Powerhead Related Info

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.'

Click below to Jump Education Sections

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

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

What is Compression?
When You Don't Fog/Longterm Storage Prepare Your Motor
Why Yearly Tune Ups are Important
The Simplicity Of Compression
Blown Head Gasket
When To Use Helicoils
What Happens When You Don't Use Oil In Your Gas
Pulling a Flywheel...The Wrong Way
Worn Connecting Rod Bearings
Broken Crank Shafts
Exhaust Mix Build Up & Bad Performance (11/26/18)

What is Compression?

Motors need 3 things to run (not necessarily run right).  Spark, compression, and fuel/air.  When you look at a used motor you want to check for compression with a compression guage.  Each horsepower and model type of motor has different compression numbers.  Generally, the higher, the better, but just because compression has dropped doesn't necessarily mean it's a bad motor. 

There are a lot of things which affect compression; cylinder moisture, temperature, the guage, carb throat opening, speed of cranking, plus more.  The general idea is to know acceptable compression numbers for the motor to run.  If it has 'low' compression, that may be an indicator that something is wrong, but it could also be a false positive.  Generally, motors 9hp and below will have a peak value around 85PSI, but that isn't always true.  The size of the cylinder, style of cylinder head, intake and exhaust ports, pistons, and connecting rods all affect compression. 

People get hung up on compression all the time.  If the motor starts up within 1-3 pulls and runs, then that's what you really want to see.  If it has 'low' compression (generally anything south of 65PSI, repeatedly, is probably in need of service), it will just fade out and die off at a low RPM.  You might see less power at the top RPM range too.  Or it may just plain not run at all.

I can say that low numbers, or uneven numbers between cylinders doesn't necessarily mean a mechanical problem.  I have seen many motors that once run, the PSI goes up, sometimes as much as 20-30PSI.  This was probably because of carbon build up in the rings, stuck rings that freed up, or with a dry cylinder, the oil helped reseal the gap between the cylinder and piston walls.  When metal warms up, or anything for that matter, it expands and softens.  This could have loosened up hard carbon build up in the rings or walls and swept them away, allowing for better sealing/compression.

Get the motor running and calibrated before writing it off.  That's the moral of the story.  Remember a 10hp motor for OMC compared to the same one for Mercury won't show the same numbers.  They are two entirely different motors.

When You Don't Fog/Longterm Storage Prepare Your Motor

Customers give me motors all the time that are for parts.  Either they are beat to hell, stuck, or just plain taking up space.  Often times folks want to get some form of trade in value if they're buying a replacement, and are often shocked at what I offer.  The motor below from the outside looked cosmetically very good.  However, it was stuck. 

ph1Now a seized motor can sometimes be saved, but it's really just a roll of the dice whether that motor will ever see regular service again.  And no, it's not just a matter of throwing some 'Mystery Oil' into the cylinders for a couple of days and magically everything is OK again.  That is just plain foolish when I hear people say that.  A seized motor can happen relatively quick too, depending on where the motor has been sitting.

So to start, we remove the cylinder head to see down the cylinders and what condition they're in.  This one actually appeared pretty much OK, no scoring on the cylinder walls, and no corrosion build up.  The motor had been lift sitting with the bottom intake/exhaust ports uncovered, which means air can pass through the motor at least on the bottom half of the powerhead.  Both up through the exhaust, or down through the intake manifold.  This is bad.  Sometimes a motor can dry seize, and in this case, filling it full of oil and then giving the pistons some jarring strikes can free it up.  This is a best-case scenario situation though.  Sometimes you can just use a long lever on the flywheel nut and this will free the pistons/crank up.  Wasn't the case with this one.

After giving it my best, it was time to do a full teardown.  You have to disconnect everything (ignition, fuel), pull the powerhead, than split the crankcase.  And guess what I find?  It's full of corrosion throughout.


After removing the crank, you can see the cylinders are badly rusted from the crank side of the motor.  The needle bearings were pitted and ruined.

ph3  ph4 

ph5After fully removing the pistons, it reveals further damage on both cylinders.  This motor is obviously toast and not worth repairing.  The pistons actually came out pretty easily after a few driving blows; the combustion chamber side was still in reasonable shape.  But since it had been stored away in a basement for years, never fogged (oiled), and worst yet, left with the intake/exhaust ports open, this allowed air/moisture to pass through the motor. 

