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Moto Mind is a technical blog written by Paul Olesen who is a powertrain engineer working in the motorcycle industry. The blog covers a wide variety of topics relating to two and four stroke engine performance, design, and optimization.

## Filling Up At The Pump

How Residual Pump Fuel Affects Your Fill Up
This week I have a quick tip I want to share with you regarding buying fuel and filling up gas cans for your bikes. I know many of you, myself included, rely on premium grade gasoline dispensed from local gas station pumps to put endless grins on your faces. One of the downfalls of gas station pumps is that fuel from the previous sale is left in the hose. According to the American Petroleum Institute, the amount of fuel left in a gas pump's hose is around 1/3 of a gallon.   Generally speaking, when two fuels are blended the octane rating of the resulting fuel is approximately the average of the two fuels. So if you had a gallon of 87 octane and a gallon of 93 the resulting blend would have an octane rating of 90. I'll be the first to admit that 1/3 of a gallon of fuel added to a two gallon gas can won't have much effect on the octane rating. For those of you that like numbers, 0.33 of a gallon of 87 added to 1.67 gallons of 93 will yield the following octane rating:   0.33 gallon of 87 / 2 gallons = 16.5% of the total mixture
1.67 gallons of 93 /2 gallons = 83.5% of the total mixture   (0.165 x 87) + (0.835 x 93) = 92 octane blended fuel   So in a two gallon can, the octane rating of the fuel has dropped a point due to the 1/3 gallon of 87 in the pump hose. Unless you have a very well developed performance engine, this isn't anything to lose sleep over. I think a bigger reasons to want to keep that 1/3 of a gallon out of your can is due to the possibility of ethanol being in the hose from the previous sale. Many articles can be found outlining why ethanol should be avoided, but the main reasons include part corrosion due to the exposure to alcohol, rubber seals and o-rings may not be compatible with ethanol resulting in swelling and failure, and some plastics deteriorate when exposed to ethanol. Not to mention ethanol contains less energy than gasoline. Again, we're not talking about a large percentage of ethanol in the overall scheme of things but I prefer to stay away from the stuff when I can.   Fueling Tip
I also wanted to invite you to check out my book on how to build four-stroke engines, which is now officially available in print form. It took a ton of work to bring the print book together and get the right help on board. The project hasn't been easy, but I'm proud to offer this book to you and can assure you it will make a great addition to your workshop. You can learn more about the book by following this link: The Four Stroke Handbook   To celebrate the arrival of the print book, I'm running a sale until the 27th of September offering all versions of the book at a 20% discount. After the 27th the sale will end and the price will go up. If you've got a build coming up now or in the future and are interested in the book, now is a great time to pick up a copy.   Thanks for reading and have a great week!
-Paul

