Modifying for Power


Now that we have a basic understanding of what each part does in our engine, lets look at getting the most out of those parts.

A couple things to again keep in mind: first, simply bolting on a Holley 850 double pumper isn’t going to get it. All parts must be matched to flow the same through our little air pumps. We also must understand exactly why we want this to be. There will always be trade offs when building power. The more you build for top end power, you're going to loose some of that on the bottom end. All the top end in the world isn’t going to help if you're stuck in deep snow.

Belt drive clutches, which we’ll get to later, just on like big sleds allow you to have a variable speed transmission, but their expensive disc or drum clutch, either stock or aftermarket, work extremely well but you need power at engagement speed as well as up top.

Plan out what you realistically want your sled to do. I have seen where more power than your kid is ready to handle has immediately flipped him on his head and mom running for the bandages. That really never has worked out to well.

Starting at the beginning.


We know the function here is to mix the incoming air with the proper amount of gas. What it comes down to here is finding the right size of carb, more specifically the amount of air it will pass. Again more air, more power assuming everything else can handle it.

To a large degree, the size of the hole running through the carb will determine this. This can range from around 3/8” stock to almost an inch on full opens. Also a factor is anything in the way of the air flow. Inside the first thing you notice is a round plate referred to as the “butterfly” or "shutter.” When closed it is near vertical and only passes enough air for your engine to idle. Full open is 90 degrees or horizontal, this is when it passes the most air possible. When looking at the shutter in the full open position, you can see how thick the plate itself is and the thickness of the shaft that holds it.

Some classes have a rule requiring the use of the stock carb and give you a maximum size for the diameter of the bore. Of course we bore it to that limit, but we also can pick up air flow by making a shutter of a thinner material and by making a custom shaft that is also thinner. Some times we go to the extent of soldering the plate to the shaft so no small bolt needs to be used. The head of that bolt also hangs out in the airstreem and disturbs air flow. On a full-out carb everything in the airstream is smoothed out and polished to provide the maximum possible flow from that carb.

In an open or no rule situation, or building a hot rod trail sled, it’s often cheaper and easier to simply replace the stock carb with something bigger. There are several options for this. Briggs and Stratton makes the Animal for the kart racing world. This comes with a 22mm slide valve carb very much similar to the Mikuni carbs found on big sleds. These are easy to adjust for jetting changes. Tilitson also makes a wide range of performance carbs for small engines. Either of these can be used for a simple performance gain but you need to understand jetting and get the correct intake manifold and throttle linkage for your engine.

We know that the carb must flow fuel as well as air to perform correctly. Fuel and air must be of course supplied in the correct amount. Sometimes this means a main jet change or adjustment. Performance carbs also offer low speed adjustments to balance this out.

The stock factory OEM carb was designed at best to simply either make this engine idle or run at 3600 RPM. Add to this the factory jetting is a compromise for this to run either summer or winter. Unfortunately, that means it’s far from "right" for almost anything we do with it.

One of the most common things people buy first for a stock sled is the adjustable main jet set up. This is simply a needle that goes up into the main jet, has a cable with a knob that you mount under the hood and can then spin the needle up or down to add some adjustability to the stock main jet system.

If you understand jetting from big sleds, or anything else, you know as the outside temperature changes so does your engine fuel requirements. This is because colder air caries more oxygen. This also varies with humidity and elevation changes. The simple adjustable main jet system will help considerably with cleaning up how your sled runs.

More often then not, a no start situation is caused by carb problems. Because these carbs are very small, a good inline filter is mandatory, simply slice the line from the tank to the carb and put in a ¼ inline filter. The bigger problem we all have is now all pump gas has some degree of alcohol mixed into it. Alcohol when contacting the cast inside areas of the carbs starts to corrode it away. The longer it sits in there, the worse it gets. I always use a product like StaBil when anything sits in storage but even a week or two can cause some problems. As the carb corrodes really small pieces of metal will float off in the fuel. It doesn't take long for these to plug up a jet or small passage in the carb. The only fix here is to remove the carb from the engine, drop the float bowl, and completely clean everything. This means carb cleaner AND compressed air to blow out all the passageways. When done well, this will fix most no start situations.

We've talked over several options on better carb choices. When making your choices, it's always better to err on the small side rather than the large side. When we get into talking about ports, we will be discussing “velocity." This is the speed at which the mixture flows through the engine. Picture this by taking your garden hose with nothing on the end and the valve full open. A lot of water comes out the end, but it only goes out a foot or so, then falls to the ground. Now, pinch down the end of the hose, it now sprays out much faster and goes much further, maybe 30 or more feet. You are still using the same amount of water (volume) but it picks up speed because the end of the hose is pinched down making it smaller.

An engine spinning 9000 RPMs keeps everything flowing very fast, but at a 2500 RPM idle it moves slowly, so slow that the air and fuel mixed in the venturi of the carb can actually separate again causing the heavier fuel droplets to fall out of suspension and puddle on the intake or port floor. This leaves you with a lean and varying fuel mixture to make power. This is the "bog" you experience when you suddenly go to WOT at a low RPM. The larger your carb is and the lower your clutch engagement is, the more prone this is to happening.


