HOW TO FIGURE OUT WHY YOUR TWO-STROKE WON’T START

August 24, 2014
Comments off
9170 Views
facebooktwittergoogle_plusredditmailfacebooktwittergoogle_plusredditmail


You might think that a pull starter, like on a Rokon, would be better than a kickstarter, but it’s not about how you spin the engine, but about the fire triangle (fuel/air/spark).

Like a temperamental car that needs to have the steering wheel smacked before a turn signal will blink, or a car starter that won’t engage until you jostle the key, a motocross bike can be temperamental. The MXA wrecking crew has had its share of bikes that won’t start. Over the years, we’ve come up with a short list of two-stroke bike-starting strategies.

THE BUMP START

If you’ve kicked until you’re blue in the face, it’s time to try the bump-start method. Get a buddy (or two) to push the bike to terminal velocity. Then, pull in the clutch, shift the bike to second gear (third on a big bore) and pop the clutch. Once the bike starts to churn over, keep it moving until you can clean the flooded gas out. Ride it around for a few minutes to clean up the plug. There’s one problem with this method, though. If the bike doesn’t start, you will most likely find yourself at the bottom of a hill.

CHECK THE PLUG FOR WETNESS

If kicking and bump starting the engine don’t work, it’s time to look for a cause of the malady. Pull the spark plug out of the engine and give it the eagle eye. Is it wet with gas? A wet plug indicates two things:

   (1) That you’ve flooded the engine by trying to start it, or that the lower end is loaded up (because you left the gas petcock on).

   (2) That there is no spark (thus, the plug is wet because the plug never ignited the fuel).

Fixing a wet plug (from a flooded engine) is easy. Remove the plug and replace it with a new one. If the plug continues to foul after riding, try using a different heat range plug. The lower the number on the plug, the hotter it is (a B8EV is hotter than a B9EV).

If you think the lower end is flooded (known as being loaded up), kick the engine over a few times with the spark plug out. This will aerate the lower end and get the excess gas out. Be careful not to start a fire or to push the bike without the plug in (electronic ignitions don’t like that).

WHAT ABOUT A DRY PLUG?


Two-stroke plugs are easier to get to for a reason.

If the plug is dry, you’ve got different problems. A dry plug means that no gas is getting to the combustion chamber. First, take the gas line off the carb and turn the petcock on to see if gas is flowing to the carb. If gas rushes out of the petcock, then the restriction is in your carburetor.

Before delving into your carb, clean the engine as thoroughly as possible. You don’t want dirt falling into the carb or intake while you’re working on it.

The quickest way to check the carb is to loosen the carb clamps and swing the float bowl far enough towards you that you can take off the float bowl (this isn’t possible on every model of bike). When you take the float bowl off, use care to try and preserve the fuel in the bowl-it might have clues in it. For example, is there dirt in the bottom of the float bowl? That dirt could be jamming up the works. Is there water in your float bowl? Water will bead up in gasoline and definitely louse up the works.

ELEMENTARY MY DEAR WATSON

The clue you don’t want to find is that the float bowl has no fuel in it. That means you can stop in your tracks and remove the carb from the bike, Once you have the carb removed, follow these basic steps:

   (1) Make sure the floats move and the needle-and-seat move freely. Occasionally, the floats will freeze because of a small piece of dirt. A minor tear-down and spritz of carb cleaner will fix the problem.

   (2) Hold the carb upright and blow in the fuel line while moving the floats up and down. If you can’t feel air moving through the system when the float bowl is all the way down, or feel it shut off when the floats are lifted up, then the needle-and-seat are jammed. Dirt, small pieces of debris or pieces of plastic flashing (from the gas tank) can clog the needle and seat. Clean it thoroughly.

IT COULD BE DEAD

Another possible reason for a dry spark plug is worn-out piston rings. Your bike relies on the vacuum created by the piston and rings to suck fuel and air into the motor. If you’ve raced your bike nonstop for six months and it suddenly refuses to start, it’s because the piston and rings have become so loose they don’t seal against the cylinder tightly enough to create suction. The solution to this problem is easy. Replace the piston and rings; not only will the bike start, but you’ll also appreciate the new horsepower.

CHECKING THE PLUG

In most cases, if your bike absolutely refuses to start, the problem is electrical. The quickest way to find out if it’s electrical is to check the plug for spark. Remove the spark plug from the cylinder head and pop it back into the spark plug cap. Hold the threaded portion of the spark plug against a cylinder head nut and have a friend kick the engine over. While the bike is being kicked over, look inside the spark plug to see if you can see a spark jump across the gap. You may have to cup your hand to provide a shadow. If you see spark, is it blue or yellow? Blue spark is good. Yellow spark is bad. If you have yellow spark, try a new spark plug. A word of caution: if you mishandle the plug, you could get a mild shock. It’s not enough to hurt you, but it will wake you up.

WHAT IF THERE ISN’T ANY SPARK?


Jeremy McGrath’s CR250 two-stroke started every time because he was the King.

If there isn’t any spark visible when kicking the engine over, follow these steps:

   (1) Check the metal cap on top of the spark plug and make sure it is tight. Give it a gentle twist with a pair of pliers if it isn’t.

   (2) Make sure that the spark plug lead wire is solidly connected to the spark plug cap. If you suspect the spark plug cap, pull the lead wire out of the cap and look at the end of the wire. If the core wire is not visible or looks burned, cut 1/4-inch off the end of the wire and reinsert it into the spark plug cap (using the proper procedure, either prong or thread-in lead).

   (3) Disconnect the kill button lead wire from the wiring harness. Kill buttons are prone to shorting out and lack of spark can often be cured by disconnecting the kill button. Obviously, you should replace the kill button before riding to avoid the ominous overtones of the button’s name.

   (4) Remove your gas tank and check all the electrical connections. Not only do they come loose, but they also get water in them. Spray the connections with contact cleaner, plug them solidly together and wrap them with electrical tape to insure that they stay put in the future.

   (5) Pull the ignition cover and check the flywheel. Look for loose stator bolts or cracked wires. If the ignition shows any signs of moisture or dirt, clean it with contact cleaner and allow it to air dry.

   (6) If you’re lucky enough to have a friend with the same bike as yours, borrow his black box. It can be pulled off one bike and plugged into another in a few minutes. If your black box is bad, putting a working one on will point you in the right direction.

   (7) Pull the seat and take a close look at your air filter. Pull it out of the bike and look for rags in the intake (it happens, often as a practical joke). An extremely dirty filter can stop an engine from starting (air is required for the spark to ignite the fuel).

   (8) If none of the above fixes give you spark, then the trouble is deeper. You will either have to keep replacing parts from your buddy’s bike until you have replaced everything or take the bike to your dealer.

WHAT YOU NEED TO KNOW

Bikes need three things to run (two-strokes and four-strokes): fuel, spark and air. This is know as the fire triangle. If you discover that you have all three, then your bike should run. If it doesn’t, sell the bike and buy a mountain bike. It only needs air in the tires.

facebooktwittergoogle_plusredditmailfacebooktwittergoogle_plusredditmail

Comments are closed.