Optimizing Your Ride

Michael Forrest /
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Optimizing an Engine

For most motocross bikes all that is needed is to increase the squish velocity and perfect the jetting.


This is needed to be perfected because due to manufacturing tolerances almost always the squish clearance is more than it needs to be which lowers squish velocity. When it comes to this topic just ignore people who talk only about the clearance. The squish velocity is all that matters and it is influenced by piston/head clearance, angle between piston and head, how fat the squish band is, and RPM. And the only way to figure out the squish velocity is to use a squish velocity calculator such as this one: http://www.dragonfly75.com/moto/sqVelocity.html Gordon Blair recommended 15-20m/s squish velocity at the peak power RPM. Too little velocity indicates too much clearance which lessens power due to the higher percentage of fuel/air mix hiding out in that area. Too high a velocity and it contributes to the possibility of detonation. Using that recommended calculator you can also see what the “hot velocity” is which can vary significantly from the cold velocity on small engines.


Almost all bikes come from the factory jetted too rich which minimizes the possibility of a new bike seizing the engine and requiring warranty repair. My write-up on this subject is at http://www.dragonfly75.com/moto/carbtuning.html The only thing I would add to that is if you are inexperienced at jetting just buy 3 sizes leaner and 2 sizes richer of main jets and try them all at wide open throttle so you can get a feel for the difference between lean and rich. Jetting by feel is better than going by plug color which can be influenced by type of gas and engine oil used.

For non-MX bikes I also do the following:


If it vibrates too much then obviously the factory did a sloppy job with the crank design. I once had a ’89 KDX 200 that vibrated so much that I had to put buckshot inside the handlebars to calm it down. The counter balance designed into the crank is ideally set for a certain maximum RPM and if it is correct and you change the design of the engine/pipe for higher or lower max RPM then you may have to change the size of the counter-balance holes in the crank. Here’s my write-up on the subject: http://www.dragonfly75.com/moto/vibes.html A vibrating engine may also lessen peak power due to its effect on carburetion.


Ignition timing has a direct effect on power and engine cooling and possibly detonation and so is important. Typically by retarding the ignition the engine may be able to rev out farther or have more power at high RPM. And by advancing the timing the mid range power is increased as well as engine heat. So a balance between the two needs to be had. According to one report the timing leaves a tell-tale sign on the spark plug (but not on the fine wire types such as iridium). On the center electrode look for a small clean ring around the electrode. Correct timing leaves its height at .5mm. If it is more then retard the timing.


Most gasolines have a certain percentage of ethanol added to them which is usually listed on the gas pump. For best power and longer lasting engine seals you want ethanol-free gas. If the pump don’t list it then just buy a $7 Alcohol Content Tester from Amazon and test the gas yourself. For amount of octane you generally want to use as low an octane as possible without causing detonation. The main thing that effects octane requirement is the engines compression ratio. A generic chart is shown at https://vintagesleds.com/bs/index.php?topic=77538.0

There’s a whole science behind engine oil selection. Believe it or not, dinosaur oils (normal petroleum oils) contribute to combustion more than synthetic oils. But the synthetics burn cleaner and smoke less. So usually I pick an oil that is a combo of both. Oil selection does influence jetting requirements so pick a good oil and then rejet your carb. My write-up on engine oils is at www.dragonfly75.com/moto/oil.html


Raising the exhaust and transfer ports raises the RPM at which peak power is produced. On my 2 stroke 100cc street bike I had to gradually find that sweet spot between maximum high RPM power and a broad powerband. Since pistons are cheaper than cylinders you may want to grind down the piston edges (making sure there is still at least 1.5mm till the ring) adjacent to the ports to simulate raising the ports. Then you can keep it that way or if you went too far then buy another piston. For some unexplainable reason by making piston ramps at the transfers on small engines (<125cc) I’ve found it also broadens the powerband. My info on this subject is at http://www.dragonfly75.com/moto/porting.html


An expansion chamber has both benefits and disadvantages. The benefit is that it increases power within a 2500 RPM range at the top end of the engines RPM range. The disadvantage is that it causes a power loss for about 800 RPM before getting “on the pipe”. It also limits how far the engine can rev out. And it is somewhat inconvenient for use on street bikes and may cause seizing when cruising while the engine is “on the pipe” due to the decreased throttle opening (less gas). Suzuki solved that problem decades ago by designing an expansion chamber for street bikes that has an extra long belly section that causes the boost from the baffle to happen at low RPM and the boost from the diffuser cone to happen in the normal top range. This gives a broad powerband that is much more linear on a HP/RPM graph. My info on expansion chambers is at http://www.dragonfly75.com/moto/ECtheory.html If you don’t like the power curve caused by the pipe then you can modify the pipe to suit you. Also if you increase the stinger diameter and lengthen it to start inside the pipe at the end of the belly section then you will cut exhaust noise in half without negatively affecting the power.

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