Mikuni VM26 (and some tips on carb tuning in general)

by **Greg** » 27 Feb 2010 02:30

Edit: Scroll down to the ninth post (titled, Carb Tuning) for tuning tips and links to tuning instructions.

Today was a little like Christmas. Not only did my POC HMF Exhaust come in, which I am totally blown away by, but so did my new Mikuni VM26mm Performance Carb Kit from Honda Trail Bikes.

The kit comes with a genuine Mikuni VM26-606 carb, intake kit for the smaller heads, 3 main jets in addition to the one in the carb, a throttle cable, a Uni knock-off(or rebranded) dual layered foam air filter, and an assortment of bolts. The bolts that came with the kit had me totally perplexed. They still have me perplexed. If you look at the picture, the 2 bolts on the right attach the rubber intake boot to the carb end of the manifold. This is the only pair of identical bolts in the kit. One of the remaining bolts works great for attaching the manifold to the engine if you don't use the black plastic heat shield, and the other works great if you do use the heat shield. Neither works in both situations. Then I am at a complete loss what the other two bolts are for, the two that are threaded on both ends. I installed it with the heat shield, the long bolt, and one of my original bolts. I'll send an email to Mike at Trail Bikes tonight or tomorrow to see if he can clarify the whole bolt thing to me.

Once through that small bit of confusion, I turned the bike on and it took about 20 seconds to get the mixture and idle set. That was nice. I threw on some layers, and took her for a spin, and my fears that this carb would be too big and I wouldn't have enough low end power on this engine were immediately dissolved. She bolts from the line. The stock pilot jet seems to be pretty much right on. With the stock setting on the jet needle and the stock main jet, everything is great until I start to get close to 3/4 throttle at which point she just doesn't want to continue accelerating. So tomorrow I'll try the other main jets the kit came with. One of the nice things about this carb is the main jet can be accessed from a plug on the bottom so you don't have to take the whole carb apart.

There are actually a couple guages I would really like to have, one that is sort of a mobile dyno that you can then plug into your computer at home and will actually show you a dyno graph, and the other a fuel/air mixture meter. The Veypor Gauge that has the dyno, also tells you your speed in mph or kph, how many g's you are doing, estimates how much fuel you have left(very handy on our bikes that have no fuel gauge), and a whole lot of other stuff. The thought of being able to do my own dyno runs, even if they are not as accurate as a shop dyno is really appealing. To be able to swap two or three parts and see which one gives better torque and horsepower and at what rpms would be really cool.

The other gauge that would be nice is an air/fuel ratio gauge which would take all the guesswork out of tuning the carb. With those little babies you can easily fine tune the carb so that idle to 1/4 throttle has an even mixture, 1/4 to 3/4 throttle is a little lean for good mileage, and 3/4 to full is a little rich to protect the engine at high rpms. Sadly these are more expensive than the Veypor and require modification to the exhaust pipe to install the O2 sensor.

Edit:

Nice accessories for the VM26:
- K&N air filter (RC-2310) and PreCharger (22-8028PK)
- Longer throttle cable

Good Links:
- TBolt USA - Simple Guide to Jetting
- ScootRS - Carb Tuning
- OORacing - Tuning Tips

by **Greg** » 28 Feb 2010 19:01

Finally broke the 100 kph mark today, hitting 102 kph on a level road. I think I need a slightly larger main jet, and maybe just perhaps I'll be able to hit the 65 mph mark with my current setup. My carb came stock with a 190 main jet. I have a 175 in now. I think either a 180 or 185 would be optimum. I ended up going with a pilot jet a notch smaller. The carb came with a 22.5 pilot stock which I found with some main jets I played with would cause the engine to want to stall as I came to a stop. I then tried the 17 pilot and it had issues at the 1/4 throttle transition. 20 seems to be great. I ordered several more main jets from HVC Cycle, the 180, 185, and a few others.

by **lurgee** » 23 Mar 2010 02:12

Just wondering how you have routed the EGR now that, i assume, you have removed all the plastic air intake.
I just bought virtually the same kit and I'm not sure whether to just block it off or leave it sticking out.

