Bergfelt Racing Enterprises
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Kid Kart Tech Tip
Tuning the SHA Dellorto Carburetor Found on Comer C-50 and C-51 Engines
No single tuning variable makes more difference in the performance of the Comer C-50 and C-51 engine than getting the carburetor tuned just right. Actually that’s true for tuning just about any engine but especially the tiny Comer engine, which has found its way onto kid karts the world over, since it has such a limited power potential to begin with.
In one sense the carburetor has a relatively straight forward task to perform; to supply a properly measured mixture of fuel and air to the engine. As far as carburetors go the SHA series Dellorto carburetor that Comer selected is as simple as float style carburetors get. The primary means of tuning the carburetor is the single brass jet that is one of the first things that the tuner will see after removing the small black plastic float bowl from the bottom of the carburetor.
Even though the Dellorto SHA series carburetors are very simple, they all do not work equally well. After tuning dozens of these engines, I have discovered that no matter how much tweeking that I’ve done on some of these carburetors, some of them simply do not work very well. I have run in to cases where the only tuning variable left that worked was to simply replace the carburetor with a new one and start over. What I find most intriguing is that after all was said and done, if you were to place the two stripped carburetor bodies side by side it would be very difficult to tell which was the good one and which the bad.
When trying to determine if you have a good carburetor or a bad one, make sure that the basic parts are performing properly. For instance, if the fuel leaks from the carburetor when the engine is just sitting around, there is a very good possibility that the float is leaking making it sink to the bottom of the float bowl holding the inlet needle valve open so that fuel constantly runs through the carburetor. More likely than that the rubber tip on the inlet needle valve could be damaged and even more likely there could be debris stuck in the small hole that the rubber tip of the inlet needle valve fits into. Other than making sure that all of the vent and fuel passage holes are clear there is little else that can malfunction other than obviously broken and cracked pieces.
One thing to look for when judging a good carburetor from one that isn’t is that with the bad ones, when the engine gets up to around 10,000 rpm fuel will “percolate” up the float bowl vents and spill over the outside of the carburetor. Some of this fuel spills down through the choke lever hole and causes an overly rich condition and a drop in rpm and inconsistent high rpm operation. Really bad carburetors do this a lot. Ok carburetors percolate a little but don’t have a big effect on performance and really good carburetors don’t percolate at all. Most of these carburetors percolate a little bit and can be tweeked to run well. Those that don’t percolate at all are rare but are the ones that are desired. I have yet to figure out why this condition occurs even though I have thoroughly examined both “good” and “bad” carburetors and can’t visually find the difference.
After making sure that your carburetor is in good shape and everything is functioning as it should the only thing left to do is make sure that it is fitted with the correct jet for your application. One thing is certain; there is no single jet size that will work best for every carburetor/engine combination. There are many factors that affect the correct jet size but they boil down to two factors; how efficient the engine is at pumping air and the density of that air. By air density I am referring to how much oxygen there is per cubic foot of air that is flowing into the engine. Air density is determined by temperature, relative humidity and barometric pressure. From my tests I have concluded that although temperature and humidity should be considered, barometric pressure by far has the greatest effect on jet size and elevation has the greatest effect on barometric pressure. Simply stated, the denser the air is the bigger the jet needs to be. Obviously less dense air requires smaller jets. An engine that runs perfectly on a cool clear day in Florida near sea level can be sent to a racer racing on a track in the rocky mountains at 5,000 ft above sea level and it will not rev very well and will be very slow on the straights simply because the jet is way too big for that elevation where the air is much less dense. A smaller jet is what is needed. It is for that reason that different size jets should be and are available.
When testing to see if you have the correct jet, the engine must be operated with a load similar to that which it will encounter on the race track. Starting an engine mounted on a kart on a stand with the wheels off of the ground is not even close to the best way to test the jet size. Too many new racers will run their karts on a stand and run them at full throttle while watching their tachometer to see what rpm it reaches. They will then incrementally try the next smaller, then the next smaller jet and so on to get the highest rpm possible. The problem with that method is that although the tuner can get the engine to scream that way with no load, it will not be able to accelerate very quickly when the wheels are on the ground and if it does manage to get up to speed it will most likely severely overheat and possibly severely damage the engine as a result. It is far better; although very time consuming, to do the testing on the race track. The best way to tune is on a dynamometer.
It’s a good idea to know what you are looking for. A well tuned engine will start relatively easy, will accelerate reasonably, and will achieve its top potential rpm on the longest straight away. Always start out the tuning session with a jet size that is presumed to be too big. The way to tell if it is too big is that once the engine is warmed up it will take off well with little or no hesitation but when it gets to half to ¾ of its maximum rpm it will start to “break-up” or miss from the ignitions inability to burn all of the fuel that is in the combustion chamber. The next step is to try the next smaller jet. The engine should rev a little higher before it starts to break up. Now try the next one and so on. Do that until the engine reaches the highest rpm possible on the straight. You will know you have reached the limit when you try the next smaller jet and the top rpm either does not get higher or even results in a slight drop in top rpm from a too lean condition. You will probably have also noted that the kart did not start off as strong either and was probably a bit sluggish starting off. Go back to the next bigger jet and use either that one or the next bigger one. Also note, if two jet sizes both wind up with the same peak rpm, use the bigger of the two. The bigger one will accelerate better and be less prone to overheating. One more thing, weather can change drastically on race days and this does affect jetting. It’s a good idea to have a small weather station with a barometer so that you can predict what jet to use. It would be a good idea to keep a small note book in your tool box to keep track of this data and how the weather relates to your jetting.
