Hey everyone, earlier this year I picked up a '94 YZ-125 that needed a bit of work. Did a new crank and connecting rod, had the jug bored for a +2mm piston by Eric Gorr, had the jug ported and the head milled for more compression by Eric Gorr, new Carbon Tech reeds, threw on a Fatty pipe and Fatty muffler, new NGK spark plug, and put a new foam air filter on.

I'm not sure of the honest driveability of this bike prior to me getting my hands on it but I assume when it was last running with the previous owner it must've ridden fine. But when I got it the jug was removed and the piston was siezed slightly, along with a rusted connecting rod from being left outdoors without the jug on it. So I never got a chance to test the bike out while it was in stock form.

Ever since getting the bike together and getting it running for the first time for me I've had problems in the very lower power curve. I tried adjusting the pilot jet air screw to no avail with the stock #15 pilot jet and couldn't get the motor to stop bogging under load at lower RPM. I was using a 30:1 fuel mixture ratio with 93 octane (though the manual claims for 95+ octance, I can't get anything higher than 93 around here).

I finally tried ordering several different pilot jets to try to tune this thing right. I've now tried the stock #15, and I've also tried a #12.5, a #22.5, and either a #18.5 or #10... I don't remember now what's currently in there since I haven't played with this bike since around April. But no matter which needle jet I've tried I've adjusted the pilot air screw every direction I could and can't seem to get the bike to stop bogging down low.

I've tried adding more fuel to the mixture, I've tried adding more oil to the mixture... and I just can't seem to get it right. For the first time in months I went out last night, drained out the old fuel, and put 1 gallon of new fuel in it. Again with a 30:1 mix ratio. This time I added around 1-3 ounces of acetone to the mix to try to bump up the octane level in hopes that maybe the low octane that I've been using was the problem. This morning I tried adjusting the pilot screw all across the board again and the bike still bogs down low. I've made sure that the carb vent hoses are not blocked, and I've tried everything that I can think of and can't seem to fix this situation.

The motor runs like a raped ape up top once you get past 1/8 to 1/4 throttle, but as soon as you fall back down in that area again it just sputters around. I still have the rest of the carburetor parts as they came stock, and I believe that the throttle valve is a 4.0.

I can't seem to determine though if the bike is too rich, or too lean. Or do I maybe need to add more acetone to try to get that octane rating up higher? I've contemplated trying to Dremel out the cut-away on the throttle valve a tad under the impression that maybe I'm too rich, but out of two experts that I've described my problem to one has told me that it sounds like I'm too rich, when the other has told me that I'm probably too lean.

Since there is no one local that works on these 2-strokes and none of the experts are able to see my bike in person and ride it, I figured that the next closest thing that I can do is share a video with you guys so you can hear it....

YouTube - M4H00291 (http://www.youtube.com/watch?v=tpB_YUmY9_A)

In this video, once I left my driveway and started making the right-hand turn into the cul-de-sac I put the transmission in second gear. I did one lap around the cul-de-sac in second gear and put some load on the motor by giving it around 1/4 throttle. You can hear how it's kind of bogging, making that "bwaaaahh" sound as it tries to accelerate.

I left the cul-de-sac switching into third gear, and as you can hear my make my way down to the end up the street at higher RPM the bike sounds just fine.

I re-entered the cul-de-sac and my driveway in first gear, and due to the incline of my driveway it bogs a little even in first gear usually unless I were to fly up the driveway a bit faster. You can also hear at around the 1:11 mark where when I give the bike a little throttle to keep it running you can hear it bog some.

So what do you all think????? Does it seems like it's running too rich??? Too lean???? I'm thinking it's too lean from what I keep reading about lean symptoms, but I've having a hard time deciding the real truth.

Thanks in advance,
Andrew

porting can cause a bog in the midrange too.

I'd try fattening up the pilot one size to eliminate that set-off bog, the trouble is that it won't idle on it's own after that, you'll have to burp it.

Hey thanks for the reply! Yeah I've already tried going up 2 different sizes higher than the #15 that was in there earlier this year once I got the bike running and wasn't able to get anything out of it. I think there's a good possibility that the jet that's in there now is higher than what was in there from when I picked up the bike, but I'll pull the carb apart later and figure out exactly which jet I still have in there.

I'm looking at the #22.5 jet right now that's sitting out, so I can always try that one too. But you agree that the bike seems to be running lean down low???? Do you think I may also need more octane than what I'm currently using?

I have no problem with the bike not idling on it's own... I'm just tired of the bogging under load. It caused me to wreck the bike and break the subframe earlier this year... I was testing it out after trying to tune it, climbing a dirt hill, when the bike start to bog going up the hill. I popped the trans down to first gear before the motor died and left me rolling down the hill backwards in reverse on this tall-ass bike.... then as it continued to bog I gave it more throttle. The next thing I know the front wheel flies up, and I fall on my butt. lol. This bogging crap needs to be addressed... I'm getting too old to fall on my ass like this haha.

fuck I hear ya there. I'm only 24 and I'm too old to be falling off like I used to.

