Fuel System

All material on this website, and these sections, is ©Runner Outboards LLC and is intellectual property.  You may freely distribute this information as long as it is NOT edited, and credit is given to the author.

Disclaimer: The information provided should never replace common sense or the recommendations of the OEM.  I do not assume responsibility for the use or misuse of this information.  The information provided is based on my experience working as a full time mechanic, on hundreds of motors over time, reading a lot of manuals, education, and consulting other experienced mechanics along with a number of retired service reps I am friends with.
If I can offer any advice from experience, it would be NOT to try and fix your own motor if you don't have a good understanding of what you're doing.  You need to have the right special tools, reference materials, and most importantly, UNDERSTANDING of what is wrong and how to properly fix this issue.  Most people do more harm then good if just messing around blindly.  The reason why I can do these repairs is I've put in thousands of hours reading, fixing, and practicing.  I learn something new everyday.  I have also gone out and acquired the necessary, CORRECT tools and reference manuals to work on the motors.  These are very important to promote correct operation of the motor.  The idea is to have a reliable motor, not just one that 'kinda runs.'

Index - Click below to Jump to That Section

Internal Combustion Essentials
The Basics, What You Need To Know About Motors

Cooling System
Gearcase Components
Ignition System
Fuel System
Mechanical Components
Trailer 101

Calibrating your Carburetor

The carburetor is responsible for metering the amount of fuel delivered to the motor.  When you pull the motor over or hit the starter button, the crank shaft spins (you see the flywheel rotate, which is attached to the crank), this causes the pistons to 'punch' back and forth, and the result is induction, compression, and internal pressure changes, hence the 'internal combustion' engine.  The induction, or suction, of the pistons moving away from the crank causes a vacuum which pulls fuel in through the front of the engine, or the 'barrel' of the carburetor throat. 

Here's a video of a 30hp Johnson, late 80's model carburetor on my test boat.  You can see the rear flap rotate up as I advance the throttle, and see the fuel getting sucked up through the high-speed main jet (brass 'pole' in the middle).  Note that I did not go to full throttle, only abou 3/4, because holding the camera in one hand, steering the boat with the tiller in the other, and than keeping my view forward along with trying to keep the sunlight (sunset) lined up so that the camera didn't cast a shadow to ruin the video shot was challenging.

When you pull the choke knob, you see a brass flap close, this is the barrel or throat of the carb.  Most motors post 1985 use a basic fuel priming system rather than a choke, which squirts fuel into the manifold and cylinders.  This accomplishes the same thing as a choke but more efficiently; it richens up the air-fuel mixture to get the motor to fire off.  If you don't see a choke knob (or flap when the hood/coweling is off) and see a plunger type knob, then it's a good bet you have a manual fuel injection type system. 

The barrel of the carburetor is also where the venturi is, or basically where it goes from a large hole down to a smaller one, which helps create an increase in air velocity and a 'suction' through a mainjet (usually seen as a vertical brass cylinder in the carb throat).  This is true regardless of if you have a choke or manual injection system.  As you advance the throttle, it rotates a second flap near the back of the carb which meters the fuel based on the timing of the ignition system (magneto, or newer motors, capacitor discharge ignition [CDI], on today's motors, it can be direct/electronic fuel injection [D/EFI]). 

On a side note, this second flap ***MUST*** be timed properly with your ignition system or your motor WILL NOT run right, or possibly not start at all.  There is an entire process for setting this up, and some nuances to getting it right on that only a mechanic will understand.  The video shows a carburetor working in action, and the pic below is a still frame of the carb advanced about 3/4 throttle (notice the brass flap isn't completely horizontal), and you can see the fuel spraying through the main jet, being sucked into the manifold, and then down to the cylinders for internal combustion.  I find this cool.

carb barrel

What a choke does by principle is richen up the fuel vapor for getting the motor started by starving the oxygen, and allowing more fuel to be induced into a cold, or dry motor.  Once the motor fires off, this entire process reciprocates and continues on it's own until you either starve the motor of air, or ground out the ignition system using a kill switch.  This is the reason why you sometimes have to leave a choke closed fully or even partially while a motor is cold, to give it more fuel until warm so it doesn't stall out.  Generally you shouldn't have to leave the choke closed completely to keep the motor running if both the ignition and fuel systems are right.  There are always exceptions though.

