dlp@akgua.UUCP (D.L. Philen [Dan]) (11/05/85)
A previous article had the following information about setting SU carbs. "On my '71 MGB with dual SU carb I can easily test if the mixture is too rich by lifting the manual piston rod slightly while the engine is at idle. If it is too rich the engine slows and does not return to the same idle speed. When it is just right the engine speed stays the same as I lift the piston rod." I tend to disagree. I know what the manual says about tuning SU carbs so I will try and also explain it with a little logic. For all cases the engine is set at idle approx. 800 rpm. 1. Carb set too rich. Lifting the piston on the carb in this case causes the engine to SPEED UP and STAY SPEEDED UP. Lifting the piston on an already too rich mixture leans out the mixture, gives better air/fuel ratio, and results in increased engine rpms. 2. Carb set too lean. Lifting the piston causes and already too lean mixture to be even leaner still resulting in DECREASED RPMS and eventual engine stalling if the mixture is really lean. 3. Carb set just right. Lifting the piston causes the engine to MOMENTARILY SPEED UP then slow down to the original speed. Leaning the correct mixture intially causes the engine to speed up but at the higher rpms it is too lean so it slows down again. In setting the carbs the draft of each carb must be set independently first. Use a UNISYN or similar suction gauge to set the amount of air that each carb draws. The linkage between the carbs MUST be disconnected to do this. With the linkage still disconnected, set the mixture for each carb. Lifting the piston only 1/32 of an inch is sufficient for the lean/rich test. Finally when all is set, reconnect the linkage and you are in business. I have done this many times on my 54MG, 65 Healy and 64 Rolls. I know it works and the previous comment simply doesn't make sense, follow the manuals, or work in practice. Enough said. From the mad mechanic at Dan's English Car Garage akgua!dlp Q. Why do the English drink warm beer? A. Because they have Lucas refrigerators!