CV Carburetor VS Mikuni flat slide explained - ep34 - Roma Custom Bike

In this episode of Roma Custom Bike we will examine the difference between a CV

carburetor

and a

flat

slide

carburetor

, i.e. a Mikuni HSR 42. Hello friends, I'm Custom Cez for Roma Custom Bike and I'm back to talk about A topic that comes up very often on all the Harley Davidson forums and chats: does a Mikuni carburetor, priced around $300, make a big difference compared to a stock or stage 1 HP carburetor? The quick answer, in my opinion, is yes! It's worth it, but let's look at why and how it works, starting with how it's different from the stock carburetor.

I already made two videos on how the original CV carburetor works and how to rebuild it, the links are in the description, but let's do a quick summary: On the CV carburetor, CV means constant speed, the throttle cable is engaged. even a butterfly valve. As it opens, it transfers the vacuum created by the engine to a chamber at the top of the carburetor that applies suction onto a membrane that lifts the

slide

and allows fuel and air to enter the combustion chambers. This type of system creates a sort of "smoothing effect" between the twisting action of the throttle and the actual opening of the slide.

As you can see from the graph, even if you try to turn the throttle very sharply, the effect of the vacuum system smoothes out any jolt in the fuel supply, providing the right amount to the engine, proportionally to the amount. low pressure generated by the engine itself. That is why it is also called constant speed: because the system is designed to maintain the air flow at a constant speed between the air inlet and the collector, preventing the slider from opening too quickly. Now let's take a look at a

flat

slide carburetor, in this case a Mikuni HSR 42, just like the CV type, we have all the basic elements of a carburetor: the float that keeps the bucket full of fuel; the pilot jet, main jet, needle and slide, to supply fuel to the engine in all RPM ranges and the adjustment screw that, unlike the CV carburetor, regulates the amount of air at idle.

In this type of carburetor, the throttle cable connects directly to the slide. When you turn the throttle, the slide opens immediately at the same rate as you turn it, with no smoothing effect. Taking a look at a graph we can see the direct relationship between throttle torque and slide opening, and comparing it to that of the CV carburetor, we can see that the driver has full control over the amount. of the fuel and the supply flow without a softening effect and regardless of the needs of the engine. It is worth noting that while on the original Harley carburetor the adjusting screw is located on the manifold side of the carburetor and regulates the amount of fuel at idle with a fixed amount of airflow, on the Mikuni carburetor the adjusting screw The idle mixture valve is located on the air inlet side and regulates the amount of air that is mixed.

Now let's look at the pros and cons of both carburetors: The stock CV carburetor is a great piece of equipment! With an injection kit you can make it work at its best. It's great for saving fuel because you never feed the engine more than it really needs. In addition to that, the smoothing effect makes driving quite comfortable, with delicate accelerations and decelerations, even with a nervous driver. Last but not on the list, CV carbs allow for "some" altitude adjustments, mainly related to engine vacuum based on the amount of oxygen in the air. Now let's take a look at the flatslide carburetor: you, the rider, will have full control over the acceleration and deceleration of the motorcycle, and this alone will make your butt stick to the seat in a whole new and exciting way!

With the accelerator at full throttle there is nothing to prevent air and fuel from reaching the engine. Another great feature of this Mikuni carburetor is that you can adjust just about everything! From the shape and position of the main needle to when and how much fuel the accelerator pump sprays into the manifold. There is a smooth and easy access point on the bottom of the bowl to change the main stream without taking anything apart! With all this freedom, you can really dial in your

bike

's settings, although most Harley guys end up with a 20 pilot jet, the needle with the clip in the middle, and a 165 main jet.

Take that as a good point. of departure. Obviously there are some disadvantages as well. The throttle control becomes very sensitive. Sometimes you don't want to take off like you're on a race track and without the “smoothing effect” it all comes down to your wrist, both in acceleration and deceleration. If you're not careful, it can be quite a difficult ride and, especially for your passenger, it won't be very pleasant! Without the vacuum system there is also no built-in atmospheric or altitude compensation and, above all, the slide makes a lot of noise due to the pressure differential between the intake cycles.

This pushes and pulls the slide in and out on the tracks creating a clicking sound. The type of cam and muffler can accentuate this effect. That being said, the bottom line is that it all depends on your driving style! In my opinion, while I find the CV to be a great carburetor, the ease of adjustment and incredible acceleration boost you get from the Mikuni are well worth the price. You have to control it with your wrist if you want a smooth, easy ride that can be tiring in traffic, but again, when you open the throttle it really lets you know where your money went.

Of course, it needs to be tuned properly, and to really shine it needs to work in combination with a good airbox, some mufflers, and maybe a performance tube, but you already knew that. Don't forget to check out our other videos on how to repair and rebuild carburetors and remember to subscribe to receive notifications of new videos. I'm Custom Cez for Roma Custom Bike and I'll see you next time.