If you listen to a car that won't start, a person lays on the starter for a long time (which is bad, by the way, easy to burn out a starter or solenoid...if you've ever had a starter solenoid die on an old vehicle all you hear is a 'click' or no click at all, which means the solenoid failed and burned out and needs replacement...a solenoid simply takes high voltage and uses a plunger type system to send high voltage where it needs to without uses that high draw on the same circuit.  It promotes the prevention of burnout with smaller electronics i.e. low voltage switches & wires...production cost reduction.

Calibrating the carb for single barrel motors isn't very complicated.  What I find is that most people have no clue where to begin with this process, yet it's one of the most important skills to have a basic understanding off.  So make sure you read this a couple of times because it will really keep the cursing to a minimum.

If you've bought a motor from me I've already calibrated the carb the best I can using a test tank, test props, and in most cases, on an actual test boat.  What I do is get them pretty close, sometimes right on, sometimes needing to be dialed in a little more.  Remember your settings may change slightly throughout the year, based on temperature.  A cold environment will generally lend itself to needing more fuel, and less fuel economy too.

First, you need to do this under load, in forward gear, with the motor warm.  Either find a big lake with nobody around, or get a spotter in the bow of your boat during this process.  You shouldn't need to do too much adjusting after you buy a motor from me, but if you think it's necessary, you can start the process from default settings recommended by manufacturers. 

I'll explain this as if we're doing it from scratch, with no settings made yet.  Depending on the motor you may have 1 or 2 adjustment needles, one for high speed, one for low speed.  The key here is that the high speed jet controls the lionshare of fuel flow.  The low speed needle is really just for idling and very low speed.  Make sure you understand how the needles work in the first place.  Imagine you have a round hole "O", and you are placing a finely machined BRASS needle into it.  By screwing/unscrewing the this needle, you are controlling how much fuel/air can pass through it.  This is a sensitive adjustment.

So when we start off, you screw the needle all the way in to shut off the fuel completely.  You want to be careful here, though.  Screw it in until it gently seats, don't try to screw it down hard because these brass needles are damaged VERY easily.  As in, a 16th of a turn too far and you just ruined it.  Or let's say you drop it on the ground.  It hits the floor and is instantly ruined.  This isn't something you're going to be able to throw on a grinding wheel or bend straight with needle-nose pliers.  It is no good once damaged. 

In the picture below notice the taper on the needle is worn, and the tip is in fact bent.  This is due to careless operator habits.  There was probably something wrong with the carb but the person kept messing with the low-speed needle, rather than fixing the carb (and real problem) to make things right.  This needle is no good.


Back the high speed needle out, generally 1/2 to 1 full turn.  Unless you own a service manual and have the exact settings, you are flying blind.  You'll need to go out and get a manual for your specific motor to know what the start settings are.  Keep in mind these are general rules of thumb for your motor HP, experience tells me they are ball park but the 'right' final setting could be very different, particularly if you have a worn needle or carb orifice.  That's where experience comes into play which is what you're paying me for.

Get the motor running.  Back your low speed needle out 1 full turn but leave it alone for now.  Let the motor run for a couple of minutes to bring the temperature up.  And one thing to consider - don't try doing this in cold water or on a cold day.  The setting won't be correct and the motor will run differently on a warm day.

Once warm, screw the high speed adjustment in slowly, say 1/8th a turn, wait 15 seconds, repeat.  You should observe the motor start to rev up, eventually until it stalls, which means it's running lean.  On the low speed needle, if you hear the motor cough/sneeze, back the needle out slightly until it revs back up.  Keep backing out until you hear it slow down, start to blubber, or even die off.  Find the 1/2 way point between these two extremes.  Now throttle up and see how it responds, even all the way up if you have a spotter.  You should see the motor accellerate.  If it doesn't, back the needle out to give it more fuel.  You'll find the 'right spot' by fiddling with it at near full speed and than again at low idle.

Keep in mind that after the high speed is set, repeat the same process with the slow speed needle.  If you have a motor with a low speed idle adjustment (in other words, something that sets the stop on the tiller so you don't stall the motor out), you may need to either lower or increase the idle stop as you get the low-speed needle set.  Newer motors have a fixed high speed jet so all you have to do is get the motor running and deal with the low speed needle.

