A little useful info before ordering the cheap Mikuni carbs

[125ccCrazy]

I see alot of people buying the cheaper Mikuni's they think are 22mm carbs.... On the taper bore carbs they are usually 4mm smaller than advertised due to the taper from the gasket surface to the venturi
(4mm taper)..... If you look on the mounting flange of the carb there will be a 2 digit number stamped in the carb like 16,18,20,22,24, 26 ect ect.... On Mikunis that 2 digit number is the actual size of the carb (venturi)..... If you want a 22mm then you'll want to get a 26mm taper bore or an Actual VM series 22mm... The actual VM series carbs will also have the MM size stamped into the casting somewhere near the mounting area of the carb...

No matter what the mm size of the gasket surface or inlet is, the carb will only flow as much air as the venturi size..

 

[125ccCrazy]

30mm

Here is an example of a "30mm" taper bore Mikuni being sold on ebay, if you notice it's stamped 26..... thats the actual venturi size and not 30mm as advertised....

 

[minidragbike]

It's still a 30mm carb. Tillotson carbs are made the same way.
My HL380 carb is 28.5mm, but only has a 25.4mm venturi. That doesn't mean it's only a 1" bore carb. The venturi speeds up the flow, and gives better velocity.

With a all out drag motor you don't want a venturi. But if your motor isn't all out you will have a bad bog at instant full throttle.

 

[125ccCrazy]

It's still a 30mm carb. Tillotson carbs are made the same way.
.

Well thats true to a sense, now the vm series are actually 22mm,24mm,26mm ect straight through, whatever the cfm figures out to be that is how much air the carb will actually flow.... I know going from large opening to a smaller venturi will compress the air and cause a stronger flow (vortex) but whatever size the venturi is, thats how much air you can actually pass through it... The taper bores are measured at the gasket surface, the venturi is actually smaller.... The taper bores should be sold as what size the venturi is...

You can take a 22mm taper bore or a 22mm VM carb and bore the outlet to 26mm, 30mm, 34mm whatever the amount of material is to work with but thats not gonna make the carb bigger as far as how much it can flow through it, it's still only gonna be whatever the venturi size is no matter how big the gasket end is....... It's kinda like blowing through a straw into a garden hose and then blowing through a straw into an exhaust pipe and then a drain pipe ect ect, the straw will only allow as much air to pass through as it's I.D no matter how big the other end is... I know you know this already but thats the main idea for porting (enlarging) ports so you can flow more in or out, if you just opened the gasket end up, the port will still only flow what it could to start with, you can cram more into it but it's only gonna flow what it could before.... same with valves, if you put a 26mm valve in place of a 24mm valve without enlarging or installing a larger seat the valve will still only flow what it could before because the bore is the same...

What my point is that a 26mm taper bore with an actual venturi of 22mm is not gonna flow the same volume of air as a true vm26mm with an actual 26mm venturi.. The air flow may be faster with the taper bore and will raise the cylinder pressure but the volume will be less....

These carbs should be rated by CFM like on cars...

 

[buttgoblin707]

I replaced my carb with a mikuni just because my float bowl broke, and I figure why replace palstic parts when you can have a metal carb with no annoying linkage. even the smaller venturi is better than a stock clone carb

 

[npatton19]

Sweet that means I have a 26mm carb:thumbsup:

 

[125ccCrazy]

I replaced my carb with a mikuni just because my float bowl broke, and I figure why replace palstic parts when you can have a metal carb with no annoying linkage. even the smaller venturi is better than a stock clone carb

these carbs are definatley better than the clone carbs but the places that sell them for "minibikes" should specify the actual size.... an engine with some mods and increased compression can handle more carb than the 22mm taper bore carbs that are being sold as "high performance"... on a relatively stock clone they will be better than the stock carb
...

Sweet that means I have a 26mm carb:thumbsup:

You have the 30mm taper bore?? If so do you know what the stock jetting is??

 

[minidragbike]

I wouldn't reccomend telling everyone that they can run carbs with no venturi. Those carbs are strickly made for full race 4 stroke, or 2 stroke motors. Not for just cruising around low dynamic compression motors.

Check out this video of my buddies motor. The carb cam from me, it's a 26.5mm bore no venturi Ibea carb. The motor is built very radical, and doesn't allow it to bog down like a mild motor would. Most of the modified motors on minibikes I see videos of sound stock.

YouTube - The BoneShakers Drag Motor

 

[npatton19]

You have the 30mm taper bore?? If so do you know what the stock jetting is??

Sorry just a 26mm taper bore, but thought it was only a 22mm, just happy to hear its got more velocity.

 

[125ccCrazy]

I wouldn't reccomend telling everyone that they can run carbs with no venturi. Those carbs are strickly made for full race 4 stroke, or 2 stroke motors. Not for just cruising around low dynamic compression motors.

