Next time you need to winterize the bike, forget the stabil. All you need to do is remove the bowl plug from the carb and let the fuel drain and air out for a few minutes. For the fuel in the tank, just pull the fuel line off at the carb and drain the gas into a gas can and put it in your truck or something. During spring time, fill with fresh pump gas and you will have a guranteed startup with fresh fuel in the tank and the carb.
THANK YOU! Do you know how many arguments I have been in with guys that ALL have different thoughts on oil/gas ratio...and every time I tell them I run 40:1 I get bashed for "that's not what the books says" or "your going to blow that thing up." Heaven for bid you mention 50:1....they loose it on you! At least I have heard now. Thanks!
mx216
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mx216 12/4/2011 7:20am
12/4/2011 7:20am
I say put it back to stock jetting and throw in a BR9EIX plug and see where you stand from there. Those plugs are definitely a little more expensive, about 12 bucks, but i have had great success with them. I know here in the Northwest around 1000-1500 ft the stock jetting ran great on my bone stock 08 yz125. If you are still fouling plugs after that you may even have a right side crank seal goin bad on ya.

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