You should notice the motor doesn't smoke as much once you have both needles set.  Also, you should be able to throttle up quickly without the motor bogging down or lagging, and throttle back fast without it stalling.  If you have either of these problems your fuel system isn't calibrated or has a problem.  Keep in mind a cold motor runs quite a bit different then a warm motor.  A lot of this has to do with the fuel 'loading up' in the cylinders and crankcase, and you may need to 'clear it out' every so often by running at high throttle in gear.  Don't ever do this in neutral, it's a great way to ruin the motor.  Many motors have a limiter for the throttle in neutral anyways to prevent this.

The nice thing about older motors is you have the ability to control the fuel mixture always with the rich/lean knobs.  When the motor is cold, you'll have to richen it up.  When warm, lean it out.  If it's misfiring, it's a fuel issue based on motor temperature most of the time.  It is possible the ignition system may need something too.

Here is a video showing what happens when you adjust the carburetor (low speed) either too lean or too rich.   The motor is a 15hp OMC, 1984 M.Y. with a fixed high speed jet (no adjustment for high speed). 

The first 10 seconds the motor is set properly.  I lean it out (clockwise) and you'll hear the idle drop to the point where the motor is ready to die.  I catch it in time and bring the adjustment back to approx. the 6 O'clock position and you hear the idle come back up.  Next I richen the mixture by turning it to approx. the 12' o'clock position.  Listen/watch closely as you'll hear the motor 'chugging/bucking/blubbering' and you can see it shaking quite a bit due to a rich running condition.  If you watch the video a few times with your computer volume up, you can hear the marked difference in idle quality by making these adjustments.

I lean it back out and the idle smooths back out.  Shows how a 1/4 turn in either direction can make a big difference.  Note that the motor was warmed up in forward gear using a test propeller (OEM) in a 300 gallon test tank (minimum recommended tank size) of proper OEM recommendations.  The throttle was not touched at any point during this video; my left hand was holding the iphone, my right hand made the rich/lean knob adjustments.

Keep in mind this motor has been fully tuned up with a strong ignition system, new fuel system components, decarbed powerhead and exhaust system, and is set up properly for correct calibration.  If your motor isn't running adjusting the carb may make no difference if there are other underlying problems.

Flooding The Motor With Fuel While Starting

What we have here, is a situation where the motor has received way too much fuel.  This happens for various reasons; the float or gaskets within the carburetor could be faulty and allowing you to prime the motor with more fuel than it's supposed to get; you may have the low or high or both speed adjustment needles set too rich; the ignition system might have failed or is too weak, leading to lots of fuel but no ignition source; you might have left the choke closed too long when trying to start the motor; or a common one I do - forget to put the plug leads back on the spark plugs because you pulled them off for servicework, then forget to put them back on when you went to evaluate the corrective action you just made; the list goes on and on...

Whatever the issue, you're probably going to see a bunch of fuel leaking from the motor, generally the 'throat' of the carburetor, but you could even see it seaping out of your fuel lines due to pressure with the primer bulb.  If you were particularly persistent, you probably even fowled your spark plugs too.

To fix this issue, first you need to figure out why you have the problem in the first place.  If the motor was running recently, and you know the rich/lean settings are dead on, or at least pretty close, then make sure you have spark and ignition.  Once this is confirmed, make sure you didn't starve the motor of air.  Keep the choke OPEN, pull it over a few times, and see if you can get a pop.  If it runs momentarily then dies, now put the choke CLOSED, and try your normal starting procedures.  It should start up. 

The common sense thing to do is if you see fuel leaking out of the carb (or if the motor cowel/hood is on, out of the bottom of the motor), then you need to let the motor breath by opening up the choke and pulling it over to clear things out.  Pull the plugs, blow them clean and use a rag to wipe any oil/gas/carbonation.  What's even better is spraying a little carb cleaner, using a wire brush, clean with compressed air, then regap and reinstall.  Once everyting is 'back to normal,' try to start it up.  It'll probably pop off within a pull or two.

Should I Use Fuel Additives?

My personal preference - NO.  I do use "SeaFoam" or OMC Engine Tuner on occasion, but mostly just to help keep the combustion chamber and exhaust housing cleaner.