DIY Moto Fix

## Leak Down Testing // Pt 2

1. Make sure the petcock is turned off and remove the fuel line from the carburetor or throttle body. Use a rag to catch any fuel draining from the line.   2. Remove the seat and fuel tank from the bike.   3 . Remove the radiator cap and pull out the crankcase breather tube.     4. Remove the spark plug cap . Prior to removing the plug blow compressed air into the plug cavity to rid it of dust and debris so that it can’t get into the engine. Then remove the spark plug.     5. Remove the crankshaft hole cap cover so that you can gain access to the crankshaft.     6. If working from the clutch side of the engine, rotate the engine over so that the piston is just shy of TDC (approximately the width of a punch mark off) on the compression stroke. There are usually alignment marks which can be used depending on the manufacturer to help gauge how close to TDC you are. On the Honda pictured there are punch marks on the crank and balances shaft gears denoting TDC. As you rotate through to just shy of TDC feel for compression building up in the cylinder. As you turn your wrench, resistance should build up as you approach TDC. If you do not feel resistance, rotate the engine over once more and realign to just shy of the TDC markings. This will take you from the exhaust stroke to the compression stroke where you should feel the compression start to build.     If working from the flywheel side of the engine, rotate the engine over so that the piston passes TDC by around a ¼ of a rotation of the crankshaft. Reverse direction and set the crank so that it is just past TDC using the applicable alignment marks. On the Kawasaki pictured you can see the alignment slots. As you came up on the compression stroke you should have felt a little bit of compression build up. If you do not feel resistance, rotate the engine over once more and realign to just shy of the TDC markings. This will take you from the exhaust stroke to the compression stroke where you should feel the compression start to build.     7. Install the leak down tester in the spark plug hole with the regulated pressure set at 0psi initially.       8. It usually takes a breaker bar and two hands to lock the crankshaft from moving once the cylinder is pressurized. This is where having an extra set of hands helps immensely. One person should focus on keeping the crankshaft in position while the other operates the leak down tester. Remember to set the piston just shy of TDC (whichever side is applicable for your application) and to lock the crankshaft in place while the piston is still traveling upwards so the rings sit in the bottom of the ring grooves.     9. Once the crankshaft is locked in place pressurize the cylinder. Slowly turn the pressure regulator up to 100psi.     10. Note the reading of the outlet pressure gauge. This corresponds to the amount of air the combustion chamber is retaining. If both pressure gauges read 100psi when they were calibrated the difference in pressure on the outlet gauge is the cylinder leakage. For example if the outlet gauge reads 95psi the cylinder leakage would be 5%.   If however during calibration both gauges did not read 100psi then the leakage will need to be calculated. For example if the outlet pressure gauge read 97psi during calibration and 92psi when the combustion chamber was pressurized, then the leakage could be found by dividing 92 by 97.   92 ÷ 97 = 0.948   0.948 x 100 = 94.8%
100 - 94.8 = 5.2% leakage   11. Open the throttle and listen for an audible hissing sound coming from the intake, exhaust, or crankcase breather. Also look in the radiator for air bubbles.   Here are the four problems that can result:   1. Air passing through the intake indicates leaking intake valves.
2. Air passing through the exhaust indicates leaking exhaust valves.
3. Air passing through the crankcase breather indicates worn rings.
DIY Moto Fix - Empowering and educating riders

## Save The Salt

I want to switch gears with this post and bring your attention to an issue that is near and dear to myself and many land speed racers. Over the past decade salt mining has caused track conditions at the Bonneville Salt Flats to become worse and worse. Years ago, the salt layer was up to 5 feet thick, today it is less than an inch in some places. Salt mining is partly to blame for the depletion of salt at Bonneville and holding racing events there is becoming extremely hard!   I know not many of you here on TT may participate at, or have ties to Bonneville, but this is great opportunity to band together as powersport enthusiasts and help out those that do. Please help your fellow motorheads by signing the following petition which asks for salt mining to stop at Bonneville. I've signed and hope you do too!   Save the Salt Petition   The salt flats at Bonneville are a special place and it would be unfortunate to not preserve them for future generations. Thanks everyone!   Paul