The condensation cycle that happens in this case can start rust in as little as just 24 hours (leave an iron wrench outside overnight and watch all the rust that appears overnight).  Keep in mind these motors are aluminum housings with iron cylinders.  Multiply this by a few months, or even years, and the motor is totally ruined. 

The crankcase and cylinders are the most important part of the motor, which is why you do a compression test on motors before buying to make sure the internals are OK.  Even brand new motors this holds true; over time compression will drop due to normal wear and tear, but if something has lasted 40 years and is OK, then chances are you're going to be OK moving forward.

Pulling the crank and looking at it showed the same abuse.  The journals (the smooth parts that the rods are connected to, must be perfect with no pitting) were ruined.  After removing the needle bearings, it revealed the crank was not salvageable.  The cranks are stainless steel, but that can only tolerate moisture up to a point.

The last picture shows the cylinders from the combustion chamber end again, pistons removed.  Again, the initial inspection of the motor in it's entirety, not just the powerhead, did not reveal any of these significant issues.  So the customer who I accepted this from knows there is a legitimate approach to the offer I made him.  It was priced as a parts motor, and was in fact just that.

  ph6  ph7  ph8

Why Yearly Tune Ups Are Important

Regardless of if you have a 2-stroke or 4-stroke motor, carbon build up inevitably happens.  The power from your motor has to do with the spark strength, fuel delivery and throughput, and compression.  You have control over all 3 factors.  Fuel delivery is pretty simple (clean carb and a good fuel pump).  Compression has to do with overall maintenence of the mechanical components (mostly just enought cooling components i.e. oil and water pump/thermostat).  Spark strength is ignition system maintenence, which is generally clockwork too.

Now the "X" factor is carbon build up, which spans all of these 3 factors, and just builds up in general.  Yes, using 'sea foam' or other fuel additives certainly holds these things as bay, but a yearly tune up insures that you reset your motor, specifically the combustion chamber, pistons, rings, and exhaust side of the motor.  But a de-carb clears all of those components out. 

dirtypiston1  dirty piston2

You can physically clean off the top of the piston by pulling the cylinder head, but that doesn't mean you've freed up stuck rings.  This is an older 18hp motor that was running fine, but had separate issues that required a tear down.  I pulled the pistons and between the top and bottom, there wasn't a huge PSI difference, but when you hold them in your hands, it's pretty obvious where the problem is stemming from. 

Both pistons had minor scoring (but still acceptable compression numbers), but the piston on the left had stuck rings due to carbon build up.  When rings stick, they reduce compression because they don't 'grab' the cylinder walls as well, allowing what is called 'blow by', or simply air/fuel mix to escape during the compression cycle back into the compression chamber. 

This is what makes your motor run OK, but have less top end.  Guess what?  Every internal combustion engine you own behaves this way.  At least electric motors work consistently, and then just stop (i.e. a starter motor, drill gun, or even your blender).

stuck rings

The Simplicity of Compression

Not a long entry, but interesting none the less.  We've talked about the basic principles of motors (whether that be an outboard, weed wacker, chainsaw, lawnmower, or snowblower), but these pics show how the major companies keep production costs lower but yet expan their product line.  The idea is to produce the same block, then modify it to create more product lines.  The reality is that spending money from a business standpoint on R&D, even today with computers and such, cost some seroius denaros.

The older 18-20-25hp motors are all the same (there are internal mods) but the most obvious change is the cylinder head.  By cupping the compression the pistons create more PSI's and gives more power.  For what it's worth, putting a 25hp cylinder head on a 18hp motor creates PSI numbers that would quickly blow up a powerhead, so don't even attempt to do this.  It might be a fun spectical but a complete waste of an otherwise good motor. 

Looking at these pictures, you can see the obvious physical differences in picture 1.  Picture 2 shows how newer models had the number '20' stamped inside the cylinder head to help folks differentiate things if they didn't understand the obvious physical differences.  The secret that most people don't know is the only 2hp increase between an 18 and 20hp OMC motor is the leaf valves. all comes down to fuel supply.  This might be the simplest HP conversion of all motors...ever.  But finding those leaf plates is uncommon, unless you want to pay for P&L with someone who knows what they're doing these days, which would exceed the conversion cost on paper.

