CV Carburetor VS Mikuni flat slide – ep34 – Roma Custom Bike

In this episode of Roma Custom bike we’ll examine the difference between a CV carburetor and a flat-slide one, namely.

In this episode of Roma Custom bike we’ll
examine the difference between a CV carburetor and a flat-slide one, namely a
Mikuni HSR 42. Hi folks, I’m Custom Cez for Roma Custom
Bike and I’m back to talk about a subject that comes up very often in all the Harley Davidson
forums and chats: does a Mikuni carburetor, with a price tag of about 300$, make a big
difference compared to a stock or stage 1 CV carb? The quick answer, in my opinion, is yes! It’s totally worth it, but let’s see why
and how does it work, starting from how it’s different from the stock carburetor. I’ve already made two videos on how the
stock CV carburetor works and how to rebuild it, the links are in the description, but
let’s do a quick recap: In the CV carburetor, CV stands for Constant
Velocity, the throttle cable is hooked up to a butterfly valve. As it opens it transfers the vacuum created
by the engine to a chamber on top of the carb that applies suction onto a membrane that
pulls up the slide and allows fuel and air to enter the combustion chambers. This type of system creates some kind of “smoothing
effect” between the throttle twisting action and the actual opening of the slide. As you can see in the graph, even if you try
to twist the throttle in a very abrupt way, the effect from the vacuum system smooth out
any jerkiness in fuel delivery, providing the right amount of it to the engine, proportionally
to the amount of low pressure generated by the engine itself. This is also why it’s called Constant Velocity:
because the system is engineered to maintain the air flow at a constant velocity between
the air intake and the manifold by preventing the slide 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 bowl filled with fuel; the pilot jet, main
jet, needle and slide, to provide the engine with fuel at all RPM ranges and the adjustment
screw that, differently from the cv carburetor, regulates the amount of air at idle condition. In this type of carburetor the throttle cable
is hooked up directly to the slide. As you twist the throttle the slide opens
immediately at the same rate as you twist, with no smoothing effect. By taking a look at a graph we can see the
direct relationship between the twisting of the throttle and the opening of the slide,
and by comparing it to the one from the CV carburetor, we can see that the driver has
total control on the amount of fuel and the rate of delivery with no smoothing effect
and independently from the requirements of the engine. It’s worth noting that while in the Harley stock carburetor the adjustment screw is located on the manifold side of the carb and regulates the amount of fuel at idle with a fixed amount of air flow, in the Mikuni carb the idle mixture
adjustment screw is located in the air intake side and regulates the amount of air to mix in. Now let’s look at the pros and cons of both carburetors: The CV stock carb is a great piece of equipment! With a jetting kit you can make it perform
to its best. It is great for fuel economy, because it never feeds the engine more than what it actually needs. In addition to that the smoothing effect makes
for a pretty comfortable ride with delicate acceleration and deceleration even with a
nervous styled driver. Last but not list, CV carbs do allow for “some”
altitude adjustment, mainly related to engine vacuum based on the amount of oxygen in the air. Now let’s look at the flatslide carburetor: You, the driver, will get full control over the
acceleration and deceleration of the bike, and this alone will stick your butt to the
seat in an all new and exciting way! At wide open throttle there is nothing in
the way of the air and fuel getting to the engine. Another great feature of this Mikuni carburetor
is that you can adjust pretty much everything! From the shape and position of the main needle
to the when and how much fuel the accelerator pump sprays into the manifold. There is even and easy access spot on the
bottom of the bowl to change the main jet without taking anything apart! With all this freedom you can really dial
in the settings for your bike 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 starting point. There are obviously some downsides too. The throttle control becomes very sensitive. Sometimes you don’t want to take off like
you are at the drag strip and without the “smoothing effect” it all comes down to your wrist, both in acceleration and deceleration. If you are not careful it can be a pretty
rough ride and, especially for your passenger, will not be very enjoyable! Without the vacuum system there is also no
altitude or atmospheric compensation built in, and, most of all, the slide makes an all
lot of noise due to the pressure differential between intake cycles. This pushes and pulls the slide in and out
in the tracks creating a clicking sound. The type of cam and muffler might accentuate
this effect. With that said the bottom line is that it
all depends on your riding style! In my opinion, although I consider the CV
to be one hell of a carburetor, the ease of adjustment and the crazy acceleration boost you get from the Mikuni is well worth the price tag. You do have to keep it in check with your
wrist if you want an easy and smooth ride which can get tiring in traffic, but then
again, when you open up the throttle it really let’s you know where your money went. Of course it needs to be tweaked properly
and to really shine it needs to work in combination with a good air box, some mufflers and maybe
a performance cam, but you already knew that. Don’t forget to check out our other carburetor
how to and rebuild videos and remember to subscribe to be notified of new videos. I’m Custom Cez for Roma Custom Bike and
I’ll see you next time.

34 thoughts on “CV Carburetor VS Mikuni flat slide – ep34 – Roma Custom Bike”

  1. I don't want to hear any harley boys complain about a $300 flatslide carb upgrade. The flatslides for the VMAX is literally 10x more expensive. Really. $3K for vmax flatslides…

  2. Thank you, this was just the video I was looking for. I am putting an HSR 42 on my '03 Sportster Sport, and this was very informative about what to expect vs. the CV carb.

  3. The bike at the end had the coolest air cleaner. I have a hsr48 on my 1275cc drag sportster and that thing is insane. The ability to tune a performance Harley is hands down incomparable. Great video!

  4. That accelerator pump on the carb is out of tune as the fuel shouldn't hit the slide when opening the throttle. Other than that, great video explaining the differences between a CV and a FCR carb. I think FCR's sound great because of their sound 😛 and everybody should know about em'.

  5. I have a 4t Gilera Runner and the cvk carb constantly blocks up so i want to change to a flat slide one ,will i need to use the original 4t gilera airbox connected to the flatslide?and would i be better off taking the jets out of the cvk and simply swaping em over?

  6. Wow my Harley chop I have owned for 3 years came with HSR fitted and it pulls away like no other bike I have had. It jolts away at every set of lights, not smooth. Thought it was a problem and now you have eased my mind in that its the characteristics of the carb. Do you have any tips on tuning it as mine runs really RICH.

  7. Cez your air cleaner on you’re badass springer bike with the apes ,is so fricking cool I can’t emphasize enough great informative video

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