Help tuning a carb

Pulled the plug...

Looks a little rich. A couple of things you might try are lowering the needle for the main jet and/or going to a smaller main jet. It sounds like when you crack the throttle and and it stutters, it's getting too much gas. if you lower the needle (move the clip up a position or two) it would lower the amount of gas coming in. If there aren't two positions above where it is now, it may be necessary to go one size down on the main jet.
Here are a couple of diagrams that may help:






If you have a Keihin carb, here's how to adjust the needle:

Bike is definitely running rich. I found the intake boot was torn up, so replaced that to get rid of that leak. Now I'm basically starting at zero trying to set this. I'll need some guidance reading the signs along the way.

Recap:

• Pre-jetted Niche VM 24
• Pilot 40
• Main 150
• Air pod on the end of the air filter hose, no other mods

I'm using the Mikuni tuning guide. I am currently on the first step setting the Pilot circuit with the Air Screw. They recommend 3/4 turn out on Air Screw. Understood. But....before setting this Air Screw, what should the Idle Screw be set at as a starting point? I can't find anything that says where it should be.

I currently have Idle Screw at 7.25 turns out (~10.50-11 is wide open fall off). And the Air Screw at 1 turn out. This is where the bike will idle around 1300-1500 RPM (per the TTR125 manual). At this setting it chokes and dies if I rev the throttle.

The next step in the manual it says to turn the Air Screw counter clockwise to raise RPMs until they peak then fall off. I got up to 3-3.5 turns out and the RPMs just kept climbing and climbing. I was kinda scared to go further than that so I backed it back down and returned it to 1 turn out.

What is this telling me so far? Do I keep turning out the Air Screw? Or do I need to change out the Pilot and go again on the Air Screw until I get the RPMs to fall off like they are suppose to?

Thanks

Edit:

Ok I'm back...been reading....by needing to add more air by opening the Air Screw that much means I have too much fuel from the Pilot jet and I need to go smaller on it, correct? If so, should I go down to 35, a size smaller or make 2 jumps down to 30?

I would think you should go one size down but I know how you feel thinking about going 2 sizes down.

Yeah you're probably right. I'll try the 35 and see how it goes.

You know, it's not "hard" to tune the carb, a lot of trial and error and setting stuff. It's constantly taking it off and on the bike to run it each time.

Monica said:

The next step in the manual it says to turn the Air Screw counter clockwise to raise RPMs until they peak then fall off. I got up to 3-3.5 turns out and the RPMs just kept climbing and climbing. I was kinda scared to go further than that so I backed it back down and returned it to 1 turn out.

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This is a dance between the air screw and the idle screw. as you open the air screw and the RPM climbs bring it back down with the idle screw and then go back to the air screw. I generally work in 1/4 to 1/2 turn increments on the air screw. I would turn the air screw 1/4 to 1/2 turn out and if RPM increases (which it did) then I would bring the rpm back down with the idle screw. Then I would go back to the air screw 1/4 turn out. Did the RPM increase? IF yes then lower back to desired idle with idle screw. Follow this until you quit having results from one adjustment. See other comments below.....

Monica said:

What is this telling me so far? Do I keep turning out the Air Screw? Or do I need to change out the Pilot and go again on the Air Screw until I get the RPMs to fall off like they are suppose to?

Thanks

Edit:

Ok I'm back...been reading....by needing to add more air by opening the Air Screw that much means I have too much fuel from the Pilot jet and I need to go smaller on it, correct? If so, should I go down to 35, a size smaller or make 2 jumps down to 30?

Click to expand...

Yes you are adding more air but, until you go back and adjust the idle screw to try to lower the RPM you can not be sure where fuel is coming from. The idle screw simply holds the slide open allowing air to pass by the slide. If air is passing by the slide so is fuel from the main circuit since the needle is lifted. Once you can no longer lower the RPM with the idle screw you will know that the slide is down all the way. Once the slide is down all the way then you can determine if you have the correct pilot jet. You may very well find that the air screw doesn't need to be that far out to achieve peak RPM.

