Technical Document Repository


Engine Related Technical Publications

The Truth About Compression Testing
   Compression testing an engine - what it can and what it cannot tell you.
My Bike is "Dyno Tuned"
   What is Dyno Tuning and Do I Need It
Keihin PWK PJ PE FCR CRS Carb Tuning
   How to Tune your Keihin Carb
Is Methanol for Me?
   What and how methanol can do for your performance. Coming soon...


Steering and Suspension Related Technical Publications

Steering Geometry 101
   All about best practices in setting up your steering geometry. Coming soon...


Mull Engineering Technical FAQ

How do I check my spark plug to check jetting?
A   First you must warm the bike up completely which takes about 10min.  That’s 3min of fast idle while blipping the throttle and 7min of easy run time.  After warm up, run the bike hard through 4th or 5th gear almost to redline with the throttle wide open.  We are looking to have the machine under max load not necessarily max rpm.  At this point where you would generally shift, chop throttle, pull the clutch, and hit the kill switch all in one motion.  Coast to a stop and remove the sparkplug.  Please note this is the procedure for main jet check.  If you have problems with the lower end or mid throttle, apply the same procedure to the throttle position in question.  ALWAYS use a brand new plug for accurate readings.  Please note that with certain racing fuels or alcohol, it make take up to 10 additional minutes to get visible color on the plug.  This is normal. 

Q   How do I read the sparkplug?
A   You can get a mountain of information from the plug.  You will look for proper color, and any abnormal wear or erosion of the plug.  The first is color.  You are looking for a medium brown to tan color.  You will take this reading from the rim of the plug where the ground strap attaches as well as the nose of the porcelain insulator.  The rim will color first and the porcelain may take up to 20min to show accurate color in some circumstances.  It is important to also go by how the bike runs as well.  If your plug shows a white or very light color, your jetting is lean.  If you have metallic specs or a light grayish color, then your motor has a dangerous detonating condition and should not be operated until richened or other items looked in to.  If your plug appears black or has fluffy dark deposits, then jetting is rich. 
After determining the color and condition of jetting, you will need to look for any erosion of the insulator or ground strap.  This will indicate a detonation condition caused by a low octane fuel being used, lean jetting, or ignition advance issues.  This condition is very serious and can end a motor’s life in seconds. 

Q   What is detonation and/or  pre-ignition?
A   These are both called uncontrolled spontaneous combustion and occur in the combustion chamber when fuel/air mixture is spontaneously ignited without spark.  If the mixture lights before the proper time, this is called pre-ignition and is generally caused by too much heat in the engine from a lean condition, cooling, a hot spot on the head or piston from carbon or pitting, too high heat range of the spark plug, too much ignition advance, or too low octane fuel.  The other type of spontaneous combustion is detonation and occurs after the sparkplug has fired and is generally caused by a hot piston crown on the exhaust side, improper cylinder head clearances, or squish velocities.  The sound that is produced is called knock and can be lethal to the engine.  This sound is two percussion waves colliding with each other in the combustion chamber.  Ideally the fuel mixture should burn centered at the top of the piston and push down.  If another wave occurs on the edge of the piston from detonation lets say, then, when the waves hit each other, it causes a severe rise in temperature and pressure in the cylinder as the two explosions hit each other.  This will eventually cause your engine to melt down or cause rod bearing failure. 

Q   What type of fuel should I use?
A   For most applications, premium unleaded is fine.  For a tuned motor with higher compression, ignition advance, and porting, then higher octane fuels will generally be needed to suppress detonation.  Octane is a rating of a fuels resistance to pre-ignition.  There are three ways in which a fuels octane values are calculated.  First, motor octane number, this is generally the lowest number offered because it involves using a test engine that increases compression to the point of pre-ignition on a given fuel.  This number is the most accurate to real world situations.  The next number is research octane. This number is a calculated number by evaluating the contents of chemicals in a given fuel.  Motor and research octane values always vary because of the different calculation methods.  The third way and the most common in the US to calculate fuel octane is the R+M/2 method or the average of the two numbers.  Always make sure of how your fuel in calculated.  You want to use as low of an octane fuel as possible without bringing in pre-ignition or detonation.  This is because as you raise a fuels octane number, you are effectively slowing the burn rate to a degree.  This is why many race fuels and alcohols require more ignition advance to compensate for the slower burnt rate. 