## Leak Down Testing // Pt I

DIY Moto Fix - Empowering And Educating Riders From Garage To Trail

## Cam Chain Wear and Replacement Tips

I hope you all are getting out on your bikes a lot this summer and are putting some time on them! This week’s post is dedicated to an engine part that is often overlooked, its importance not totally understood, and its service specs minimal. I’m talking about the timing chain. I want to discuss and share with you some signs that the cam chain is worn out.   Just like the drive chain, timing chains elongate, fatigue, and wear out. Luckily, they are not subject to dirt and mud, are bathed in an oil bath, and their overall environment is much better. Before I get into it, one misconception I want to clear up right away is that the timing chain doesn’t technically stretch. Instead, the pins and rotating elements of the chain wear. When the pins wear they become smaller and their mating holes grow larger leading to increased clearances and chain length.   When an engine is run with a worn timing chain engine performance is compromised and the likelihood of related failures is greatly increased (think chain tensioner). Cam timings that are off several degrees will result in a loss of power and the cam chain tensioner will have quite a job trying to take slack out of the valvetrain. When a timing chain elongates it may not do so in a uniform way and parts of the chain may be tighter or looser than others. While automatic cam chain tensioners have proven to be reliable on the majority of engines, some model years, brands, and individuals have fared better than others. A worn timing chain which adds extra slop and inconsistent chain tension to the valvetrain certainly won’t make the tensioner’s job any easier. So it makes a lot of sense to keep tabs on the condition of the chain itself from time to time.   When I was working on a Kawasaki KX250F engine build I took the time to do some comparisons which illustrate the differences you will see between a new and worn out cam chain. First, with the worn chain installed I checked the cam timing. Then I installed the new chain and rechecked the timing. In the table below you can see the intake cam timing was retarded by 6.625° and the exhaust by 9.50° when compared to the new cam chain timing values. For the average weekend warrior this may not seem like much but in terms of performance engines this is miles off the mark!
Next I looked at chain elongation. With both the new and old chain lying on the workbench it is easy to see a noticeable difference in chain length. The old chain is around 5mm longer than the new one.     You can also easily see how much more flexible the old chain is in comparison to the new chain as well.     These comparisons are all well and good but when do they become practical? Good question, since most of you won’t be checking the timing or removing the chain to compare it to a new one. Unfortunately, very few service manuals provide specifications or guidance on cam chains apart from the “inspect and replace as necessary” phrase commonly found throughout manuals. You may get lucky and find a pin to pin measurement spec you can use from time to time but it is not the norm. My suggestion is to document the applicable attributes of a new chain the next time you have one before installing it into your engine. This way you’ll have some tangible specs to compare to as the chain wears.   Method 1 - The Pin to Pin Measurement
One of my favorite methods to gauge chain wear is to compare pin to pin measurements of new and old chains. To do this you will need to know the total number of pins on the chain. It is easiest to count the number of pins prior to installing the chain, however, pins could still be counted with the chain installed in the engine by marking one of the links, rotating the engine around, and counting.   Once the total number of pins is known a measurement across a set number of pins is taken with the chain installed. I like to span around 6 - 8 pins between the sprockets on a twin cam engine. Unicam engines are trickier and the number of pins you can measure is usually less. It is important to try and measure across multiple pins because the variation between new and old chain measurements will be more pronounced this way. With the new chain installed I measure 1.846” (46.89mm) across eight pins. This is my benchmark measurement and is what I will compare all future measurements to.     Over time, the chain will wear and it will elongate. To keep tabs on chain wear it is never a bad idea to check the condition of the chain whenever you check valve clearances. In the image below I replaced the new chain with the old and repeated my pin to pin measurement across eight pins. This time I measure 1.860” (47.24mm).     The difference in length between the new and old chain can be seen in the table below.
A chain length increase across eight pins certainly isn’t earth shattering at first glance but that is only a fraction of the chain. To get a better idea of the total chain length increase and the severity of the problem some math is required. My cam chain totals 114 pins and since I only measured eight pins I will need to divide 8 into 114 to determine my length multiplier.   8 pin segments which fit into a 114 pin chain = 114/8 = 14.25   Once I have determined how many 8 pin segments fit into a 114 pin chain I can multiply this value by the change in length between the two chains to determine the total length the chain has increased.   Total Chain Length Increase (inches) = 0.014" x 14.25 =0.200"
Total Chain Length Increase (mm) = 0.36 x 14.25 =5.13mm   The calculated increase is accurate since this is about the difference in length I saw when I laid the chains side by side and measured them. If you aren’t familiar with what changes in length are acceptable this example can be considered one on the far end of the spectrum. The engine had been seriously neglected and wasn’t in real good shape.   Method 2 - What Does the Chain Tensioner Say?
The second way you can observe chain wear is by comparing cam chain tensioner plunger position throughout the chain’s life. To do this the tensioner is temporarily installed, set, and the engine turned over at least four times to allow the tensioner to self tension. Once the chain tensioner has absorbed the chain slack it is removed and the plunger position is noted. Remember to either install the chain tensioner stopper tool or remove the center bolt and spring, depending on tensioner design, so that the plunger doesn’t extend as the tensioner is removed.   In the image below I marked the plunger position with the new chain installed. As you can see the plunger is hardly extended and the second tooth on the plunger is engaged.     As the chain wears the plunger will have to extend out further and further to take up the slack. When I set the plunger with the old chain installed the ninth tooth was engaged on the plunger. Observations like this can be used to gauge cam chain wear and to determine when the chain should be replaced.     Method 3 - Alignment Marks and Feel   Every engine will have specific marks designed into components to aid in the timing of the engine. These marks can also be used to get an idea of the condition the timing chain is in. When correctly timed all alignment marks should be positioned nearly perfectly. As you can see in the image of the Kawasaki engine the two punch marks on the cam gears are aligned with the machined surface of the cylinder head. Both punch marks are visible and in the correct spot.     As the chain wears the tensioner will absorb slack and the cams will retard. In this image with the old chain installed the exhaust cam punch mark is far too high in relation to the cylinder head machined surface and the intake cam is not even visible. This is a good indication that the chain is worn and should be replaced.     Lastly, a basic feel of the tension of the chain between the cam gears can be performed if the engine is of twin cam design.
Here, with the old chain installed there is roughly 6-8mm of slack when I pull up on the chain.     Pushing down I get around the same 6-8mm of slack.     With the new chain installed I get around 3mm of slack when I pull up.
Finally, pushing down I also get around 3mm of slack.     I hope this write up gives you a few ideas of how you can gauge cam chain wear and that it makes you consider the condition your chain is currently in. I know these methods require a new chain to obtain quantifiable measurements, however, even if you don’t currently have anything to compare your chain to, this write up can be used to give you an idea of where your chain is at in its life. The examples I have provided are at the two extremes of the spectrum, being brand new and severely worn, and anything you encounter should fall between them. For those of you replacing your chain in the near future be sure to take a couple measurements so you have useful info to go off of down the road!   If you have additional tips and tricks relating to cam chain wear I’m all ears and am sure the TT community would love to hear them. For those of you who want to know even more about your engine and are performing your own maintenance check out my book on engine building by clicking here. The Four Stroke Dirt Bike Engine Building Handbook is full of practical engine building knowledge you can use on your next major or minor overhaul.   -Paul Olesen
DIY Moto Fix - Empowering And Educating Riders From Garage To Trail

