tuning challenge - off throttle issues

fatcaaat

fatcaaat

Well-Known Member
Hey all,

I wanted to see if I could get some folks to chime and so I can get this bike tuned. The bike in question is my xl125 with the xr200 motor. I'm experiencing something I cannot seem to tune out.

Overall, the jetting seems pretty spot on. Starts easy, idles clean without missing, and revs out clean. Throttle response from top to bottom is pretty seamless except off idle. When pulling out from a stop the engine wants to die. Just that tiny turn of the throttle to get started. Once you pass that everything is perfect across the range. As a matter of fact, once the bike is warmed all the way up, you really have to be careful or it will stall.

So, not sure what to try. I've already tried leaning the pilot jet and it made it hard to start and made the idle a little rough. I have moved the needle richer, but didn't notice anything different. I'm going to try and move the needle to the leanest setting and see if that corrects it.

Any other thoughts? On this bike, I could also try advancing or retarding the timing...worth a try there?
 
kirrbby

kirrbby

Well-Known Member
I always call that a "off idle LEAN spot" ??
Caused by a too big carb, or too small pilot jet, etc. Richen the pilot circuit with the air/fuel screw...try a bigger jet...smaller carb.
 
6

69ST

Well-Known Member
With zero experience working with this engine, all I can offer is generalized tuning suggestions.

First thing I'd try is richening the idle mixture, to the absolute limit. The only parameters that come to mind, regarding the lean spot, are reversion and low vacuum...which would mean lazy fuel circuit(s) and poor atomization. Low vacuum means low flow velocity.

As I recall, you stabbed-in a hairier cam; that may require a high curb idle rpm. Back in the day, Honda used constant-velocity carbs; perhaps a CB750 carb would be a "natural" fit for the breathing characteristics. I would suggest an MJN24 but, 26mm strikes me as being the lower limit for this displacement & HP. Sourcing a carb that has an accelerator pump might work, but would bring its own issues. Going "sky's the limit" brainstorming...EFI. And, no, I don't see that as practical at this time.
 
fatcaaat

fatcaaat

Well-Known Member
Thanks. I am going to go up one on the pilot and see what happens and keep the needle in the middle for now. The xr200 engines are not all that much of a performance piece. I added in a cam that has slightly more lift and duration, but nothing radical. And the compression is slightly increased due to piston shape, but again, nothing radical. I did not touch the ports on the motor...they are pretty big IMO. I'll mess around with this VM26 for a bit until i exhaust all options and then i'll likely go see if i can find a carb with the pumper. Might be the easiest thing to do is just swap the carb and call it a day.
 
6

69ST

Well-Known Member
Just to be clear here...I'm talking about taking the idle A:F mixture into pig-rich territory, only as a temporary/diagnostic measure. If the lean spot is improved, or goes away, you'll know where to start looking, i.e. getting more fuel earlier in the throttle opening/rpm curve. FWIW, my first foray into VM22 tuning began by testing the as-delivered 30/100 jetting. Of course, we know how bizarrely off-the-charts that combo is...now. But, for the initial firing & around-the-block test ride, it was instant cold-start and EFI-like off-idle throttle response. I presume you recall, clearly, how the taming-of-the-VM22 (with apologies to Shakespeare) saga played out from there. My point is to make use of the lesson learned from starting off with a pig-rich idle-1/8 throttle A:F ratio.
 
fatcaaat

fatcaaat

Well-Known Member
Ok, here's some results from today's tuning.

Baseline jetting = 20/180 on the mikuni vm26, needle one clip lean from center. In this position, it idles and runs out top to bottom clean all the way. It starts easy. However, this is the off throttle that will make it die. Changing the a/f mixture screw impacts the idle speed and how well it idles, but won't clean up the off throttle stumble. Idles best at 1.75 turns out.

17.5/180. Hard to start, didn't notice any change in the off idle issue. Idles a little loopy.
22.5/180. Easy to start, removes the off throttle issues. Creates new idle issue that is rough and stinky. Bike now stalls at idle and getting the idle adjust screw is really finicky.

