How *I* Diagnose a Scooter That Does Not Run

Your scooter doesn’t run. That’s a problem. I can solve it. This is how *I* diagnose broken scooters. There’s nothing secret about my methods.

I encourage you to try some of this stuff on your own, to see if you can fix your own scooter. I can also offer something like free technical support over the phone.

Your scoot is powered by a 2 or 4-stroke engine. It needs fuel, spark, and compression to run. Let’s talk about how all of that works, starting with a few simple tests.

Checking for spark

The “carburetor cleaner” test checks for spark. If the bike runs on carburetor cleaner, the bike has spark. There are other ways of checking for spark.

The best way to check for spark is to remove the spark plug, reinsert it into the boot on the coil wire, turn the key, and kick or spin the engine over with the bottom of the plug grounded against the engine.

You should be able to see sparks jumping the gap on the electrode every time the engine spins over. If your bike doesn’t spark, you’re going to need further electrical diagnostics, a new CDI box, a new stator and pick-up, a new coil, or some combination of those four things.

The carburetor cleaner test

One of the best ways to check for spark, and to a lesser extent compression, is to shoot some carburetor cleaner into the intake and then attempt to start the scooter.

You don’t even need a fresh battery or a working starter to try this test.

Just get a can of carburetor cleaner, spray some into your airbox, and attempt to start the scooter using the kick-starter. Most scooters will start and run even with a dead battery.

If it runs on carburetor cleaner, even for just a second or two, then your scooter has spark. If the scooter runs on carburetor cleaner but fails to run on gas, there’s an issue with how the fuel is being delivered to the engine.

Your scooter may need a new petcock. It may need its carburetor cleaned. It could also need a fuel tank flush. Keep reading to learn how to test the petcock.

Testing the petcock

Most scooters use a vacuum actuated petcock. That means fuel flows from the fuel tank to the carburetor only when the engine is cranking or running.

To test the petcock, disconnect the fuel line from the carburetor. Disconnect the vacuum line from the petcock. You can use a fluid syringe to produce vacuum in the line. You can also use your mouth to suck on the line if you don’t have a syringe handy.

If fuel comes out when you put some vacuum on the petcock, the petcock is working properly. If the bike runs on carburetor cleaner, but not on the gas in the tank, you can be about 99% sure that the pilot jet inside the carburetor is clogged. Your carburetor needs to be cleaned.

The bike still runs poorly after cleaning the carburetor…

If the gas has been in the tank for a while, there’s a good chance it’s bad. The next step is to try draining the gas and replacing it with some fresh 91 octane. I recommend using 91 octane in all motorcycles, scooters, and ATVs unless the markings on the machine indicate otherwise, or the owner’s manual clearly states that low or mid-grade gas is OK to use in your machine. When in doubt, pour 91 in it.

One “trick” I use to avoid flushing fuel tanks is to dilute the bad gas in the tank with fresh gas. This works best when the tank is nearly empty, but the gas in it has gone bad.

If it still runs poorly with a clean carburetor and fresh gas, it’s a good idea to make sure there’s a fresh spark plug in the engine. That’s why one of the first things I do when diagnosing a scooter or motorcycle, or anything with an internal combustion engine, is to install a fresh spark plug or plugs.

If the scoot has a fresh plug, fresh gas, and a clean carburetor, but it still doesn’t run quite right, then it’s time to dig a little deeper.

Diagnosing compression issues

If your scooter runs poorly even after cleaning the carburetor, installing a fresh spark plug, and replacing the bad gas with fresh gas, then you need to look at compression next.

A “quick and dirty” way to check compression is to remove the spark plug, cover the hole with your finger, and then spin the engine over using either the electric starter or the kick starter. If you hear a loud “pop” as the piston pushes the air past your finger, then it’s likely your engine is making enough compression to run well.

A better way to check compression is to use a compression tester. Compression testers have their limitations. They will tell you that you have a problem, but the source of that problem may remain ambiguous. For instance, if you have a 4-stroke 50cc Chinese scooter with 60psi of compression, we know that the engine internals are going to need some attention. What we don’t know is whether it’s the piston, the rings, the cylinder walls, or the valves that are the source of the problem.

Most mechanics simply tear down the bad engine and check each component individually, replacing broken or worn parts as necessary. There’s a better way yet.

The best way to determine the health of your engine is to use a leak down tester. A leak down tester introduces compressed air into the combustion chamber.

If your intake valve is leaking, I’ll hear air rushing out of the carburetor and intake manifold. If your exhaust valve is leaking, I’ll hear air rushing out of the exhaust. If the rings are worn, I’ll be able to hear the air coming out of your dip-stick hole. If you have an issue with your head gasket, I’ll be able to see the leakage around the seam or hear it coming out of your radiator cap.

A leak down tester is a clever, precise tool. Not only do I have one in my van, I also have a source of compressed air large enough to perform the test on most scooters and small motorcycles. Using a leak down tester in addition to the finger and compression tests allows me to pinpoint the source of your problem without relying on guess work or the dreaded “process of elimination” that many mechanics use to justify their fees.

Tuning issues

Let’s recap. At this point you’ve checked for spark, made the engine run on alcohol (AKA carburetor cleaner), made the engine run on gas, verified compression, verified the condition of the engine’s internals… and the bloody thing still doesn’t run right.

It’s time to take a step back.

Is there really something wrong with the way the scooter is running? Does it hold an idle? Does it accelerate smoothly? Does it go as fast as it should go? Does it hold that speed without sputtering or stalling? If “Yes!” to all of these questions, your scooter is running exactly as it should. If “No!” to one or all of these questions, then your scooter has tuning issues that we can definitely sort out with a little more work.

Tuning issues are beyond the scope of this article. Let’s just say it’s complicated, tricky, and not something that can be outlined or described quickly and easily.

Other issues

This article scratches the surface, and it’s already running long at 1000+ words. There are as many different “scooter issues” as there are scooters and parts for scooters.

Chassis issues having to do with brakes, steering, and suspension are generally pretty easy to diagnose. Just find the part that’s loose or worn out and replace it. Things like dry-rotted valve stems, intake manifolds, and tires are easy enough to diagnose and replace. Leaks are generally a matter of bad gaskets or ill fitting, worn out parts. Bodywork and plastic can be difficult or impossible to find and replace.

As always, I’m available on the phone to talk you through your scooter issues. I’m also available 7 days a week, from sunrise to sunset, to diagnose and repair your scooter. Just call (719) 565-9295 to talk or to schedule an appointment. I do my very best to return calls within 24 hours. I’m also available via email at! Note that I’m not terribly good at sending text messages with the work phone, so if you need to communicate via text email is best! Otherwise just call!





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