18/20a  18/20b

Blown Head Gasket

The head gasket serves the very important function of maintaining a seal between cylinders, confining the compression-detonation cycle between cylinders.  I have had situations where I've gone to bench test a motor only to find it was starting hard, or stalling out at anything much above idle.  After evaluation the ignition and fuel systems to no avail, I check compression to find that seems to be OK, and than finally deciding to pull the cylinder head to make sure there are no internal issues only to find that the last person who laid hands on the motor had forgotten to torque the cylinder head bolts down to the specified ft-lbs.  Well what this does is lets the compression escape out of the cyilnders to adjacent cylinders, or even out of the motor.  A loose spark plug can also do this too.  Remember, it takes compression, spark, air, and fuel to make a motor run.  Any 1 of these 4 components being off, and you're going to have a problem.

Below is a head gasket that was partially blown.  The 2nd picture is another head gasket that separated upon removal of the cylinder head, and it was obviously in need of renewal.  On the first picture, you can see the break in the metal rings between the top and bottom cyilnder, and the wire mesh showing through where the fiber gasket material had broken down and been swept away into the cooling system.  The motor was showing compression numbers below 30 on both cylinders.  I could turn the flywheel by hand with both plugs in with little to no resistance.

Closer investigation of pulling the head revealed that the cylinders looked good, but the gasket had failed.  Replace the gasket and the numbers were restored to 130PSI+, very good numbers for a cold bench test.

I've seen situations where a cylinder head was in fact warped (concave).  So as the motor warmed up, the head would expand and create a very small gap between cylinders, causing loss of power.  It's possible that the motor had gone through a minor overheat at some time prior, as after replacing the cylinder head and gasket the compression numbers evened out and no further problems were observed.

A cylinder head does not need to show any evidence of having been overheated and you can still see low compression numbers.  So when you apply extra heat to aluminum there really is not rhyme or reason to what may or may not happen.  It's best just to avoid that from happening in the first place by keeping up with proper maintenence of your cooling system!

blown head gasket  gasket 2

When to Use Helicoils

Helicoils make sense for a practical application in terms of functionality.  However, applying them to a stripped plug hole could possibly cause issues (unless you're underpropped for a vessle, which would cause excessive RPM's and mechanical failure).  Predetonation is a situation where the combustion chamber is too hot too early, causing fire when it shouldn't be, and almost instantly causing the pistons to get so hot that they melt.

checkit  checkit2

Let's say part of the helicoil breaks off, or say a 'save-a-thread' boot is installed, but the person who does it doesn't bother to pull the head, and some metal fleck sticks to the head of the piston and randomly engrain's itself in some carbon build-up.  Well, that could possible light up like a x-mas tree in about 2 seconds, causing predetonation.  This is an instant catostrophic failure, and the motor will be toast.  One of my first motors did this, and it actually started right back up (after 3 consecutive seizes from overheat...cooling system was fine BTW) and ran fine up to the mid range, then would go into an instant overheat. 

What Happens When You Don't Use Oil in Your Gas

The #1 & #2 ways of killing a 2-stroke motor are running straight gas with no oil, and running it with a malfunctioning cooling system.  The oil you put into the fuel serves as lubrication and a method to reduce friction of moving internal parts, while tolerating the extreme heat produced by an internal combustion engine.  Some of the oil is burned off during this process too, which is why you see smoke along with the fumes produced by combustion of gasoline (which of course causes and explosion and drives the pistons away from the cylinder head).  A faulty cooling system is another story to be reviewed in the cooling system section.

Most outboards are made with cylinders that have iron sleeves (or another strong, heavy metal) which are fitted to an alumimum motor block.  Here is a picture of an older 25hp block with pistons removed, and you can see the 'teeth' of the iron cylinder sleeves fitted into the aluminum black via the intake ports.  I had just fogged this block, which is why you see oil slung all over the cylinders.  This particular motor had a bad crank and required replacement, along with inspection of the pistons and replacement of both rods (why I removed the pistons).   After rebuild, the motor still had very good compression and ran perfectly.


When a motor is run without oil mixed into the fuel, bad things happen in short order.  The pictures below are of a motor (25hp) that had been run without oil.  Keep in mind at low, low idle, most motors are turning 600 RPM, or 10 RPM per second.  That's fast!  The block itself is aluminum and, much like your laptop, cell phone, and desktop computer, aluminum does a pretty good job of dissipating heat.   By the time the block is showing signs of an overheat, you can bet the internals have heated up far more than what you see/hear/smell on the block.