I'm drawing my conclusions...that the idle and main jets are probably right at 20/180. I guess the next thing to do is start looking at needle tapers. I have about 6 different needles here so really I just need one that is a little thinner for the off idle and that might clean it up so it richens right there.
 
fatcaaat

fatcaaat

Well-Known Member
Ok, here's an interesting one. I don't recall where I got this carb from, but it is sporting the wrong needle in it. The VM26's take a 5-series needle and this one has a 4 series in it. It is also about 10mm shorter than the correct size one. So, the good news is i have 4 different correct sized needles here and they are incrementally spaced across the lean-rich spectrum. So, I'm going to make a good bet that i'll be able to solve this issue. I'll start first with the stock needle that supposed to be in there as it sits in the middle of the road and start the tuning. I'll report back. I remember buying and working these needles to get my TRP176 dialed in completely. I'll report back.

But, for those that have that off throttle lean (or rich) sudco has 20 different needles for your VM26 and 13 for your VM20 or VM22. For those that want the ultimate in dialing in, you might consider some new needles to play with.
 
6

69ST

Well-Known Member
Sudco lists them but, in my experience, probably doesn't have them. I ran into that dilemma years ago. They had 5 available jet needles... more holes than list. If the situation has improved, I'll happily stand corrected. Similarly, if they can once again, supply slides with different sized cutaways, that could make a huge difference. I'd go for .5 & 1.0. There are also different needle jets listed...and those can have a HUGE impact on main jet sizing.

Interesting that you're reaching the rich limit with those <ostensibly> small jet sizes. The main could be due to the needle jet. However, that has zero effect on pilot jet sizing and even #22.5 seems small, for the displacement. That indicates, to me, that your motor is pulling healthy vacuum.
 
fatcaaat

fatcaaat

Well-Known Member
Its super interesting when you consider the differences in jetting on similiar sized motors. My TRP176 Dax with all the fixings, 30mm intake, 25.5 exhaust, s25 cam, porting, etc...using a TM28 that one is jetted 17.5/200. And when I say it's dialed in, this one is dialed in more so than any other bike I have. However, recently spending some time jetting a yx195 with the superhead+r on it, and also one of the stock Daytona 190s with the same carb, the differences are vast. I think I ended up on the yx195 with a 27.5 pilot and 230 main and needed a really rich needle. It runs crazy good now, but that's where we ended. The Daytona also runs a 27.5 pilot and a 210 main.

I did run out to the garage and swap in the stock needle. It took me about 10 seconds to realize that things were much better as soon as I kicked it. I was able to adjust the idle quickly, and it rev'd nice and clean. Obviously I'm going to have to heat it up before I really start messing with it, but things are already promising. We may end up having to rejet the main...who knows?
 
6

69ST

Well-Known Member
With the 20 & 22 carbs, the jet needle affects fuel flow including WOT mixture.

Frankly, I view the VM26 as a really good choice on an engine of this displacement & output, for road use. And you know me when it comes to carburetion...conservative. I expect you'll get crisp, virtually gap-free, throttle reponse, with ~85mph horsepower, maybe a little more; the Mikuni rep I spoke with said the VM26 will support 25hp+.
 
C

CT70_Scotts

Member
Has anyone ever played with the emulsion holes in the side of the pilot jets? Seem like years ago when I was playing with Methonal on one of these that was the ticket for both pilot and main.

I have to say though I built my 106cc about 18 years ago and after having a 140cc Lifan this little 106 will about kill that 140. And for the first time ever the 106 has a stock carb and intake put on for a show, throttle response is unbelievable.
 
6

69ST

Well-Known Member
CT70_Scotts said:
Has anyone ever played with the emulsion holes in the side of the pilot jets? Seem like years ago when I was playing with Methonal on one of these that was the ticket for both pilot and main.

I have to say though I built my 106cc about 18 years ago and after having a 140cc Lifan this little 106 will about kill that 140. And for the first time ever the 106 has a stock carb and intake put on for a show, throttle response is unbelievable.
Click to expand...

The smaller the venturi, the higher the airflow velocity through it. Since carbs work on pressure (vacuum) differentials, higher flow velocity translates into higher efficiency, manifesting as improved throttle response, fuel atomization and fuel consumption numbers. It's moving toward EFI-like operation...up to a point. Just as there is clearly an upper size limit, there is a lower one, also. Go too small and you'll lose horsepower, at best. Beyond a certain point, vacuum levels exceed design parameters and you'll get excessive "pullover effect", where overrich mixtures become difficult-to-impossible to tame...and there'll be a knife-edge separating the effects of tiny changes in tuning/jetting.