Any tips would be great

by **Greg** » 23 Mar 2010 08:58

I put a block off plate over it that I got from Moto-Scoot.

by **Greg** » 13 Apr 2010 16:50

For anyone interested, here is the Mikuni VM Carburetor Super Tuning Manual.

by **MadAssGuerrillia** » 26 Apr 2010 19:30

ghouston wrote:For anyone interested, here is the Mikuni VM Carburetor Super Tuning Manual.

you know this manual is all we got, i think Mikuni would come up with something better.

by **Greg** » 10 Jul 2010 16:16

So I am posting this basically in any of the forums where I've been talking about the VM26 or my bog issues with it more specifically. I finally got rid of the bog today.

After an exchange of emails with Vince at TBolt last week I installed the largest pilot jet I had, a 25, on my VM26. This got rid of about 70% of my bog if I snapped the throttle hard off idle, so I ordered three larger pilots.

Those jets came in today, and I figured before fooling with them I'd give Andy at MotoScoot a call and see how he has these bikes jetted. It turns out he is using the biggest pilot I ordered, and which I almost didn't order just because it seemed so large. I'm glad I called him because it would have been the last one I tried.

So for a 142cc MadAss with the Mikuni VM26, here is a good starting to point for your jets and needle clip:

Main: 180
Pilot: 32.5
Needle clip: 2nd or 3rd position (from the top)

This will vary of course based on the temperature and your altitude, but this is a good starting point. The kicker here is the pilot. It's a big pilot.

I still think a 24mm carb would be fine (probably optimum) on the 142cc engine, and most definitely on the stock engine, but the VM26 will work properly once you get it tuned.

by **Ven0m0uS** » 28 Aug 2010 16:59

Hey Greg,

I've read a lot of your posts and I'm never dissapointed. You have a lot of good information on just about all areas on the MadAss. I'm an Auto Technician and I'm new to the 2 wheel world. I haven't had much practice with carburetors, I pretty much deal with fuel injection all day. I'm looking over details on the carburetor for adjustment for optimum running condition. I found a lot of good info so far.

Thanks,

-Johnny

by **Stewped** » 06 Dec 2010 21:37

WoW -- Just an awesome post {you get a big "Likes" response on this one}

I been having some performance since the installation of the yx160 engine kit from Fast Trails Performance -- I dont know anything about carburators / jetting and/or tuning them however after reading this post many of the symptoms {low thottle crackling / popping / backfiring} I have been suffering from appear to be carburator related. I believe many of the issues can easily resolved by changing the pilot jet.

The yx160 kit came with a 14 dollar 26mm Molkt carb and what I understand is the stock jetting is 15 on the pilot and 80 on the main jet {way too lean}
Many of the forums state to start with a 25 pilot {which is the same jet fitment for a Mikuni vm22/210} and a 105 to 115 main jet {some keihin/OKO will work}
I didnt want to mess with all that so I purchase a 7 piece jet kit just for the Molkt carb -- comes with 3 pilot jets {25-27.5-30} and 4 main jets {106-108-112-115}

Though almost everyone agrees that the Mikuni VM26 is the way to go -- Thanks for the post Greg I think this is going to work out well -- I keep you informed

Note: Just a thought here -- It might be better if you could rename this post as Carb Tuning and Jetting I almost didnt read it because of the title

by **Greg** » 06 Dec 2010 22:21

Note: If you have never tuned a carb at all or need a refresher, skip to the bottom of this post and check out the "Good Links".

Yeah, carb tuning can be a little frustrating the first few times, but then you'll start to get a feel for it, and just know what needs adjusting as the weather changes.

Here's some random tips from another post I made:

A clue that the bike is running a bit lean is if you are riding and when you let the throttle off the bike, the bike very abruptly begins engine breaking, or if you are riding along at low throttle and the bike starts engine breaking. Though I don't recommend it, when going 45 or so and coming to an intersection, once you release the throttle you should be able to take your hands off the controls entirely ("Look Ma, no hands"), and coast a bit before you need to start downshifting. When the carb is tuned right, the bike will start engine breaking when you let off on the throttle, but it isn't abrupt, like it wants to immediately bring the bike to a stop.

When engine breaking, if the exhaust starts popping, it's probably running lean. [An additional note: This symptom tends to start at a leaner point than the abrupt engine breaking. So this is like your second warning that it's time to rejet.]

If you have put a larger carb on your bike, it may need a fairly large pilot jet in order to not have any bog when you snap the throttle from idle. You may find then that having the right size pilot and right size main jet for your current weather conditions will cause the bike to stall when you come to a stop after speeding along. If this is the case, then moving the needle clip up a notch or two will smooth everything out. Move the clip up one notch at a time.

