Two-Stroke Trouble Shooting Guide

Nov. 25, 2008 By Rick Sieman

[Editor's note - this was written from the motorcylist's point of view, but if you ride any powersports machine with a two-stroke motor, it's info that could save you a lot of headache. So listen-up!]

There's no doubt about what caused this bike to stop running.

Like most other first-time riders, I took my first bike for granted. Never did much in the way of maintenance or tuning. After riding it, I would just stop by the old 25cent car wash and blast away until it was all clean and shiny. Most bikes will take a certain amount of abuse before they break, and mine was no exception. But when mine went, it went all the way.

There was a large hole in the cases, and part of a rod was peeking through the metal. Immediately, my keen mind told me this was not as it should be. Something was visibly wrong.

After a trip to the local unfriendly, non-smiling dealer (accompanied by many bucks), I was back on the trails again—a tiny bit wiser and much poorer. Things went along smoothly, until one day, many miles from civilization, the bike quit running. That's right. Dead. Hmmmm. It was running smoothly a second ago, so why did it stop?

This was very disconcerting. I could understand something like a hole in the engine making a bike run a hair off, but everything was intact and there was no mechanical noise. It just flat quit. Right then and there, I realized the reasons for the existence of a how-to-do-it guide.

A situation like that calls for that most necessary of all skills: trouble shooting. Anyone who wanders off normal roads should have at least basic troubleshooting skills to fall back on when the engine malfunctions.

Trouble comes in two basic flavors: The bike won't run at all, or the bike runs poorly. These evils can be subdivided for easier troubleshooting, starting with the most common ailment first.

If the engine fails to start after a reasonable number of kicks, say 50 or 60 good boots, then it's time to do some poking around. First, and most logically, pop off the gas cap and check for fuel. Don't laugh; it's happened to the best of them. And not only because they forgot to add gas, but because a leak may have developed.

Most of the time, your ordinary spark plug is the culprit.

When a two-stroke fails to start, most riders usually yank the plug, a very logical thing to do. But something could be making a perfectly good plug foul, and that might be the real cause of the engine's failure to start.

Checking the plug is easy. Remove the plug from the head and look at it closely. If it's clean and dry, ground it against a fin and push the kickstarter through. Watch for a Zap! at the tip. The spark should be strong and blue/white. If the plug is wet and oily looking, clean it thoroughly before checking for spark.

Even if you don't have a lot of tools with you, the offending plug can be reasonably cleaned. The tip can be lightly scraped clean against a sharp, thin edge on the bike (fender tip, fin, anything), and a slender twig can be run down inside the plug. Attach a piece of lint from your pocket to that twig, and you have a first rate plug-dryer-offer. Check the gap. A cover from a book of matches will give you a ballpark measuring device, about .030. Also, that striking surface makes a good tip cleaner.

If, once the plug has been cleaned properly, you still don't have a good spark, check the plug wire. All connections should be clean and tight, and no corrosion should be present. Check the other end of the plug wire. This leads out of the coil and sometimes vibrates out of contact.

If, after all this, you still feel there is no juice going to the spark plug, try the good old acid test. Grab hold of that plug in your bare hand and push the kickstarter smartly through. If you are then forced to pick yourself up off the ground and say "wha happen," there really is juice flowing and you'd better recheck the wire and the plug. The plug itself might be bad (cracked insulator), or the plug cap might be sloppy loose and faulty. Additionally, any moisture around either end of the plug wire will short out an intact system. Everything should be clean, dry and tight for a strong spark.

See Page 2 for Fuel Supply Issues

No fuel, could be traced to a simple clogged petcock

Once you have determined that there is indeed fuel in the gas tank, remove a gas line and turn the petcock on. If the gas flows out smoothly, the trouble is somewhere else. If not, you have a clogged line or petcock. Dismantling and cleaning will solve this, but if you're stuck out in the boonies, blowing in the gas line will sometimes temporarily clear the debris.

