How to Tune a 125cc Pit Bike Engine

Most pit bikes, which are small-engine dirt motorcycles, make great, simple transportation around the large area of a motorcar race track preparation area. However, because pit bikes don't necessarily get run at their full speed in practice, the engines don't warm up well and instead build up gunk over time. Many use two-stroke engines, which make matters worse with oil mixed into the gasoline. A bit of tuning can clean the engines out, making the bike reliable again at your next track race.

Instructions

1

Park the pit bike in a place where you can work on it. Remove the ignition wire on the engine spark plug. Use a socket wrench to remove the old plug. Hold it in your hand with a shop rag to examine the tip of the spark plug. Replace it if it's black and oily. Install a new one and tighten. Re-install the ignition wire.

2

Make sure the fuel line valve is in the off position. Use a screwdriver to disconnect the hoses on the carburetor of the bike. Dismount the carburetor from the engine. Disassemble the carburetor and spray out all the parts with carb cleaner or a similar spray solvent. Reassemble the unit with a new carburetor kit, including gaskets and rubber seals. Re-install the carburetor onto the engine and tighten the fuel lines back onto the unit.

3

Roll the bike out to an open area where you can drive it for a distance. Turn on the bike and warm it up. Take it for a ride and pull on the throttle to make the bike run at full speed. Repeat the process a few times for about 20 minutes. Ride the bike back to your starting point and turn it off. Pull the ignition wire and check the spark plug again. Confirm the spark color is a nice chocolate brown.

4

Replace the spark plug if it is still showing an oily black color, but use a hotter-grade spark plug this time. Use a cooler-grade plug if the tip looks chalky white. Run the bike again at varying speeds. Repeat the spark plug test until the bike produces a chocolate-colored spark tip. The pit bike engine is now tuned.

5

Check your tire pressure on both tires with a tire gauge. Add air with a bike pump if a tire is below the minimum level recommended for your model pit bike.

6

Check your brakes by riding forward and applying the front brakes first. Then do the same test for the rear brakes. Replace either set of brake pads on the wheels if the stopping power is not fairly immediate.