Mikuni HSR42 Flatslide vs. CV40


The CV40 is a good carburetor; we've had numerous customers break 100rwhp using a stock, rejetted CV. Likewise, the Mikuni HSR42 is a popular aftermarket carburetor used on many high performance projects.


We often get asked if the Mikuni is needed or worth the money. Well, on several occasions we've dyno tested the CV against the Mikuni HSR42. Here are the answers.



On a mildly modified bike like this one (883 to 1250 conversion with stock 883 heads), the Mikuni just brings a little bit to the top of the dyno sheet. We've seen this result over and over, anytime we test it on a mild bike, it's hard to justify the cost of the Mikuni with the dyno sheet.


When the mod level goes up, however, the story changes:



This motor has a 1250 kit and a set of Stage 2 '04 XL1200 heads, as well as a set of SE .536 cams. The gains are still at the top of the dyno sheet, but picking up more than 5hp at this level is HUGE, and it's some of the cheapest horsepower you can buy.


What the dyno sheets don't show you is that in either case, the Mikuni gives noticeably quicker throttle response. Twist the grip and the motor reacts right now, giving the impression that your wrist is connected to the rear wheel. Also, the Mikuni is a tuner's dream, it's main jet is accessible in seconds via a plug on the bottom of the carb, instead of being a 20 minute job as it is on the CV. There are a lot more adjustments available on the Mikuni and although they're dialed in very well out of the box, the obsessive tuner can really get one perfect with all the adjustments available to him. So even if your bike is mild and your dyno sheet doesn't stand to benefit a whole lot from a Mikuni, it may be a worthwhile investment anyway. I've never heard of anyone who bought a Mikuni and regretted it.


What manifold do you need? Well, a Mikuni HSR42 slips right into a stock CV40 manifold. A Screamin Eagle manifold is also available to accommodate the HSR42, and it comes with an additional rubber booty to allow the fitment of an HSR45 or CV44. But do you need this manifold for the HSR42, or will the stock one work just as good?



As luck would have it, we've tested the HSR42 with the SE manifold against the stock manifold, multiple times, and each time the SE manifold brought nothing to the party; save your money, you don't need it unless you're using the HSR45 or CV44.


With the CV air cleaner adaptor installed, the Mikuni's air cleaner surface ends up in the exact same place as the CV's. A standard carb support bracket made for a CV works fine, and everything bolts right up. Late model Mikunis even work fine with your stock throttle cables. About the only thing you may have to do is clearance the top fin on each cylinder just a little, you're more likely to have to do this on aftermarket big bore cylinders. But overall, installing a Mikuni is largely a "bolt-on" operation.


Mikuni carburetors are readily available from NRHS, see our carburetion page.


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