Check out this video of my buddies motor. The carb cam from me, it's a 26.5mm bore no venturi Ibea carb. The motor is built very radical, and doesn't allow it to bog down like a mild motor would. Most of the modified motors on minibikes I see videos of sound stock.

YouTube - The BoneShakers Drag Motor

no I never said only run a carb with a straight bore, I said that the taper bore carbs are not true to the size as they are advertised as and wont flow the same amount of air as a true vm series will....For a cruiser the taper bores are fine but be sure to get one with the correct size venturi to what mods the engine has....... Your buddys bike if it has a 26.5mm venturi then it's gonna flow nearly the same amount of air as a vm26mm straight bore but if you compare it to a cheap mikuni 26mm which actually has only a venturi of 22mm the cheap carb isnt gonna flow as much even though it's advertised as a 26mm, they get the 26mm reading from measuring across the outlet (gasket surface) of the carb which has nothing to do with the actual flow of the venturi........You really can't compare a Tilly to a mikuni either, they are two totally different carbs with different characteristics and tunabilty... Tillys are mostly an all out carb, they like to run wide open........

 

[buttgoblin707]

carbs

I have never ran a tilly but they look alot like a gigantic walbro carb with low/high settings, they look very easy to tune.

 

[minidragbike]

no I never said only run a carb with a straight bore, I said that the taper bore carbs are not true to the size as they are advertised as and wont flow the same amount of air as a true vm series will....For a cruiser the taper bores are fine but be sure to get one with the correct size venturi to what mods the engine has....... Your buddys bike if it has a 26.5mm venturi then it's gonna flow nearly the same amount of air as a vm26mm straight bore but if you compare it to a cheap mikuni 26mm which actually has only a venturi of 22mm the cheap carb isnt gonna flow as much even though it's advertised as a 26mm, they get the 26mm reading from measuring across the outlet (gasket surface) of the carb which has nothing to do with the actual flow of the venturi........You really can't compare a Tilly to a mikuni either, they are two totally different carbs with different characteristics and tunabilty... Tillys are mostly an all out carb, they like to run wide open........

Your not understanding what I'm saying.
The carbs are the true size they claim to be. They just have a venturi that's all. Most of all the kart classes have tillotson carbs and all tillotson carbs have venturi's. I have a HL380 that has a 28.5mm bore, with a 25.4mm venturi. I can't go around saying it's a 25.4mm carb because it's not. The only reason a tilly isn't used in fun riding is because it has a butterfly.

With the mikuni it works the same way. I also run Lectron carbs. They come standard in what ever size you want. I have had a 34mm regular model, then they have a HV model. The HV has a venturi in it. It starts off 36mm, then goes down to 32mm, then finish at the back at 34mm. This allows a rush in flow and no drop off on a instant full throttle.

It all depends on what the carb is used on. The flatslide mikuni's are normally used on 2 stroke motors with a much higher air speed. They don't need velocity, because there is no compression stroke. Or if your motor is built full out like a drag motor is will also work well. But if it's like a kart motor or less it will need velocity to keep the air speed up. Or it will bog, or be sliggish out of the hole, or starting line. The only way that would correct this would be to run a smaller straight bore carb on same motor, much bigger motor same carb, or a carb with a venturi.

The carb on my buddy bike in the video is 26.5mm and no venturi. The carb is a slide and works just like a flatslide mikuni. Not like a butterfly tilly carb, but easy to tune like one. As you can see and hear for yourself, the motor is very modified. And doesn't sound like a regular modified motor you hear on a kart. It's not the carb making it sound that way, it's the other mods done to it. Those same 26 venturi carbs come on briggs WF and make 15hp. Most modified minibikes don't even make that much power.

 

[125ccCrazy]

Your not understanding what I'm saying.
The carbs are the true size they claim to be. They just have a venturi that's all. Most of all the kart classes have tillotson carbs and all tillotson carbs have venturi's. I have a HL380 that has a 28.5mm bore, with a 25.4mm venturi. I can't go around saying it's a 25.4mm carb because it's not. The only reason a tilly isn't used in fun riding is because it has a butterfly.

With the mikuni it works the same way. I also run Lectron carbs. They come standard in what ever size you want. I have had a 34mm regular model, then they have a HV model. The HV has a venturi in it. It starts off 36mm, then goes down to 32mm, then finish at the back at 34mm. This allows a rush in flow and no drop off on a instant full throttle.

It all depends on what the carb is used on. The flatslide mikuni's are normally used on 2 stroke motors with a much higher air speed. They don't need velocity, because there is no compression stroke. Or if your motor is built full out like a drag motor is will also work well. But if it's like a kart motor or less it will need velocity to keep the air speed up. Or it will bog, or be sliggish out of the hole, or starting line. The only way that would correct this would be to run a smaller straight bore carb on same motor, much bigger motor same carb, or a carb with a venturi.

