Today I will discuss tuning of SU carburetors. Tuning includes mixture adjustment, idle speed adjustment, and synchronization. It is assumed that your carbs have been properly rebuilt and the jets centered; if not, stop here, because your carbs are likely not tune-able. Instead, go study one or more of my posts on SU rebuilding, starting with:
The particular carbs in this post are installed in a 1967 Volvo P-1800, owned by a guy in Waterford, Maine whose grandparents bought the car new in Sweden, so it’s a family treasure. The carbs were rebuilt by the owner, with some assistance from Joe Curto.
Although these particular carbs are HS-6s, on a Volvo P-1800, the procedures described in this post are generally applicable, with only minor variations, to virtually all SU carburetors.
Here’s a photo of the car and its current owner.
And here’s a shot of the carburetors. These are the nice “3-hole” variety, with threaded holes for screws to hold on the air cleaners, vice the earlier “2-hole” ones, which used bolts and nuts, a lot less convenient.
Here we have a photo of my ancient PSW Tool, which is designed for tuning SU carburetors. It contains a pair of tubes and wires for balancing the carbs, a jet adjustment wrench, and a (completely useless) “jet centering” tool, which may be seen peeking out of the plastic pocket, just to the left of the jet adjustment wrench.
The jet centering tool is useless because it does not work, nor can I think of any way it could be re-designed to work. That’s because jets and needles, and jet bearings, are all a bit different, and they really must be centered individually, using the actual parts to ensure proper centering. Typically, the holes in the jets are a bit off center, so just use your own jet, and your own needle, and center the thing, as shown in my posts on rebuilding.
A tool set similar to my PSW tool is available as item “B” from:
It’s worth having, particularly if you have Type H carbs. And the wrench, item “C”, is really nice to have. In my opinion, the “Uni-Syn”, item “A”, is worthless; I can’t think of a less useful tool for adjusting SUs. And, for reasons noted above, I don’t believe that item “D” (for jet centering) would be of any value, either.
Next is a photo of the PSW tool in use.
But, not everyone has a PSW tool, so I’m going to show you how to make your own synchronization tool from a wire coat hanger. The homemade one actually works better than the PSW tool for synchronizing and balancing; that’s because the stiffer wires don’t vibrate as much.
Nevertheless, the PSW tool has its uses; the oval tube (at the right in the photo above) being just perfect for adjusting Type H floats, for example. And that short jet adjustment wrench is really nice to have.
Note that the center-to-center distance between this pair of SUs is about 8-1/2″.
So, cut and bend your coat hanger so it looks something like this. Notice that the sharp wire ends are bent inward to prevent scratching the dashpot bore holes. You should read the rest of this post before you make your own synchronizing wires because I’m going to show you what I consider a better design at the end. But the ones shown in the next photo are the ones I used on this car, so here they are:
The first thing you have to do is to start the car and get it warmed up to normal operating temperature. If you’re installing carbs that I rebuilt, the jets adjustment nuts will be turned all the way up. That’s to ensure that the jets are properly centered.
You should turn each jet adjustment nut down about 2-1/2 to 3 turns, then pull your “choke” and start the car. As the car warms up, push your choke in to lean out the mixture. Do whatever you have to do to keep the car running at a reasonable speed while it warms up.
Shut the engine off and loosen the linkage between the two carburetors so that each throttle shaft will turn independently. For Volvo HS-6s, loosen the two nuts that are facing upward, one on each end of the center linkage shaft, as shown in the next photo. When installing your carbs, make sure those nuts on the linkage face upward; you’ll regret it otherwise.
For other types of SUs, and other makes of cars, the linkage is different, but the principle is the same; you want the two throttle shafts to turn independently, and you don’t want any input from the accelerator pedal.
Also, if necessary, back off the two “fast idle” screws, one on each carburetor, so they don’t contact the “choke” cams and thereby prevent the throttle plates from closing fully.
Now, with the engine still off, put the wires into the carbs as shown in the next photo and align the pointers so they’re both at the same height.
Now, re-start your engine, and adjust the idle screws on the carbs to obtain about 1500 RPM; the exact value isn’t critical, but it should be in the range of 1200-2000 RPM. You might also check the timing at this point, just to make sure that it isn’t grossly off. For Volvo B16, B18, and B20 engines, the timing spec is usually given at 1500 RPM, so that’s a good engine speed for checking your timing and for starting to adjust your carburetor mixture.
In the next photo, I’m adjusting the idle screw of one of the carburetors to achieve 1500 rpm while keeping the pointers at the same height to ensure equal flow through each carburetor. Obviously, you have to adjust both idle screws, alternately, to achieve the goal.