Some more tips:

The engine will run leaner as it gets colder. You may need to put bigger jets in during the winter. Likewise, the bike runs richer as it gets hotter out. You may need to put smaller jets in during the summer.

If there is just a small window where setting the idle screw gives you a workable idle, you probably need to change the pilot jet. So if you turn the idle screw out just a notch and the bike wants to die or if you turn the idle screw in just a notch and the idle starts racing, and you have tried different air screw settings to no avail, try a different pilot jet.

If you consistently have trouble with getting the idle and snap off throttle tuned just right, you may want to consider a smaller carb. When the carb is the right size it will be a lot easier to tune, and you'll have to fool with the needle position a lot less. The needle basically balances the pilot and main jet.

Running lean can cause damage to your engine. It's better to error on the side of running a bit too rich, particularly with the main jet since it's the one with the greatest influence at high RPMs where the engine is most vulnerable. Running rich can dramatically decrease your gas mileage however.

A rough guide to carb sizes:
50cc – 18mm carb
88cc – 20mm carb
110cc – 22mm carb
125cc – 22/24mm carb
140cc – 24/26mm carb
160cc - 26mm carb
172cc - 26/28mm carb

Good Links:
- TBolt USA - Simple Guide to Jetting
- ScootRS - Carb Tuning
- OORacing - Tuning Tips

by **chrink** » 05 Apr 2011 20:44

I did the MotoScoot stage 1 tune up with the asspipe and I am having a real hard time getting it dialed in. Currently it will run but if I snap the throttle at idle it will die and I have to ease into it. At the same time, after riding a few minutes it will just die and it takes half a minute or so for me to be able to get it running again. Almost feels like the engine is getting flooded which would lead me to believe I'm running rich. I checked my plug and its not black so I'm perplexed. Running the stock carb on the stock engine but these carbs don't have any other jets to try.

Anyone have any ideas? I really want to get out on the road again. 4 weekends with no riding is giving me withdrawls.

Thanks,

by **Illberato** » 29 Jun 2011 09:45

Do you guys foresee any compatibility issues with this? :
http://cgi.ebay.com/ebaymotors/Mikuni-2 ... 500wt_1182

by **Greg** » 29 Jun 2011 15:00

Illberato wrote:Do you guys foresee any compatibility issues with this? :
http://cgi.ebay.com/ebaymotors/Mikuni-2 ... 500wt_1182

Being that it's on eBay, there is no guaranty that it is actually a Mikuni carb or that it is actually 26mm. Also, it's hard to say whether or not that intake manifold will fit.

by **Tobyg** » 18 Dec 2011 00:56

Thanks Greg!

I just tuned my carb after fitting the POC HMF. Haven't changed jets yet, that's the next step. I just fixed the idle as after fitting the exhaust it was jumping all over the place (obviously running lean).

I richened the idle mixture using the mixture screw and she idles smooth as now!

Cheers for your help!

Toby

by **wrc323** » 18 Mar 2012 19:09

Hey guys,

A question on carb tuning. I've just changed my exhaust to a POC HMF, with the stock carb, and had my first ride to work on it. It crackles and pops on deaccelleration. Quite a bit and loudly! :lol:

I did notice it crackling before the exhaust change but the POC amplifies it. Also after coming to a stop, sometimes it will die but I am able to start it again.

So, going by what's written above does this mean the engine is running lean? Should I start by changing the clip on the needle to one step richer? Adjust the idle in slightly too?

by **MilwaukeeMadass** » 27 Apr 2012 12:56

Thinking I'm finally going to pull the trigger and bin the stock snorkel and carb and go with the Mikuni VM26 to help the stock engine and AssPipe breathe a bit better. And before I do the damage I thought I'd see if anyone might have some suggestions, tips or things to keep in mind for a knuckle-dragger like me before starting.

by **MilwaukeeMadass** » 27 Apr 2012 15:02

So do you oil these suckers the same way as any other foam pod filter?

by **pjrey** » 29 Apr 2012 08:25

what kind of oil should i use to oil my filter?

by **MilwaukeeMadass** » 29 Apr 2012 08:36

I've been told that you shouldn't skimp on the filter oil and get something quality like K&N filter oil

by **Greg** » 29 Apr 2012 12:56

If you get an actual K&N filter, they come pre-oiled, and you know you have a quality filter.

by **pjrey** » 16 May 2012 17:38

can someone please help me out.

i have the stage 3 from motoscoot

it is a mikuni 26 carb

is this an air screw on it, or a fuel screw?

which way do i turn it to richen it? i thought it was counter-clockwise to lean it, clockwise to richen...

but i am not seeing any info, even after reading the mikuni super tuning 1964-lookin manual

by **TheRain** » 16 May 2012 21:57

pjrey wrote:is this an air screw on it, or a fuel screw?
which way do i turn it to richen it? i thought it was counter-clockwise to lean it, clockwise to richen...