If, however, the gas line flows well, the next logical step is a close inspection of the carb. Here the problem can be anything from dirt in the jets, to a stuck float, to a broken needle. Rather than rip completely into the carb, initially remove the float bowl (or float bowl cover on some carbs) and see if the float is moving freely. For a quick spot cheek, turn the gas tap on and let the gas run freely. Then push up on the float lightly with your finger. The flow should stop. You will be able to watch the float needle start and stop the flow by doing this. If this works OK, look a little further into that mysterious little carb.

A main jet clogged with water or debris can stop a bike cold

Disassemble the carb carefully, taking note of the sequence of parts. Anything that is bent or broken is the obvious culprit. More on the sneaky side are the small holes in jets and orifices. These can be clogged by even the smallest piece of dirt. Remove each jet and hold it up to the light. If you find crud, blow it out. Don't gouge it out with a piece of wire. Jets are made of soft metal and damage easily.

Using a little lung power, blow through all of the little channels or orifices in the carb and make sure they're passing air. Once you have checked the entire carb and found no malfunction, you can pretty well bet that it's an electrical problem.

Most people recoil from the words "electrical problem" like they just found a snake in their boot. Without getting into deeper electronics, there are a few things anyone can do to isolate and possibly cure the problem.

In order for a motorcycle to start, a number of things must be happening. You've gotta have gas, the machine must spark, and it must spark at the right time. Assuming there is no real mechanical problem (hole in piston, broken rings, etc.), you can readily isolate the reason the bike won't start.

If none of the jets are clogged, the carb is in good working order, gas flow is good and you have a fat spark, then the logical place to look on an older bike is the points. If the spark is intermittent or weak, then it is almost certainly a point or timing problem. Most conventional older dirt bikes have a flywheel magneto ignition and either have the points exposed (a la CZ or Maico) or have the points visible through a slot in the flywheel magneto. Initially, make a visual check of the contact points. If there is any dirt, oil or grime, this is probably the culprit. Any bad pitting on the points will also contribute to non- running and should be removed.

A dollar bill makes a decent point cleaner if you're away from the tool box, and that old pack of book matches (the striking area) is ideal. After the points are thoroughly cleaned, check the gap. This is in your owners manual and you should know it. But if you don't, try 13- to 15-thousandths. That should at least get the bike started and running halfway decently.

On a newer bike, incorrect timing can prevent a bike from starting. Many riders use an old trick to enable them to make a field check without going through the entire hassle of dial indicators and whatnot. A small scratch is put on the mag and on the mag base. If these scratches do not line up, then the timing has moved. Lining up these scratches should allow the bike to at least run.

A total visual check of the ignition area is a must. Any wires that are frayed and bare can short out an otherwise good system. Loose connections are a distinct possibility. Poke, prod and feel each and every connection thoroughly and examine each wire, especially where it comes near a metal surface or a sharp edge. All wires in the entire electrical system, including the kill button lead, must get the same careful check. It only takes you one small bad contact to prevent the electrical system from functioning. The ground must be clean and tight. Any corrosion here can stop you.

A good general rule of thumb to remember when that bike won't start is that at least nine out of 10 times, the problem will be electrical. Everyone likes to blame the carb, but that rarely is the case, just remember: check for gas flow, spark and electrical shorts—in that order.

This condition is even more frustrating than not being able to start the engine. Generally, the malfunction is more subtle, or of an intermittent nature. These are the hardest to trace down. All indications are that everything in the bike's various systems works, but one or more things are not doing their job fully.

Starting again with the fuel/carburetion area, any number of things can cause a problem. The important thing is to break the possible trouble areas down, and the best way to do this is to determine whether the problem in running is constant, or intermittent. In other words, does the engine run just fine one second, then roughly the next? Or is it just plain ratty all of the time?

The most logical step to take with a constant malfunction is a sequential check. The fuel line or gas pet-cock may be partially blocked. This would allow just enough fuel to let the engine run, but run poorly under any sort of load.