BTW, see that fuel filter, just beneath my right hand? Get rid of it. Do not use any fuel filter on your SUs. Totally unnecessary. There is nothing that will go through your fuel pump that won’t also go right through your SUs without doing any harm. Fuel filters on SUs are, quite simply, a breakdown waiting to happen. You’re better off without them. And putting a fuel filter upstream of the fuel pump is particularly detrimental, as it can cause vapor lock, starving your engine for fuel.
Well, it’s pretty hard to see in the next photo, but the Tach/Dwell meter shows about 1500 rpm, and the pointers are both at the same height.
Next, adjust each carburetor jet mixture nut while watching the Tach/Dwell meter. Turn the nuts up and down alternately until you find the position of each nut that maximizes the engine RPM. It’s not necessary to keep the RPM constant at 1500 RPM. But you should try to keep it in the range of 1200 to 2000 RPM.
You may have to turn one, or the other, or both, idle screws to maintain equal flow through each carburetor as well as the desired RPM.
I like my analog Tach/Dwell meter much better for adjusting carbs than a digital one. The analog meter stays relatively steady during adjustment, whereas the digital one I recently tried jumped around all over the place, making it hard to see the effect of small adjustments of the jets.
Forget about those lift pins under the pistons; they’re useless. That’s my opinion, anyway.
After maximizing the RPM with one nut, switch to the other and adjust it the same way, then go back to the first one. Keep at it until you’ve found the point of maximum RPM, where moving either nut, in either direction, causes the RPM to decrease.
You have to wait a few seconds after each adjustment to give the engine time to stabilize at its new RPM; so be patient.
Okay, here we are. RPM is maximized; pointers are at the same height, and the engine speed is around 1440 RPM, as shown in the next photo:
Note that this analog Tach/Dwell meter has an RPM scale (at the top) for 8 cylinders. The reading must be doubled for use on a 4-cylinder engine. So 720 x 2 = 1440 RPM.
Now that you’ve found the point of maximum RPM, you should probably turn each jet adjustment nut downward (i.e., richer) about 1/6 to 1/3 turn (one or two flats). This seems to help prevent hesitation on acceleration. So, wait until after you’re able to try your car out on the road, and then decide if you need to richen the mixture by 1 to 2 flats on each carburetor.
Now, adjust the idle speed to about 800 RPM, using the idle screw on each carburetor. Be sure to keep the pointer wires at the same height to ensure equal flow through each carburetor.
Remember those two linkage rod nuts you loosened a while ago? Here’s the photo, again:
Well, now it’s time to tighten them. Unfortunately, I don’t have hands enough to do the job and take pictures too, so I’ll show you two pictures of the process, whereas the job was actually done in one step.
Take a screwdriver and push one of the linkage arms downward until it just touches the bottom of the fork on the end of the throttle shaft, as shown here:
At this point, rather than take a picture as I did, use your other hand to hold a nut driver and tighten the nut while holding the lever arm downward. See the next photo:
As I said, you really need to hold the lever down at the same time as you’re tightening the nut. I just don’t have enough hands to do that and take photos too.
Now, do the same operation at the other end of the linkage:
And, of course, you tighten the nut such that the lever arm just touches the bottom of the fork on the end of the throttle shaft.
For Type H carburetors, which have a different center linkage arrangement, just tighten the linkage, being careful to ensure that neither throttle shaft is moved during the process.
At this point, you should grab the throttle linkage at the middle and use it to open both carburetors a bit. The two pointers should move up simultaneously and at the same rate and height. If not, recheck your linkage and readjust to ensure that both carburetors open at the same time.
Now it’s time to add oil to the dashpots. Use Mobil 1, 15W-50:
Nothing else I’ve ever tried works as well. Nothing. For more information, refer to:
Fill the dashpots to about 1/4″ below the top of the inner tube. If you put too much in there, it won’t do any harm, but it will get squeezed out and wasted when you install the dampers.
After installing the dampers, lift each of the two pistons all the way up and use a rag to wipe up any excess oil that gets squeezed out.
Now, before re-installing the air cleaners, go out for a road test, and see how it runs. You may want to richen the jet adjustment nuts by 1 to 2 flats.
At this time you should also adjust your fast idle screws so they don’t contact the fast idle cams with the “choke” pushed all the way in, but such that they do make contact with the cams as the “choke” cable is pulled out. Where you leave these screws is pretty much up to your personal preference.
Now it’s time to re-install your air cleaners.
Now, as promised above, here is a photo of my later-design adjustment and synchronization wires:
The bends are at the top of the inner dashpot tubes, rather than at the bottom. This configuration works a bit better because it holds the top of the wires more steady, with less vibration. It also makes it less likely that the sharp cut end of a wire might scratch the dashpot tube inside the carburetor. Also, you can easily cut a little off the bottom end, if you need to, to adjust the height.
Here’s a shot of this later design in use on another P-1800.
Note that the ends of the wires don’t overlap; rather they’re cut so that they almost, but not quite, touch. That works better; it’s easier to tell if the carburetors pistons are at equal heights.