New 125 Sputters through all throttle/idle

[69ST]

I only address starting points with genuine Mikuni carbs. Anything else is "terra incognita"...unknown land.

With knockoff anything, it's blind guesswork.

It's not that all cheap carburetors are problematic; some work as they should. My approach is geared toward consistency, over the long term. The VM20 is a good value for $80...easy to tune, consistent quality and parts support. That matters when you need a small part, say, 2 years down the road. Mikuni is still there, as are the major suppliers...like Sudco...long after the ebay sellers are onto selling cheap knockoff POV camera drones.

The L125 motors out there should be making close to 9hp and a VM20 fitted with a 140 main should be close.

 

[TNStreamerDreamer]

I only address starting points with genuine Mikuni carbs. Anything else is "terra incognita"...unknown land.

With knockoff anything, it's blind guesswork.

It's not that all cheap carburetors are problematic; some work as they should. My approach is geared toward consistency, over the long term. The VM20 is a good value for $80...easy to tune, consistent quality and parts support. That matters when you need a small part, say, 2 years down the road. Mikuni is still there, as are the major suppliers...like Sudco...long after the ebay sellers are onto selling cheap knockoff POV camera drones.

The L125 motors out there should be making close to 9hp and a VM20 fitted with a 140 main should be close.

I wish I had taken your advice on the genuine VM20 4 months ago. I've dropped over half that cost on sub-par knock-offs. I finally have a VM22 close running well enough through the full range of throttle but it just isn't "smooth". I may end up dropping the $100ish anyway for the quality of life improvement if the genuine VM20 would help clear that up. Through replacing cheap/unwanted parts, my mistakes, and having very little to start with, I've already got close to $1,900 in the bike anyway. This could be a nice way to ring in the 2K mark!

 

[69ST]

VM22 really needs more like 140cc to deliver seamless throttle response across the entire range. 125cc is the low limit of acceptable, using off-the-shelf tuning parts. 110cc is the absolute minimum displacement and it's a mutha to tune.

The problem is lack of airflow at low rpm...and the resultant lack of vacuum. Seen, in realtime, via O2 meter the stumbling is obvious...transient lean spots, lazy fuel circuits & poor fuel atomization. Holding steady throttle, these self-correct in 1-3 seconds, an eternity when you're in traffic. And it gets old in a hurry.

It'd be easy to correct...smaller slide cutaway, different jet needle taper and a different airbleed orifice. Unfortunately, none of those parts are available, despite what the Sudco catalog sez. I used to re-profile the VM22 jet needle...saw about 85% improvement. But, it's almost a black art, working in 0.0001" increments and getting the right section of the needle. All it takes is a couple of ten-thousandths too much material removal and you get a steady-state rich misfire at that, specific, throttle position. Those jet needles aren't always available...without a carburetor attached.

On the other hand, the VM20 is very easy to tune behind this displacement. All that is needed is the right size main jet and a little tweaking. Without those gaping lean holes to cover, I typically see 15% better mpg numbers...which makes a difference with a whole .62 gallon fuel capacity. If there's any reduction in top speed, it's a couple of mph...above 60mph...traveling downhill with a tailwind. Usable power is actually a little better, without musician-like throttle technique required.

Cost-wise, it's oftentimes cheaper from the get-go, once the motor is performing as it should, vs the "cheaper" alternatives. Then, of course, there's the part support angle when some little item like a seal, gasket, needle & seat need replacement, eventually.

Next step up the proverbial ladder is an MJN22 carb...~$350 by the time you add the intake, jets & shipping.

 

[TNStreamerDreamer]

VM22 really needs more like 140cc to deliver seamless throttle response across the entire range. 125cc is the low limit of acceptable, using off-the-shelf tuning parts. 110cc is the absolute minimum displacement and it's a mutha to tune.

The problem is lack of airflow at low rpm...and the resultant lack of vacuum. Seen, in realtime, via O2 meter the stumbling is obvious...transient lean spots, lazy fuel circuits & poor fuel atomization. Holding steady throttle, these self-correct in 1-3 seconds, an eternity when you're in traffic. And it gets old in a hurry.

It'd be easy to correct...smaller slide cutaway, different jet needle taper and a different airbleed orifice. Unfortunately, none of those parts are available, despite what the Sudco catalog sez. I used to re-profile the VM22 jet needle...saw about 85% improvement. But, it's almost a black art, working in 0.0001" increments and getting the right section of the needle. All it takes is a couple of ten-thousandths too much material removal and you get a steady-state rich misfire at that, specific, throttle position. Those jet needles aren't always available...without a carburetor attached.

