Cure carb problems
Get your bike running sweetly with these top tips
1 Sticky but sweet A well set up carb can be just as sweet, if not sweeter, than modern fuel injection. But part of the problem with carbs is that their many internal parts need to be kept clean, especially if the bike isn’t used very often. Prolonged periods of storage will cause poor running, and refurbing the carb internals is the first step on the road to recovery. 2 Lift the tank In order to work on them you need to completely remove the carbs from the bike. On many bikes, such as this Yamaha RD350 YPVS, the carbs are located beneath the fuel tank. To remove the tank, undo the bolt at the back and remove the two fuel lines from the tap. Make sure the tap is not in the prime position. If it is, it will leak fuel. 3 Tops off Undo the tops of the carbs to free the throttle slides; some simply twist off but other types may be retained by a couple of screws. Undo the clamps that secure the carbs to the airbox and inlet rubbers. Once these are undone, completely remove the screw and the clamp without distorting it too much. 4 Wrestling rubber Carb rubbers become hard over time, making them tough to get on and off. One solution is to apply gentle heat, using either a hair dryer on a hot setting or a heat gun on a low setting. Apply the heat universally and the rubber will become supple again. Rock the carbs backwards and forwards with a twisting motion and they’ll pop off.
5 Work on one carb at a time Place the carb in a tray and undo the bottom of the float bowl. Remove the bowl, but be prepared for some fuel to spill out. Remove the delicate float valve by pushing out the pin at the pivot, before undoing the main jet and pilot jet. Check new parts alongside the old to ensure they will fit. 6 Remove the tube Now remove the emulsion tube by lightly tapping it out. With the tube removed, check the holes on the sides and the main bore are free from debris (this example is typical of a bike that has been stored). Use a carb cleaner and compressed air, or carefully use a wire to ream out the holes. 7 Blast before you fit The float valve and seat are usually to blame for leaks because stale fuel creates deposits that prevent the valve from sealing correctly. Before fitting any new parts, give the body of the carb a blast with cleaner and an airline. Fit the new float valve assembly, making sure you don’t forget the O-ring around the outer side of the brass float valve. 8 Get the correct jet Some carb rebuild kits are supplied with new main and pilot jets, but do not fit them without cross-referencing the sizes, either with your manual or the old jets. The size is the embossed number stamped into the jet. If the jets in the kit don’t match the ones from your carb and your bike was running fine before, use the old ones but make sure they are clean. 9 Niggling needles Reassemble the carbs, use a new gasket for the float bowl and screw the bowl on with even pressure on each screw in turn until they are firmly in. Carb rebuild kits sometimes contain replacement needles, so make sure they match the originals. Replace the needles by squashing the slide spring and moving it to one side so that the needle can be popped out. 10 Refit and fire up Reinstall the carbs using the correct clips and make sure all the link pipes are connected as well as the oil feed pipes. Tighten the carb slide tops, refit the tank and connect the fuel pipes to the tap. Pour fresh fuel in the tank and start the bike, when it is warm check the tickover speed and adjust as necessary.