But ya it sounds/looks like a lean bog to me, either that or a dip in the curve from the porting. You can sometimes "cheat" your way out of bad wave action by pounding the fuel to it and getting some heat out into the pipe tho.

It's not pinging, atleast not that I can here over the video, but it wouldn't ping at that rpm anyhow, you're gonna need to get her well rung out before you'll hear ping. Just listen for the little man in the engine with the tuning fork trying to smash his way out with it.

Thanks for your reply again man! Yeah I'm only a year older than you but I guess we're getting to that age where we need to be more careful with our bodies haha.

Well I got the carb apart a couple hours ago and had a #17.5 jet in it, which is one size richer than what the bike came with. When I ordered jets earlier this year I was told that there was no #20 jet to be had, so I didn't have one of those. I instead to the #22.5 jet that I had and installed that.

I added a little more acetone to the gas tank and took the bike out for a spin down the road a little ways to get it warmed up. It seemed like the bog improved a tad before I started fumbling with the pilot air screw, and once it got warmed up I still couldn't get any better results down low. I'm thinking I still need more octane, but I'm no expert at this and I don't want to hurt anything from too much octane.

As much as I don't want to pay someone else to do this I'm getting really close to just dropping the bike off with a shop and letting them figure it out. This dirt-bike has cost me so much time and money as it is lol. I really don't want to pay someone a bunch of money for something probably so simple, but I'm just getting tired of fumbling with this and want to be able to actually take it out on some trails and stuff for once.

Well, any other ideas/suggestions?? Haha.

skip the acetone and mix you fuel with stuff from the pump.

Look again at your manual and it probably calls for 95 research(RON) octane or higher regular unleaded (87) usually has a RON of 91-92, "super"(93) is usually 97-98 RON.

In the US we use the (R+M)/2 formula to call out octane. (Research Octane Number + Motor Octane Number) divided by 2.

It will be printed clearly on the pump.

Have you messed with the needle and air screws at all? if so what happens?

on the air screws its pretty common to get varnish buildup in the internal passages which can cause low speed bogging. try running the screw out 3-3.5 turns and then in to the seat and note the differences (if it runs better seated you need a bigger pilot, at 3 or more smaller.
the needle can also be the issue. try moving the clip up and down. moving the clip up effectively delays the event creating a leaner transition between the pilot and main circuit. moving the clip down causes this event to happen sooner in relation to the slide position creating a richer transition.


try moving the needle 2 positions each way (if possible) and record the results.

another issue may be fuel level. make sure that this is correct or you will never stop chasing the low speed jetting issues.

Is that thing running a Mikuni or a Keihin?

I f00king hate Keihins, I can never get 'em right in the bottom.

and on the getting old bit, ya, I hear ya. Even a night out drinking hurts a lot more than it did even a couple of years ago.

As far as the fuel, I'm with Bpro, don't mess with acetone, if you've GOT to add something for octane boost, kerosene is a better bet. It can sure mess you up in a hurry tho, I played with Toluene a bit for a power adder in fuel too, but it sure didn't help fend off detonation, and at that time I may as well have been ordering pistons by the dozen and Nic was cheap.

Yeah I just looked at my Clymer manual (don't have the regular owner's manual) and it calls for "Pump gasonline with an octane rating of 95 or higher" and calls for a 30:1 mix ratio. But yeah, it doesn't state anything about the RON rating, it just gives me the impression that it wants a higher octane fuel than 93 pump gas.

Even though the jet needle doesn't necessarily affect the range that I'm having problems in yeah I have previously tried adjusting the needle up and down with no different results.

I also failed to mention that I have replaced the pilot air screw months ago. The old one was a little dull on the tip, but it was clean with no gunk or anything. The new one is still completely clean as well. But yeah I've twisted the pilot air screw to no avail with every adjustment that I make with anything else and can never seem to find any sort of sweet spot.

I'll definitely trying what you suggested about back the screw out 3-3.5 turns. What do you mean though by "if it runs better seated you'll need a bigger pilot, at 3 or more smaller?" I'm kinda confused at the "3 or more smaller" part.

I'll also try moving the needle clip up and down two notches again and see what differences I get. And I've got probably just shy of 1 gallon of fuel in the tank right now, so it's got plenty in it. I've tried with the tank nearly full and never got better results. Thanks for your advice and I'll let you know of my results!

Is that thing running a Mikuni or a Keihin?

I f00king hate Keihins, I can never get 'em right in the bottom.

It's a Mikuni TM flat-slide carb. Seems like with this carb things should be pretty simple lol.

and on the getting old bit, ya, I hear ya. Even a night out drinking hurts a lot more than it did even a couple of years ago.

As far as the fuel, I'm with Bpro, don't mess with acetone, if you've GOT to add something for octane boost, kerosene is a better bet. It can sure mess you up in a hurry tho, I played with Toluene a bit for a power adder in fuel too, but it sure didn't help fend off detonation, and at that time I may as well have been ordering pistons by the dozen and Nic was cheap.

