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Testing SU carburetor throttle shafts for wearFirst Things First   

If your car is not running the way it should, and you suspect that your carburetors are to blame, be sure to check that your plugs, points, valve clearances, and particularly your ignition timing, are set correctly before you do anything else. Do a compression check; the cylinders should be within 10% of each other. Many problems blamed on the carbs are actually the result of ignition, or other engine issues. In fact, many of the carbs we receive, do not need rebuilding; the problem(s) lay elsewhere.    

IMPORTANT: Before you even consider rebuilding your carbs, or having them rebuilt, please be sure to check everything else on the engine FIRST! If not, when you finish your own rebuild, or get them back from a rebuilder, the engine will almost certainly run just the same as it did before.......

Click Here to download a DIY checklist that covers all the basic checks referred to above.

You can also take a look at our FAQ section in the About Us page.

LEGAL DISCLAIMER: All of the information on this Web site, is provided in good faith to assist the enthusiast owner. If you do not feel confident enough to tackle any of these issues, then don't; solicit the help of either a knowledgeable friend, or a trained professional. Always refer to the appropriate workshop manual for your vehicle as necessary.

Working on motor cars is inherently dangerous, and British Classic Motors accepts no responsibility for any damage, or injury, resulting from the use of this information.


Once you are sure that you have thoroughly checked all of the items in the engine checklist, we can move on.

One of the most common problems is rough running at idle, and before it is assumed that a rebuild is required there are a couple of easy things to check, which may solve the problem. The most likely culprit is a vacuum leak, and these account for the majority of issues reported to us by owners.

You need to check that all of the tubes and hoses connected to the carburetor(s) and the inlet manifold are connected properly, and are not damaged, or perished. And although it sounds a bit basic, check that the inlet manifold nuts are tight; you'd be surprised at just how often that is the problem. If everything looks OK, one more check is needed. With the engine running, take a can of starting fluid and carefully spray all of the hoses, their connections, and the inlet manifold gasket area. If the engine speeds up at any time, you have found a vacuum leak. Fix that leak, or leaks before you do anything else. You MUST still check the entire system after you find the first one; there almost certainly will be more than one!

If your car has a brake booster, don’t forget to check the hose that runs from the check valve fitting on the manifold, to the booster. Check the condition of the hose, and each end for leaks. Spray along the whole length of this hose; they tend to perish, and develop cracks.

Even the smallest leak in any of these areas will cause problems. If the engine is drawing air in from anywhere except through the carburetor, the mixture will be excessively weak; and you will NOT resolve it be adjusting the carburetors(s).

CAUTION: Please remember that starting fluid is extremely flammable - and ALWAYS be extremely cautious when working around a running engine!!

SU carburetor dashpot oil levelIn response to a large number of inquires about exactly how much oil should be used in the dashposts, I have added this image.

It should be about 1/2" below the top of the hollow piston rod, NOT the threads in the top of the suction chamber.

And, as I state elsewhere in this site, whatever else you may have read, or been told, the correct oil for all SU's is SAE 20, period.

Once that’s done, it’s time to check the throttle shafts for wear. Grasp the end of each shaft and try to move it back and forth. Test both ends of each shaft. If you find anything more than absolute minimal movement, they may need work. To confirm this spray the starting fluid onto the end of each shaft, again, if the engine speeds up the shafts, and/or the bushes, should be replaced. Testing for wear on SU throttle shaft bushes

The next thing to check is the oil in the suction chamber. If the oil is either the incorrect grade, or the level is too low, you will experience a stumbling effect when you accelerate from idle.

Everyone you talk to will give you a different opinion as to which oil you should use; everything from Marvel Mystery Oil, to EP 90 Grade gear oil. The grade of oil recommended by SU is, and always has been, is SAE 20, and this is the one we use, and the grade we supply in a small bottle with all rebuilt carbs. Is it absolutely critical? Not really, and if all you have on hand is 5w-30, it will do at a pinch. Do not go as high as 20w-50 though, it is too thick, particularly when cold, and will give you an over-rich mix on acceleration. If you feel a distinct hesitation when you accelerate, it means the piston is rising too quickly; try a slightly heavier oil.