Too much fuel can be as bad as too little fuel, resulting in loading up and whiskered, or fouled plugs. A clogged vent on the gas cap can pressurize the gas tank and upset flow.

A sticking needle will load the bike up with raw gas. Quite often a stuck float can be remedied by tapping lightly on the float chamber. This does not mean that you should beat it to death. Just tap.

Keep Reading - Page 3 has more on Intermittent Problems

Sometimes a rotten-running engine can be caused by no genuine malfunction, but being improperly set for the altitude. A bike that runs great at sea level will fall on its face at 5,000 feet. If you are going to ride at any appreciable difference in altitude from normal, carry along a selection of jets to compensate for air density. This should be taken into consideration before you go blindly ripping the bike apart.

After checking the fuel lines and the carb for partial clogging or jamming and if everything else so far checks out, move on. We are assuming, of course, that while you are doing all of this troubleshooting, the air filter is clean. Many riders wonder why the bike isn't performing right, only to find out later on that a filthy filter was choking the engine to death. You wouldn't be that dumb, would you?

While we're on the subject of things you take for granted, make sure you have the correct spark plug in your machine. Too high a heat range will make the engine run as if it were overly rich; again, check your owner's manual.

Leaking base gasket or loose carb intake is a possible cause

We're assuming that the engine is in normal mechanical condition for this troubleshooting session, because a worn set of rings, bad reeds or leaky seals can cause you untold quantities of grief. These things, however, don't happen overnight and are usually of a gradual nature. This guide is for those 'little things" that happen, not for overhauling the entire bike.

Two additional things that most people never bother to check when the engine suddenly starts to run sour are the following all-too-common bummers: Motor mounts work loose and vibration makes the gas in the carb froth. This will give every indication of a poorly adjusted carb, and will drive you batty. Check all of those motor mount bolts. Secondly, check for loose head or barrel bolts. Accompanying these will be a blown head or base gasket. While you're checking gaskets, sneak another peak at that carb and make sure that the carb base gasket, or hose, is intact and no air leaks are present. Simple things like a poor-fitting hose can cause an engine to run bad.

As previously stated, if the malfunction is of an intermittent nature, chances are something is loose and jiggling around. This can be anything from a piece of dirt fluttering around through the carb, to a wire that occasionally brushes against bare metal. This is actually the easiest type of malfunction to trace down.

An initial check of the plug is in order. If the plug is clean, look elsewhere. A small number of particles around the base, or a buildup on the tip, can cause a temporary pause in an otherwise clean running engine. A heavily carboned-up engine can cause detonation, as hot particles of falling residue reignite and post-ignite, spoiling the fresh charge of gas and air. Often, crud will accumulate on the plug, get hot and fall off, causing the same thing.

The air cleaner should be checked to see if it is allowing dirt to pass. Also, look at the air cleaner base and air hose for possible dirt leaks. These leaks will allow crud to enter, cause the engine to misfire and then get blown out the exhaust ports, leaving a great mystery as to why the engine misfired.

Here, you'll find a handy reference chart that you might want to cut out and carry with you when you're on the trail. It is not meant to be a treatise on mechanics, but a good general guide that can help isolate the problem and get you running again. Just remember: When trouble strikes, don't start ripping things apart. Analyze the situation and eliminate one possibility after another, until you find what ails your bike.


No gas
Clogged petcock or line
Dirt in carb or jets
Vent hole in gas cap clogged
Fouled or whiskered plug
Electrical short
Bad or dirty points
Slipped timing
Loose connection

Partially clogged petcock or lines
Carboned up plug
Sticking float or needle
Improperly jetted for altitude
Incorrect point gap
Dirty or pitted points
Dirty air filter
Incorrect heat range on plug
Loose motor mount bolts
Loose barrel or head bolts
Blown base or head gaskets
Leaky carb gasket or hose

Dirt or water floating in carb
Dirty plug
Heavily carboned engine
Air cleaner or hose passing dirt
Loose electrical connection
Frayed wire shorting out somewhere
Sticking parts in carb Newsletter
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