On the other hand, the VM20 is very easy to tune behind this displacement. All that is needed is the right size main jet and a little tweaking. Without those gaping lean holes to cover, I typically see 15% better mpg numbers...which makes a difference with a whole .62 gallon fuel capacity. If there's any reduction in top speed, it's a couple of mph...above 60mph...traveling downhill with a tailwind. Usable power is actually a little better, without musician-like throttle technique required.

Cost-wise, it's oftentimes cheaper from the get-go, once the motor is performing as it should, vs the "cheaper" alternatives. Then, of course, there's the part support angle when some little item like a seal, gasket, needle & seat need replacement, eventually.

Next step up the proverbial ladder is an MJN22 carb...~$350 by the time you add the intake, jets & shipping.

Tuning a VM22 in the manner you mentioned is welllll beyond my level of skill, understanding, or patience. Reading that makes me that much more thankful for guys like you on here who can offer educated recommendations for me.

If I was running a nice honda engine, I would consider the $350 carb option. My whole dilemma has been carb cost vs that of the motor itself.

I haven't put enough time on my fake VM22 yet to make a decision on going for a VM20 yet or not. Your suggestion has not fallen upon deaf ears, I assure you. I may end up dropping a Piranha 140 Semi Auto in this thing some day to give a riding experience even further from my street bike so I'm also weighing that in my decision.

 

[69ST]

I wasn't suggesting that you pop for an MJN series Yoshimura carburetor. It is the next step up the ladder from a genuine Mikuni. My point was that the VM20, for your application (or similar) is a lot of value, for a fraction of $350...and really not much more than a cheapie knockoff. Thus, it's a logical option.

 

[TNStreamerDreamer]

Hey @racerx , general carb tuning question for you...
I seem to be a little rich on my idle circuit as it starts best half choked. This doesn't bother me.

I run great at WOT, getting 56mph on flat out of my 125cc. Hoever, the middle 50% range is pretty awful. I have my needle on my richest setting and it seems to be best there. I was concerned if I jetted up, I may mess up the top end?

 

[69ST]

Hey @racerx , general carb tuning question for you...
I seem to be a little rich on my idle circuit as it starts best half choked. This doesn't bother me.

I run great at WOT, getting 56mph on flat out of my 125cc. Hoever, the middle 50% range is pretty awful. I have my needle on my richest setting and it seems to be best there. I was concerned if I jetted up, I may mess up the top end?

First off, the cold-starting, the pilot (idle) fuel circuit and the main fuel circuit. Consider those 3 discrete (separate) things, with very little overlap.

As long as the engine idles well, once it's up-to-temp, and there's no lean-stumble when transitioning from idle to ~1/8 throttle, the pilot circuit is properly adjusted and has the right size jet. Cold-starting, if needs full choke, partial choke or no choke, doesn't really matter if that first set of conditions has been met. Mikuni carbs don't have an actual choke...only a priming circuit, that's used with the ignition switched off.

Having the jet needle C-clip in the lowest groove is the richest setting, just to be clear. If that's how you've got the carb, at present and raising the C-clip to a higher groove makes things worse, then I'd order the next larger #main jet...and the next one above it. Without going into a long dissertation, IMO, it sounds like you're near the lean limit. Oftentimes, when the engine will rev-out cleanly & make full power with 2 different sized main jets, the smaller jet/leaner needle setting will deliver a richer part-throttle mixture. But you're indicating that it's still too lean. That's why I'm recommending the next two larger main jets. You're seeing, firsthand, why I like to find the rich limit, then tune/adjust/jet back toward the lean direction until the richest setting that allows the engine to rev-out cleanly is found. That will get you to within one needle jet height setting (or one jet size) of optimal.

Rich mixture is associated with strong throttle response, but misfiring at WOT, under full power. In the middle range, it'd manifest as a steady-state misfire. Lean mixture is associated with stumbling/bogging, weak throttle response. A transient lean spot...at part-throttle or/and light throttle tip-in that goes away if the throttle is held steady, usually indicates an oversized carburetor.

 

[TNStreamerDreamer]

First off, the cold-starting, the pilot (idle) fuel circuit and the main fuel circuit. Consider those 3 discrete (separate) things, with very little overlap.

As long as the engine idles well, once it's up-to-temp, and there's no lean-stumble when transitioning from idle to ~1/8 throttle, the pilot circuit is properly adjusted and has the right size jet. Cold-starting, if needs full choke, partial choke or no choke, doesn't really matter if that first set of conditions has been met. Mikuni carbs don't have an actual choke...only a priming circuit, that's used with the ignition switched off.