SU Carburetor Setup, and Tuning Guide

To download a .pdf version of our Tuning & Setup Guide which we include with every set of rebuilt carbs, please click the link.

Now you have gone through all the steps above, let's finish off with the correct way to adjust your carbs, and as most of you will have a twin carb setup, that is what we are using here. I have used the H/HS-Series as the example, and where there is a difference with the HIF's, this is referred to in italics.

1) Bring the engine up to normal operating temperature

2) Loosen both the screws on the interconnecting linkage; it's extremely  important that each carburetor is able to operate independently of the other. If you don't do this, when you go to adjust the second carb, you undo the adjustments you made on the first.

3) Using a tachometer, either the one in the car, or a hand-held unit, adjust the idle speed to factory specifications. Make sure you keep track of your adjustments, and turn the adjusting screw only 1/4 of a turn at a time, adjusting each carburetor equally.

4) Now for the fuel/air mixture. Raise the jet adjusting nut, on one carburetor at a time, until the engine speed just begins to fall off. Now, lower the jet adjusting nut until the engine speed begins to rise again, and then raise it again just 1/6 turn (one flat of the nut).

HIF NOTE: Raise or lower the jet by using the mixture screw on the side of the carb. Screw in for a richer mixture, out for a weaker mixture. Remember, just 1/4 of a turn at a time.

5) When you have set the mixtures on both carbs, re-adjust idle to factory specifications.

6) It's now time to test those fuel mixtures. Using either the lifting-pin (if installed on your carbs) or a thin-bladed screwdriver, carefully lift the piston about 1/32". If the engine speed increases and remains elevated, your mixture is a little rich. In this case raise the mixture adjusting nut by one flat at a time, and re-check the mixture.
If the engine speed drops when the piston is lifted your mixture is too lean, and in this case lower the mixture adjusting nut, again, by one flat at a time. When the engine RPM initially raises as the piston is lifted, and then settles back to the original speed or a little higher, your mixture is right.

7)  Again, re-adjust your idle speed if necessary.

8)  We now need to synchronize the carburetors using either a balancing device, such as a Unisyn, or, as will use here, a length of fuel hose. Hold one end of the hose just in front of the carburetor inlet and listen to the 'hiss' of the air as it enters the carb. It is easy to notice any difference in airflow by the intensity of the 'hiss'. It is best to reduce the idle speed of the 'faster' carb and increase the idle speed of the 'slower' carb until both produce the same 'hiss'. Then, evenly adjust both idle speed screws to reach the desired idle speed. Re-check fuel mixture, and carburetor synchronization.

NOTE: If you are unable to get your idle speed down to where it should be, and you can't find anything else to adjust, e-mail me at barry@britishclassicmotors.com telling me which carb(s) you have, and I will give you the solution.

9)  Adjust the pegs on the ends of the interconnecting linkage until there is approximately 1/16" inch of clearance before the peg contacts the slot on the carburetor tab.  Tighten all the screws on the interconnecting linkage, being careful not to move anything.

10)  Pull the choke until the jets are just at the point of moving but have not yet moved. Screw both the fast-idle screws in until they just touch the fast idle cam. Alternately turn each fast-idle screw until the idle speed is increased to 1,000 RPM with the engine warm. Push the choke back in.

Congratulations, you're Done! Take a test drive, and enjoy the feel of a correctly set up pair of carburetors.

Can I help?

If you have any questions about our Do-It-Yourself section, or you simply have a question about your carburetors, please don't hesitate to e-mail me; I will help in any way I can.

We are currently closed; please use email if you have a question.
You can e-mail me at barry@britishclassicmotors.com

British Classic Motors - More than 40 years of SU Carburetor Rebuilding & Restoration experience..............