Jeep Engines - Oljeep

41 6 MOPAR PERFORMANCE PARTSSPECIAL CARBURETOR CONSIDERATIONSHeaders vs. CarburetorsHeaders are one of the most common performancemodifications. Thirty years ago, the installation of headersnetted an instant performance gain. Today, thc installation ofheaders doesn’t always result in a gain except at theracetrack itself. The headers haven’t really changed-thecarburetors have.Pre- 1970 carburetors were richer in fuel-air ratio. Headersalways lean out the fuel-air ratio of an engine. With theextra fuel available in richer, older cngincs, the headersleaned out the mixture level to the point that the engine wasstill running well, and the lower backpressure availablewith headers resulted in an increase in horsepower.The emissions laws since 1Y70 forced productioncarburetors and inlet packages to have almost perfect fueldistribution, but with much leaner fuel mixtures. The bctterfuel distribution is a plus, but the leaner mixtures leave noroom for modification. The installation of headers on tothese “emissions” engines leans them out too far. Thismeans that the gain the engine got for the decrease in backpressure was canceled because of the engine output loss ducto the too lean fuel mixture.The solution is quite simple: enrich the carburetor. Thisusually requires richer jets to be installed. This allows thefuel-air ratio to be corrected. The headers with thc richercarburetor will then result in better output (horsepower).Carburetor cfmThere always seems to be a lot of questions relating tocarburetion, especially questions such LIS which carburetorto use or how much cfm the engine should have. Theproblem with answering these questions is that the answersvary with engine size and tuning rpm. A related problem isthat all carburetor manufacturers don’t use the same ratingsystem for their carburetors, and some don’t rate theircarburetors’ “cfm” at all. Cfm stands for cubic ,feet perminute, which is used for rating the air flow of a carburetor.OE manufacturers (such as DaimlerChrysler) don’t testcarburetors for air flow. Since the advent of emissions laws,OE engineers test carburetors for fuel flow because it relatesto the richness level in the engine which relates to emissions.Air flow relates better to horsepower, high rpmperformance, and inversely, driveability. To use it, first youhave to obtain air flow data. The only good sourcc isHolley, but don’t try to compare 2-Bbl. to 4-Bbl. data.Check and record the accelerator pump shooter size. Alsocheck the accelerator pump and adjust it for maximumtravel. Rod adjustment to the hole closest to the pivot isrecommended as the initial starting point. Bc sure themetering rods move up and down smoothly by pressinglightly in the center of the metering rod bar.The point at which the throttle opening opens the air dooris determined by the number of turns on the spring. Toadsjust the spring, two screwdrivers (or a special tool) arerequired. The adjustment is typically on the driver’s side.The outer slotted ring is the locking screw and must beloosened before the spring can be adjusted. Once loosened,the air door will hang vertically. Adjust the inner screws(which tensions the spring) until it just closes the air door.Thcn turn the screw and tension the spring an additional 1 -I/:: to 2 turns. Once set, always use the same technique forfuture adjustments. The fewer turns, the quicker the air dooropen- typically used on faster vehicles. The more turns,thc slower the door opens- typically used with automaticsand street tires.Thc amount that the air door opens can be adjusted at the trackwithout disassembling the carburetor, This adjustment canchange the amount of air flow that goes through the carburetorand also change the fuel-air ratio (richness level) of the mixturedeilivered to the engine. Before you make any adjustments youmust know how to accurately measure the amount of opening.(R’efer to the proper service manual for details.)Remember, the first thing to do is to measure your specificcarburctor and record its reading. Once the vehicle has beenba’selined at the track at this setting, adjustments can be made.Always measure the air door opening using exactly thesame technique. Adjust as follows: open up to increasepower and lean out the mixture; close down to enrich. If itdevelops a miss in high gear at high speed, you haveopened it up too far and it’s too lean. Close it back down forbest performance.If all else fails, keep in mind the golden rule of carburetorselection -don’t over carburete!Lean CarburetorOn any given performance engine, the odds are the engineis lean. Since every engine performance increase requiresmore fuel, the carburetor should be rejetted for everyperformance modification that we make. However, thisdoes not happen most of the time. Therefore, the enginetends to be lean (or close to it).One of the most common modifications to a performanceengine is to install a performance ignition system. Thesesystems typically include a performance distributor. Thesedistributors have a fast advance curve for betterperformance. (For more information, refer to ‘IgnitionSystems’ section of this chapter.). ..