Types of fuels vary greatly by manufacturer so never mix them.  Most race fuels are synthetic blends and pump gas is a petroleum based fuel.  Alcohols are made from many sources, but corn is the most common.  Race fuel can generally offer better performance on even stock engines because of the more complete burn.  This is because of better quality controls in race fuels. Race fuels also contain vary few additives and detergents for cold weather and such that take away from the total parts of mixture burned in the combustion cycle.  Race fuels and alcohols also make more power by making an engine run cooler.  This is because race fuels generally have lead in them to raise the octane number and lead also helps to lubricate and pickup heat and transfer it out of the engine.  Alcohols are hygroscopic fuels meaning they pickup water out of the air.  Because they always contain water, this water turns to steam in the combustion chamber and collects heat in the same way as race fuels.  It may be required to use a different heat range of plug to retain more heat with some race fuels.

Q   If I change fuels, will I need to re-jet?
A   It is always best to check jetting with every fuel change.  This is because the amount of fuel that actually burns plays a big part in jetting.  There is always a percentage of fuel that does not contribute to combustion like stabilizers and other additives.  Other factors include the specific gravity and density of a fuel.  If a fuel is more or less dense than a previous fuel, it will throw your jetting.  The biggest change in jetting comes in when you switch to alcohol.  The most common alcohol used is Methanol for race engines because it offers better performance that Ethanol.  Methanol contains 49.9% Oxygen and Ethanol only contains 34.7%.  This means you can put more Methanol to a motor without running into a rich condition.  The Oxygen content is why you MUST rejet and generally switch carbs to run alcohol.  You must put about 3 times the amount of fuel in to get a proper burn.  This is good for horsepower, but bad for fuel consumption!  You cannot just switch from Methanol to Ethanol either because of the differences in Oxygen content.  To sum up, never mix different fuels or oils and always check jetting with every fuel change or oil ratio change. 

Q   Will more compression make my bike faster? 
A   In most cases in two strokes it will offer some added performance, but not from the compression alone.  By either decking the head or top of the cylinder, you are bringing the head to piston clearance or “squish clearance” down.  This forces more of the fuel/air mixture to the center and away from the hot edges of the piston for a more compact and turbulent charge in the “dome” of the head.  This offers better combustion and a more centered burn on the piston.  This clearance reduction actually reduces the chance of detonation by keeping the fuel charge from the cylinder walls and exhaust edge of the piston, where all the heat is.  This is primarily where detonation will occur.  More compression will help primarily with bottom end power, but if too much compression is added with the wrong porting, your motor will end up working against it self and max rpm will not be achieved.  IA 230psi static compression with stock porting will not equal a speed demon only a heat demon.  Another thing to look at in added compression is heat.  Anytime you raise compression, you will gain more heat.  Fuels can pre-ignite from the heat as well as the added pressure.  A good rule of thumb is 160-175psi static compression should be ok with stock porting and ignition timing.  Any more modifications should see race fuel to handle the extra compression.  Keep in mind that ambient temperature, humidity, elevation, etc, all have an effect on how much compression you can run before you run into detonation. 

In regards to four strokes, compression has a more noticeable affect on power mostly because four strokes run a much longer power stroke and a stronger explosion has more time to act on the piston.  It is always advisable to raise compression on four strokes, but if cams are added, you must be careful of piston to valve clearance issues.  You generally will not deck a head or cylinder on a four stroke because you will throw off the cam timing and timing chain tensioner specs.  High dome pistons are the most common way of increasing compression and most manufacturers offer a standard and high comp. piston that requires race fuel. 

Q   How do I tell if my bike is over heating?
A   Well the easiest way is to install a heat gauge.  Either digital or analog will work.  We sell a digital model that will mount anywhere and has a back light for illumination during the night.  It costs 160.00 installed and could save hundreds in repairs.  Many bikes we see have been overheated and the sure sign is a severely scorched exhaust skirt on the piston on two strokes and gaulded cams and burned valves in four strokes.  Most bikes around our area in Kansas see a lot of time in the dunes, and the dunes can KILL motors!  This is because it takes much more power to turn your tires in sand and this in turn causes heat.  Couple this with an average speed of about 15-20mph, and this will cause certain overheating in any built motor.  Generally you can feel an engine “fall off” on power when very hot and this is because when the fuel charge heats up as it touches the engine internals, it expands thus making itself less dense.  This is not good for horsepower.  We want the fuel charge as dense as possible.  We have marked not only good engine protection with our gauge, but an edge in drag racing.  This is because a hot motor is a slow motor.  By simply watching your water temp go up and down, you will know when to sit on the side lines and wait for horsepower to “build”.  With this mod., you can rest assured of no more failures due unknown temperatures. 

