"FastC" Method Of Diagnosing Any No Start / Runs Poorly Engine

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FastC Method for Diagnosing No Starts:

The FastC method of diagnosing a no start or a poor running condition on your scooter, motorcycle, or other engine is the ultimate way to figure out what is wrong. If you follow all the FASTC steps you’ll be well on your way to being an expert at diagnosing no starts and poor running. The FastC method for diagnosing a no start on your scooter stands for checking for fuel, clean air, strong spark, proper timing, and good compression thereby spelling “FastC.”

I learned about the FAST acronym method from reading this article and added the “C” part for a compression check. It is best to think of your gasoline engine like a recipe, you need each one of these components in order engine to run well or the cookies to taste right.

Work through this list, and I guarantee you will figure out why your scooter/car/truck/motorcycle or other small engine isn’t starting.

It may take more parts to fix it, or it may even take a complete engine overhaul, but at least you won’t be guessing anymore. This method applies to cars, chainsaws, and anything with a gasoline engine (slight difference for diesel).

The “F” in FastC stands for Fuel:

The best way to check for fuel is to start from the beginning and most obvious. Make sure there is gas in the tank.

I like to think of this as the trickle down test for checking fuel.

Start at the very beginning of the gas or fuels’ journey and follow it making sure there are no problems along the route.

Add fresh gas to the tank
Check that all lines are clear and unplugged
- Check the fuel line at the carb.
  - Make sure you are getting gasoline free-flowing while trying to start the vehicle or with the fuel petcock “on”
- If you do have a vacuum regulated fuel petcock then check to make sure the fuel isn’t being drawn through the vacuum line.
- Find any and all filters and check to make sure gas is flowing freely through them.

There may also be a vacuum regulated fuel petcock, or manual fuel petcock, and you want to make sure gas flows freely through both of these.

The vacuum regulated fuel petcocks have their own set of issues. They can cause many problems including leaving your engine not starting, running richly, or starved for fuel.

My vacuum fuel petcock failed on my 1985 Honda Elite 150 Scooter and caused the cylinder to fill with gas overnight.

It took me a while to diagnose because I forgot how finicky these parts can be, and how weird the problems can seem when they do fail.

To read my story of how I found my scooter literally dripping gasoline out of the air filter and piston click here. I cover it pretty in depth how to diagnose and fix this issue.

Follow the fuel lines checking along the way to make sure they’re flowing freely, and that any and all fuel filters are replaced.

They’re cheap just do it if you at all suspect they’re clogged.

Warning about old gas:

Please, make sure you have a full gas tank with good clean gasoline first.

You do not know how many people have brought their motorcycles, chainsaws, scooters to mechanics only to get sent home with a $100 reminder to check their gas first.

If it is a two-stroke scooter or motorcycle make sure you’re actually properly mixing fuel.

A lot of people end up damaging their equipment because of a failure to mix fuel properly and at the right ratios.

Second check for Air and Spark (A&S part of the FastC Method):

Just as an engine needs fuel to start, it also needs a spark and air.

You need to first pull the spark plug from the engine and make sure it is the right one for your engine.

This cannot be stressed enough. You do not know how many times I have seen the wrong spark plug, or improperly gapped spark plugs cause problems.

To figure out what is the factory approved spark plug for your engine just google “factory spark plug–insert your model here.”

For example, on my 1985 Honda Elite 150 scooter, it is an NGK DPR7EA-9. I just found that out now in less than 10 seconds.

If you’re battling a no start situation it is always worth just swapping a plug out, because they’re cheap and it is great to have extras.

How do you check to see if you have spark on a gas engine?

There are a couple of methods and each work with different levels of ease/safety.

The backyard way literally involves pulling the spark plug and grounding it out while you start the engine If you see spark then you know that you have spark.

The other method is to use an inline spark tester.

I personally love using these because you do not even have to remove the spark plug to first verify if there is even Spark.

If there is no spark happening in the inline spark plug tester then you know that the problem lies elsewhere.

But you have now narrowed down your search. You no longer need to worry about the other parts of the FASTC method, you can solely focus on fixing your no spark condition.

If you are not getting spark at the spark plug you need to start checking the other components.

Most common components that are bad if you have no spark on a scooter or small engine:

CDI
Wiring Shorts
Kill Switch Problems
Stator Issues
Timing

How to diagnose a no spark issue on a scooter:

These same ideas apply to other makes and models as well. Vespa’s, Piaggio’s, Aprilia, Husqvarna, Tao Tao, will all use some form of these type of components.