Too Rich = starts up easily, takes off very strong, accelerates well but breaks up before reaching maximum rpm. This won’t hurt the engine but you won’t win either.
Too Lean = hard to start, sluggish starting off, may need to be choked to get moving, revs high but gets hot. If excessively lean it won’t even reach maximum rpm. One way to test for too lean is to put the choke on a little bit. If running the engine with the choke on improves performance even a little, the jet is too small. Running an engine too lean can cause severe damage to your engine.
Just right= starts easily, accelerates well from a dead start, revs to full rpm without missing.
The air filter makes a huge difference. The aftermarket fabric air filters, regardless of brand, flow a lot more air that the stock plastic filters with the foam element. By all means, use them if the rules allow. If you must use the stock filter only use one foam piece and make sure it is clean. Of course you will get more air into the engine with the stock filter housing with no foam at all but that will result in premature engine wear and possible severe damage. Since the aftermarket filters allow more air to flow it is logical that they require bigger jets, and that is true. Sometime 2 or 3 jet sizes will be needed when switching from a stock air filter to an aftermarket filter. By the way, the aftermarket fabric filters will pass plenty of air. Make sure that they are properly oiled, with the correct filter oil, to make sure abrasive dust does not get into your engine. They also need to be cleaned and re-oiled periodically.
Promoters have found that the restrictor pill is good for limiting the performance of the Comer C-50 and C-51 engines for inexperienced drivers. Those restrictor pills are very effective at limiting air flow into the engine and there for limiting the power output. These pills present a tuning nightmare for people who don’t know what to expect. Since they restrict the amount of air entering the engine, logic would dictate that a smaller jet is in order. Actually the opposite is true. A larger jet size is needed when switching to the restrictor. From testing I have found that the rule of thumb is two jet sizes bigger are needed for proper operation with the restrictor compared to proper operation without the restrictor. There is good reason for that. Carburetors are designed with a restriction in them called a venturi that is used to speed up the air and consequently lower its pressure. This lower pressure coupled with atmospheric pressure above the fuel in the float bowl causes the fuel to travel up the nozzle and enter the air stream. When the restrictor is placed behind the carburetor, it becomes the venturi where the air pressure is lowered. Unfortunately that is not the place where the carburetor nozzle dumps the fuel into the air stream. The actual venturi in the carburetor becomes much less effective and consequently does not work very efficiently so very large jets are needed to compensate for the lack of efficiency.
Quite often, when tuning a Comer C-50/51 with a restrictor, the tuner will find that even with the 58 jet, the biggest that stock class rules allows, seems to be too lean. Guess what, it probably is! This situation is not very likely at high elevations, but racers at lower elevations will run into this. I know I have, at lot, and I do most of my testing at an elevation around 1000 ft above sea level. One alternative is to reduce the amount of oil that is in the fuel mixture. I’ve heard of some racers using as little as two ounces of oil to a gallon of fuel to get around having a jet that is too small. I advocate using much more oil than that so jets larger than the “legal” 58 jet size are needed to do that. Technically, if you do that you are cheating under some rule sets, but the engine won’t run right if you don’t. How the reader deals with that dilemma is his choice. Dellorto does make jets larger than 58 and Bergfelt Racing Enterprises, LLC does stock them.
There is one other fine tuning variable that I’ve played around with. That variable is the float level. A person with even extensive knowledge of small engine carburetors would say that with the plastic float that these carburetors have that is not possible without some complicated measures. Actually it’s very simple. It’s done by tilting the carburetor on its round mounting spigot. To make the mixture slightly richer, tilt the carburetor toward the side that the plastic fuel inlet strainer cover that the fuel hose attaches to is on. This will have the same effect as lowering the float on a carburetor equipped with a brass float. To effectively raise the float level, making the operation effect leaner, tilt the carburetor away from the fuel inlet strainer. Please note that this is a very fine tuning measure. Sometimes it makes a noticeable difference and sometimes it doesn’t. This is a good method to adjust the carburetor on race days to tweek for varying weather conditions.
I hope this article has shed some light on the mysteries of tuning the Dellorto carburetor found on the Comer C50/51 engines. I am interested in learning if this is helpful to the reader. Please e-mail firstname.lastname@example.org
For those racers who race at tracks that have very few rules regarding the kid kart class, our outlaw carburetors WILL provide better performance. I have developed a set of modifications that dramatically increase the flow of air through these carburetors. I won’t describe those modifications here because I worked very hard to develop them and I don’t want to give that information away to competing engine builders. I also will not build a carburetor for someone and ship it off to a customer by itself. As I stated earlier, there are a variety of factors that come into play in proper tuning and sometimes even brand new carburetors percolate badly as I described earlier. The only way I will sell one of my “outlaw” carburetors is if I have the engine that it is to be used on so that I can tune the carburetor and engine combination together. Just to give an idea of the difference in flow through these carburetors, they require much larger jet sizes, some as large as #68.
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