delbene@homxa.UUCP (K.DELBENE) (10/29/85)
A year or so ago, I detoxed my '79 MGB, removing the catlytic converter and Zenith carb in favor of dual SU HS-4s and a stainless exaust system. Not wanting to go overboard, I left then valve train in tact. Well, last night I got to wondering about the carbs' applicability with the stock pollution-control-influenced valve train. The question I have is whether the size 6 needles that I put in are perhaps a bit too rich for the valve timing in the later model Bs. I thought perhaps I'd try a number 21 instead. Has anyone had experience with this modification? Any recommendation on needle sizes. While you're thinking about that one, try this one on for size: Sometimes when I accelerate hard off the line, I get a sharp knock from the passenger side rear. I've checked the brakes and spring shackles, but nothing seems amiss. I've had the problem since I bought the car 2 years ago, but never experienced it on any other B's I've owned. Having just had the car repainted 6 months ago, I told them to check for any frame cancer, but the prognosis came back negative. Anyone ever experienced such a problem? Any help is much appreciated. Kurt D. Del Bene Bell Labs - WB ...!ihnp4!homxa!delbene
gxm@raybed2.UUCP (GERARD MAYER) (11/01/85)
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 have found this to be a sensitive method with a little practice. Noise: I had the same problem and in my car it turned out to be the vapor collection canister in the trunk was loose enough to hit when I took OFF!!! Gerry Mayer uucp linus!raybed2!gxm
delbene@homxb.UUCP (K.DELBENE) (11/05/85)
On the point that one can lift the piston of the HS-4 carb. and check the effect to determine whether or not the mixture is proper, you're right and you're wrong. For background in case anyone else cares, the SU, Stromberg, and Zenith carbs are designed so that the needle is afixed to the end of a damping piston. When the throttle plate is open, the inrushing air causes the piston to rise. Since it is tapered, this in turn increases the amount of fuel that mixes with the air. The piston is dampered with an oil cylinder and piston arrangement, so that the changes in air/fuel ratio are not so erratic. Now, back to the point. The test you do implies that you are starting with the proper needle. I don't think it will tell you if you have the proper needle for the characteristics of your engine. That is the question that I've raised. It really belongs to a more general question: If one changes the characteristics of an engine, how do you find the best carburetion for the new set-up? Other stories I've heard lead me to guess that my suspected problem of overcarburetion is quite common. Another point on setting air/fuel mixtures in general: I've found this and other methods very inexact for setting mixture. About 6 months ago I bought a tool called Colortune and have been very pleased with it. It's really just a see-thru spark plug with instructions on what color of flame you should be seeing. You simply richen out the carb and back up until the flame turns bunsen blue. It works fine on both single and multi-carb set-ups. Kurt D. Del Bene Bell Labs - WB ...!ihnp4!homxb!delbene