Choosing an Aftermarket Carburetor
If you’re having a hard time finding a replacement carburetor for an antique or custom bike, aftermarket carbs can be a real lifesaver. I’ve found that Niche Cycle has one of the largest selections of Mikuni carburetor parts in the country. They are a fantastic resource for me because I restore a lot of vintage Yamahas, Kawasakis and Suzukis. These bikes primarily used Mikuni carbs. The jets in Mikuni carbs have been standard for years and if you know the number of the jet you need you can replace it. Keep in mind there are different sizes and shapes for main and pilot jets, so you need to get the right one. The Niche website is great because they give you the dimensions and provide really good pictures of all the jets. You can easily match your old jet with one they’re selling.
Niche also sells the complete line of Mikuni aftermarket (public release) carbs. They have the popular VM round slide carbs (similar to those on vintage Japanese bikes), the TM flat slide carbs, the HSR carbs for Harley’s, as well as the RS line of banked carbs for four cylinder bikes.
I especially like the VM carbs for use on vintage bikes. Even though there are subtle differences, they look correct for the year. For instance, if you were rebuilding a Yamaha DT1, Vintage MX racer, a new VM carb would be a perfect choice. The biggest difference is the injection port on the carb for the oil pump and racers almost always removed those pumps anyway. So a missing port is no big deal.
Aftermarket carbs can also be a real asset to anybody building custom or racing machines. These carbs offer outstanding value. They’re relatively inexpensive and often provide quicker starting and improved performance over most stock carbs.
That being said, you also need to understand these new aftermarket carbs were not built as a direct replacement for any OE (original equipment) carburetor. For the past 60-plus years Mikuni has worked with OEM factory engineers to adapt their carburetors to a host of motorcycle models. Each OE carb has special features particular to bike they were designed for. These carbs are identified as OEM (original equipment manufacturer) parts - (i.e., Mikuni for Yamaha). By agreement, Mikuni can only supply those carbs and parts specifically designed for that model to OEM parts distributors. The parts for those carbs are only sold by OEM authorized dealers. For instance, if in 1980 Suzuki put a high exhaust on the right side of a particular model, Mikuni would have been contracted to design a carb with the idle adjustment and mixtures screws on the left. This particular carb would have it's own part number specific to Suzuki and this model. Yamaha or Kawasaki would not share that part number and neither would other Mikuni carbs. So, while the round-slide aftermarket Mikunis look similar to many OEM carbs, there are differences. As mentioned, you may find the idle, mixture screws or choke levers in a different place than the factory carb. And, none of the aftermarket carbs have vacuum ports for petcocks or nipples or oil injection. But the good news is, these aftermarket carbs are inexpensive and there is usually a work around for any problem you might find.
A word of warning though: there are really cheap (different from inexpensive), copycat Mikuni and Kehin carbs on the market. These are made in China and India and do not work well, if at all. If you see a new carb on eBay for $20-$90, rest assured it isn’t a genuine Mikuni or Kehin. Trust me, you don’t want it.
Niche also sells the complete line of Mikuni aftermarket (public release) carbs. They have the popular VM round slide carbs (similar to those on vintage Japanese bikes), the TM flat slide carbs, the HSR carbs for Harley’s, as well as the RS line of banked carbs for four cylinder bikes.
I especially like the VM carbs for use on vintage bikes. Even though there are subtle differences, they look correct for the year. For instance, if you were rebuilding a Yamaha DT1, Vintage MX racer, a new VM carb would be a perfect choice. The biggest difference is the injection port on the carb for the oil pump and racers almost always removed those pumps anyway. So a missing port is no big deal.
Aftermarket carbs can also be a real asset to anybody building custom or racing machines. These carbs offer outstanding value. They’re relatively inexpensive and often provide quicker starting and improved performance over most stock carbs.
That being said, you also need to understand these new aftermarket carbs were not built as a direct replacement for any OE (original equipment) carburetor. For the past 60-plus years Mikuni has worked with OEM factory engineers to adapt their carburetors to a host of motorcycle models. Each OE carb has special features particular to bike they were designed for. These carbs are identified as OEM (original equipment manufacturer) parts - (i.e., Mikuni for Yamaha). By agreement, Mikuni can only supply those carbs and parts specifically designed for that model to OEM parts distributors. The parts for those carbs are only sold by OEM authorized dealers. For instance, if in 1980 Suzuki put a high exhaust on the right side of a particular model, Mikuni would have been contracted to design a carb with the idle adjustment and mixtures screws on the left. This particular carb would have it's own part number specific to Suzuki and this model. Yamaha or Kawasaki would not share that part number and neither would other Mikuni carbs. So, while the round-slide aftermarket Mikunis look similar to many OEM carbs, there are differences. As mentioned, you may find the idle, mixture screws or choke levers in a different place than the factory carb. And, none of the aftermarket carbs have vacuum ports for petcocks or nipples or oil injection. But the good news is, these aftermarket carbs are inexpensive and there is usually a work around for any problem you might find.
