Tuesday Tip: Four-Stroke Carb Tips

We recently checked in with Honda’s Eric Crippa, and he gave us a few fuel- and carb-related tips to pass along. While these were aimed for Honda’s big-bore thumpers, we’d guess that they also apply to a lot more riders.

First off is with any kind of oxygenated race gas, he suggests,  “I don’t like leaving these oxygenated fuels in the carburetor for more than the time you ride it, because they can leave gum or deposits in your carburetor. Either pull the drain plug on the bottom of the carb, or let it run dry. If you do pull the drain plug on the bottom of the carb, that’s a good time to check that it’s really clean, because it could have water in it from washing the bike, or have dirt in it. It’s designed to catch all that. Of course, it’s also the main pickup area for the main jet, so make sure it’s really clean.”

Eric also noted, “I’d also run regular gas through it to flush it out. Those fuels also don’t like moisture, or a motor that’s re-breathing oil.” How would your engine be re-breathing oil? “If you don’t drain the breather tube, and it fills up with oil. the excess oil can end up going into the airbox, and then come through into the carburetor.”

If you’re having trouble starting your bike, Eric has a couple suggestions. “With oxygenated fuels, or even if you let your bike sit with conventional fuel, the pilot jet can pretty easily plug up.”

To clean it you don’t have to remove the carb. “Loosen the clamps, so you can rotate the bottom of the carb toward you. You have to turn your bars to get the most slack on the throttle cables, and make sure you don’t kink the cables. A long thin screwdriver will get you into the area with the pilot jet. It¿s good if it has rounded edges to prevent catching anything in the carb.” Contact cleaner will help remove any offending residue.

Of course, don’t forget to straighten out the carb after you reinstall the pilot jet and float drain. Also, make sure that once you straighten up the carb and retighten the clamps that the throttle works smoothly.

Eric also had a couple suggestions for the fuel screw. “Every carb is flow-tested by Keihin, so on the 450s they’ll come with like 1.6 to 2 turns out. That’s a good thing to fine-tune. Like in the morning you might want to open it up 1/8th of a turn, or in the afternoon when it’s really hot you can go in 1/8th. But that’s an important adjustment without having to get into the carburetor.”

“I don’t recommend opening the fuel screw more than 2 1/2 turns. I’ve seen in amateur races where they open it up more than that, and with the vibration, they lose tension on the spring and it actually falls out. If your bike is really popping and feels like an air leak, check your fuel screw¿it may be gone.”

Eric’s last suggestion? “It may sound silly, but read your owner’s manual. There’s a lot of good info in there.”