Yeah I'll probably stop with trying to get higher octane into the fuel for now then... I definitely don't want to be having to tear this thing back down again because I fudged something up.

Okay so I just got back from trying to play with the jet needle and the pilot air screw some more. I first went out with the needle where it has been for a while now. It was with the clip at the highest position. I turned the pilot screw out 3.5 turns. The bike was running like it usually does (ass), it got slightly better as I turned it clockwise, and then begins to get worse again as the screw gets closer to bottoming out.

I then lowered the clip all the way down (since I know I had once had the clip in the center position it couldn't get much out of it). The bike ran like total crap. Bogged like crazy down low, and took even more throttle than usual to get it to stop bogging. Adjusting the pilot air screw didn't seem to make much of a difference except for at the extremes (all the way in, or all the way out).

I put the clip to the middle and the bike ran a bit better than it did with the clip at the bottom position, and maybe a TAD better than at the top position. Still same luck with adjusting the pilot screw.

All this time I ended up leaving the #22.5 jet in. Any suggestions what jet to try now based on these results? Again, I have the following pilot jets to try:

#12.5, #15, #18.5, #22.5

Thanks!

see if you can get your hands on a 27.5

fuck those jets are small. On the mikuni 33's I'm used to rocking 45-55 for pilots.

that's on big race sleds tho, 250cc/hole minimum.

see if you can get your hands on a 27.5

fuck those jets are small. On the mikuni 33's I'm used to rocking 45-55 for pilots.

that's on big race sleds tho, 250cc/hole minimum.

I'm definitely up for trying a richer jet, but is there a better way to pin-point if the bog is a rich bog, or if it's a lean bog?? I can be forever throwing away money on jets at this pace lol. Any other tricks other than maybe adding more fuel to the mixture to richen it up?? Or is that my best plan of action to try again? Thanks!

looking at the results of the mixture screw/needle I think you need to go up at least 1 more size on the pilot, but first I really suggest getting the carb body ultrasonically cleaned (or do it the old fashioned way with dip and compressed air.

The mixture screw (act air screw on that carb) is an air bleed into the pilot circuit. the further you turn the carb in to the seat the more fuel you will be able to pull through the circuit. turning it out leans out the system. based on your results you are lean, but you have not listed any mods that should have made that big of a difference. I suspect a bit of varnish is in the circuit and giving you fits.

Scratch that I just noticed that you had it bored .080 over. definitely go big on the pilot. I have done this to my YZ250 and went with a 55 pilot (48 stock in a keihin) which is 3 sizes up from stock. In my nephews SM KX65 which is also bored over I went up 4 sizes from stock.

By that I would suspect that you will want at least a 25 if not a 27.5. this is backed up by the bike running better with a "richer" needle setting.

big bores need lots of pilot. Beyond that tho, going overly large on the pilot won't make it bog unless the fuel has built up in the bottom end. Which is shouldn't be able to do if you're burping it to keep it's throat clear.

this may be way off in left feild but i was having the same problem in my 250 and it turned out to be the power valve was broken fixed it and put it back together and it fixed the bogging down problem:dunno

that's a solid point, it COULD be a stuck valve.

Hey thanks for the info guys. Yeah I'll try the larger jet before I give in and fork $200+ over to the local dealer to try to figure it out.

The valves and all should be good... this was a virtually new jug and Eric Gorr swapped my valves and stuff from my old jug into this one and cleaned everything up before sending it to me.

Eric had also tore the carb apart and cleaned it, and I've cleaned all passages and stuff several times with brake parts cleaner since I've been trying to get this thing to run. So I'm hoping an ever bigger pilot jet solves the problem. I'll let you guys know when I get around to trying that.

Thanks again,
Andrew

Did the larger jet fix the problem?Having the same problem on 95 bored out.Thanks zach

Did the larger jet fix the problem?Having the same problem on 95 bored out.Thanks zach

Nope. I'm back on here again 2 years later because I still had the problem when I gave up working on in about 1.5 years ago lol. About to tackle it again. Might just try with a whole, new carb.

Hey all. new to the forum and been away from bikes for some time and fixing to get back into the sport with a 84 yz250. Anyway. Yo might check and make sure you don't have a small vacuum leak around the manifold somewhere. This would create a lean condition.
Also try adjusting the mixture screw for best running conditions first with the needle clip set in the middle, then go onto the nest adjustment. This way your not chasing down two adjustments at once. Have you by chance checked to make sure that everything is good with the ignition timing. If everything is good and you still can't get it try taking a ziploc bag with the bottom cut out of it and cover part of the air filter. This will restrict the air and create a rich condition. adjust by exposing more or less of the air filter. This will help you determine which way to go with your jetting.
Hope this might help. Like I said, it's been awhile and there's lots of cob webs.
Good luck.

Did a little research on porting the other day and it sounds like a bad porting job. Sounds like they may have raised the ports to high increasing the duration to a point that low end sucks. Kind of like having a all out drag cam in a car that is built for daily commuting.
Just a thought.