Adjusting SU Carburetors

Some people say that adjusting a pair or three SU carburetors takes a combination of a little knowledge, experience and a dash of Voodoo. 

Not so! it is all Voodoo. ha! The following is the method I have used in several MG, A-H, Triumph and Jaguar dealerships that I have worked in since 1960. My teacher and mentor use to teach tech school for Jaguar and was at the time a service manager for a Jaguar / Fiat dealership in Jax FL. The method is different than you will read in the books. But it has worked well for me for many years now.

This is just the adjusting and tuning procedure not overhaul or rebuilding.

To start with it is a waste of time to adjust SUs on a engine that has not had a valve adjustment, compression test and the ignition timing set FIRST!!!

First --- confirm the fuel pressure at 1.5 to 3 PSI depending on what kind of car it is. Then set the float level as per specs of the type of SU you are working on. The old fork style float arm was a 7/16" bar under the fork, the flat metal arm attached to the float or the all in one float with an arm were mostly a 1/8" plate placed between the top and the float. The HIF or SU with the float inside the body has a 1mm setting for the float. 

Now --- Loosen the throttle linkage so that the throttles will operate individually. Back off the throttle stop screws and the fast idle stop screw so that the throttle plates hit home (closed all the way).  Now carefully lock the throttle linkage back together so that there is a little free play before the linkage will open the throttles and that as you start to open the throttles, they both open at EXACTLY the same time. 

Take a thin piece of cellophane and place it between the throttle stop screw and the stop that it seats on and adjust the screw until you feel a drag on the cellophane but not enough to open the throttle at all. Do this on both or all three SUs. 

Now you have both throttle plates closed all the way, both stop screws adjusted so a piece of cellophane is snug under each throttle stop screw and if you move the throttle linkage there is a little free play before it tries to open the throttles and when you do contact the throttles they will open at the same time. 

Remove the top pots and look at the shoulder of the needle in the piston to see that it is flush with the bottom of the piston. (BE SURE NOT TO MIX UP THE PISTON AND TOP POTS AS THEY ARE A MACHINED MATCHED SET) Adjust the jets so they are flush with the surface of the bridge in the Venturi.  The old SUs like on early MGAs etc will be a 5/8" hex nut under the center of the carb. HIF with the internal floats will have an adjustment screw in the base at about 45 deg from the front and the HD type used on E-type Jaguar and some A-H had an external adjusting screw on a arm to lever the jet up and down.  On this type it is good to use the cellophane between the adjusting screw and it's stop to get an exact starting point on each carb. Adjust the jet down about 3/16" below the surface of the bridge and count the number of turns of the adjusting nut or screw so you can exactly duplicate that amount on each carb.  This is just a preliminary setting but it needs to be exactly the same on each carb. Now adjust each throttle stop screw down one and a half turns exactly. This too is just a preliminary setting but must be exactly the same on each carb.

Replace the top pots and check to see that you can lift the piston all the way to the top and let it drop and it will hit the bottom. Be sure there is enough oil in the top pots. Unscrew the top and lift the pin up and push it down. You should feel hydraulic resistance as you push the pin down. I use engine oil but some prefer ATF and others use silicone oil.

Now start the engine and set the fast idle screw as as to keep from 1200 to 1500 RPM. As the engien warms up the RPM will raise but don't readjust the fast ilde screw but just push the choke in to keep the 1200 to 1500 RPM.

When the engine warms up set the idle stop screws so you have a 800 to 1000 RPM idle. When you turn one stop screw be 100% sure to turn both screws exactly the same number of turns. (or number of flats since many of the adjustment screws are hex shaped)

Now with the air cleaners off you can use the Unisyn gauge if you have one. If not you can use a piece of vacuum hose to listen to the "Hiss" sound of each carb to balance them so they sound the same. This is the only time you will adjust each stop screw individually, once this is done you will always adjust both screws the exactly the same amount.

Now it is time to sent the jets. With the engine warmed up and idling lift the adjustment pin under the lip of the front carb and note the response. If the engine speeds up and stays fast the rear carb is too rich. Never adjust the jet more than half a turn and rev the engine to clear it and lift the pin again and adjust again. If the engine dies the rear carb is either too lean or you did not get the throttle plates set correctly. If the engine slows and starts running like an old John Deere tractor it is exactly correct. Lift the pin on the rear carb and adjust the front exactly the same way. If after adjusting the mixture you may need to readjust the throttle stop screws but remember to adjust both the exact same amount.

The end result should be that you are idling (800 to 1000 RPM) and if you lift either the front pin or the rear pin the engine will drop off and sound like an old two cylinder tractor running.

When you lift the rear pin you adjust the front carb and when you lift the front pin you adjust the rear carb. This is not what most books tell you but it has worked well for me for many years.

Good luck



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