## How To True A Dirt Bike Wheel Yourself

In my last blog post I covered how to lace up a wheel assembly with new spokes. This week I’ll discuss how to properly true the rim. Truing the rim is actually not too difficult. Once you understand the interaction between the spokes and rim, you will make quick work of the job.   To get started a truing stand of sorts needs to be set up. This doesn’t have to be anything special and I used a bench vice, adjuster block, rear axle, spacers, a series of old bearings and washers, and the axle nut. The reason I went to the trouble of clamping the hub in place was to eliminate any possibility of the hub sliding back and forth on the rim, which would make my truing efforts difficult.     This is by no means the only way to create temporary truing stand and you can use your imagination to come up with alternatives. Temporarily installing the wheel back into the swingarm may work equally well if you don’t have a bench vice.   Next, some sort of gauge will be needed so the amount of runout can be seen. I used a dial indicator attached to a magnetic base, however more simple solutions could easily be fabricated.     It isn’t absolutely necessary to measure runout, especially right away when major adjustments may need to be made. Instead you only need to see how the gap between the end of the pointer and rim changes as the wheel rotates. A coat hanger, piece of welding rod, or even a pencil could all be used to the same effect as the indicator shown.   Axial (side to side) runout will be corrected first. Here you can see there is a noticeable difference in gap size between the rim and pointer through a full revolution of the rim.     The goal is to tweak the tension in the spokes so that the gap between the rim and pointer is even as the rim is rotated.     To do this the gap can either be increased or decreased depending on which spokes are tightened or loosened. To decrease the gap, tighten the spokes originating on the side of the rim where you want the gap to decrease. In the previous photo I’m tightening the right side spokes, and in doing so I am pulling the rim to the right. An ⅛ to ¼ turn of the nipple is enough to induce a change. For the given area of the rim that must be pulled over, evenly tighten at least three of the surrounding spokes on the side being pulled. If the rim needs to shift a lot, loosen the opposite side spokes the same amount you have tightened the pull side spokes. This will help keep even tension on all spokes and help to shift the rim.   The process of tightening and loosening the spokes to pull the rim from side to side can be performed at all the high and low points surrounding the rim. Continue to turn and rotate the rim around until the gap between the rim and pointer evens out. Some areas may require tightening the spokes and pulling the rim one way while other areas may need to be loosened to allow the rim to move back the opposite way. Take your time and make small changes as you go. As I mentioned before, it doesn’t take much to see a significant change in rim location as the spokes are tensioned.   As the rim is fine tuned for side to side runout, the pointer can be moved closer to reduce the gap. Reducing the gap as the rim is trued will make it easier to see smaller differences in runout. To really fine tune things I like to use a dial indicator, setting the contact point up on the outer edge of the rim. Again, this isn’t absolutely necessary and similar accuracy could be achieved with a simple pointer.     Here I’ve snapped photos of the high and low points on the rim. The total runout is the difference between the high and low points. In the left picture the needle is 0.0075” (0.19mm) to the left of my zeroed point. In the righthand picture the needle is 0.008” (0.20mm) to the right of the zero. This gives me a total runout of 0.0155” (0.39mm). Most service manuals suggest a max runout of 0.079” (2mm) so I’m well within spec! Quite frankly I was very pleased to get the rim to 0.0155” since the rim is old and slightly dinged up.
The rim I was working on is centered on the hub. Some rims will be offset and it will be more important to pay attention to the relationship between the edge of the rim and a feature on the hub (usually the brake disc machined surface or the machined surface for the sprocket). Your service manual will provide specs for measurement points and specify how much offset should be present. Setting the offset correctly is important because if the offset is off, the front or rear wheel will not be inline with the other wheel. This can make the bike's handling very interesting! I don’t think a little misalignment is too noticeable on dirt, but it is definitely a problem on asphalt.   A straightedge can be used to measure from the indicated surface, outer edge of the sprocket, or brake disc to the edge of the rim. If measuring off the sprocket or brake disc, you’ll need to subtract the thickness of the sprocket or disc from your measurement.     If the rim is not quite positioned right after all the side to side runout has been corrected, it can be shifted at this time. To pull the rim one way or the other, simply evenly tighten all the spokes on the side you are trying to pull the rim to. The opposite side spokes can also be loosened to help allow the rim to shift over. Once the rim is set where it needs to be, half the battle is over!   Next, the radial runout must be corrected. To do this move the pointer so that it sits past the outer edge of the rim.     The gap between the pointer and outer edge of the rim will be monitored and tweaked to achieve evenness throughout the rotation of the rim.
DIY Moto Fix - Empowering And Educating Riders From Garage To Trail