Most people look to use an additive with expectations that it will stabilize their fuel from going bad.  Well what happens to your motor when you run bad fuel through it or let the fuel sit in the lines, pump, and carb?  You get 'gunk' that stops the motor dead in it's tracks in as little as 1 day. 

Don't believe me?  Then take an old glass jar with metal cap, fill it with gas, and let it sit undisturbed for a few days.  Even better, expose it to sunlight (loosen the lid so expanding gas can escape so you don't have any hazards - and make sure their is airflow where ever it's sitting).  Watch what happens to the fuel and how long it takes (sometimes just 1 day). 

Fuel goes sour like milk. As I say over and over, I can't tell you if fuel stabilizing will work or not, but I can tell you what works 100% of the time - RUN YOUR MOTOR OUT OF GAS after each use.  All you have to do is pull the fuel line while it's still running and leave it in neutral at low or slightly above idle.  If there's no fuel left, then you don't have to worry about the ethanol from doing it's harm.

I suppose if you have deep pockets, you can buy 'marine' fuel, but I don't know why just running the motor out of gas should be such a big deal.

Why You Should Run Your Motor Out of Gas

Below are some pictures of things I commonly see on motors that have been left sitting, and whoever used it last, didn't bother to use up all the fuel.  The reality is that gas, even 'stabilized' gas, is bad to be left sitting in a carburetor.  If you ever want to see just what happens (and how quickly), take an old pickel jar or glass jar from something in your kitchen and pour some gas in there.  Let it sit somewhere safe and watch what happens. 

Depending on where a motor is left to sit, worst things can happen too.  The first picture shows an older style cork float, which originally had a coating to allow it to float and not absorb fuel.  Over time they fail, but when left sitting in today's fuels, they become caked up, and look like this.  The 2nd picture shows a newer style foam float.  These are quite a bit better than the original ones, and in many cases can be the only part of an abused carburetor that survive.  With a cleaning, they can be reused in almost all cases. 

The picture to the far right is a 9.5hp carburetor that was so full of build up of old fuel and crystals from moisture and condensation, that it took several days of soaking and an hour of hand cleaning.  Leaving a motor in an unfinished basement, barn, or out in the elements leaves it exposed to moisture cycles.  This results in white build up similiar to efflorescence you see in your basement walls. 

The bottom two pictures are what old fuel turns into when left to sit in the carburetor bowl.  Either it turns to a gooey sludge, a brown mush, or sometimes a white frosting.  All can be a pain in the butt to remove, and every aspect of the carb needs to be cleaned, in particular the tiny passages and fuel orifices which if clogged will basically prevent the motor from running correctly.  This isn't something where you're just spraying some solvent into the carb and away you go.  This is a full teardown situation and you really need to understand exactly how the carb works to fix it correctly and recalibrate as needed.

bad float   dirty float 2   9.5 filthy dirty bowl   dirty bowl 3

Here is what happens to fuel when it sits in your fuel lines for a longer period of time.  In this instance, the same white powder/gunk has formed inside the lines.  So while your carburetor may still be clean, fuel remants can be flushed downstream and clog up the works.  Run your motor out of fuel and drain the lines if possible to avoid a lot of headaches.  If your fuel lines are getting vulcanized/hardened, replace them right away too.  Externally they may look OK, but internally they very well may be breaking down and sending bits and pieces throughout the carburetor and motor.

fuel line

How The Fuel Manifold Works

When the motor turns over the pistons move back and forth.  As the pistons move away from the cylinder head, it creates a suction through the manifold.  The manifold in most small OMC motors has 'leaf' valves.  They are called this because, well, they look like leaves.  These valves have been used for several decades and help to further meter the amount of fuel the motor receives.  In many cases, the difference in horse power has to do with how much fuel a motor is receiving; giving it more fuel and air gives it more power output. 

Here are two manifolds for a 15hp motor.  The one on the left is worn, notice the rounded off edges to the orifices.  This is a rare situation, but can happen after a motor has been used for a long time and the leaf plates just slowly wear away the manifold.  This particular motor would not run right, and after evaluating the ignition system, carburetor, and fuel pump, I suspected the only thing of explanation was a worn manifold.  Sure enough, after a full teardown, this is what I found. 

