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Setting up a new pit bike engine

If you have followed our advice and preserved your engine this season, we hope this article will not be necessary for you. Otherwise, you will have to order a new pit bike engine, endure the agonizing wait for delivery, and break it in. Contrary to the common belief of "install and ride," things are not as straightforward as they may seem at first glance. We have previously discussed how to assemble a pit bike and prepare it for its maiden ride. Today, we will talk about the initial setup of a new pit bike engine before starting it.

We will need:

  • The engine itself. For example, let's take the YX 140 cc.
  • An 8 mm box wrench
  • A ratchet with 8 mm and 10 mm sockets
  • A set of feeler gauges with sizes 0.08 mm and 0.1 mm
  • A container with a capacity of at least 1 liter (for old oil)
  • 1 liter of semi-synthetic 10W-40 motorcycle oil for 4-stroke engines
  • A funnel

1. Remove the generator cover​

It is located on the left side of the engine, secured by three 8 mm bolts. The bolts are of different lengths, it is crucial not to mix them up, as it may damage the crankcase.

2. Assemble the intake manifold​

Ensure to install spacers with all gaskets between the engine and the manifold, and between the manifold and the carburetor. It is attached to the engine with a pair of long bolts and to the carburetor with short ones. In this case, due to the angle of the intake pipe, one of the bolts did not fit, so we ground down the head on one side. Remove the plug from the intake port, ensuring no foreign objects enter. Assemble the sandwich of the pipe, spacers, and gaskets, and attach it to the engine. Tighten the bolts alternately with even force.

3. Adjust the valve clearances​

On the right side of the engine head, unscrew the long bolt securing the camshaft sprocket cover. Do not lose the washer. Align the T mark on the generator bell with the mark on the crankcase so that the mark on the camshaft sprocket (circle) aligns with the mark on the head. Remove the intake and exhaust valve covers by unscrewing two bolts each. Insert a 0.08 mm feeler gauge on the intake valve side. The gauge should not fit with excessive force, nor should it be loose. It should be slightly "pinched." Loosen the locknut and adjust the pushrod until the clearance is set, then secure with the locknut. Repeat the same procedure with a 0.1 mm feeler gauge on the exhaust valve. After adjusting the clearances, replace the valve, camshaft chain, and generator covers.

4. Change the oil​

With the engine mounted on a stand and a container placed underneath for oil, unscrew the drain bolt. Drain the transport oil. To speed up the process, unscrew the filler cap-dipstick. Once all the oil is drained, screw the drain plug back in place. Do not overtighten, as there is a risk of stripping the threads on the crankcase. Remove the filler cap, and using a funnel, pour exactly 900 grams of fresh semi-synthetic 10W-40 oil. Check the level with the dipstick. Mount the engine on the pit bike, connect the electrics, install the carburetor with the air filter, and the exhaust pipe. Begin the break-in process.