So I recently bid on and won a keihin 38mm carburetor off a 1999 CR250. This is the second time I nabbed one of these carb's for around $100 shipped to me. The first one was pretty dirty so Cooper and I pulled it apart and scrubbed it clean. We then swapped the needle, slide, and jets to what I thought was a good starting point for his fresh engine. We did a bit of tuning and got it dialed.
So I was eager to try to get another one and do a cleanup and rebuild with my other son Gavin. I was patient and found the deal I was looking for. I was surprised to see some kind of swirl pattern in the inlet end of the carburetor. I did not see them when it was on ebay.
I wonder if anyone else has seen this pattern. The add did not advertise this carburetor as modified. I'm going to say that this was either an aftermarket job or possibly something keihin tried. Look at the pictures and let me know what you think.
FYI going this route can save you some money....
Thanks Loren.
Loren:
If it was something that Keihin tried and gave positive results, the new feature would have been incorporated in some of their new carbs. If the carb runs well and is not difficult to tune or is not real sensitive to altitude and ambient temperature changes, run it and forget that the carb bore has swirl groves.
From a engineering or theoretical point of view, I do not think physics would support there being any positive aspects to creating swirl in the intake on a reed valve engine. I also think the intent of the person that carved the swirl pattern in side of the bore of the carburetor, was to create swirl but the effort may have been in vain and did nothing but move money from the consumer of the snake oil to the creator of the snake oil.
For those that are technically minded I have listed a some information that some may find interesting that has come from my years of research and development on two stroke engines than relate to air flow and fuel vaporization.
1. Swirl causes turbulence, turbulence reduces fuel droplet size, turbulence reduces airflow, reduced air flow reduces horsepower, reducing horsepower makes me sad!
2. If the swirl has a very high rotational speed, fuel will move to the outer walls of the intake tunnel like mentioned in an earlier post. The bend in the rubber intake boots on almost every offroad bike and quad built since the advent of the single rear shock, causes the fuel to move toward the outside of the bend as the mixture travel through the rubber manifold and enters the reed valve. This phenomena cause the mixture of air and fuel in the crankcase to be richer on the right side and leaner on the left side on a engine that has the carburetor on the left side of the shock. The mixture in the right side of the crankcase will travel through right transfer ports and fill the right side of the cylinder. The same process occurs to the mixture in the left side of the crankcase causing the mixture in the left side of the cylinder to be leaner. The design compromise of putting the carburetor on the left or right side of the rear shock causes the air fuel ratio in the combustion chamber to also be leaner on one side and richer on the other side. This problem becomes more evident when watching the top of the piston when tuning an engine on the ragged edge. When developing and engine on an engine dyno we have the luxury of using/making intake manifolds where the mixture enters the reed from the left or right or straight in. The detonation traces will always be on the opposite side of the piston (left or right side) that the outer wall of the bend is in the intake passageway between the carb and reed.
3. Reed petals tips do not open evenly on the engines that have a bend in the intake track. If the outside of the bend in the intake tract is on the right side, the right side front corner of the right side reed petal will usually open first and then makes contact with the reed cage seat first on it's right corner tip Reed petal tips landing on one of it's corners cause that corner to erode away.
4. Fuel is put into the air stream in the carburetor bore as small droplets. The majority of the atomization/vaporization occurs inside of the engine not in the carburetor or intake tract. We want the droplets to be as small as possible, making it easier to absorb heat so it can change states and become a vapor. Fuel has to become a vapor before it will burn. The smallest fuel droplets turn into vapor first as they make their way through the crankcase, transfer ports and into the cylinder. During the compression stroke the mixture temperature rises to 600F to 800f before the spark begins the combustion process. During the combustion process the temperature continues to rise converting the majority of the remaining remaining fuel droplets to vapor so they can burn. In a high performance two stroke engine peak combustion temperatures will exceed 4000 deg F and peak cylinder pressures can easily exceed 1500 psi.
5. Exhaust Gas Temperature (EGT) probes measure the average temperature at the probe over one engine revolution not the actual exhaust temperature entering the exhaust port as the piston opens the exhaust port the first few degrees. A EGT probe placed 6" to 8" from the piston may give a 1200 deg F reading when the actual exhaust temperature leaving the cylinder and entering the exhaust port will be well over 2000 deg F.
6. My experience indicates that Dyno operators that are jetting/tuning a two stroke engine using a oxygen sensor do not not have a very in depth understanding of two stroke engines and or what an oxygen sensor is actually measuring!!!
Show Posts