## How To Properly Replace And Install Spokes On A Dirt Bike Wheel

How many of you become disheartened when spokes break, bend, or a rim becomes permanently damaged necessitating a rebuild of the wheel? I know a lot of people think rim building is a black art and are willing to shell out serious dough to avoid the job altogether. This week I want to debunk the black art of wheel building and provide you with an overview of the process, allowing you to take on your next wheel build yourself. Next week, I’ll cover the second half of the project by showing you how to true the wheel.   As you can see I have a great example of a wheel assembly that is way past its prime. The spokes are bent, loose, and the nipples are mostly all stuck. On top of that, the rim is cracked in a couple spots necessitating further repairs.     Before getting started disassembling the wheel, measure the distance from the rim to the ground. When the wheel is built the rim will need to be blocked up at approximately this height. Blocking the rim up will make the wheel much easier to assemble.     The spokes will be offset from one another. Often times this offset necessitates the use of different length spokes. The spoke kit I received came with two different length spokes and there was no indication of which went where. If there are no instructions provided with your spoke kit and your wheel features spokes of different lengths you will need to determine the correct layout of the spokes. This can easily be done by removing two of the old spokes, measuring them, noting their lengths, and positions.     Once you have determined the spoke length you can go to town cutting the rest of the spokes out of the rim using a cutting wheel or other suitable tool.     Remove all the old spokes, then closely inspect the rim for damage. On my rim I had two nice size cracks I had to deal with.     Once the rim has been replaced or repaired, preparations for lacing can begin. Since the wheel will be exposed to dirt, mud, water, and whatever else nature throws at it, I like to coat all my spokes with anti-seize before assembly. The anti-seize will provide a little extra protection against corrosion and help keep the spokes turning freely for a long time.