The manifold pictured on the right was a replacement and you can see it is in good condition with 90° edges, which allowed the leaf plates to properly seat and meter fuel intake.  The motor ran fine after this replacement service what carried out.  When you try to start a motor that is not fuel injected (this started being introduced in the 80's), it can take 3-5 pulls of the manual starter to charge the cylinders with enough fuel mixture before it will start up.  This is completely normal in a dry motor.  You have to pull fuel into the motor first.

worn manifold  manifold

Below are pictures of a good manifold but the leaf plates are worn and not seating properly.  On the left is the worn leaf plate, with a small gap between the manifold and leaf plate (about 1 mm).  This may seem insignificant, and yes, the motor will run, but it won't run right, and it also ran all over the place (bad power curve).  The reason being is the fuel mixture was inconsistent at all RPMs, so nothing you do with the carb settings, timing, or anywhere else is going to correct this.  The right picture is good leaf plates which sat properly on the manifold.  Once these are worn you replace them.  They are not something you try to repair.

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Here is a video of a 9.5hp motor I took with the intake functioning as it would when the motor is running (it's not easy holding a drill gun, steadying a motor, and filming and getting light into the manifold...sorry for the camera wobble).  Notice the 'owl whooo' you hear when I turn it over a little more slowly with the drill gun.  This is the suction occuring and is normal.  You can see the leaf valves fluttering as suction and pressure changes occur in the cylinders/crankcase.  If there was a carburetor installed, it would pull fuel from the carb and send it into the combustion chamber.  This is also why there are warnings usually on the airbox of the motor saying "keep clothing, hair, and garments away", because they can easily be sucked in.  If the motor is running and you get caught with the flywheel, well, "sianara" as they say.   Introduce some spark (i.e. ignition/spark plugs) and you have yourself an internal combustion motor.


Here is another 9.5 motor that had been sitting in a basement for many years.  Notice the intake manifold is full of what appears to be sand.  This is that same build up of efflorescence-like material.  This motor is sitting in the big lake in the sky now.

manifold dirt

Chipped Manifold

Recall in the prior section we talked about how important an undisturbed manifold and good fitness leaf plates and lead stops are.  Oddly enough, it is possible for the actual manifold plate to not only get worn, but to become chipped.  This may have been a casting imperfection that found it's way past quality control, or a small chip that broke away sometime during the earlier days of the motor.  The picture on the far right is with the leaf plates removed, and I placed an orange slip of paper behind it to make the chip more visible to the camera.

Whatever the reason for this problem, this tiny chip was allowing the motor to pull more air into it than it shoud, and caused the motor to run erratically at all ranges.  If there is anything I've learned over the years, is that if you have gone through and eliminated all the possible sources of a problem where you've looked, than the logical remaining possibility should provide an explanation.  The trick is understanding that there are a lot of different things that can cause a problem, and it can be particularly frustrating when there are MULTIPLE things going on at once!

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Broken Leaf Plate

Here we have a situation where one of the actual leaves from the plate snapped off.  Now, this is not an obvious problem because even with the carburetor removed, you may or may not necessarily be able to see this problem.  After refurbishing a motor, I was able to get it to start but it was very obvious that something was wrong.  The motor would not run without the choke closed, and was a hard start.  The motor would not idle down and was not able to get to full throttle RPMs. 

I pulled the plugs to see the cylinders were soaking wet, so the next step was to pull the powerhead and inspect the water jacket, looking for leaks into the cylinders.  Not being able to find evidence of where the water was getting in, I was left scratching my head.  Another thought was perhaps the fuel pump was leaking fuel through the diaphragm, but even with switching that out, no improvement was noticed.  After 10 hours of pulling things apart and putting them back together, and having to replace all the gaskets that were destroyed along the process, I put the motor away for a good night of sleep and a warm dinner. 

Than, AH-HA!  Could it be the manifold?  Pulling the carb and carefully shining a flashlight into the manifold revealed my smoking gun.  One of the leaf plates was completely gone!  I have seen instances where the center screw had loosened up and fallen off, generally resulting in odd running characteristics (this allows more fuel/air mixture), but this truly was an oddball situation.  After replacing the leaf plate, the motor ran completely normal.