Remove Air Filter entirely from the Carb. Allow free flowing air to run into the carb, and try starting now. Does it start? Probably a problem in the air filter if it does. Take it apart and inspect.
Check air filter for debris and clogs. Tap it out, or blow it out with a compressor (always blowing from clean side out to the dirty side)
Pull the spark plug and ground it out while starting the scooter or kicking it over (or pulling it if it’s a lawnmower or chainsaw). Do you see a spark? Does it look healthy?
Check the gap on the plug and adjust. Set it to factory settings!
Is the spark plug the factory plug? If not, replace. Suspect it at all? Replace it.
If you have an extra spark plug check the gap, and replace the other one just to be sure.
Spark plugs and air filters are cheap. They’re parts that are okay to through at it because in reality you should always have backups for these anyways. Don’t feel bad buying a plug and an air filter if you aren’t sure that is what is wrong, you can use them in the future if they were okay.
Once it has factory spark plug and good clean air (with clean blue strong spark) move onto the (T & C) part.

Still no start? Move onto checking the timing and compression (T & C of FastC):

I recommend starting with compression if you have any suspicion of the state of the engine.

It is as easy to check compression as it is to check for good spark and should be done at the same time to be efficient.

For someone like me who has heard plenty of engines that don’t have compression trying to start, you can just hear when an engine has no compression.

Just listen to this Honda 80 Scooter with no compression.

It just sounds like “nothing” is happening in the engine. Like a faster whirling than a normal scooter starting. That is because the starter motor is not having to work nearly as hard to move the cylinder over (no compression means no pressure pushing back against the piston).

Once you get a sense for the sound, you’ll never forget it and be able to hear it from a mile away.

Especially on cars that have broken their timing belts, this is the most classic way to hear what a no compression engine sounds like.

What no compression sounds like on a Scooter (with any engine):

What does no compression sound like on a car?

Answer: Watch the video below. It sounds easy and faster, like a high pitched whirring. Hard to describe, but once you hear it once you’ll always remember what it sounds like.

How do I test compression on a scooter with a compression gauge?

You need to pull the spark plug
Install the compression fitting that fits the spark plug’s thread size and pitch
Use compression gauge like this one, it will come with its own set of adapters for different spark plugs.
Make sure that the fittings are tight, but no need to torque them down.
You can damage your compression gauge if you go too tight on the o-rights.
Next, with a full battery and a good starter crank the bike over with your hand holding full throttle.
Keep cranking or kick-starting the bike until the reading on the compression gauge stops increasing.
That is your compression reading. On my 1985 Honda Elite Scooter, it should be around 180psi to be good. Search online for your vehicles “normal” compression amount and check it against what you got.

Low compression will definitely result in weird performance issues, and if bad enough a complete no start / dead scooter, motorcycle, or car / truck. Every engine needs good compression to run well.

If you do find you have low compression or no compression you will need to do a further diagnosis to figure out what is causing it.

How to diagnose the problem after finding low compression:

First, check Compression.
Learn what an engine sounds like or the kick starter feels like when it has no compression. Watch videos above.
If kick starter feels super easy to do, it may have little or no compression causing the no start condition or running poorly.
Buy a compression gauge kit or rent one.
Remove spark plug
Install compression gauge into spark plug hole and kick or start the bike while holding the throttle fully open.
Keep going until the gauge on the compression tester stops going higher.
Record the number.
Check the number you get (i.e., 150psi against what normal is specified in the manual.)
Anything lower than normal or very low may cause poor running or a no start condition.
To check Timing refer to the service manual for your exact engine, or google search “how to check timing on a —— fill in the blank—– engine.” Some may not have adjustable timing, and this is honestly one of the last things I check as it is usually not the issue. 9 times out of 10 it is going to be fuel or spark issue.

Low or no compression on scooters is typically caused by:

Worn Piston Rings Not Creating A Good Seal In The Cylinder
Cracked Head or Other Cylinder Damage
Valve Problems or Adjustment
Gasket Issues (Blown Headgasket for Example)

When diagnosing a no start or running poorly use the FASTC method:

You will save a ton of time and money by not throwing parts at things. Once you master this checklist and remember it, you can apply it to any gasoline engine. The same ideas apply to a brand new subaru as to a stihl chainsaw. All gas engines need air, fuel, compression and spark to work.

Hope that helps you figure out what is wrong quickly and easily. Please comment if you have any more tips and tricks.

Here are videos I have made where I don’t discuss FASTC specifically, but you see how if I had been using that method I would’ve figured out the problem a little quicker.

One last pro tip. On anything with a carburetor check for vacuum leaks:

If you’re working on anything that has a carb you need to check for vacuum leaks! They might not be super apparent at first, but trust me they can cause all kinds of weird issues.

ALWAYS CHECK VACUUM LEAKS WITH CARB ENGINES. Also important on fuel injected engines.

Watch my video below to see how certain tao tao scooters are notorious for vacuum leaks, which lead people to throw all sorts of parts and money at the scooters.

When in reality it would’ve been fixed by tightening a hose clamp.