We will include the intake manifold here because in reality it is simply an extension of the port to mount the carb to. Ports are the passages in to and out of the cylinders.

From the factory only one thing dictates how there ports are made: how simply they can do it. Performance is not an OEM consideration. Basically all they do is bore in from one side of the head, then bore up from where the valve is in the combustion chamber. Two simple processes and done. This is about as far from performance orientated as you can get. About the worst example of this is the Polaris/Robin engine. Not in the port itself which exits the head right above the recoil starter fairly close to the valve. To fit their industrial applications, they wanted the carb mounted in the back of the engine so they cast an intake manifold. They made it with a dead 90 degree bend right after it bolts to the head. From there it goes back several inches to mount the carb behind the engine. Because this is a thin wall casting there was very little we could do with it to improve. Racing rule makers recognized this as a serious draw back in that engines design now allow us to cut off the flanges from the factory intake and weld up a short tube placing the carb straight out the port and right above the recoil starter. Without this rule change the Robin had a serious drawback compared to the other engines.

What to do with the ports these days is simply determined by one tool: a computerized flow bench. A flow bench is simply a very elaborate shop vac. You mount the carb, intake, or head to a fixture. It then draws or blows air through it and very accurately measures its volume. We use a Superflow brand bench just like NASCAR, NHRA, or anything else. It has a very useful feature called a velocity wand. This is a 1/8 inch plastic tube with a wand attached. This allows you to move it around inside the port and while the bench is gathering data it will record the speed at which the air is moving anywhere you choose inside the port. When done all info is put out on a spreadsheet for analysis. When taking in info from the bench, because we use cams and valves unlike a two stroke, we have a fixture that holds the valve open at preset valve lift measurements to show us flow under all conditions.

Designing a port for a racing application is far from a simple standard operation. In all cases where maximum power is wanted, because of the way the factory builds them, we weld up the inside of the port in the bowl area, the area above the valve where the holes come in from different directions. This area is always too big for the rest of the port runners. Once we decide what type of use the engine will see we rough the new port in.

Going back to our garden hose story, if the engine will be used for drag racing and have CVT clutches we know the engine will do almost all of its work at max RPM, say 9000. We can then make the ports for max volume that the carb we choose can supply. The velocity will be kept up by the high RPM the engine turns.

Now if were building an engine for cross country, Snow-X, or a fast trail application, especially if it has a single speed clutch, we know that much of its life will be spent pulling through the entire RPM band. Here we know that low and midrange is king and a smaller port design, maybe a smaller carb, will keep velocity up, keep the engine clean, crisp, and much stronger then something with monster ports.

The exhaust side of things follows the same theory, but somewhat simpler. What we are looking for here is a clean and free flowing path for the spent gasses to reach the pipe, which is where the tuning magic happens. We will save that for a later chapter.

The last thing to cover here deals with surface finish. The surface finish plays into several things. On the intake side a somewhat rough surface helps create turbulence, which helps keep the fuel and air mixed. The downside is the rougher the walls are, the thicker the boundary layer is. The boundary layer is the outer layer of the mixture as it flows through the port. Because this outer layer is in contact with the walls, they create drag which slows the mix touching them down compared with the center areas. The rougher the walls are, the thicker the boundary layer becomes so a larger amount of your total flow gets slowed down. As a general rule of thumb finish the intake ports with a 240 roll, then run a red cross buff through to even things out a bit.

On the exhaust side, we have no mixture of fuel to keep suspended, simply spent gasses. Here we can use a much more polished surface. This does two things. First it keeps the boundary layer as thin as possible to keep up flow and speed. Second, especially if using gas as a fuel, it helps to keep the build-up of carbon from sticking to the port walls.

The rest of the job follows standard porting principles and common sense. No humps or bumps in the walls, keep it flowing straight. Porting doesn’t start and end with the head, make sure the ports mate up with the intake and pipe. Check the fit when compressed of any gaskets so there’s no steps in the path.

Honestly to study flow, one of the best things you can do is walk along a river. Study the path the water wants to follow see where it wants to go, see what speeds it up and what slows it down.

We’ll end this section with two warnings. The first is common sense: wear safety glasses and use a dust mask. Understand and be comfortable with the tools. The cheap high-speed ¼ inch air grinders that are everywhere now are not the best tool for most of this. One slip with a carbide cutter and you can put in a groove or make something too big and the only fix is a new head. If you're not really used to porting, I would start with a 1/8 inch Dremel tool and go slowly. The last warning I can give you is simply a reality we see far to often. When a customer sends in an engine or head for work that already has been worked on often I have to tell them, "Sorry we need a new head, this one is too big to do what we need." Plan well before you start.

Cams and Valve Train

I will try to keep this as simple as possible, but by far this is the most complex area of engine building with these engines. For the home builders out there I would definitely stay with the commercially available grinds available. We do have some specific custom grinds, some of billet, some with lightened drive gears, but the cost far out weighs the benefit to anyone other than the top level race engines.