Q   Will a Coolhead keep my engine cooler?
A   Not necessarily, the Coolhead or comparables only hold more coolant for more heat absorption in the head and no where else.   In order to dissipate this heat, you need more airflow or a better cooling system to cool the water more efficiently.  The majority of heat is concentrated around the exhaust port anyway so basically it will take longer to get hot, but generally will only run a couple degrees cooler once sustained equilibrium temperatures are reached.  Coolheads do, however, generally offer an improvement over the OEM stock head design with better squish angles for a more efficient head.  However, the OEM head can be machined by Mull Engineering and a few others to perform as good or better than a Coolhead. 

Q   Is it normal for modified engines and stock engines to foul plugs constantly?
A   No!  This is a sure sign of a jetting problem.  More than likely, the jetting is rich.  Please note that jetting could be off in a certain throttle position causing the fouling.  If you have a 1/8 to 1/4 throttle problem say, never change the main jet to try and correct this or you could damage you motor.  If you don’t know, get with us or someone who does.  Engines of extreme tune still should not foul plugs, but it is a good idea to change plugs every 10-20 hours of use.  If you have two or four carbs, it may be necessary to sync the carbs to get rid of the fouling problem.

Q   What type of two stroke oil should I use and what ratio?
A   Well that really depends on what kind of engine and fuel you are using.  Generally the manufacturer’s oils are sufficient for stock trim motors.  We have found yamalube Y-R to be a very good oil at a good price.  There are many types of oil blends but mainly four types of oil compositions.  First, the petroleum oils which are pretty much obsolete any more offer great lubrication, but at the price of too much carbon deposit in the motor and a lot of smoke.  The second is a semi-synthetic.  This type of oil is the most common from OEMs and offers some petroleum properties and some synthetic properties.  These oils are very good and offer great scuff resistance and low carbon and smoke.  The most common of these types are Yamalube Y-R and Hondalube H-2.  They offer the best of all worlds including cost.  The next oil is the full synthetic.  This oil is 100% lab engineered and carries none of the petroleum properties.  These oils can generally run at very lean ratios with good protection and will almost not break down under extreme heat and will leave an engine looking almost new inside.  The next in line is the vegetable or castor bean oils.  These oils have shown to offer the best in scuff resistance in high output motors, but can gum up and cause ring sticking over time.  These oils also are some of the only oils that will mix with alcohols. 
So which is better?  Well they all have their strong points, but generally the bean oils win simply because of the better protection and increased ring seal.  Bean oils do have a down side. They generally need frequent agitation to stay suspended in fuel and can fall out of solution in cold weather.  This is why full synthetics have been very popular in motocross.  They offer great protection, but generally like the leaner ratios which can lead to a slight drop in ring seal meaning loss of power.  I am convinced that the positives override the negatives with synthetics for a recreational machine.  The Y-R offers near the same amount of protection and works with rich ratios well so for stock motors, it is generally is the way to go.  Oils change frequently so always know what you are buying and call the manufacturer for compatible fuels.  So to sum up, we prefer the Y-R semi synthetic in stock to light mod engines because of the low carbon properties of the synthetic and the anti scuff properties of the mineral base and the price.  For alcohol bikes we are using the Klotz oils right now.  The super techniplate bean/synthetic seems to offer great resistance to wash down which happens a lot because of the huge amount of fuel being delivered to the engine. 
So what ratio?  Now that really depends on what oil.  Synthetics run as lean as 50-60:1 and petroleum based oils as rich as 20:1.  Contrary to most opinions, actually the more oil the better in a two stroke.  This offers better ring seal, better power, more protection, and better cooling.   Generally the Y-R runs good at 32:1, but the heavy mod motors sometimes need about 26:1 to keep from scuffing.  We run the alcohol oils rich at 22-24:1.  This is again because of cylinder and piston washing.  Synthetics seem to do there best at the recommended richest ratio.  Generally about 45:1.  This is great for the stockers or recreation motors.  No matter what oil you use, never mix different oils!  This can cause you major problems and expenses if dissimilar oils make each other fall out of solution.  And as a general rule of thumb, the longer you hold the throttle wide open, the more oil you should have in the fuel. IA motocross bikes should run leaner oil ratios while road race and flat track bikes will need much richer ratios to survive.

