Tech How-To
There are few things more frustrating to a rider ready to ride than a motorcycle that won't start. But a silent machine need not stay that way. If an engine has fuel, spark and compression and the timing is reasonably close, it has to run as long as the Earth's atmosphere still contains oxygen. The key is to be methodical and see which parts of the combustion equation are missing in action.
Start with history. When did the engine last run: When I parked it six months ago? Until I turned it off a few minutes ago? Right before I worked on it? Right before I put these trick parts on or right before I crashed? The past will reveal the direction you need to take in the present. Where you start looking after a crash is very different than where you begin with a bike that has been parked for months or years. If you bought a bike that didn't run and you need to read this story, you're an idiot or a compulsive gambler, but maybe you'll get lucky.
1. Whether the bike is a two- or four-stroke, it's almost always easiest to check the fuel supply first. If you're pretty sure there's fuel in the tank, don't take off the cap and look in. Remove the fuel line from the petcock and see if fuel flows easily when you turn it on. If the fl ow is slow or stopped, open the cap. If you get fuel with the cap off, there is a problem with the vent in the cap or with the vent hose.
2. If no fuel comes out of the petcock but there's gas in the tank, you need to get out the tools. Take off the tank and carefully empty the fuel inside into an approved container. Gasoline is extremely flammable and the fumes are explosive. Make sure you work in a well-ventilated area and away from any source of combustion. When the tank is empty, remove the fuel petcock from the tank.
3. An off-road bike will have a long strainer like this one while a motocross bike will have a shorter one. If the petcock is messed up, more than likely you'll see crud on the outside. The screen won't look clean like this. It'll look chalky or like it has jelly on it. If the mess is bad enough, you'll need to replace or rebuild the petcock. Clean the screen and parts with carburetor cleaner.
4. If you did have fuel leaving the end of the fuel line, then open the fl oat bowl drain on a fourstroke, or remove the fl oat bowl cap (usually a 14 or 17mm hex on the bottom of the fl oat bowl) on a two- or four-stroke. Turn on the petcock and see if fuel runs out of the overflow lines or out of the fl oat bowl. If the carb is getting fuel, normally you would move on to compression and spark before removing the carb.
5. We will assume that you have spark and compression for now and continue following the route of fuel into the engine. Because it's easy right now, check the carburetor vent hoses to make sure they aren't blocked. Check more than the ends and especially on a four-stroke. The hot exhaust pipe can easily melt vent lines shut. If the carb has blocked vents, it won't allow in enough fuel.
6. Start by taking the carburetor off the bike and removing the float bowl. The general condition of the bowl may hint at what you'll find. Usually, if you're going to see plugged jets or passageways, you'll see some evidence of gunk, a varnishlike coating or even dry, white powder. Here, the only telltale sign is the yellow (normally white) color to the plastic main jet shroud.
7. Remove the actual jets (there are three in this CRF450X carb) and look through them while holding them up to the light. If the jets are clogged or even completely plugged, clean or replace them. In the case of an emission-controlled four-stroke like the 450X, the pilot jet has extremely small openings, and it might be easiest to replace it. Carb cleaner and a tool for cleaning welding tips works well for cleaning jets.
8. Look at the float valve next. Get to it by pulling the pin holding the float. The fl oat needle will probably be attached to the float. You can see some gook around the float valve here. Wear eye protection and spray carb cleaner into the fl oat valve opening. In there is a screen that isn't removable on Keihin FCR carburetors, so follow the cleaner with compressed air to clear the opening and screen.
9. By now you should be sure the engine is getting fuel. Start checking for spark by removing the spark plug. Inspect the electrode. Is the porcelain cracked or damaged?