Article 7 – Adjusting the Carburators

Technical Article 7 Tuning the Carburators

If you have been with me for my previous articles on troubleshooting the XK engine, you will know that we have throughly gone over the cam timing, valves, and the ignition system. I have saved the carburators for last. There is an adage that says “most carburator problems are electrical”. Although that may sound strange, the point that is trying to be made is that many folks try and place poor engine performance on the carbs when the real problem most likely lies in your ignition system. So I have left the carbs for last because if the cam timing, valve adjustment, and ignition system aren’t in great shape first you will have the devil of a time getting your carbs to adjust properly.

I am going to concentrate on the SU carbs that were placed on the XK engine up to late 60’s. I have no direct experience with the Stromberg carbs that were used later due to increased pressure to comply with US emmisions regulations. But most of the principles described will apply to the Stromberg carb. There are 3 SU carbs on the E-Type. The SU carb is a “constant depression” carb. Simply, this means that no matter what the throttle position, they maintain a constant value of low pressure (depression) in the throat area of the carb, where fuel is drawn up and into the airstream. There are many excellent articles on the Internet describing the operation of SU carbs. Some that I would recommend are listed at the end of this article.

Your carbs must be in reasonably good mechanical condition before final tuning. Good quality overhaul kits are available from Joe Curto (www.joecurtoinc.com) in Brooklyn, NY. You can also send your carbs to him for overhaul. Joe is probably the most highly recommended expert on SU carbs in the US. The SU (Skinners Union!) company is now owned by Burlen Fuel Systems in the UK (www.burlen.co.uk) from which you can order all manner of SU parts.

There are basically 4 adjustments that can be made to the SU. They are standard idle, fast idle, fuel mixture, and synchronization. I will start with standard idle. If your E-Type engine can’t be slowed down to an idle speed of around 600 rpm, you may have some mechanical issues that need to be resolved first. Looking into the opening of the carb, the throttle butterfly plate must seal well in its opening or air will leak by and elevate the idle speed. Sometimes gunk exists at the throttle plate that can be cleaned up to solve the problem. If the throttle shafts are very worn, air will leak by them and cause a fast idle. If this is the case, the fitting of new throttle shafts and seals will be required. The idle speed screw may be worn or leaking. Finally, the linkage may artificially hold the throttle open. I like to get the engine up to operating temperature and then loosen the throttle linkage at each carb. This way, you can check each carb independantly to get the lowest possible idle. Once the 3 carbs are each set such that a low idle speed is achieved, carefully reconnect the throttle linkage. Next I like to check the synchronization. You will need a Uni-Syn tool or similar, which is an airflow gauge that fits over the opening of the carb. It is assumed that the air filter assembly is off. You can first syncronize air flow at idle by adjusting the idle speed screws. I personally don’t worry too much about synchronization at idle. What I like to be fussy about is getting the air flow synchonized at a modest rpm of say 1500 rpm. This will require that the crimp bolts on the linkage be tightened down in a trial and error process. It will be clear when you look at your linkage that there is a U shaped opening in the thottle linkage that engages the shaft on the carb that opens each individual carb thottle butterfly. There is a fair amount of slop in the opening of the U. The trick is to get each U to engage the throttle shafts at the same time so all 3 throttles open equally. Again, I like to open the throttles to a speed like 1500 rpm and check the air flows there. The exact speed isn’t important. You will find that this is a finicky step that may take you some time until you get the balance between the 3 carbs reasonably the same.

Setting the mixture should be the next step but let’s look ahead at the fast idle. Fast idle is set by the choke lever in the cockpit. Pull open the choke to the Hot position and you will see the 3 throttles all move. The work shop manual describes a process for setting the fast idle but suffice it to say, the fast idle screws should not be engaged with the choke off (Run position) and with the choke in the Hot position the idle speed should be about 500 to 750 rpm faster. With the fast idle engaged, you can use your Unisyn to balance the fast idle using the fast idle screws. Finally, with the choke lever in the Cold position, you should see further movement of the throttles and the choke levers at the bottom of the carbs should clearly engage and rotate. Finally, move the lever back to Run and everything should release and you should go back to a nice slow idle.

As I have said, setting the mixture is tricky. There are various schools of thought on how to do this. I haven’t had great luck with any of them so I won’t promote one over the other! The Bentley shop manual decribes the process of lifting the suction chamber piston by a small amount and listening for a drop or increase in idle speed. See the Bentley manual for details. Some folks adjust the mixture for fastest idle speed. Other folks swear by (or at) a tool called the Colortune. It is available from a variety of parts houses. Here is a link to an article about its use. (http://www.xs11.com/tips/maintenance/maint34.shtml) You can get fancy like I did and buy a fuel/air meter. This is a wideband oxygen sensor that drives a small graphical meter. It requires that a bung be welded in your exhaust pipes. I thought it would be a great way to go but the ones I bought (you need two) for several hundred dollars don’t seem to be all that accurate. But it is fun and instructive to watch as you drive around! If you go this route, you want to stive for a fuel/air ratio of around 13:1. The XK engine absolutely will not run well at the optimum stochiometric mixture of 14.7:1. I have tried!

