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DRZ 125 Fuel Screw and Carb Tuning

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Rebuilt the carb and petcock on my '03 DRZ 125 (what a comedy of errors). Fresh spark plug, new Moose air filter nicely oiled up, valves adjusted, tensioner "tensioned", fresh gas, everything including air box and plastic intake surgically cleaned, carb disassembled and soaked in Sea Foam. Has a Turbine Core II sparky on it, cut open the top of the air box. 

 

Note: I did not replace the "diaphragm" in the carb, was not able to source the part locally, will replace it as soon as I get a chance to order it online. No holes, rips, tears in it, but it does look "old", the rubber is not very rubbery if you know what I mean. 

 

Re-jetted as per some recommendation I found on this forum: 

 

Pilot: 20

Main: 110

Fuel screw: 2.5 turns out

Needle: replaced with TTR 125 needle, clip on lowest slot (raised needle up to highest setting)

Float height set as per specs

 

Put it back together, it started right up, 2nd kick! Idles nice and smooth. Yeah me! 

 

Doesn't want to rev up though. Major stumble off idle, then revs, but is not smooth. Back fires when throttle is chopped. Settles back to idle nicely though. 

 

Went to adjust the fuel screw before I got into fiddling with the needle setting. 

 

What da?! 

 

Can't get to the thing while the bike is running! What a pain! 

Edited by Woods_Rider

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Started researching online, couldn't find any aftermarket fuel screws that fit the Mikuni VM22, was very surprised. Kept looking, looking, looking. Nothing.

 

Finally decided I would have to McGiver my own solution. Looked around the garage, rounded up all my misc screw drivers, driver bits etc. Couldn't find anything suitable.

 

Took the fuel screw down to my local hardware store, found a driver bit that fit the screw slot nicely. Bit is about 2" long, I think its a 1-3 size(?).

 

Bought it home, tried to adjust the screw with the carb on the bike but couldn't get a good grip on it.

 

Looked around my junk nut and bolt bucket and found an old brass screw with a knurled end, I think it was part of an exterior light fixture, the screw shaft part is hollow. The driver bit fit perfectly into it.

 

Hoarding pays off. Sweet!

 

Cut the brass screw to size and JB welded the driver bit into it. Let it sit over night.

 

IMAG3916-M.jpg

 

IMAG3918-M.jpg

 

IMAG3919-M.jpg

 

Will be adding a white mark on it so it will be easier to count turns. 

 

Going out to the garage shortly to try it out. Would like to be able to adjust it with the bike running so will probably need to wear some gloves so I don't fry my hands, its a pretty tight fit in there so not sure how that will work out. Also thinking about placing a couple of o-rings on it to see if I can keep it from dropping out of the carb while the bike is running. Just need something to snug it up in the carb body temporarily while tuning. 

 

If the o-ring idea doesn't work I'll just place something under it, between the case and the bottom of the screw to hold it up. Only problem is it might interfere with getting my fingers on the knurled end. Will see. 

 

Hopefully this will work and I can get the fuel screw setting set and then I can get on to tuning the needle height. 

 

I will report back on how this whole thing works in a bit. 

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Hey guys , I have a drz 125 2003 , and ireally need a rear wheel for it. Can't find one anywhere please help

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Ok, tried my little tool out, works pretty good, but can't really adjust with the bike running. Bummer. But at least I don't have to loosen up the carb and twist it around to make the adjustments. 

 

Could use a larger knurled knob, its difficult for me to get my fingers on the little knob and get it turning. And at my hands are too big to get in there without getting fried on the exhaust and hot engine. Gloves are not providing enough heat protection, plus make it impossible to grip the screw driver and make the adjustments. 

 

O-ring on the tool shaft idea also did not work, the screw driver bit doesn't extend far enough into the screw hole to allow it to secure it to the carb. Its too bad, forced me to reach around the exhaust header side with one hand to hold the bottom of the screwdriver forcing it up into the adjusting screw, then I had to reach in the other side with my other hand to make the adjustments. 

