Suck It Up
Have you heard the saying "Bad advice is worse than no advice?" In this business of building high performance engines there is a serious problem "would-be" mechanics sit behind a desk and hand out all sorts of bad advice. Now, this bad advice is not given out intentionally but rather by mistake. The law of physics has two sides-the side that is studied in the classroom and the side that applies to the real world. Unfortunately, too much classroom technology is being passed on to unknowing customers.
The biggest and most expensive problem facing two stroke engines lies in the misadjustment of the carburetor. There is nothing that can do so much damage in such a short time as an engine running with a carburetor in an overly lean condition. You can avoid this by following a few simple rules:
Rule #1: Do not ever use some one else's carb settings on your machine. There are many variables that affect an engine's fuel needs and a dangerous practice to use settings not appropriate for your engine.
Rule #2: Always, always start carb setup from the richest setting and then work your way to a leaner setting. Take your time, make small changes and test, test, test. There are no shortcuts where it comes to testing.
Rule #3: Whenever you replace a part, no matter how insignificant, always go back to a rich, four-stroke setting and work our way back. An engine's fuel needs change dramatically when changes are made to the engine. These changes range from porting, compression changes, exhaust systems, ignitions, flame arrestors, intake manifolds, atmospheric pressure, air temperature, cooling capacity, the specific gravity of fuel, and go on and on.
The horsepower produced in any engine is related to the amount of air/fuel mixture delivered to the combustion chamber. Maximizing this fuel charge is the goal of every tuner. This is accomplished when all of the engine components are well put together and complement each other. It is important that one component not limit the performance potential of the engine. For example, if your porting specs allow your motor to run at 7500 rpm, but your exhaust system only runs well at 6800, you have a problem. Your engine's performance will be limited by the component with the poorest performance. This is comparable to the saying, "A chain is only as strong as it's weakest link."
(Above) This is the inside of 44 mm Mikuni square pumper at 12:00 is the main jet, directly below is the is the low speed jet. On the left is the fuel valve and arm spring which controls fuel delivery.
(Above) These are tuning components of Mikuni carburetor. Top left: slow speed jet, main jet, needle valve body, arm, arm spring and needle valve.
(Above) This chart displays approximate amount of fuel delivery that each circuit contributes to total fuel flow of the Super BN's. LOW SPEED CIRCUIT contributes fuel all the way to full throttle. TRANSITION CIRCUIT controls the fuel flow for a smooth "transition" to the high speed circuit. HIGH SPEED CIRCUIT controls the fuel flow in addition to the LOW speed and TRANSITION circuit. High speed adjuster controls as much as 1/2 of the fuel delivery.
(Above) This chart indicates verity of possible fuel delivery combination. Adjustment begins by inspecting pop of pressure. Step one: make sure that valve dos not leak. Connect pressure gage, fill up valve with kerosene, apply pressure very slowly, observe needle for leakage. There should be no air leek up to the point when valve pops open, repeat this several times to assure correct readings. Note: it is best to use plaxi glass between carb and your face, other wise you will get wet.
BABY STEPS
There are some clever salesmen out there who will lead you to believe that by changing or getting rid of venturis you will see some sort of astronomical power gain. Well, ask yourself this question Why wouldn't the manufacturer have done that in the first place? Radical induction mods do very little for performance. My belief is that if your engine really needs more power, buy a bigger carb. In highly modified engines you can install twin 44s 46s and 48 ts. These carbs are each capable of handling the fuel/air needs of a 200 horsepower engine! Instead of falling into the "bigger is better" trap, spend the time mastering your stock carb and get the most performance it can provide. In fact, it's better to learn the intricacies of your carb on a stock machine before jumping into over-sized twin carbs on a highly modified machine. When you are really trying to dial in your carbs, making adjustments of only 1/4 turn can have a significant impact on the performance and longevity of your machine. Go too rich and you have a poor performer; go too lean and you have a large repair bill from a seizure. Because there are a multitude of factors that affect carb adjustment, many different ideas and inventions have surfaced that simplify the tuning process. For instance, electronic fuel injection controls the fuel/air mixture at varying atmospheric conditions automatically. Someday this will become affordable, but it is still very expensive for the average consumer.