Having the jet needle C-clip in the lowest groove is the richest setting, just to be clear. If that's how you've got the carb, at present and raising the C-clip to a higher groove makes things worse, then I'd order the next larger #main jet...and the next one above it. Without going into a long dissertation, IMO, it sounds like you're near the lean limit. Oftentimes, when the engine will rev-out cleanly & make full power with 2 different sized main jets, the smaller jet/leaner needle setting will deliver a richer part-throttle mixture. But you're indicating that it's still too lean. That's why I'm recommending the next two larger main jets. You're seeing, firsthand, why I like to find the rich limit, then tune/adjust/jet back toward the lean direction until the richest setting that allows the engine to rev-out cleanly is found. That will get you to within one needle jet height setting (or one jet size) of optimal.

Rich mixture is associated with strong throttle response, but misfiring at WOT, under full power. In the middle range, it'd manifest as a steady-state misfire. Lean mixture is associated with stumbling/bogging, weak throttle response. A transient lean spot...at part-throttle or/and light throttle tip-in that goes away if the throttle is held steady, usually indicates an oversized carburetor.

That was such a top-notch explanation. Thanks so much. I haven't found such clear explanations of the lean vs rich misfire/bog conditions. Based on what you said here and my experimentation, I am definitely towards the lean end and will try the larger jet(s).

You never let me down and I really appreciate it @racerx !

 

[TNStreamerDreamer]

First off, the cold-starting, the pilot (idle) fuel circuit and the main fuel circuit. Consider those 3 discrete (separate) things, with very little overlap.

As long as the engine idles well, once it's up-to-temp, and there's no lean-stumble when transitioning from idle to ~1/8 throttle, the pilot circuit is properly adjusted and has the right size jet. Cold-starting, if needs full choke, partial choke or no choke, doesn't really matter if that first set of conditions has been met. Mikuni carbs don't have an actual choke...only a priming circuit, that's used with the ignition switched off.

Having the jet needle C-clip in the lowest groove is the richest setting, just to be clear. If that's how you've got the carb, at present and raising the C-clip to a higher groove makes things worse, then I'd order the next larger #main jet...and the next one above it. Without going into a long dissertation, IMO, it sounds like you're near the lean limit. Oftentimes, when the engine will rev-out cleanly & make full power with 2 different sized main jets, the smaller jet/leaner needle setting will deliver a richer part-throttle mixture. But you're indicating that it's still too lean. That's why I'm recommending the next two larger main jets. You're seeing, firsthand, why I like to find the rich limit, then tune/adjust/jet back toward the lean direction until the richest setting that allows the engine to rev-out cleanly is found. That will get you to within one needle jet height setting (or one jet size) of optimal.

Rich mixture is associated with strong throttle response, but misfiring at WOT, under full power. In the middle range, it'd manifest as a steady-state misfire. Lean mixture is associated with stumbling/bogging, weak throttle response. A transient lean spot...at part-throttle or/and light throttle tip-in that goes away if the throttle is held steady, usually indicates an oversized carburetor.

Hey @racerx ,
I'm revisitting this as I was just troubleshooting my 125cc.

I do think that both my idle and main jets were at least mildly lean.
I've got great throttle response through acceleration and holding WOT.
HOWEVER, the engine vibrates so hard and really just sounds like it wants to rattle to death when I get to a cruising speed or go downhill and maintain that middle of the road throttle.
This same effect occurs on my 140cc but not nearly as severe.

Both have really good throttle response and run smooth from 0 to WOT on the accelerate, really don't get a terrible bog when I punch it or creep it up. The only problem, be it what I feel a serious one, is that downhill or cruising scenario in the mid range.

Any diagnosis? Thanks again for all the help.

 

[-Nate]

4.6.2023

Reviving an old dead thread because I had questions about my 1984 Honda B125S (e) from Brazil with the factory KEI-HIN PD carby .

First and foremost I want to say THANK YOU to the older hands here who are willing to share thier extensive time earned knowledge .

My Honda is one of those emissions equipped bikes, it runs lean and has a fuel tank & carby vapor recovery charcoal filter and myriad hoses .

The Shop manual says it should have a 108 main jet but it came with a 102.5, the pilot jet was so tiny the idle fuel mixture screw made -zero- effect until it stalled from too far leaned out and the engine was incredibly cold blooded .

As a Dealer / shop Mechanic I never had to deal much with fine tuning the carby, just make sure it's all clean (a rare thing indeed) and no crud in the emulsion tube blah blah blah .

My son listened to it run and said ' the pilot jet is way too small', I hunted through my box 'O jets and found one that's visibly larger, installed it and Lo ! . the engine now starts on one kick and idles well .

I was going to try a 105 main jet but here I see talk about larger than 125 main jets ! .

I'm dealing with a 100 % bone stock engine, I think I'll go home and install a 125 main jet and see what transpires .

Good to see some familiar names from the old long gone YAHOO CT90/110 forums .

Looks like Lee's going to have another CT Tech Session this Spring .

Any comments on the advisability of slapping in a 125 main jet are very welcome .