The carb on my buddy bike in the video is 26.5mm and no venturi. The carb is a slide and works just like a flatslide mikuni. Not like a butterfly tilly carb, but easy to tune like one. As you can see and hear for yourself, the motor is very modified. And doesn't sound like a regular modified motor you hear on a kart. It's not the carb making it sound that way, it's the other mods done to it. Those same 26 venturi carbs come on briggs WF and make 15hp. Most modified minibikes don't even make that much power.

Well I just don't see how making the outlet (gasket) end of the carb bigger than the venturi will make the carb flow more air or as much air as a carb with a straight through bore of the same size..... I understand about the venturi funneling the air and when the throttle is wrapped real quick you dont get a big gulp of air like a straight through carb but what I'm saying is making the end bigger doesnt change the amount of air that can pass through the middle..... Maybe if they would rate them with CFM instead of MM it would make more sense..... I'm just comparing a true MM rating with a straight through bore to a carb that only has the end opened up to that size.... If I'm wrong about all this so be it but someone is gonna have to show me how to pass as much air through a smaller hole without pressurizing it with a turbo or blower on the same engine....

2 strokes do have a compression stroke, the piston comes up after taking fuel in through the intake port (reeds unless it's piston ported) and compresses it, plug fires and drives the piston back down but since they do everything in 2 cycles they have a much faster rev and will rev higher... They just don't have 4 seperate cycles ((1)intake, (2)compression, (3)power and (4)exhaust) 2 strokes are (1 intake and compression) (2 power and exhaust)

 

[minidragbike]

The flow test has been done about 10 years ago in jr dragster racing, and kart racing. The 28mm Mikuni that everyone know and loves was tested against a HL380 tillotson carb. The 28mm Mikuni is 28.5mm all the way through. The tillotson carb is 28.5mm at the back, but with a 1"/25.4mm venturi.

At instant part throttle the mikuni flowed more air
At instant full throttle the mikuni flowed more air
At constant part throttle the tillotson flowed more air
At constant full throttle the tillotson flowed more air

The reasons why the test went this way is just like you said. The mikuni flows more unrestricted flow at instant throttle. But at constant throttle the venturi speeds up the flow. Something like going up hill entering the carb, then coming down hill into the bore.

Now at the dyno session:
The motor test was a 20+hp jr dragster flathead motor. The 28mm Mikuni carb worked very good at part throttle, and full throttle. The down falls of the carb is that the carb had to setup on a medium setting. Meaning that you was 100% sure if the carb was setup as best as can be. It was setup to only please the dyno guy. The carb didn't bog, hesitate, or sputter. So that's what matters when setting up a miknui multi jet carb. If you can crack the throttle and it doesn't bog, and revs up your basically satisfied with it as is, not knowing if it's as best as can be. Mainly because there isn't a way to tune for bottom end, and top end.

Now with the HL380 tillotson carb it was setup to work 100% correct to the motor. Reasons why is because it has it's own separate jet/needle for the high and low. So it ran perfect on both bottom and top. Giving it more bottom end TQ, top end HP, and more RPM from having more air. Being able to tune on the fly is key to getting the most possible RPM/HP you can get.

Now I use to run a 29.5mm Ibea carb back 3 years ago on my modified gx200 motor. The motor turned 10,300 RPM. At the time no one believed me because it was unheard of. The guys I was building half midget motors for have never got there motors to turn over 9,600 with 28mm Mikuni's. But the Deco 1/4 midget guys were turning over 12,000 RPM with Ibeas on them.

Ibeas are just like the tillotson carb, but MOST of them DON'T have a venturi in the carb. But they are only used on full all out high RPM race motors.Usually 2 stroke kart motors.

The 2 best carbs ever made are the Lectron, and the Ibea.

 

[125ccCrazy]

The flow test has been done about 10 years ago in jr dragster racing, and kart racing. The 28mm Mikuni that everyone know and loves was tested against a HL380 tillotson carb. The 28mm Mikuni is 28.5mm all the way through. The tillotson carb is 28.5mm at the back, but with a 1"/25.4mm venturi.

At instant part throttle the mikuni flowed more air
At instant full throttle the mikuni flowed more air
At constant part throttle the tillotson flowed more air
At constant full throttle the tillotson flowed more air

The reasons why the test went this way is just like you said. The mikuni flows more unrestricted flow at instant throttle. But at constant throttle the venturi speeds up the flow. Something like going up hill entering the carb, then coming down hill into the bore.