Q   What type of air filter is the best for my motor?
A   This is one of our most common question and reasons for engine failure so listen up!  Most people really like the pleated gauze type filters like the K&N.  This type has been well tested and proved to flow more air than stock.  However, once dirty, they can lose as much as 40% of their flow capacity and actually restrict the engine.  The major problem with these filters is there inability to catch the fine sand particles found at sand dune riding areas.  They are however very easily cleaned and maintained and will last many years.  Not forever though!  This statement comes from street type filters which see drastically less dirt and cleaning.  Oversizing the gauze filter is usually recommended for offroad applications to compensate for the lack of flow when dirty.  Mull Engineering uses a custom made gauze filter that has additional pleat layers to increase the filtration ability. 
The next type of filter is the foam filter.  This is actually the best you can buy for protection.  This is because these filters have a network of small holes that interlace but don’t  connect directly causing particles to become separated from the air as it hits the filter.  This would be much like a maze for particles to travel through so plenty layers of protection to catch particles.  The pleated styles only have one chance to stop particles and this is rarely enough for off road vehicles.  The foam filters also flow comparably to pleated styles when new and clean, but lose less of their efficiency as they get dirty making them the real off road winner.  Which ever way you go remember that failure to maintain your filter WILL result in severe engine damage.  Damage is generally more severe in two strokes because the air that goes through the filter passes over the crank shaft and main bearings where particles can become lodged as well.  If your filter looks dirty, then it is!  Even if you only have a couple of hours on a fresh cleaning. It is not uncommon to see racers changing filters between events. 
As long as either are kept clean and oiled properly, they should both offer good protection and better performance. Take special note that clean filters save engines. 

Q   How do I care for my air filter?
A   For pleated gauze type filters, you want to lightly bump off the loose soil, then wash in warm soapy water.  We use dish soap.  Never use solvents or gasoline.  This will destroy the filter and then your engine if used.  It may take two bowls of soapy water to get it clean.  You want to swash the filter around like the agitation cycle in a washing machine.  Be careful not to scoop the soil out of the water and into the inside of the filter so as not the further contaminate the inside.  After the warm soap bath, run a cool bath and then just run under cold water.  Then sling the filter to get most of the water out.  Never use heat or compressed air.  If you do, replace the filter!  Simply let the filter dry in the sun or a dry place and spray with a pleated type spray oil lightly when dry.  Don’t coat to the point of saturation, this will only shorten the cleaning intervals.  Always use a pre filter screen the help block the bigger particles.  Always insure good seal on the air box and check that all connections are tight.
For Foam style filters, you will wash in the same order with soapy water as above, but you will need to dirty your hands and squeeze the filter to get the particles to let go.  Generally the foam filters are a two stage where the out filter is dry so don’t wash them together and don’t apply oil to the outer filter.  If oils get on the outer filter, it will cause it to collect too much dirt.  Foam filters require a special type of oil for foam but transmission oils can be used in an emergency.  You want to pour the oil on the inner filter and wring out by hand. 
With either application, you must apply oil.  We have seen many engines fail from a lack of oil on the filter so remember that when in doubt, too much oil is always better than not enough.

How do I break in my new or rebuilt engine?
This really depends on many factors so we will go through steps for new engines from OEM and fully rebuilt engines.  OEM engines are run minimally at the factory and are never fully broke in.  Anytime a new or rebuilt engine is started, it is best to bring it up to operating temps (around 10min) keeping the rpms around 2k, then allowed to cool to room temp.  This allows the first uniform expansion of heated parts and allows stress areas to “relax” and seal better.  Also a great time to re-torque bolts and check for leaks.  The reason we keep the rpms up is to allow better lubrication and in four strokes, and reduce cam loading or face loading on the lobes which can cause cam lobe flattening and valve lash to get out of spec. 
      After the initial start up and cool down, you may ride but cautiously at first.  Part throttle should be used for the first 20min of ridding.  Afterwards you may allow short bursts of full throttle but do not allow the engine to reach full rpm.  Select taller gears and allow the engine to “tractor”.  The larger throttle openings put more force on internal parts and force a firmer ring seal.  The reason we don’t want max rpm is because when an engine is fresh is has many micro machining errors that the reciprocating and rotating parts have to rub off and smooth through normal wear.  The rough edges are most easily seen on newly bored or hones cylinders.  These errors or rough edges increase friction thus increase heat in an engine.  So to sum that up, new engines run hotter!  Allowing the engine time to smooth these rough edges in will allow tolerances to stay tight and your engine will last longer. 
The “break it in like your going to ride it” or “engines that are not broke in are faster” theories are easily disproved through real physics and engineering data.  If an engine is not broke in properly, it will overheat and glaze the cylinder walls causing a lack of lubrication since the scratches in the cylinder walls and piston are there to retain oil for lubrication.  Another consideration is the small edges we talked about earlier may be ripped off rather that lightly filed down causing an out of tolerance engine.  We have seen engines with only a few hours run time that are simply worn out due to a lack of break in. 

            In two strokes, we do not advise to add more oil if you are planning to do jetting because it WILL throw off your jetting.  In general it takes about 1.5 hours or 1 tank of fuel for an ATV or dirt bike engine to break in properly.  In street  bikes, we have seen as much as 2-3k miles to break in because of very tight OEM tolerances.