Oh ok, that makes sense with the Air and Idle screws. I'll will try that.

I found a video with a illustration that made me say "OHHHHHHHHHH". (I'm draw it in crayons kinda learner)

[ame="https://youtu.be/O53c1HxCAuI?t=3m29s"]How to adjust a carburetor, jetting and mixture explained! - YouTube[/ame]

Is that guy accurate?

I would say that is a good description and visual.

Ok cool, good deal

Well, I went down 2 sizes now on the pilot jet to 30 and it's getting closer. Took notes like crazy and could see the numbers slowly start walking it down . The Air screw is at 2.5 turns out and the Idle screw at 5.5, but idle RPM is still a little high. Going to order a couple more smaller Pilot jets and probably smaller Mains too just in case for when I get to that step. I'm using a new spark plug with each jet change too.

So the idle screw will not lower the idle anymore?

It does, all the way up until it dies.

Currently with a size 30 Pilot, the Air Screw goes out to 4 turns before it levels off.

Alright, back for more after a busy last few months. Got the bike idling now. Tonight Dad and I installed a 17.5 Pilot and 105 Main, set the Air Screw at 1 turn out, the clip at 3rd slot from top of needle and a new spark plug. Bike fired up without needing the choke. It's idling fine and the throttle smoothly rolls through the range while sitting on the stand. Have not taken it for a spin yet to get it through the gears, hopefully tonight after traffic dies down.

Problem: bike dies if I crack open the throttle hard....? I have a video but no idea where to upload/link.

edit, ok here's what I got...
https://youtu.be/r-pNSVLNqwE

Monica said:

Alright, back for more after a busy last few months. Got the bike idling now. Tonight Dad and I installed a 17.5 Pilot and 105 Main, set the Air Screw at 1 turn out, the clip at 3rd slot from top of needle and a new spark plug. Bike fired up without needing the choke. It's idling fine and the throttle smoothly rolls through the range while sitting on the stand. Have not taken it for a spin yet to get it through the gears, hopefully tonight after traffic dies down.

Problem: bike dies if I crack open the throttle hard....? I have a video but no idea where to upload/link.

Click to expand...

And make sure you don't have any air leaks/ cracks around the cylinder intake etc which plays havoc on the mix.
Best of luck...fingers crossed

StromXTc said:

And make sure you don't have any air leaks/ cracks around the cylinder intake etc which plays havoc on the mix.
Best of luck...fingers crossed

Click to expand...

Oh definitely. I put a new one on, the rubber thingy from the carb to the cylinder was cracked.

Monica said:

Oh definitely. I put a new one on, the rubber thingy from the carb to the cylinder was cracked.

Click to expand...

Just taking stab in the dark but can you put that carb on an independent fuel source with a new fuel line just in case it is being starved at wot. Also, Haven't read the thread but assumed you flushed out the carb?

You may want to cut bait and look for a new cheap carb online but like I said, haven't looked at what all you have done

It dies if you snap the throttle wide open or just if it goes to wide open at all (even slowly)?

If it idles fine but dies as you gradually roll on the throttle, but will run if you snap the throttle past the mid range to wide open, it is likely that there is an issue with the idle/pilot jet/circuit being partially clogged. I used to have this problem with my KLR 650 and KTM 530 if I let them sit for more than a few weeks with gas in the carb. A new jet usually fixed it. It was a fuel starvation issue.

If it runs as you gradually open the throttle but dies if you snap it to wide open, then it might be flooding. Basically it is pulling in gas faster than it can pull in more air to maintain a mixture that can burn. If you do it slowly and it runs, that is because you are giving it more time to pull in the air needed to keep the mixture in the combustion range. The easiest thing to try to just lower main needle one notch (move the clip toward the top of the needle). This should not affect your idle, but it will reduce the gas flow as you open the throttle.

After it dies, does it smell gassy? That would be an indicator of flooding.

It dies if I snap it open.