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Foam in Manifold

In the last section I showed you pictures of a manifold with 1 leaf plate missing.  Well on another motor, I was able to get it started but it was running lousy.  See prior to me running motors for the 1st time I have already gone through the entire motor, so if they don't run right the first time is usually means there is some sort of oddball situation.  In this case, part of the insulation foam that is glued to the inside of the cowel had been sucked into the motor by the prior owner, and unbeknowst to me, was lodged into the manifold.  Amazingly, the motor would still run, even up to near wide throttle, but it was running erratically.

Once I pulled the carb to inspect closer, I could see the yellow foam mashed down in the manifold.  I carefully pulled it out to find out this little foam was more like a 2 x 2" block of foam!  The motor ran totally fine once removed.


Common Carb Issues

Carburetors, at least OMC ones, aren't terribly complicated, but do need to be set exactly right to work correctly.  Recall that they are responsible for metering the fuel flow to the motor, while also being syncronized with the ignition system.  Many DIYers think that 'the motor will run if the carb gets cleaned.'   That is almost never the case.  There are many, many different things that have to be correct for a motor to run right.  Don't forget there probably is more than 1 thing wrong in many instances, not just the fuel system, but other areas of the motor too.

needleHere is a float needle that has become worn out.  The point should be a straight point, not have those divots (which is where the needle seats on a brass orifice which also needs replacement at this point).  What this does is turns fuel on and off inside the carburetor based on how much is being consumed. 

When you prime your motor with the hand fuel bulb, you are filling the carb bowl with fuel.  The float is connected to this needle.  When the bowl fills, the float rises, and when filled up this needle shuts off fuel so you don't flood the carb.  This is why all the operator's instructions say 'pump bulb until it feels hard.'  If you continue to force fuel into the motor, you can damage the needle and fload the carburetor.  In this instance, you'll need to read the section about a flooded motor I wrote.

If you see fuel pouring out of the mouth of the carb it's because either the needle has worn out and isn't sealing correctly, the float is no good, or there could very well be several other issues leading to this. 

Now you may still get the motor running, but it probably won't run right because fuel can enter in an unregulated manner. 

Another common issue is a worn ignition cam follower.  As you accelerate a motor and give it more fuel, you have to increase the timing of the spark to account for this.  DFI (direct fuel injection) motors are computerized with sensors and handle this electronically.  Carbureted motors use a cam that opens the carburetor flap based on the timing of the ignition (either your tiller, or remote throttle).  When these wear out, it changes the timing of the motor and it will not run correct.  People often 'cob job' this fix which is a mistake, a band-aid at best.

cam follower

Debris in your carb will make the motor run erratically.  All it takes is a small piece of lint or hose material to dramatically change the way the motor performs.  It's important you always take a look inside your fuel tank and where possible, pull the intake filter of the gas tank and your fuel pump and check for clogs or just accumulations of materials.  What can happen if you don't do this, is eventually the 'junk' will get past the fuel filter screen(s), and make their way into the carburetor.   For fuel injected motors, this can clog up injectors which will turn into a very costly service procedure, to the tune of $200-400 on a good day.  At the very least you'll see a fuel restriction which will either make the motor stall, have a difficult start, or just not have a lot of power when running at higher RPMs due to the restriction.

Here is a picture of just how little it takes to make a motor not run right.  Some small pieces of bugs and fuel line had clogged up the high-speed jet of the carburetor (seen as the black stuff in the middle of the carb).  These can also get caught in the float needle and seat.  So the motor will start and run, sometimes a little longer to get it to kick over, but is only getting a trickle of fuel. Anything much idle will have you itching your head and possibly fiddling with the rich/lean settings.  Frankly nothing you do with the rich/lean adjustment is going to fix this without disassembly of the carb and inspection.


Leaving fuel in the motor for an extended period also accelerates deterioration of the carb bowl.  Here are pictures of what happens when you leave fuel in the carb for a long time.  You can see the float bosses are badly corroded, and the float bowl itself destroyed.  This carb had to be thrown out.

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It's also possible that a bug may have climbed into your carburetor over night and built a nest.  Here is a picture of a carb I pulled and was inspecting as part of my normal routing, prior to putting gas to the motor to try and start for initial calibrations.  A common mistake people do is put fuel to their motor and try to start it after it's been sitting for some time, hoping they will get lucky and it will just work fine.  In many cases, this actually cause more damage by sucking debris into the manifold and possibly damaging the cylinders.

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