It's an AIR screw. Good rule of thumb- if any given carb's screw is on the air filter side of the carb, it's an air screw. If it's on the motor intake side, it's a fuel screw.

Out (counter-clockwise) = More open = more lean (with an air screw).

by **pjrey** » 16 May 2012 22:08

that is exactly what i thought... but today while reading over the forum, i read someone post about turning the screw counter-clockwise would richen the mix.. there were many follow up posts, and no one corrected him.. so i thought... hmmmm.. maybe i got this all backwards...

i am having one heck of a time tuning my carb.. i have raised and lowered the needle clip... fiddled with the air/fuel screw... still get sputtering... when i finally get it working ok.. i notice my speed decreases.. and my spark plug is totally white... (i have the clip on the lowest setting on the needle...)

i did a bunch of tests today.. with the air screw and needle positions..

i have the stage 3 kit from moto scoot.. 17/39 sprockets
i would think i could get better than 62 mph on flat ground

(another quick question, this isnt a carb issue.. but what is the feeler gauge clearance with the stage 3? i just check and it is currently at .05 and .08 exhaust...
should it be different for the stage 3 than stock?)

thanks rainman for your answer!

p

by **Amahoser** » 17 May 2012 01:16

MilwaukeeMadass wrote:So do you oil these suckers the same way as any other foam pod filter?

I know this answer is late but hopefully it will help. There are two types of oil for filters. That is a gauze type element like K&N. Foam pods require a different oil. Make sure you get either K&N oil or oil for a gauze filter.

Jose Soriano

by **Amahoser** » 17 May 2012 01:43

pjrey wrote:that is exactly what i thought... but today while reading over the forum, i read someone post about turning the screw counter-clockwise would richen the mix.. there were many follow up posts, and no one corrected him.. so i thought... hmmmm.. maybe i got this all backwards...

i am having one heck of a time tuning my carb.. i have raised and lowered the needle clip... fiddled with the air/fuel screw... still get sputtering... when i finally get it working ok.. i notice my speed decreases.. and my spark plug is totally white... (i have the clip on the lowest setting on the needle...)

i did a bunch of tests today.. with the air screw and needle positions..

i have the stage 3 kit from moto scoot.. 17/39 sprockets
i would think i could get better than 62 mph on flat ground

(another quick question, this isnt a carb issue.. but what is the feeler gauge clearance with the stage 3? i just check and it is currently at .05 and .08 exhaust...
should it be different for the stage 3 than stock?)

thanks rainman for your answer!

p

When you checked your plug, was it after a plug chop (full throttle run, kill the engine and coast to a stop with the clutch pulled in)? That is how you check your main jet's jetting. A white plug means you are too lean. But pulling the plug without a plug chop wont tell you what is too lean, main jet, needle or pilot. Sounds like you need to buy some jets...

Pilot Jet. You should be able to adjust the air screw to around two turns out. If it is more than three turns out at best idle, you need a smaller pilot jet (too rich at idle). If you are 0-1 turns out at best idle you need a bigger pilot jet.

Main jet. A plug chop is a good way to check your main jet. Warm up the engine by driving it around for a few miles. Install a new plug and do a run a full throttle. Kill the engine while at full throttle and then remove the plug. If it is white, main jet is too small. If it is black, you are rich and need a smaller jet. Coffee or chocolate brown is what you want.

Needle. The needle adjust air fuel between idle and full throttle. This should only be adjusted after you have properly selected the main and idle jets. You adjust the needle by throttle feel. It should be smooth throughout the range. If it bogs mid throttle, you are probably rich, if it races then smooths out at full, you are probable lean. Raising the clip on the needle LOWERS the needle into the jet making it leaner and vice versa. After all is set, you should be near the middle of the adjustment range, if the clip is at one end or the other, buy the next size needle.

Jose Soriano