This squirt fills in the fueling “gap” created by the associated drop in air speed through the carb, which becomes momentarily inadequate to pull enough fuel up through the jets.
The more directly performance-oriented mods involve changes to the accelerator pump, which shoots a fixed-duration burst of extra fuel into the carb throat when the throttle is opened further from any position it’s especially useful when the throttle is opened quickly at low rpm and the engine may take a long time to reach the rev “target” set by the new throttle position. One lets fuel in, the other lets air out. Notice that the stock pump cover (background) has only two holes opening into the reservoir. The other parts come in the Honda rebuild kit, but aren’t necessary for this mod. Look closely and you’ll see that the new plunger (black, right) has a taller post than the stock one beside it. I chose the Flex-Jet because I didn’t want to wedge my fingers into a very cramped, hot engine bay. Several companies make replacements for the stock screw that simply extend straight down and have a finger-operated wheel at the bottom. Less expensive alternatives are available if your have enough clearance underneath your carb. Given that a lot of trail riding is done with light throttle, having this screw set properly can make a significant difference in power and rideability. The fuel screw has the most influence on mixture during the first 25% of throttle opening, but continues to make a contribution throughout the entire range. And it’s more resistant to vibration-induced rotation than the stocker. While technically not a performance mod, the Flex-Jet makes it easy to optimize mixture for varying altitudes and temperatures, allowing you to keep your bike running its best through changing conditions. Voila! Now adjustments can be made trail-side, without even getting off the bike! This end has a small knurled wheel that allows remote adjustment of the needle by hand. The Flex-Jet’s other end is hung out in the open with a tiny bracket at a rear corner of the carb (all mounting hardware is included). The original screw is simply backed out and replaced with the identically shaped end of the Flex-Jet.
The R&D Flex-Jet is simple to install and allows making mixture screw adjustments a snap without you having to remove your carb.The Flex-Jet makes mixture tweaks just as easy as idle-speed adjustments, without the need for any additional tools, and installation is a snap. Even small/routine adjustments for altitude and temperature shifts required almost completely removing the carb–a major hassle! On our 450XCR-W, there wasn’t enough clearance to use such a tool anyway. Such tools are pricey, though, and don’t fit in fanny packs. On some, it might be possible to use a special tool that turns a tiny screwdriver bit at a right angle from the handle assembly. The fuel screw on FCR carbs resides face-down in front of the float bowl, making it difficult or impossible to access with a screwdriver on most motorcycles. Most anyone with an FCR carb will benefit from the convenience mod, regardless of whether they do either performance mod, so I’ll begin with R&D’s Flex-Jet. R&D’s Flex-Jet makes something ridiculously difficult ridiculously easy. Nevertheless, I think the principles below have broad applicability. Various engines may respond differently to the modifications that follow, so it’s best to check with your own local expert regarding your particular machine. Keihin’s FCR carburetors come fitted on a wide variety of dirt and dual-sport bikes, not just those from KTM. My local KTM guru advised that even better throttle response could be obtained with a couple of easy mods to the KTM’s Keihin FCR carburetor, and added that mixture adjustments could be made much more convenient with a third. But, as is so often the case, I just couldn’t leave well-enough alone. I really had no complaints about my 2008 KTM 450XCR-W test mule’s carburetion, at least not after jetting and adjusting it for local conditions in my home state of Tennessee. With a few mods, the Keihin FCR carburetor found on many carbureted KTMs, Hondas and Yamahas can be tailored for better throttle response.