Colder air temperatures mean more oxygen in the air to burn in our engines. A change in temperature, say from 70 to 90 degrees, has a significant impact on performance, more noticeable in highly modified machines. You have to be able to adjust your carbs to compensate for such atmospheric changes or suffer the consequences. If the temperature goes up, then your motor runs richer and you lose power. If the temperature goes down and you don't adjust, you run a serious risk of seizure. We also need to realize that elevation changes also affect performance. Higher altitudes mean less oxygen available. While most recreational riders don't operate at different altitudes, racers on the circuit face this situation. I have a graphic illustration of the altitude effect on carburetion. This summer I was fortunate enough to travel to England and experience the racing scene over there. I was assisting a racer with his modified X-2 and we were able to achieve a consistent 48 mph in the sea near his home. However, when we traveled north to a race on an inland lake, the situation changed for the worse. The X-2 was running so rich that it was unable to race. Lake was at an altitude of 600 feet above sea level. Adjustments were made to the carbs and we were on our way to a successful race finish. What does this all mean? Well, in order to be successful at our sport, we must be able to compensate for changing conditions and enjoy the day racing.
(Above) Many times the carburetor is blamed for damage that was caused by another problem. This melted down piston had its top and a hole eventually formed by the exhaust side of the crown. There are several reasons for such an occurrence, but in this case the operator bought fuel that had alcohol in it. Whenever the combustion chamber is subjected to water, this type of damage will almost always occur. Another possible cause of such damage is the use of octane boosters or aviation fuel. These products have no place in a high performance engine. As a rule, stick to buying your fuel from one reputable manufacturer. When in doubt, throw it out!
(Above) This photo shows the damage caused by a cold water seizure. In northern climates, this problem surfaces when the water temperature drops well below normal. Too much cooling water running through the cooling lines running through the hot cylinder will cause this condition. Many times dual cooling lines are run during the summer, but these must be either restricted or removed when the water temperature begins to drop. This condition becomes noticeable when a loss of top end power occurs or the engine doesn't want to idle smoothly any more.
(Above) This is another cold water seizure. In this case, the rings actually became lodged into the piston. You probably wouldn't have noticed this problem if operating under 3/4 throttle. However, if you were to go to full throttle, the engine would shut down or self-destruct. The best way to detect if too much cooling water is flowing is to use temperature probes. You would mount the probes on each cylinder and watch the gauge. NOTE: Unfortunately some engines with cold water entering head first, do not allow this type measurement The temperature would be normal at slow speed, but as you got faster, the jet pump would begin to flow too much cold water through the engine and the temperature would drastically drop on the gauge. The piston is still expanding due to more fuel being burned yet the cylinder is contracting due to more cooling water flowing through the jacket. This conflict would only last a few seconds before the engine failed. You may richen up the fuel mixture and use best oil available, but nothing can save your engine if piston becomes larger then bore diameter.
SOME OF THE GUIDELINES OF CARB OPERATION:
1. The fuel/air ratio must be correct for proper performance.
2. The fuel/air ratio influences engine cooling.
3. Running rich is better than running lean.
4. Large carburetors require larger jets.
5. Operating at higher altitudes requires smaller jets.
6. Temperature changes will affect carburetion due to the differing oxygen content of air.
7. The starting circuit of carburetion requires a ratio between 1 and 3:1.
8. Idling requires a ratio between 8 and 10:1
9. Low speed requires between 12 and 14:1
10. Ordinary operation between 15 and 16:1
11. Heavy operation between 11 and 13:1
12. When tuning your carb, find the main jet setting that shows just a touch of richness and then go one step leaner. This is where maximum power will be.
13. An engine's performance is related to the total weight of air drawn into the engine, which depends on air density and volumetric efficiency of the engines.
14. The fuel flow through a jet is determined by volume and density and area of the jet, not diameter.
15. The air/fuel mixing process depends on velocity created by a venturi, tuned length of the intake system and air filtering devices.
GET THEORETICAL
Since carbs are going to be around for quite a while, let's see how they work. Fuel is draw into the carb with a pump that is built into most carbs today. Theoretically, the fuel pressure should be the same for an engine running at 1000 rpm as it would be running at 8000 rpm. However, this is not the way it works in real life. For this reason, the fuel valve opening varied in its diameter to supply fuel for differing engines and rpms. Additionally, variations in pop off pressure, jet sizes, and fuel return lines are necessary for proper operation. Less sophisticated carbs have two adjustment screws, one for idle and one for high speed. On certain carbs, there a three adjustment screws, which, unfortunately, add to the difficulty of adjusting the carb properly.