A word of warning though: there are really cheap (different from inexpensive), copycat Mikuni and Kehin carbs on the market. These are made in China and India and do not work well, if at all. If you see a new carb on eBay for $20-$90, rest assured it isn’t a genuine Mikuni or Kehin. Trust me, you don’t want it.
A CB450 I built with my son. This exact kit is now sold though Niche.
If you’re thinking about adapting a new carb to your bike, you might want to do a little Internet research. You may find that someone has already done the hard work for you. For instance, if you need a carb kit for a Honda CB350 or CB450, a Yamaha XS650, SR500, or maybe a BMW R100, you need look no further than Niche Cycle. They offer dozens of kits already made up for a host of vintage bikes. If you’re looking specifically for British Bikes like Triumph, BSA or Norton, you’re best bet is MAP Cycle. They’ve been building and selling Mikuni carb kits for British bikes for over 40 years. Nobody does it better. Whether you buy from Niche or MAP, you’ll find these high quality kits come jetted properly and in many cases, with a matched manifold adapter, cable and air filter. If they’re selling the kit you can bet they’ve worked out the kinks. It’s usually a bolt-on operation that’s worth every penny.
But let’s say you’re working on an old Guzzi V65 or maybe a Suzuki T500. You’ll quickly find nobody’s made a kit for these bikes and if you want one, you’ll have to do it yourself. That being the case, I’ve listed below some points you’ll want to consider when selecting an aftermarket carb.
What size is the right size?
You want to be in the same ballpark as the stock carburetor you're replaceing. In the carb world, bigger isn’t necessarily better. Too big a carburetor won’t make your bike faster – it’ll make it run worse. If you don’t know how big your carb is, you can measure the ID (inside diameter) of the venturi where the carb attaches to the manifold. On a Mikuni VM34, the ID is exactly 34mm. If the Dellorto on your Guzzi is 34mm, then in most cases, you'll want to replace it with a Mikuni VM34. That is unless the carb is a diaphragm carb. Diaphragm carbs are usually larger then concentric carbs like the VM style. Usually by 4-6 millimeters.
If you're putting on twin carbs, remember that a twin is really just two single engines attached to the same crank and firing at different times. That's why you often see a 500cc twin using the same sized carb as a 500cc single. Generally, with a lot of the vintage 500cc bikes, you'll usually find them using carbs in the 28-32mm range. With 650cc models, the carbs will probably be in the 30-34mm range and the 750cc twins in the 32-36mm range. But size really depends on how many mods have been made to the engine. Go with the stock size (usually smaller in the above ranges) unless you've got a big bore kit, custom performance head work, bigger valves, performance cams, pod-style air filters and less restrictive exhaust. Remember, bigger isn't better unless you have performance parts designed to handle an increased fuel/air mixture.
So you've determined the size and purchased the carb. Does that mean you’re going to slap the carb on and be riding in an hour? No, even if it’s a perfect fit for the bike. You’ll still need to dial in the jetting and that takes time. You can find out more about setting up carbs and jetting in the Mikuni Tuning Guide on this website.
Unfortunately, unless your old carb is a Mikuni of the same style, the jetting cannot be compared. So, you’ll need to do some experimenting. Quite often, if you seek out information about carb conversions in forums for your bike, you’ll find someone has done this mod before. If they’ve been successful, they’ll usually provide tips and jetting. If not, there are tried and true methods to find a jetting solution (Mikuni tuning guide). I’ve also found this website with jetting for a whole host of motorcycles: http://www.postdiluvian.org/~mason/moto/jetting.html
Here are a few questions you should be asking BEFORE you buy an aftermarket carb:
But let’s say you’re working on an old Guzzi V65 or maybe a Suzuki T500. You’ll quickly find nobody’s made a kit for these bikes and if you want one, you’ll have to do it yourself. That being the case, I’ve listed below some points you’ll want to consider when selecting an aftermarket carb.
What size is the right size?
