----- Original Message ----- From: "Colin & Bev Rainey" <[email protected]> To: "KRnet" <[email protected]> Sent: Saturday, April 23, 2005 1:25 PM Subject: KR> Carb on top
Any builder looking to run with carb on top needs to run one that is similar to an auto carb in that it has a good needle and seat style control for regulating fuel into the bowl. The problem that I have seen with alot of carbs being adapted to the KR engines is that they are purpose built, just not for our purpose. A carb used on our engines needs to be one designed for a moving vehicle, not adapted from an industrial application, or a farm tractor, or something else. A good candidate is the Weber series of carbs. Another are the Mikuni carbs from 4 cylinder engines (not from seadoos, not large enough) like the Honda Protégé or Accord. These carbs are designed to run daily with engine heat, so on top is okay, and they run with fuel pumps, so that is okay. Use the book AUTO MATH BOOK by John Lawlor to calculate the necessary cfm requirements for your application and then match the carb to it, always choosing to the low side of the cfm requirement. Example: if your engine calculates to needing say 368 cfm at your peak rpm, then choose an available 350 cfm carb, or 325 cfm whichever is readily available. This is one time where bigger is NOT better. Larger will cause hesitations, surging, and mixture problems through all ranges except peak rpm (which is what the carb would be selected to work at only in this case). Testing has shown that an oversized carb will actually cause the engine to slow down, or make less power not make more. Also make sure that it is a factory style carb and not after market like a Holley, which will require jetting and adjusting for all ranges and changes in season. Acquire a Carter brand electric pump which uses a better pumping mechanism to prevent flooding and has more volume flow without the pressure (see the Weber Carb site). The Posa requires too many modifications to make it acceptable. I think Jim Faughn is still running one on his 2180 VW. He goes into alot of detail on his site about it, as does Orma. Good reading. And here is a little tip alot of builders are missing: if you use a carb of late model design, say after 1975, it will not only be designed for best lean mix which will yield best performance, but will NOT require mixture control as it will be equipped with a barometrically controlled diaphragm that will automatically lean the mixture as you climb or atmospheric conditions change, the same way that the new FADEC engines alter the mix with electronic control by leaning the mixture. You will need to set the idle mixture and then the rest is preset mechanically based on factory adjustments internally. This is why the carb MUST be matched properly to the engine. They also have an accelerator pump eliminating the need for a primer system, AND making acceleration smoother due to fuel being added as the throttle is opened at the same time, something that even some of the aviation carbs do not have which cause the typical popping back and hesitation when the throttle is opened rapidly from idle. I have heard counter thoughts on this but we cannot replace the decades of development that have gone into the successful carbs of later cars. Use as is including the choke. The electric choke will do 2 things: make cold starts a non-issue, and will allow for an automatic richening of the mix if the incoming air is too cold. Power is applied to the choke to keep it hot, so the carb temp has to get really low for it to come on, but if it does, the mix will richen to compensate for low temp of the incoming air for proper fuel atomization. These enhancements took many years to develop and cured the mysterious surging, flat spots, "vapor locking" and other drivability problems associated with the pre-emission carbs. Just a quick note: long skinny runners for intake are more rpm sensitive and provide a better idle "signal" for smoother acceleration; short fat runners are better for overall rpm performance and make more power, but do not provide a good "signal" to the carb at low to intermediate rpm. More on this later.... Colin [email protected] http://kr-builder.org/Colin/index.html KR2(td) N96TA Sanford, FL Apex Lending, Inc. 407-323-6960 (p) 407-557-3260 (f) [email protected] _______________________________________ Search the KRnet Archives at http://www.maddyhome.com/krsrch/index.jsp to UNsubscribe from KRnet, send a message to [email protected] please see other KRnet info at http://www.krnet.org/info.html