## How To Repair Your Clutch Basket Dampers For Less Than \$30

DIY Moto Fix - Empowering And Educating Riders From Garage To Trail

## Precision Measuring For The At-Home Mechanic // Part 1

This week I want to provide you with some in-depth knowledge on the world of precision measurement. As at-home mechanics who want to take their rebuilding skills to the next level, learning about precision measurement and how to properly use precision tools is the final frontier. This post comes right out of The Four Stroke Dirt Bike Engine Building Handbook, the engine building book I am currently writing for my fellow riders who want to bring their engine building skills into a professional realm in their own garage. This post is part one of three that will cover the correct use and implementation of precision measurement tools when rebuilding your own engine. The world of measuring is so complex by nature that one could get wrapped up writing an entire book on the subject and still not cover it all. This is not my intention with my engine rebuilding book. My aim is to provide you with the principles on how measuring works, what the most important takeaways are on the subject of measuring, and an overview of how to use the tools correctly. Once informed you can then delve further into the intricacies of measuring for your own needs. There are three terms that are important to the fundamental understanding of measurement. These terms are often mixed up, confused as meaning the same things, or used incorrectly. These terms are accuracy, precision, and resolution. Understanding these three terms will go a long way in ultimately understanding measuring and the capabilities of measurement tools. ACCURACY Accuracy is how close a given measurement is to the “true” value of an object. For example, if a valve stem was exactly 0.1969” (5.000mm), accuracy would quantify how close the measurement tool was to the true value. PRECISION Precision is a measurement of repeatability. For example if an object was measured five times, precision would quantify how close the five measurements are to one another. Another way to think of precision is the finiteness of which a measurement tool can be read repeatedly and reliably. RESOLUTION Resolution is the smallest distinguishable value of a measurement tool. If a ruler is divided up into tenths of an inch then the resolution of the ruler is one tenth of an inch. A micrometer that can be read to one ten thousandth has a resolution of one ten thousandth of an inch. Just because a measurement tool, such as a micrometer, has a very fine resolution doesn’t mean it will be accurate or precise to that resolution. This will be explained more shortly. Distinguishing the difference between accuracy and precision is most easily done with a set of pictures. Four scenarios can occur when measuring. A measurement can be both accurate and precise.
A measurement can be accurate but not precise.
A measurement can be precise but not accurate.
A measurement can be neither accurate nor precise.

## When to Rebuild Your Engine

Engine Power has Diminished - This could be due to restricted fuel flow in the carburetor or throttle body, a clogged air cleaner, the clutch slipping, worn valves and seats, worn rings, a stuck valve, leaking gaskets, or ignition issues.
The Top End is Noisy - A noisy top end could be caused by a loose cam chain, out of spec valve clearances, a worn cam chain guide, or worn cam bearings.
The Bottom End is Noisy - A worn clutch basket which has started to rattle, damaged or stuck bearings, a worn bushing and needle bearing between the clutch basket and primary shaft, or gears which are improperly lubricated may all contribute to bottom end noise.
Blue Smoke - Blue smoke occurs when the engine is burning oil. Either the valve seals are allowing oil to leak past them or the piston rings are no longer sealing properly. Once the engine is warm very little blue smoke should ever be seen.
White Smoke - White smoke is emitted when the engine is burning coolant. This typically occurs when a head gasket starts leaking.
The Engine Consumes Oil - Oil is getting into the combustion chamber any time the engine consumes oil. Oil can either enter into the combustion chamber from worn valve seals or worn piston rings.
The Engine Oil is Creamy - Whenever the engine oil is creamy in color moisture is getting into the engine oil. While some moisture getting into the oil is normal excessive amounts are a cause for alarm and may indicate that a water pump seal is leaking.
The Engine Oil has Large Pieces of Metal in It - Metallic particles are common in engine oil but if larger metal pieces are found in the oil this is a cause for concern and should be associated with damaged components. An example of this could be finding fragments of chipped gear teeth in oil.
The Engine Vibrates Excessively - Excessive engine vibration may be caused by an out of true crankshaft, worn crank bearings, worn counterbalance bearings, a mistimed counterbalancer, or a loose clutch.

## Riding Year Round in The North

The free wheeling nature of the two-stroke allows for a little more natural corner entry.
Four-stroke bikes are a little more forgiving when entering a corner in the wrong gear and are more capable of tractoring out of the corner.
450cc size four-stroke and 500cc size two-strokes wear out tires faster than smaller displacement machines.
Two-stroke bikes have a lighter feel to them.
Four-strokes are a little more forgiving when tuning for cold weather than two-strokes.