You want to be in the same ballpark as the stock carburetor you're replaceing. In the carb world, bigger isn’t necessarily better. Too big a carburetor won’t make your bike faster – it’ll make it run worse. If you don’t know how big your carb is, you can measure the ID (inside diameter) of the venturi where the carb attaches to the manifold. On a Mikuni VM34, the ID is exactly 34mm. If the Dellorto on your Guzzi is 34mm, then in most cases, you'll want to replace it with a Mikuni VM34. That is unless the carb is a diaphragm carb. Diaphragm carbs are usually larger then concentric carbs like the VM style. Usually by 4-6 millimeters.
If you're putting on twin carbs, remember that a twin is really just two single engines attached to the same crank and firing at different times. That's why you often see a 500cc twin using the same sized carb as a 500cc single. Generally, with a lot of the vintage 500cc bikes, you'll usually find them using carbs in the 28-32mm range. With 650cc models, the carbs will probably be in the 30-34mm range and the 750cc twins in the 32-36mm range. But size really depends on how many mods have been made to the engine. Go with the stock size (usually smaller in the above ranges) unless you've got a big bore kit, custom performance head work, bigger valves, performance cams, pod-style air filters and less restrictive exhaust. Remember, bigger isn't better unless you have performance parts designed to handle an increased fuel/air mixture.
So you've determined the size and purchased the carb. Does that mean you’re going to slap the carb on and be riding in an hour? No, even if it’s a perfect fit for the bike. You’ll still need to dial in the jetting and that takes time. You can find out more about setting up carbs and jetting in the Mikuni Tuning Guide on this website.
Unfortunately, unless your old carb is a Mikuni of the same style, the jetting cannot be compared. So, you’ll need to do some experimenting. Quite often, if you seek out information about carb conversions in forums for your bike, you’ll find someone has done this mod before. If they’ve been successful, they’ll usually provide tips and jetting. If not, there are tried and true methods to find a jetting solution (Mikuni tuning guide). I’ve also found this website with jetting for a whole host of motorcycles: http://www.postdiluvian.org/~mason/moto/jetting.html
Here are a few questions you should be asking BEFORE you buy an aftermarket carb:
Will this carb fit on my bike?
Check the dimensions of your current carb and compare it to the dimensions of the carb you are considering. Size is important! You need to know the height and length of the carb. You’ll also need to compare the spigot size where the carb bolts to the manifold adapter to see if they’re the same size. And, you’ll need to compare the intake bell where the carb meets with the air box boot. Of course, if you’re planning on using a pod-style filter(s), the size of the intake bell is not a problem. That's because there is a wide variety of pod filters on the market to fit most any carburetor.
If the carb you’re considering has a different size spigot than your current carb, you can always have an adapter ring machined by your local machine shop. Sometimes you can just use a different manifold adapter. Niche carries every manifold adapter Mikuni sells.
Check the dimensions of your current carb and compare it to the dimensions of the carb you are considering. Size is important! You need to know the height and length of the carb. You’ll also need to compare the spigot size where the carb bolts to the manifold adapter to see if they’re the same size. And, you’ll need to compare the intake bell where the carb meets with the air box boot. Of course, if you’re planning on using a pod-style filter(s), the size of the intake bell is not a problem. That's because there is a wide variety of pod filters on the market to fit most any carburetor.
If the carb you’re considering has a different size spigot than your current carb, you can always have an adapter ring machined by your local machine shop. Sometimes you can just use a different manifold adapter. Niche carries every manifold adapter Mikuni sells.
You can get this fitting kit from Niche.
Will my stock throttle cable work?
Unless you’re replacing a Mikuni with a Mikuni of the same type (round slide with round slide) chances are the old cable will need to be modified. But even if you're replacing a Kehin, Bing, Dellorto or Hatachi, you still might be able to adapt the stock cables just by replacing the fitting on the end. It’s really not that hard to build a cable either. Please check out my instructions on how to build a cable.
You can also get a nice little package of assorted cable fittings from Niche Cycle to help you modify your cable.
Can I get to the choke, idle and mixture screws?
As I mentioned before, Mikuni worked with factory engineers to make sure their carbs were perfect for specific models. Using an aftermarket carb, you don’t have that luxury. A few Mikuni carbs, like the VM32, VM34 and VM36 offer models with idle screws on both sides, but the mixture screw will always be in the same place, whether or not you can get to it. Getting to mixture screw is often the biggest problem I face with these carbs. Sometimes they are facing the inside of the opposite carb or they are less accessible because of a breather box, MX pipe or scrambler exhaust. Since it isn’t something that I’ll be adjusting too often, I usually modify an old screwdriver (to create a long offset, or super skinny driver) or make some kind of special tool to reach the screw. Yeah, it can be a pain, but you usually only have to adjust those idle or mixture screws a couple of times and its a small trade-off for having a new carb.