 

What a royal pain in the ass. Start bike, warm it up. Note how its running. Shut it off, reach in there, try not to fry my hands. Make adjustment, start up bike, make sure its warmed up. Note how its running. Shut it off, reach in there, try not to fry my hands. Make adjustment, start up bike, make sure its warmed up. Note how its running. Shut it off...

 

Got the fuel screw adjustment set up pretty close for now but its bogging bad when I whack the throttle and backfiring when I let off the throttle. 

 

Going to adjust the floats to put more fuel in the bowl, see how it does then. If that doesn't help, then onto fiddling with the needle height. 

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Ok, feel like I'm talking to myself here, but I will keep posting, perhaps it will help someone in the future. 

 

Have not been able to get the bike running properly, backfiring, I thought it was a lean condition w the carb, or that I had fouled up the carb rebuild. Found this the other day when I was double checking things: 

 

IMAG3988_c-M.jpg

 

Thought I found the root of the problem, put another plug in there and promptly destroyed that one too. Finally decided to check the timing and woodruff key (I know should have done it right off the bat) and sure enough the woodruff key is destroyed. 

 

IMAG4002_c1-M.jpg

 

IMAG4002_c2.jpg

 

I'm hoping the key was sacrificial and the crank end and stator/flywheel are ok. I just picked up a flywheel puller so will be digging into it this afternoon. I will report back what I find. 

 

Fingers crossed! 

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How did you rebuild the petcock? I couldn't find anything for rebuilding a petcock?

 

I also looked for aftermarket fuel screw and found nothing. I even contacted Mikuni directly. They cannot sell me a screw, it MUST come from OEM. Kinda weird...

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How did you rebuild the petcock? I couldn't find anything for rebuilding a petcock?

 

I also looked for aftermarket fuel screw and found nothing. I even contacted Mikuni directly. They cannot sell me a screw, it MUST come from OEM. Kinda weird...

 

I bought 2003 Yamaha TTR 90 parts at a local shop that had them in stock cheap since the Suzuki parts are not available separately. I bought the thick rubber part, its called "valve packing", the o-ring, and then the gasket for between the petcock and gas tank. They are the exact same dimensions, probably the exact same parts. 

 

Unfortunately, mine is still leaking, but very slowly, like one drop every now and then. And by leaking, I mean its allowing fuel to flow to the carb, its not leaking out of the petcock or anything. I suspect the aluminum is worn. I'm sure it will be fine the way it is, but I want it to be right so I will probably just buy the Yamaha TTR 125 petcock at $20 vs $94!!! for the Suzuki one the next time I place an order with Rocky Mtn. They are all the same size and will bolt right up to the tank.  

 

And Rocky Mtn lists a petcock "repair kit" for the TTR 90 which has the same parts that I bought plus two bolts for $17: http://www.rockymountainatvmc.com/Parts-Finder/44/Dirt-Bike

 

And yes the fuel screw situation is a royal pain. Every time I want to make an adjustment I have to practically pull the carb. 

Edited by Woods_Rider

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Update: turns out the woodruff key, the crank end, and the stator/flywheel are all ok! Whatever I saw must have been a metal chip or something because when I pulled the stator everything was perfect. Yeah me! 

 

IMAG4010%20-%20Copy-M.jpg

 

Put it all back together, and pulled the valve cover, confirmed the timing was correct, it was. Double checked the valves, they seemed to be out of spec, so adjusted them but then saw that the crank had turned so not sure I was measuring and adjusting correctly. Put the crank back to the correct spot, secured it so it would not budge and adjusted the valves again. Triple checked the valves, rotated the engine between checks, etc. 

 

Started it up, it was running better then before, but still poorly. Would not idle without the choke, hanging idle, backfiring. 

 

Edited by Woods_Rider

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Checked the spark plug just to make sure I did not destroy another one, it was fine, put it back in. Adjusted the fuel screw from 1.5 turns out to 2.5 out. Running much better, but still not perfect. 

 

Pulled the slide, its a Yamaha TTR 125 slide, it was on richest setting (clip in the 3rd/lowest notch), put two small washers under the clip to raise it some more.