That said, with a street machine, even losing a little bit of peak power really won't register on the seat-of-the-pants dyno. In fact, average power (a.k.a. output "below the curve") might be better. Thus, you might sacrifice 2-3mph at the absolute top end...but...how much of the time does that matter? Torque is unaffected and that's pulling power + acceleration. If throttle response is EFI-like and the machine pulls itself along with effortless ease, within the range where you spend 99% of your riding time, it can be the better compromise.

As for enlarging jets and emulsion tube orifices, unless you have ultra-precision tools/machines and many years experience with this type of modification, the odds of success are low...while the odds of turning parts into scrap metal are high. If you're dedicated/motivated enough to try this, at least source replacements; that way you can bail yourself out, if need be. I've modded jet needles, believe me 0.0005" can make the difference between a slight stumble and a steady-state rich misfire, at part throttle and once you've removed metal, there's no going back. OTOH, getting it right is very satisfying. And there aren't many tuning parts available for carburetors below ~32mm.
 
C

CT70_Scotts

Member
Lol I must be some wizard then. Because on my 106cc I did a 38 pilot and took a unmarked 65 jet and drilled it out with my pin drill set up about 2 drill sizes. Seems to run darn good could use alittle tunning one way or the other but close.
 
6

69ST

Well-Known Member
CT70_Scotts said:
Lol I must be some wizard then. Because on my 106cc I did a 38 pilot and took a unmarked 65 jet and drilled it out with my pin drill set up about 2 drill sizes. Seems to run darn good could use alittle tunning one way or the other but close.
Click to expand...
There's one thing with which I'll never argue...results. IOW what works, works, end of story. If you're happy with the results, consider it a success. I've been working with carburetion for close to 50 years now, various flavors of Holley, Carter/Edlebrock AFB, Rochester Q-jet, Mikuni, Tillotson, Keihin, Xinda, etc. With the larger automotive stuff, drilling jets & airbleed orifices is a lot more forgiving. These little carbs are super finicky. I've seen far more ruined jets than successful outcomes, your results may be different. I don't claim to be the last word on carburetor tuning, for sure.
 
fatcaaat

fatcaaat

Well-Known Member
I've thrown in the towel on tuning the VM26 on this bike. None of the needles correct the off-throttle bog. I could start sanding the needle, but I don't want to invest the time. Seems that I'm not the only one, as my research shows others that have tried to put one of these on the motor experience the same issue. One was successful according to the forum, but he didn't share exactly what he did.

So, rather than invest countless hours to try and make this work, i'm going with the same carb that's on my CRF150 converted 230f engine which is tagged as a 30mm but is actually a 26mm, oval bore with accelerator pump. I do know a guy that has these on two of his xr200 engines and they run flawlessly, so I may as well hit the easy button.
 
6

69ST

Well-Known Member
CT70_Scotts said:
@racerx Point taking and I have half the years of your experience. Always enjoy reading your threads and can tell years of experience behind the logic.
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If somebody comes up with a new idea, or happens to possess a talent that I don't have, I say go for it...by all means. That's what keeps the scene moving forward. The last thing I'd ever want to do is discourage anyone. I try to share my experience, which may others in making informed decisions. When it comes to experimentation/modifications, I like to go in armed with as much info as possible...including a way to reverse the change(s) if the fail. There's nothing inherently wrong with failure, that's the basic foundation of learning anything. It's just a lot less painful when there's a fallback, "plan B".

fatcaaat said:
I've thrown in the towel on tuning the VM26 on this bike. None of the needles correct the off-throttle bog. I could start sanding the needle, but I don't want to invest the time. Seems that I'm not the only one, as my research shows others that have tried to put one of these on the motor experience the same issue. One was successful according to the forum, but he didn't share exactly what he did.

So, rather than invest countless hours to try and make this work, i'm going with the same carb that's on my CRF150 converted 230f engine which is tagged as a 30mm but is actually a 26mm, oval bore with accelerator pump. I do know a guy that has these on two of his xr200 engines and they run flawlessly, so I may as well hit the easy button.
Click to expand...

I don't blame you. The VM26 is not the most tractable carburetor (note the sarcastic tone) out there. I half expected you to pop for an MJN carb. That said, when there's a known solution to a problem there's little reason to swim against the current. The best solutions don't necessarily come with the biggest price tags.
 
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