Hey, what do I do with these leftover vacuum lines?
Sometimes you'll find some OEM carbs used a vacuum port coming off the carb to provide vacuum for a vacuum-operated petcock. I found a couple of workarounds for this. First, you can use the petcock as is by turning it off and on manually, but depending on the design of the petcock, reserve may not work anymore. You can install a non-vacuum operated petcock. On occasion, I've found petcocks of the same size or bolt pattern that do not require vacuum. Or, if you have the room, and the proper tools and skill, you can add a vacuum nipple to the manifold.
There may be other ports as well. Some manufacturers, particularly Yamaha, used an oil-injection pump on their two-stroke models. In many cases, the oil line ran from the pump directly to a nipple on the carb. The easiest workaround for this is to not use the pump and just premix your fuel. Again, if you are skilled mechanically and have the right tools, you might also be able to drill out the port on the carb (if it is in the same location) and add a nipple to the carb. I haven't tried this personally, but in theory it should work. Keep in mind, once you modify a new carb or add fuel to it, no parts supplier will take the carb back.
The skinny...
These aren’t all the problems you’ll face when installing an aftermarket carburetor but they are the one’s you’ll see most often. The huge upside is you won’t find yourself rebuilding a carb that is worn out – a carb that even after rebuilding still may not provide optimum performance. In the case of the Mikuni aftermarket carbs, they’re readily available, inexpensive and you can get any jet or part they make with just a phone call. If you’re replacing old Amals, Bings or Kehin carbs, you'll find that once you have the Mikunis jetted properly, the bike will usually start faster and have better overall performance.
Unless you’re replacing a Mikuni with a Mikuni of the same type (round slide with round slide) chances are the old cable will need to be modified. But even if you're replacing a Kehin, Bing, Dellorto or Hatachi, you still might be able to adapt the stock cables just by replacing the fitting on the end. It’s really not that hard to build a cable either. Please check out my instructions on how to build a cable.
You can also get a nice little package of assorted cable fittings from Niche Cycle to help you modify your cable.
Can I get to the choke, idle and mixture screws?
As I mentioned before, Mikuni worked with factory engineers to make sure their carbs were perfect for specific models. Using an aftermarket carb, you don’t have that luxury. A few Mikuni carbs, like the VM32, VM34 and VM36 offer models with idle screws on both sides, but the mixture screw will always be in the same place, whether or not you can get to it. Getting to mixture screw is often the biggest problem I face with these carbs. Sometimes they are facing the inside of the opposite carb or they are less accessible because of a breather box, MX pipe or scrambler exhaust. Since it isn’t something that I’ll be adjusting too often, I usually modify an old screwdriver (to create a long offset, or super skinny driver) or make some kind of special tool to reach the screw. Yeah, it can be a pain, but you usually only have to adjust those idle or mixture screws a couple of times and its a small trade-off for having a new carb.
Hey, what do I do with these leftover vacuum lines?
Sometimes you'll find some OEM carbs used a vacuum port coming off the carb to provide vacuum for a vacuum-operated petcock. I found a couple of workarounds for this. First, you can use the petcock as is by turning it off and on manually, but depending on the design of the petcock, reserve may not work anymore. You can install a non-vacuum operated petcock. On occasion, I've found petcocks of the same size or bolt pattern that do not require vacuum. Or, if you have the room, and the proper tools and skill, you can add a vacuum nipple to the manifold.
There may be other ports as well. Some manufacturers, particularly Yamaha, used an oil-injection pump on their two-stroke models. In many cases, the oil line ran from the pump directly to a nipple on the carb. The easiest workaround for this is to not use the pump and just premix your fuel. Again, if you are skilled mechanically and have the right tools, you might also be able to drill out the port on the carb (if it is in the same location) and add a nipple to the carb. I haven't tried this personally, but in theory it should work. Keep in mind, once you modify a new carb or add fuel to it, no parts supplier will take the carb back.
The skinny...
These aren’t all the problems you’ll face when installing an aftermarket carburetor but they are the one’s you’ll see most often. The huge upside is you won’t find yourself rebuilding a carb that is worn out – a carb that even after rebuilding still may not provide optimum performance. In the case of the Mikuni aftermarket carbs, they’re readily available, inexpensive and you can get any jet or part they make with just a phone call. If you’re replacing old Amals, Bings or Kehin carbs, you'll find that once you have the Mikunis jetted properly, the bike will usually start faster and have better overall performance.