Once you get your mixture set to your liking, you are about done. One final tip. Do not sneer at the factory air cleaner. It works quite well and if you go with an aftermarket system, you will most likely have to use different fuel needles to obtain more fuel to go with the increased air flow. That said, if anyone has a spare factory air cleaner setup for a triple SU carb setup, please contact me as I would be interested in purchasing one.

And there you have it. Now your XK engine should be operating in top notch form! Happy motoring.

Disclaimer – Automotive work can be dangerous if proper safety procedures are not followed. In homage to our litigious society, I must state that I cannot be held responsible for any real or perceived mis-information that may be contained in this article. A good shop manual is mandatory before you attempt any work. Read the safety section of your manual.

Required Tools: Uni-Syn, Vacuum Gauge

Recommended Reading:

Constant Depression Carburators Theory of Operation (http://www.team.net/www/morgan/tech/carbs/cd-carbs.html)

JCNA Technical Library – SU Carburators Notes (http://www.jcna.com/library/tech/tech0006.html)

Tuning Your SU Carb (http://www.team.net/sol/tech/su-tune.html)

Tuning vs Setting SU Carburators (http://www.team.net/www/morgan/tech/carbs/tuning.html)

2022 Update- Below is a nice summary regarding how to adjust your carbs, courtesy of Ray Livingston from the Jag-Lovers forum. After a poster stated that he inspected the colors on his spark plugs after a long drive and found them too white for his tastes, Ray responded:

Pretty much the worst possible way to do it, for many reasons. First, with modern, non-lead, oxygenated fuels, spark plug color is nearly meaningless. Mine ALWAYS come out white, and have for the 20+ years/50K miles I’ve been driving my E. And it runs great! Second, looking at plugs after a “trip” is ALWAYS meaningless. The ONLY way to do a “plug cut” is to drive at constant speed for some time (10 minutes or so, then switch the engine off, coast to a stop, and then look at the plugs. Looking at them after some random drive tells you pretty much nothing.

Needles make a HUGE difference, and UMs are often a lousy match for modern fuels. It is often IMPOSSIBLE to get proper mixture under all conditions with UMs. When adjusted for proper idle mixture, they will be dead lean at cruise. When adjusted for proper cruise mixture, they will be massively rich at idle. A needle that is considerably richer at higher speeds/load is often required. UBs for some engines, UEs for others, and, rarely, even UOs. The only way to know which is best for your engine is to try several, and see which gives the best result.

As indicated above, setting mixture at idle can get you in the ballpark, but will almost never give you ideal mixture. For that, you have to drive it, and see how it feels, using your butt-dyno. Generally, too lean will lead to poor top-end performance, and “lean surge” at cruise. Too rich with also lead to poor top-end performance, and poor gas mileage. Properly tuned, you should have good throttle response across the board, no missing, no surge, and should be able to get ~20 MPG on the highway. Incorrect idle mixture, whether rich or lean, will result in a rhythmic miss on the offending cylinders, and “puffing” on the exhaust. That puffing will tell you which carbs are off. Hold you hand a few inches behind the exhaust. If it “puffs” only from the RH pipe, the rear carb is off. If only the LH pipe, the front carb if off. IF both, at the same time, then the center carb is off. If any pipes exhaust is too hot to hold you have behind it for long, you a too rich. Same if it sounds “burble-y”.

I usually start with the jets set 3 turns below the bridge. This will be pretty rich, but guarantees the engine will start easily. One carb at a time, go leaner in 1/2 turn increments, until the idle (for those two cylinders) becomes smooth, and there is no missing. Do all three carbs this way. Now adjust each carb in turn 1 turn richer, then leaner. In both cases, idle should get worse. Narrow the spread between those two points, to find the spot where the idle degrades, both lean and rich. Set the mixture between those two points. Repeat for the other carbs. Finally, go for a drive, and see how it is at higher RPMS/loads. Generally, adjust all three by the same amount richer or leaner to get the best overall performance across the board. Again, if you can’t make it right across the board, you need different needles.

Some have suggested setting float level by measuring the fuel level in the jet. Complete and total waste of time. Due to surface tension, it is near impossible to get anything close to a good reading. And, fuel level is NOT terribly critical. Set it using the factory method, make sure all three are set the same, Make sure the floats are parallel to the bowl lid when the valve is closed, then leave them alone. Everything else is done by adjusting the jet height. If you cannot get it running right at idle and at speed, then the needles you have are simply wrong for your engine, and you need to try the alternates.