Now at the dyno session:
The motor test was a 20+hp jr dragster flathead motor. The 28mm Mikuni carb worked very good at part throttle, and full throttle. The down falls of the carb is that the carb had to setup on a medium setting. Meaning that you was 100% sure if the carb was setup as best as can be. It was setup to only please the dyno guy. The carb didn't bog, hesitate, or sputter. So that's what matters when setting up a miknui multi jet carb. If you can crack the throttle and it doesn't bog, and revs up your basically satisfied with it as is, not knowing if it's as best as can be. Mainly because there isn't a way to tune for bottom end, and top end.

Now with the HL380 tillotson carb it was setup to work 100% correct to the motor. Reasons why is because it has it's own separate jet/needle for the high and low. So it ran perfect on both bottom and top. Giving it more bottom end TQ, top end HP, and more RPM from having more air. Being able to tune on the fly is key to getting the most possible RPM/HP you can get.

Now I use to run a 29.5mm Ibea carb back 3 years ago on my modified gx200 motor. The motor turned 10,300 RPM. At the time no one believed me because it was unheard of. The guys I was building half midget motors for have never got there motors to turn over 9,600 with 28mm Mikuni's. But the Deco 1/4 midget guys were turning over 12,000 RPM with Ibeas on them.

Ibeas are just like the tillotson carb, but MOST of them DON'T have a venturi in the carb. But they are only used on full all out high RPM race motors.Usually 2 stroke kart motors.

The 2 best carbs ever made are the Lectron, and the Ibea.

Ok I can kinda understand that but what I dont understand is if you take a 26mm taper bore with a 100 stock main jet and you take a 26mm VM with a stock 190 main jet, if the taper bore flows as much air at full throttle because as the vm because of the venturi why such a huge difference in jet size, wouldnt the taper bore have the same amount of air to atomize the fuel with running a 190 jet at WOT?? How many jet sizes can you go up from 100 before you have too much fuel and not enough air flow??....... Not trying to be difficult, it's just some things that don't quite make sense to me..... I sure don't claim to know it all, I'm learning all the time....

 

[minidragbike]

I understand your point, but the air is mixing much better with the venturi. That's what I'm telling you. The straight through bore has to be setup as a instant throttle setup. The gas has to get in just as fast as the air.

I don't know if you know about V.E.? Volumetric Efficiency
AT WOT(wide open throttle) as your motor is climbing up in RPM your actually using less air. The RPM is slowing down, and your making less HP and TQ. At WOT with the straight bore carb at let's say 8,000 RPM. The carb is only using 60% of it's 180 main jet. But at instant throttle the motor is pulling at lot more air at low RPM so it's using the full 100% of that 180 main jet. It has to, or it will bog, stumble, or sputter because there will be to much air, not enough fuel.

Now with the venturi carb if you open it instant throttle the air is not movng as fast because it has to go around the venturi. Which makes for better velocity. Velocity is the air SPEED the motor is demanding. This Vacuum force is high at low RPM, and get's weak at high RPM. Now at high RPm when it starts to slow down the velocity is still much higher in the motor than the straight bore carb. BECAUSE the carb is letting in less gas, be keeping the air/fuel mixture at the proper setting longer.

The motors air speed demand is very important when picking a carb. If your motor is a high RPM super modified motor, or 2 stroke than the straight bore carb will work best. But if not the venturi carb will be more user friendly. Save more gas, and allow you produce more bottom end TQ and top end HP.

That's why I say you have to find a medium with the straight bore carbs. If it can instant throttle and not bog, and run good on top end then you just have to settle for that.

 

[125ccCrazy]

Ok that is starting to make more sense.... so with my clone which is fairly built...
milled ported head .080
stock size SS valves
26lb springs, locks, retainers
arc rod
arc flywheel
285-250 nr cam
flat top piston
.010 head gasket
According to NR compression chart it should be around 12.4:1 compression

I have a 24mm flatslide for the carb...... Do you think this carb will be sufficent for this build and how rideable will it be?? I'm not looking for a WOT ride everytime I start it like a strictly race bike....

also for the exhaust, should I run a pipe with an I.D the size of the exhaust port or start out a bit smaller and flare it to match the port size and then bump it up 2 or 3 sizes (steps)?? I know the bigger the pipe the less low end but you have but more top end power to a point....

 

[minidragbike]

The 24mm flatslide carb works good. A lot of it depends on the size of the motor itself. The header should be 1" O.D. that normally means the I.D. will be around .875"-.920". Most of the robertson torque tube headers are already this size. I even run the little RLV header. It's a single stage that's 1" O.D. from begining to end.

I'm gonna have to video my little street riding bike. It soud very radical just like my buddies do. It also has a 27mm straight through Ibea gasoline carb. We was riding good on fathers day, until my motor threw the chan off. :doah:The good thing I was only a few city blocks away from the starting point.