The first picture is of the needle bearings and retainer cage used to reduce friction at the connecting rod end of crank (good condition).  This same configuration is used at the piston end as well.  When no oil is supplied, these tiny metal needle bearings heat up very quickly, because it is metal rolling on metal.  Imagine rubbing your fingers on a piece of fabric non-stop.  You burn your fingers in a matter of a few seconds doing this due to the heat created by the friction.


Here are pictures of the same cage that has been burnt out in a motor where no oil was used. First picture is the cage still installed with the connecting rod cap removed.  You can see the cage actually melted, and the needle bearings melted too, into the cage, and onto the rod.  Picture #2 shows this also once the  crank was removed (which fortunately, survived the overheat without significant damage).  Picture #3 is what's left of the cage and needle bearings laid out for display.  I had to use a small punch to remove these pieces from the rod, which was no longer usable due to scoring, melting, and distortion sustained on the overheat.

2 3

Upon removal of the top piston, it was clear that this powerhead suffered catastrophic damage with the scoring of the cyilnders, pistons, rings, and just general distortion throughout.  Again, somehow the crank survived this mess and crank bearings were OK too on all 5 journal surfaces, surprisingly.  The scoring was so bad in the cylinders I could catch my fingernail on the grooves that were carved in due to grinding metal.  It should be noted that initial testing of the motor revealed 70 & 120 PSI on the top/bottom cylinders respectively.  120 is still OK for this motor, but 70 was clearly indicating a problem.  There was also a 'catch' in the normal turnover of the crank, indicating something mechanically was wrong.  I'm impressed the motor turned over at all, given the top needle bearings on the connecting rod were essentially melted down to nothing.


Pulling a Flywheel...The Wrong Way

If you have to service your older style ignition, you have to pull the flywheel.  This is a relatively easy process, and you can use a steering wheel puller from your local automotive store to accomplish this.  Newer motors (say, post 1976) require a beefier version specifically made for outboards.  You might be able to get the job done with a harmonic balancer (garden variety automotive store puller), but that is a gamble at best. 

Now, knuckleheads don't use pullers.  Correct pullers apply a relatively universal application of physics to a vertical pulling force to the flywheel by inserting 3 screws into pre-drilled flywheel holes, then hitting the flywheel with blunt force suddenly.  The shock wave jars the flywheel loose off the crank taper.  Most people don't realize the woodruff (flywheel) key is NOT what keeps the flyhwheel on the's the taper of the flywheel and crank.

What knuckleheads do is use pry bars or mallets to try and remove flywheels, rather than correct physics.  Don't get me started on Youtube...there is so much mis-information on there, it is about as gossipee as a politial presidential election, which as of this post (1/16), is a pretty common climate.  What using these WRONG methods does is distort the mechanical components of the powerhead, and basically ruin everything.  See what a large portion of the R&D department of these companies do is research the mechanical physics of a sustainable, reliable motor.  I know this because I have 2 degrees in biomechanics (which is the human application of physics).  If you're off by a few foot pounds in engineering, you have a short term ticking timebomb.

That is the difference between big companies and posers.  Here is a crank I yanked from a good motor that was  destroyed by a DIY'er & his neighbor who tried to perform a repair that was beyond their knowledge base.  Fortunately, I saved this good motor by swapping cranks and putting the motor back together the way it is supposed to.  It's still out there putting smiles on it's owners face today.  I saved this motor from being scrapped despite one person's ego quest.

They actually managed to yank the entire flywheel off with exception of the taper.  It is actually more physically difficult to do this, than to pull the flywheel off itself.  They literally ripped metal in half.  When I pulled the crank it was actually in mint condition, but getting this 'knub' off separately would have been nearly impossible and impractical to try and do in terms of labor hours.  I had a spare crank to use instead.

mucked crank

Worn Connecting Rod Bearings

Here we have a situation where over time, or perhaps due to a lean mixture (too little oil) situation, the bearings on the connecting rods (which connect the pistons to the crankshaft) have wallowed out and have some play.  If you watch the video I shot, you'll notice a pop/click noise when the pistons are at top dead center and bottom dead center respectively, but no noise when they are mid powerstroke.

