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Carburetor, Intake, and Exhaust / Anyone looking to trade carbs?
« on: April 05, 2014, 11:59:22 AM »Quote from: Jerry Hall;30876
The following may sound like a Keihin bashing session but it is just some of what I have experienced after working with many different types of carbs over the years made by Keihin, Mikuni, Delorto, Bing, Amal and the many different versions of the Lecton type one to 3 circuit carbs.
There is nothing sacred about most of the various carb designs that have come about in the last 35 years. The key is fine tuning what ever carb you have not some special shaped slide or carb that has a vane or some other marketing gimmick to sell everyone a new design every few years. The PWK, PWM, Air striker, TM or TMX are all excellent carbs if you can get the tuning part you need or be able to adjust the circuit that is out or tune.
Selecting the right combination of needle, pilot jet and slide is what separates the men from the boys when it comes to fine tuning carbs. It should not take more than about 30 minutes to find the right main jet if you have a long steep hill or dyno. Running a bike down a flat road in high gear will usually cause one to select a main jet that is too lean. Finding the right blend of slide, pilot and needle taper requires patients and a lot of different needles and expensive slides.
Sometimes the needle you need may not be available especially for the Keihin carbs on some engine combinations. Most of the time I prefer to use a Mikuni if I have to change carburetors for a special project. Mikuni two stroke carbs offer 3 more tuning circuits than any of the Keihin two stroke carbs. Mikuni offers tuning parts and a machined passageway to allow changing the needle jet, float valve and air correction jet. Another refinement that Mikuni has is multi-angle needles. I do not recall seeing any multi-angel Keihin needles in any of their two stroke carbs unless it was an OEM carb. The absence of multi-angle needles is often the missing link for getting some carbs to work perfect for a specific application.
A replaceable needle jet is a huge plus for me especially if you are building alcohol carbs. Multi-angle needles being available is another plus. Changing the air correction jets should be reserved for those with considerable tuning experience and has a dyno. Changing the float valve is not necessary unless your fuel pressure is in excess of about .75 psi.
Needle jets eventually ware out on any carb. I think it is a crime that Keihin uses a bronze needle in a brass needle jet. Similar material rubbing one another ware much more quickly than dissimilar metals rubbing each other. I think that Keihin does this just to sell everyone a new carb every few years. You have probably seen 5 guys that have the same engine build, they are using the same carb but have different jetting specs when they get their carbs tuned well. It is not uncommon to observe the pilot jets may be one to three jets different, at least one turn different on the air screw and the last number of their needle is seldom the same. The discrepancy in jetting is often caused by the needle jets being in a different state of ware amongst the different carbs. With a Mikuni you can replace a $20.00 needle jet and be back on the road to your original crisp throttle response.
Have you ever seen a bronze needle in a OEM two stroke carb? No, they usually use an anodized aluminum needle because the engine manufactures want the carb to stay in tune during the warranty period and beyond?
I guess Keihins philosophy of less circuits and single taper needles makes carb tuning easier for the average two-stroke guy. The FCR and some of the other carbs Keihin makes for the 4 stroke world are nice and sophisticated and offer a lot more tunable circuits and tuning parts than some of the Mikuni's 4 stroke aftermarket carbs.
Keihin has the technology, why don’t they give us more of it in their two stroke carbs?
Slow golf clap..........very well said!