"When in doubt, turn it out!" Always begin carb tuning from a rich setting and work your way leaner. As mentioned earlier, you must readjust your jetting whenever you've made a change to your engine or associated hardware. This policy holds true when you've replaced the rings in your engine. We all know that it takes several hours for a new set of rings to break in. When rings aren't fully seated, a lot of fuel/air mixture blows by the rings and consequently leaves a lean condition in the combustion chamber when ignited. This results in higher temperatures which can lead to damage or destruction. So when you replace rings, make sure to enrich your carb settings and properly break in your rings. It is very important to break in the rings properly. Once you've enriched the mixture, take your machine out to the lake and start it.
Make sure you don't warm up the machine on the trailer and then put it into the water. The drastic change in temperature can cause serious damage to the cylinder. Once you've warmed up the engine properly, start riding around. You must operate at varying engine speeds during this break in period, from low to mid to wide open in 30 second increments. This variance in speed ensures that glazing of the cylinder wall doesn't occur. Do this for 25 minutes and then shut down your motor. Replacing just the rings takes a relatively short period of time. When replacing pistons and rings, the process may take several hours. New pistons must not be over-heated due to their expansion rate. When a fresh bore is combined with new pistons and rings, the process becomes a bit more complicated. Start your motor, warm it up, and ride around for 15 minutes at varying speeds not to exceed half throttle. Shut down and cool off the motor completely. For the second session, repeat the above but stay out for 20 minutes at varying speeds not to exceed 3/4 throttle. Shut down and cool down the motor completely. For the third session, repeat the warm-up procedure, but now take 2-4 second blasts at full throttle every 2 minutes. Do this for 30 minutes. Now you are ready to begin adjusting the carbs. This whole process takes several tanks of gas to complete properly. You will know the rings have seated when the machine starts to run very, very rich. The fuel/air mixture is not being allowed past the rings and more full charge is present in the combustion chamber. It is important to note that much patience is needed for this process. Don't be tempted to go blasting around the lake at full throttle after only a short break-in period. Take the time to do this right and your engine will reward you with excellent performance.
MAKING IT WORK
The best tool you could possibly have for carb setup is a temperature gauge which has probes mounted under each spark plug. Having this instrument will save you thousands of dollars in damage and hours of setup work. It is very important to set up both cylinders to run at the same temperature. This allows for maximum power delivery, longevity and indicates how the cooling water is flowing in the cylinder head. Without this device it is quite difficult to get both cylinders to run alike.
Nowadays the trend seems to be having machines that are run at high speeds. Just about every carb will suffice; even a single 38 mm carb will achieve speeds of 50+ mph. However, it will take a long, long time for your machine to reach top speed. When you are a racer, your needs are different. You must have that top speed available, but you must be able to accelerate to that speed in a very short distance. This is when your carbs need to get bigger, badder and more complex. Every racer is running twin carbs which range from the standard 38 mm to 44 mm to sometimes even 48 mm. It's obvious that when you jumping from a single 38 with two adjustment screws to twin Super BN's, the complexity of setting them up increases tremendously.
Engine carburetion is affected by many, many variables, including temperature, fuel, compression, porting, etc. For this reason, we can't have one magical carb setting that will work for every rider. Engine modifications or changes affect jetting in various ways. Cylinder over-porting will reduce the suction effect at low throttle openings and will cause problems at idle and lower ranges. An aftermarket exhaust flow better than a stock system and may cause a reduction in pressure which will cause jetting problems. Intake systems and fame arrestors, if mismatched, can cause problems in high speed operation. Atmospheric pressure changes due to altitude or temperature must be compensated for. Elevation changes of 500 feet or temperature changes of 20 degrees demand jetting and compression changes.
The first adjustment must be on the idle circuit. Set up the idle speed around 1200 rpm. Next, the low speed must be set up to allow smooth transition from idle through 1/4 throttle. Maladjustments here cause hesitation off the starting line, which will make you play cat instead of being the leader of the pack. The high speed circuit must be adjusted properly for maximum to end speed. The adjuster screw usually controls 1/2 the flow rate of the jet. I always start with 3 or 4 turns and 2 or 3 sizes larger on the main jets. This setup should cause the engine to run in a seriously over-rich condition and reduction in several hundred rpm. If turning the screw in to the bottom doesn't correct the richness, go to the next smaller main jet. Turn the screw out to 3 turns and repeat the process. Yes, this is a tedious work but it's the only way you will be assured safe and accurate carb settings.
George Grabowski HPT Sport USA