24th July 2009, 11:31 PM
Join Date: Oct 2006
Re: Weber IDF, Unabridged
<B style="mso-bidi-font-weight: normal"><U><SPAN style="FONT-SIZE: 14pt"><FONT face="Times New Roman"><FONT size=4>Weber Carburetors, Tuning. June, 2009<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com
<o:p></o:p> The adjustment procedure pretty much followed the various books and articles. We started with the mixture screws out 1 ½ turns and the throttle plates opened about a half turn from where the screw makes contact with the linkage. (Pat Braden, Weber Carburetors, recommends starting the mixture screws about a half turn out and adjust them out a quarter turn at a time. Bob Tomlinson’s Original Weber Tech Manual recommends starting about two and a half turns out and adjusting the mixture screw in. Haynes Weber Carburetor Manual recommends pre setting the mixture screws two turns out. I doubt it makes much difference which way you go, as long as you end up in the right spot). After the engine was thoroughly warmed, the balance was checked.
<o:p></o:p> The two primary carbs (connected to the bell crank) were adjusted first, then the two dependent carbs.
<o:p></o:p> It is interesting how the idle speed quickly smoothes out just by getting even air flow. A fraction of a turn of the stop screws makes a noticeable difference in the idle speed. Our idle speed was set around 800 RPM.
<o:p></o:p> There was a slight variation between the throttle bodies on two of the carbs. The lower reading was brought up to the higher with the air screw adjustment. When the job was finished, all eight throttle bodies were within a needle width of each other on the air flow meter.
<o:p></o:p> We have had a persistent problem with one, sometimes two, cylinders not firing. An infrared temperature gauge is so handy for quickly identifying a non firing cylinder. It works much better than a finger. Typically the temperature of the header runs around 200 degrees or more. If one is running around 150 degrees or less, we know it is not firing. That cylinder’s mixture screw is then opened until we hear the RPMs jump, and we then know it is firing. The header temperature quickly rises. We have found that a couple of the mixture screws need to be out about a half turn or more farther than the others. Once properly set, they have been firing reliably.
<o:p></o:p> Making sure that all cylinders are firing and getting a good balance between the carbs seems to be the most important part of this process.
<o:p></o:p> The mixture screws were adjusted. First the idle stop screw that controls the idle speed was turned in to slow the engine down to around 600 - 700 RPM. Next the mixture screws were adjusted by turning them in a quarter of a turn at a time until a drop in the RPMs could be heard. Then it was backed out a quarter of a turn and that cylinder was done.
<o:p></o:p> After the mixture screws are set, the balance is rechecked. Then we are done, for the moment.
<o:p></o:p> Keeping a record of what is changed is helpful. We made up a little chart to document modifications and changes. A fresh sheet is used for each ‘session’. (We have quite a stack already filled out). When the plugs are pulled, we set them on the chart and snap a picture for future reference.