 

Bingo, its running great now! Holy cow, what a ball buster. Oh well, its running really well now, took it around the neighborhood, ripped around the back yard for a bit. I'm happy, learned a lot.  

 

 

Final tuning setup: Stock '03 DRZ 125 with Powercore IV can, airbox top cut out. 

Pilot: 20

Main: 110
Needle: Yamaha TTR 125 needle, 3rd clip from top, plus two washers (raising it up as high as it will go plus two washers). TTR 125 needle is from Moose Racing Carb Repair kit: 1003-0330
Fuel Screw: 2 1/2 out
Float: set to factory spec: .74" from bowl lip
Edited by Woods_Rider

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Nice write up. Thank you for the response. My petcock leaks when in the "off" position. Its really annoying.

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Woods Racer, so it seems that the mid (needle) was too lean, correct?  Was this cond caused by the aftermarket pipe or do think the needle is not quite right for this carb?  How do you think the stock carb setting will work with the Pro Circuit full pipe I just ordered for my klx125?

 

Thanks for the posts, BTW. Good stuff.

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Hey Thump, I'm no jetting expert, I just went with some recommendations I found here. But I think my poor running was due to both my valves not being adjusted correctly and an overall lean condition that started with my fuel screw at 1.5 out, and then the Yamaha needle needing to be raised even higher then it was. Plus I had just cut the top of the airbox open. 

 

I don't think they run poorly from the factory, but I only have this one bike to compare to, its the only one I've owned and been around. When I got it last year it ran fine with the Powercore spark arrestor and stock jetting. 

 

So, it was running pretty decent last fall, but would not start after sitting all winter.  

 

After I rebuilt the carb and adjusted the valves, that's when it started running really poorly. 

 

The final round of changes were, in order:

1. Cut open the top of the airbox. 

2. Got the valves adjusted properly.

3. Went to 2.5 turns out on the fuel screw. - bike was running much better but was still lean, backfiring, hanging idle as shown in first video. 

4. Raised the Yamaha needle with 2 small washers. - This was the final touch, its running great now. 

 

If I was you, I would wait until you see how the bike runs with the new pipe setup. If its showing lean, I'd start with the fuel screw, then pilot, then main, maybe see if you can raise the stock needle. If that doesn't get where you need to be, maybe consider the Yamaha needle. 

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Thanks, Woods.  So I cleaned the carb and removed the cap from the fuel screw. Still have the stock pipe on so I turned the fuel screw to 1-3/4 turns out and raised the stock needle with two small washers. Throttle response seems to be improved.

 

However, I am hesitant to cut holes in the airbox or even remove the snorkel because I have to think that there's a good reason its in there or else why would they invest in the time and money to build it that way. I assume it has something to do with manipulating airflow but then I'm no expert. 

 

I will get the new pipe on this weekend, and play with jetting, then report back.

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Right on, no reason to cut the air box or otherwise do any not easily irreversible mods. Keep us posted on how its running. 

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Did you ever have any trouble with the float bowl flooding? After I run it, the float will flood. The bowl height is set at .6x"

 

Thanks

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Not really. I did fiddle around with the float height a bit but set it back to approx .74" and it seems to be fine. 

 

Have you looked closely at the rubber tip on the float needle (or whatever its called) to make sure its not worn? 

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Hey so I got the new PC T4 pipe on and messed with the jetting. Man this pipe is loud. Here's what I started with-

 

main: 110, 

pilot: 20,

stock needle: shimmed with 2 small washers (@ 1mm).

fuel screw: 2 turns out

Seemed way too rich and had very soft rev. Would also cut out or bog at times. Wrong times. 

 

Second try-

 

main: 105

pilot: 17.5 (stock)

stock needle: shimmed with 1 sm washer (@.5mm)

fuel screw: 3 turns out

Ran better but still has a bog off idle.  (does this mean the pilot is rich or lean?) Also hangs on high idle before slowly settling, at which point it will stall if I dont blip it.

 

Not sure what to try next but I think I'll order a TTR needle and give it a go.

Edited by Thumpythumperton

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