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Mikuni Carb Tuning

This article was originally written by Dime City Cycles. We found it extremely useful after purchasing our dual Mikuni VM30s and CFC wanted to share this information with you!

Mikuni Carb Fitment - We get a lot of folks looking to ditch the rusted and crusted carbs found on most of these old bikes. In many cases you can, with massive amounts of work and precision tuning, get the older stock CV style carbs to work, but in the end...you'll end up wishing you'd just tossed them and grabbed a set of Mikuni's. In short, adding a set of Mikuni's is one of the best upgrades you can do to your little ground pounder. They have gains in engineering and performance since the CV style carbs are cable driven, which makes a huge difference in performance and daily reliability.

We sell a few different sizes and here's a basic rule of thumb guide for what will work for your bike:
 Regardless of what size you purchase, you will also need to grab one of the universal 2-into-1 cables (you can modify a stock one) along with a set of choke pulls because in most cases the flip levers get in the way and end up needing to be discarded. Aside from that, we recommend getting a few different sizes up and down on the main and secondary jets along with the Mikuni tuning manual as it's worth its weight in gold. (We list what they come with in the description.) And finally, a set of velocity stacks or pod filters depending on what you're shooting for. Velocity stacks do give a slight performance gain, but if you're in a dirty or dusty climate you may want to consider filters.

The VM30's are good for 350's, 400's and 440's, though some folks prefer VM32's. For 450's and 500's the VM32's are the ticket in our minds. Most guys think they need VM34's because they've built their motor's with a bore-over kit and some light porting, but in the end, even with custom cams and all the gold you really don't need VM34's. Those should really only be used in custom applications where you're combing two intakes into one or doing a wild turbo setup or some other kind of forced induction.


Jetting Basics - One of the most common questions we get asked here at Dime City is - "If I get a new muffler and pod filters for my bike will I need to jet it and if so, to what?" It's as though The Gods of Speed have a dark sense of humor, we feel. Because in the end, if you have two identical bikes and do the same modifications to intake and exhaust there's a good chance you'll find yourself having to jet one just a tad different. Maybe a bigger jet or a different position on the needle clip. Regardless, with these old bikes something a simple as cam wear difference can change the jets required.

So, the quick and dirty answer is - "Yes, you might need to jet your bike." The secondary part of that though just isn't as cut and dry, and in an effort to help guide you here's some basics we've come up with to help get your little TON-UP speed machine heading down the road as fast as possible...

Before you even get to the jets you'll need to do a couple of quick things to ensure you're starting off with a good clean slate. The most important thing here is timing - you MUST make sure your bike is properly timed. Refer to your owner’s manual for instructions and make sure it's tick-tock. From there, you're going to need to remove the filters on your carburetors and check to make sure the cable is pulling them up in sync. Otherwise, you'll be getting miss-matched air fuel at different ranges and the bike will never run right.

Most carburetors have adjuster nuts on the top where the cable goes through the slide. You'll want to turn these all the way and then back them out slowly in unison until you feel the slack gone when you first roll the throttle. From there, get a good look at them both and rotate your throttle, both slides should raise at the exact same time and at the same rate. If you can't see both of them at the same time an alternative method is to look at one and put a finger on the other and "feel" it come up.

Alright, now on to the jetting!

First things first - you need to make sure the bike is running right in stock (current) trim. If it's not, changing your pipes or filters isn't going to fix it. Once you have a solid base line and you know it's runs well take your carbs apart and note the numbers on the main and secondary (could be called main and pilot) jet's. Once you have those it's down to basics, and by that we mean if you're adding more air (pod filters or velocity stacks) and increasing the exhaust flow (mufflers) you're going to need to add fuel.

The best suggestion we can offer is to purchase 2 sizes down on both jets and then 3 sizes up. Once you install your new filters or v-stacks and exhaust take a note of how the bike is running. Get a piece of paper, back of a cereal box or a friends back and write down the jet configuration and the response the bike gives you through the entire range of throttle. Does it blow black or white smoke? Do you smell a lot of fuel? Is acceleration flat or sporadic in low, mid and high range? All of these things need to be noted and recorded so you can refer to something when you move on to the next step which is changing out the jets.

You'll want to go ahead right off the bat and up your main and secondary by once size and take it for a spin. How does it run? You may get lucky and not have to a do a single other thing. If that's the case, go buy a lotto ticket because today's your lucky day, pal. If the bike feels like it's bogging a bit on the low end you probably didn't need to increase the secondary, if you get to the top end and it goes flat or doesn't accelerate like it should, you probably need to go up one more on the main jet. So swap out the main for one larger and go back to stock on the secondary. Take it for a spin...

Repeat this process with the different combinations recording your notes until you find one that the bike runs pretty well at, well enough to ride if you had to. From there you'll want to work with your needle clip in the slide. By lowering the needle you add more fuel to the entire range of the throttle. Raising it allows less fuel. See where we're going with this? If you adjust it up and the bike gets better you're going in the right direction -  the same for lower. Got it?

Once you have it setup to where the bike runs good and pulls well through the entire range it's time to go for a nice long ride. But first, be sure to pull your plugs and clean them with a wire wheel and some sand paper; you really need a clean pallet to work with for this next step. You're going to want to run for about 50 miles consecutively and then check the plugs referring to the chart below to determine if the bike is running properly, or too rich or lean.


park-plug-gaps


If the bike is too rich, turn the air adjuster screw to the left to allow more air to enter the carburetor and vice versa if it's running too lean. Be careful though, on most carburetors turning just a 1/8 of a turn can make a huge difference. We recommend making an adjustment, cleaning the plugs and then taking another nice long ride repeating until you get the plugs looking nice and brown or light gray.

In the end, the only way to get your bike tuned perfect is to take it to a professional tuner who can use a dyno while monitoring the air/fuel ratio. You can expect to pay upwards of $150 to have this done, but it is worth it. As a final step, we also like to have the bike tuned once we get it as best as we can via touch and feel. 


A Note on stock Mikuni Jetting:

VM30 Standard
250 Main, 40 Pilot,  6F5 jet needle, 2.0 Air Jet, 159 P-5 needle jet

VM30 pre-jetted for CB350's
150 main jet, 3.0 slide, P-0 needle jet, 2.0 air, 50 pilot, and 6F4 needle 

VM32 standard:
200 main, 35 pilot,  6DP17 jet needle, 2.0 air jet, 159 Q-6 needle jet

VM32 Pre-jetted for CB450's
185 main,  35 pilot, 6DH7 jet needle, 2.0 air jet, 159 P-5 needle jet,