The previous setting had the needle lower (clip one notch higher) but it cut out in the 1/4-1/2 throttle range, it wasn't getting enough fuel. So I moved the clip down one and it resolved that.

You know, the bike starts with no choke, which indicates a little rich. I may move the pilot down one size to 15 and see what that does.

Your struggles are why I want to get rid of my carbed KTM 530 and get a newer fuel injected KTM 500. I am soooo tired of jacking with jetting issues! Unfortunately, I can't really afford a newer KTM 500 right now, so I will just keep messing with the jetting...

Tourmeister said:

Your struggles are why I want to get rid of my carbed KTM 530 and get a newer fuel injected KTM 500. I am soooo tired of jacking with jetting issues! Unfortunately, I can't really afford a newer KTM 500 right now, so I will just keep messing with the jetting...

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I've been having the same thought too about getting out of a carbed bike, this is ridiculous. I just want to ride, not be a good bike mechanic

This morning I put in the 15 Pilot and tried to start it w choke. It wanted to fire but nah. It was cold this morning so I really hoped a good start would indicate it's all good. I finally got it going while adjusting the idle screw and holding slight throttle until it ran on its own.

I'm still chasing this down and I'll take it out this weekend to see if I can nail it down.

Bought a honda 230 for GKid a few years ago and it had carb issues , had to be cleaned every time we wanted to ride it . The screws were worn out from taking it apart and putting it back together . Got ready to sell it so I bought new carb from Honda installed it and all cured , ran just fine happy new owner all is good . Why the jap carbs are so finicky and my 450 KTM could go from sealeval to 12,000 feet without ever having any issues for the 5 ears I rode it and same for my Beta , haven't touched the carb on it since I bought it 8 years ago . It's a hard pill to swallow but maybe the best way to make it run right is to buy a new factory carb , I've done that several times over the years and it worked every time . Then I could start fresh and tune it to the performance I wanted .

Cagiva 549 said:

Bought a honda 230 for GKid a few years ago and it had carb issues , had to be cleaned every time we wanted to ride it . The screws were worn out from taking it apart and putting it back together . Got ready to sell it so I bought new carb from Honda installed it and all cured , ran just fine happy new owner all is good . Why the jap carbs are so finicky and my 450 KTM could go from sealeval to 12,000 feet without ever having any issues for the 5 ears I rode it and same for my Beta , haven't touched the carb on it since I bought it 8 years ago . It's a hard pill to swallow but maybe the best way to make it run right is to buy a new factory carb , I've done that several times over the years and it worked every time . Then I could start fresh and tune it to the performance I wanted .

Click to expand...

The thought has crossed my mind. I hoped not to go back to the stock carb. The VM24 is an upgrade that, among other benefits(I've yet to see), eliminates the choke cable running to the bars (its a known failure point for this bike). It's definitely a hard pill to swallow being that I just bough this carb, new, at the beginning of the year. I looked at new ones, they've gone up $20! I've got the stock carb on the shelf, I'm halfway considering putting it on, but I'd need to fix the choke (I lost the little parts to the cable end when I removed it).

I'm not ready to give up on it yet...apparently, I'm hard headed and stubborn.

there is the issue of float height
often during a carb rebuild the tangs get bent which, if the float is too high, can cause the float to shut the fuel flow and the carb instantly runs out of fuel

nbsdave said:

there is the issue of float height
often during a carb rebuild the tangs get bent which, if the float is too high, can cause the float to shut the fuel flow and the carb instantly runs out of fuel

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Good point! I forgot about that even though I had that very issue once on my KLR 650 carb Once I got it set right, it worked great.

Got back home from the weekend, took the bike down to my parent's place where I had some room to run it around without the risk of being in traffic or in the middle of nowhere while testing adjustments.

I think I got the settings squared away on the carb - 15 Pilot, 105 Main, clip in the 4th slot from the top. It starts up nice with and without choke, and the spark plug is looking pretty close after some runs around the loop.

Still a few bugs to smooth out but what a relief to get the carb settings into the ball park.