There are several things done to a cam to effect how much and where in the RPM range it will produce more power. The two main things that effect the power potential of a cam design: lift and duration. Lift which is difference between the base circle of the cam where the valve sits completely closed and the highest point on the lobe where the valve is at its max lift. The lift number represents the maximum height the valve is lifted off its seat. Duration is the measurement from the point where the lifter first starts to open the valve, continues around past max lift, and back down the other side until once again the valve is closed.

Clearly increasing either or both of these will feed more mix to the engine and give us more power. Sounds pretty simple, but of course many other things come into play. First, this happens thousands of times per minute. Second, to take the circular motion of the cam down in the block and transfer that up to the top of the engine and across to open the valve takes several other parts. The more you stress those parts and the faster you make them move, the more likely they are to not want to play nice anymore.

There are some things we can do to reinforce the valve train components and reduce the stress. We use stronger studs and stud girdles to reinforce them. The other area is using lighter weight parts to lessen the load. Problem here is titanium parts are very high dollar even for the most serious racer.

There are a few products in this area I have also learned to stay away from. Billet roller rockers are one. They're definitely necessary on big car engines where we might see valve spring pressure in the 700 lb range. Our engines even with fast ramp cams are down in the 20-25 lb range. I see the much greater weight of the roller rockers as more of a disadvantage then the extra strength they supply. If anything on the most aggressive cams we reinforce the rockers with a little welding.

Like carbs on cam choice, study well or consult a builder. Too much cam will kill your bottom end power. Also remember that more cam will require stronger valve springs to keep you out of the dreaded valve float. Valve float is when the cam is moving so fast that the spring can no longer control the valve and it bounces off its seat. It takes very little float to damage the valve train which can quickly lead to complete engine destruction.


Compression is what the full volume of your cylinder is squeezed into as the piston comes up. Basically, the more you have, the bigger the bang. Easy enough said, but some things limit this detonation is our enemy. This is the uncontrolled firing of the gasses in the cylinder. It comes from too much compression or too much timing in the engine. Our tool to help control this is the octane rating of the fuel we use. We’ll cover that next.

The bad news here is there is quite a bit of variance from engine to engine as they come from the factory. Again this is a result of being an industrial use engine, basically it's just not an issue they care about. The short story on this is each and every part made for an engine has a tolerance limit. This means it can be slightly smaller or slightly larger then the next run of the same parts. As engines come down the assembly line they just come together from the random parts as it moves down the line. Tolerance stack up is what you get when all these parts are put together. An example would be, and exactly what were talking about here is; some engines when being measured at TDC may have the piston at .030 - .040 in the hole. Another engine may have the piston at .010 out of the block at TDC. That’s a difference of 40 or 50 thousandths. That's huge. The same holds true for the depth of the combustion chamber in the head which also determines compression. When we build engines, we deck both the head and the block to get compression right where we want it. We do this on a milling machine. Although it’s a bit of a project, it can be done at home by using a piece of sandpaper glued down to a surface plate or piece of glass and sanding the two surfaces down. Use plenty of WD-40 and your arms will hurt.

Related to the limits of compression we can use is...


There is much confusion out there regarding gas and octane rating. Many people believe that using a gas with a higher octane rating will produce more power. Actually the exact opposite is true. The octane rating of a fuel reflects its ability to resist detonation, nothing more. This simply means 93 octane or premium gas will not detonate as easily as 87 octane or regular gas The real story here is engine builders know that when an engine will be run on high octane gas, they can then build the engine with more compression and more timing giving you more power. It does not come from the fuel itself.

As long as were on liquids, let's take a minute on oil…


The biggest reason we see for engine failure is using the wrong oil. Never do we use a oil designed for cars, not even the synthetic types like Mobil 1. Car oils are made for a system that uses a pump and a filter. Our engines simply use a dipper on the rod to splash the oil around on the inside of the engine. Sounds primitive, but it works extremely well. We drive our kart engines around the big tracks like Daytona or Road America all day at 110 MPH and the engines last all season with no damage.

We use and recommend only one oil: FHS Hurricane light. This is the single most important thing you can do to keep you engine healthy. It is available from us or any kart supply shop or by mail order.


Unlike our 2-cycle brother engines, on a 4-stroke we do not need both a converging and diverging cone, we have no need to try a cram fresh charge back into the cylinder. We do hope to use some of the same principles to help evacuate the cylinder.

Your factory pipe is simply made to keep it quite. For performance we're looking for a smooth, fast flowing pipe to get rid of the gasses. We spend days on the dyno testing various pipe sizes, both inside diameter and lengths. Once this is established the job becomes fitting it into the chassis and getting it to exit out the bottom. You will notice most pipes will have several stages or larger sections as you look down the pipe. These sections get larger as the gasses flow to help pull the gas as it travels down the pipe. Their size and placement is critical.

There you have it, what we do to make power. I know we didn’t cover the bottom end but most things there require very accurate tools and big machines, beyond what most home builders have.

If you have any questions, just ask in the forums and spread the word on this site.