Both of these are great carburetors and you will be more than happy with the improvement delivered by either. The 42mm is just a little more finished version, delivering way more power and better fuel economy, understanding where they came from really makes it all clear.
In the late 80s Mikuni American released the HS40 (TM40-6) as an amazing new carburetor for the Harley Davidson market, literally knocking all its competitors out of the park. We were at the San Jose Custom Chrome Show when it was released and become part of that buzz as a distributor. This slide type fuel metering device basically made the competitors butterfly type carbs seem like lawn sprinklers.With the huge increase in Harley Davidson sales this became a very lucrative market. Recognising the potential of this market and being aware of the efforts of competitors to try and grab back their share of it made Mikuni realise the necessity of replacing the 40 with a carburetor to be the be all and end all of all carburettors. The target was to deliver unrivalled peak performance combined with smoother throttle operation and lighter throttle pull by replacing the metal on metal slide action with a slide running on roller bearings. Also a larger fuel bowl capacity was provided but most of all, superior venturi design and fuel atomisation resulting in more precise mixture ratios to not only increased horsepower and torque but also to enable the fitment of a larger carb to smaller capacity engines without losing valuable bottom end power, where in fact instead gaining more. In 1994 the HSR42 was born and just like you will if you fit one to your bike, all we could say was WOW. Instant throttle response delivering 25% more horsepower and torque over the stock CV carb before even putting on a real high flow air-cleaner. As the badge says 20 years later Still the best performance carburetor on the planet. Unfortunately, the TM42 only had limited sales in the Japanese single market due to it only appealing to the custom fitter as there had never been a cable set or air-box adaptor available off the shelf. With the slowing of the Harley market due to fuel injection on new bikes, here in Australia, Mikunioz realised the opportunity to offer Mikuni's best Carburetor to the big Jap single market and in late 2015 we released a range of ready to roar direct fit up kits with correct jetting and a few trick mods to make them work better than anything before on big Jap Thumpers. The fuel consumption is way less but when comparing with the stock carbs, it all depends on how having a lot more power and torque effects your riding habits. If you can manage to back off and just have your bike perform at its present level youll be surprised by the fuel economy, but of course when the same amount of throttle twist becomes front wheel lifting, breaking traction action, well its hard to not want to go and play. If you have more horses in the paddock and you want them all to perform, you have to feed them more hay! You will enjoy this new carb but do be sure to hold on tight on your first test ride as she may try to spit you off if you nail it to hard.
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The item "Honda XR600 XR650L Mikuni Carburetor, TM42-6 42mm Flatslide Pumper Knob Choke" is in sale since Monday, January 18, 2016. This item is in the category "Vehicle Parts & Accessories\Motorcycle Parts\Air Intake & Fuel Delivery\Carburettors & Parts".
The seller is "mikunioz" and is located in Townsville, Queensland. This item can be shipped worldwide.