This is because while they are mid-stroke, the 'slack' between the journals on the wrist pins (at the piston) and connecting rod are taken out.  HOWEVER, when at TDC and BDC, this excessive play/spacing creates a clunk, which is brought out by the minor compression you can create even when just manaully spinning over the crank by hand at a crawl-along speed.

This motor ran fine, but was loud as hell at anything much below mid throttle.  The reason is that at 4000 RPM, you could hear the clang/bang of the rods whacking against the wrist pins and crank journals.  If not for switching out the rods, this motor could have very well have blown up internally and been destroyed in the near future.

Broken Crank Shafts

This can happen for any number of reasons, but generally it's 1 of 2 causes.  Either the motor was run without enough oil (overheated), or it was just failure of the component itself (uncommon, nothing you can do about this).  Believe it or not, motors will still run with broken crankshafts!  They will be loud as hell and it will be very obvious something is wrong.  If you turn the flywheel over manually it might present as a clunk or 'sticking' at the same point as the motor turns over.  If the motor is continued to run then it is a guarantee of permenant damage.  Once the cylinders are scratched up from pieces of metal, then the powerhead probably will need total replacement.

Exhaust Mix Build Up & Bad Performance (11/26/18)

Internal combustion engines rely heavily on their ability to 'breath.' In other words, air/fuel mix flow through the engine, the speed, and the quantity dictates power produced. There are of course, several other factors and this is an over simplified concept I am presenting, but fundamentally this is what we're going to consider for now.

Think of a motor as a donut. Fuel enters through one end, combusts, then exits through the other end of the donut. In greater detail, the mix enters via the fuel manifold (leaf plates), passes through the crankcase, continues down the air intake side of the combustion chamber, is compressed and ignited during the power cycle in the cylinders, then the burnt exhaust mix (which still has some unburned fuel) exits via the exhaust ports in the cylinders. It continues along down the exhaust housing, and most modern day motors post 1968 have an internal tube used to tune or accelerate the exhaust leaving the motor, directing it the air flow away from the powerhead.

Remember that when air enters a chamber, and equal amount needs to exit otherwise the chamber is pressurized. Think about blowing air into a soda bottle; you can inflate the bottle slightly with the strenght of your lungs but not much because the soda bottle is rigid plastic. You can, however, inflate a rubber balloon easily but you still see resistance.

In a motor, the throughput of the air/fuel mix makes a big difference. If it encounters resistance this will cause a power drop in the motor because it simply can't breath. Consider blowing air through a straw. Not that easy, right? Well that's because the tube is very small and the air encounters friction against the walls of the straw. Now if you try the same thing on a garden hose, it's easy to blow air through it because the inside diameter is larger.

In outboard motors the air/fuel mix isn't completely burned, meaning some of it is sent out of the motor mixed with water (at least, not in carbureted motors). Over time, and if the engine hasn't been tuned for awhile (i.e. running cold, running rich, etc.), this fuel mix with exhaust cakes up inside the engine. Below are two pictures, the first one shows a 15hp powerhead that had been removed from the lower motor pan and exhaust housing, you can see the long black tube attached to the bottom of the powerhead which is used to tune the exhaust and squeeze more power out of the engine by allowing it to breath better. Well, this motor ran, but was a bit of a dog at the top end and sluggish at the bottom. Closer inspection revealed about a 1/2 inch of caked up exhaust inside the tuning tube, which meant the engine simply could not breath the way it should and was loading up at low RPM and starved at high RPM. A mechanical cleaning of this tube and normal operation was restored. This was caused by not running 'engine tuner' through the motor periodically.

1  2

Here is a video of me running a 70hp, 3 carburetor Johnson.  These motors are sensitive to trim height because they are loop charged.  The beginning of the video shows the motor trimmed high enough where most of the exhaust is out of the water, allowing the motor to breath normally. The lower I trim the motor, the lower the RPMs go, and fully trimmed down it starts smoking like a chimney and you actually hear a couple of lean running 'sneezes.' Well, this is because the motor can't breath because it is almost swamped, and the oil in the carburetors are partially blocking fuel flow. When I trim the motor back up, it starts idling faster and smoother. Pay attention to the audio on the video clip and you can really hear the difference. It should be noted that this sensitivity can be amplified by a cold running motor, or one that is very high hour and getting worn out. The motor ran fine otherwise, but would load up/stumble/choke itself out when sitting so low on this heavy hull boston whaler from the 1960's.

©2019 Riverside Outboards LLC.   All Rights Reserved.