The MGA With An Attitude
ENGINE TUNE-UP The Casual Way - CF-120

After years of resisting the temptation to re-write the shop manual, I am posting this by popular demand. This is a home brewed tune-up procedure written in layman's terms for the do-it-yourself mechanic with a limited tool inventory. It is intended to assure the average Joe that there is nothing mysterious or tricky about SU carburetors, and that anyone can do a decent tune-up on the MGA.

Pre-Tune-Up Checks:

If the maintenance history of your engine is unknown you should do some preliminary checks or adjustments to assure all is right with the engine.

  • Check valve motion (lift distance), set valve clearance, and do a compression test to verify good compression on all cylinders.
  • Check and adjust the ignition parts. Set the points gap and spark timing, and be sure you have strong spark to all four plugs.
  • Give the distributor rotor an anti-clockwise twist to verify the mechanical advance mechanism is free to move.
  • Draw a strong vacuum on the distributor to assure the vacuum advance mechanism will move and the diaphragm does not leak.
  • For static timing start with 12d BTDC. If you have a strobe light, try 20d BTDC at slow idle. The original specification of 7d static is sort of a lost cause intended to satisfy worst fuel conditions for the whole world in the 1950's. (More on spark timing later).
  • Check for adequate fuel delivery and no air bubbles in the fuel line.
  • Check for fuel dripping from the carburetors and fix that if necessary.
  • Pull the choke full on and release it. Reach under the carburetors and verify that the main fuel jet has returned to the full up position. If you press up and the jet snaps up to the rest position, it was stuck in the lowered condition, which must be corrected before proceeding with the tune up.
  • Open the dashpot covers, lift out the air float piston, remove the long fuel regulating needle, and check the numbers on the shank of the needle to verify use of the correct needle for your application. The needle should be installed with the shoulder flush with the bottom of the piston. The piston should rise and fall freely (firm thump when it hits bottom of travel).
  • If the dashpot piston hangs up, clean the piston and the dashpot cover with solvent and soft rag. Never use abrasives, and keep the pistons and covers in matched sets, because these parts are lapped to fit with minimal working clearance for good air seal around the outside.
  • The main fuel jet must be centered to lie concentric with the needle so it will not rub on one side. There is a jet centering "pin" included in the SU tool kit which makes short work of this alignment. With some personal attention and a little fiddling it is possible to center the jet without the special tool. Procedure is to loosen the BIG hex gland nut on the bottom to allow the jet gland to float, center the jet, and tighten the gland nut with the jet centralized. This is to be done once with initial assembly or during a rebuild, but should not need to be touched thereafter.
  • Check that there is oil in the damper tube on top of the dashpot. Oil level should be no higher than the top of the smaller inner bore of the moving piston. Lift the plunger a bit and press down. If you feel resistance to motion there is enough oil present without even looking. Any type of oil from ATF to 20W50 will work. When in doubt, thicker grades may work a little better. If you want to spend time fiddling with this you can tweak the oil viscosity to suite your driving preferences after the tune-up is finished. Purpose of the damper is to prevent fuel mixture from going lean on acceleration, which affects throttle response and possible hesitation.

    Carburetor Adjustment:

    Run the engine to normal operating temperature prior to adjusting the carburetors. The easy concept for adjusting dual carburetors is to do one at a time. There is a balance tube in the intake manifold which allows the engine to run at low speed on one carb with the other one completely closed. Loosen one of the accordion clamps on the throttle shaft between the carbs so one carb can operate independently from the other. Back off the idle screw to close one carb completely, and set the idle with the second carb only. Then it is very simple to tune the carburetor as a single carb engine with no confusion from the other carb.

    With engine running on one carb, always adjust idle to stay below 1000 rpm during tuning. Turn the mixture nut downward (clockwise from top, anti-clockwise from bottom) to richen mixture (more fuel). If you can put your right hand under the carb, turning clockwise like tightening the nut is going more lean. Go more rich until the engine will gallop at idle (which is always easy to do with any MG). Then turn back toward lean. As it goes more lean the engine will smooth out and pick up speed a little. Reduce idle speed to under 1000 RPM as necessary. Going more lean it should eventually get to a point where the engine will start to shake a little, indicating too lean and slight misfire.

    If you cannot adjust the carburetor to go to the too-lean condition, stop and fix this problem before going on with the tune-up. Running constantly rich may be caused by a worn aperture in the main fuel jet, or a worn needle, or a sticking air float, or a fuel float jet sticking open, or a float setting with fuel level too high. (See Misfire and Troubleshooting). There may be cases of odd maladjustment of things where it is difficult or impossible to get the carbs to run lean. Correct fuel level will have the fuel in the float chamber at the same height as the top of the fuel jet, which should be slightly below the flat "bridge" in the bottom of the carb throat. If you remove air cleaners, look into the throat, and lift the piston a bit, you should be able to see fuel at the top of the main fuel jet. It may be easier to remove the dashpot cover and air float piston rather than removing the air cleaner. If fuel level is too high it not only runs rich but can also overflow fuel into the throat of the carburetor when the fuel pump is on with engine not running. If the fuel level is too low you may still be able to adjust for proper fuel mixture at idle speed, but it may run lean at higher speed with a slight loss of power and maybe mild temperature rise. If the float valve sticks open it will constantly flood badly. This is very noticeable at slow engine speed but may go unnoticed at full road speed. Just because it runs pretty good at full chat doesn't mean it's right. Too much fuel will wash oil off of the cylinder walls to promote accelerated wear of piston rings and cylinder walls. Longer term flooding may result in fuel mixing with oil in the crankcase to dilute the oil, which could ultimately result in crankshaft bearing failure.

    Given that you can adjust the carburetor to the too-lean condition (engine shake), turn slowly back toward rich just enough to make the shake go away, and then go another 2 flats (1/3 turn) toward rich. The last bit is to get it away from the tricky threshold where temperature variations could throw it on the wrong side of a too-lean condition. That's all. Then close the throttle on that carb and open the other one, and follow up with the same mixture adjustment on the second carb.

    After both carbs are adjusted to the right mixture you need to "synchronize" the carbs for equal air flow. This is a lot easier than it sounds, you don't need any special tools, and you don't even need to remove the air cleaners. Put one finger on the throttle arm next to the idle adjustment screw. Back off the idle screw until the throttle is closed (solid stop). Turn the screw back and forth a few times to determine the exact position where the throttle arm just starts to move. From there turn the idle screw clockwise 1/2 turn to open the throttle for slow idle. Do exactly the same for the second carburetor. Then tighten the accordion clamp on the throttle shaft to lock the two carbs together. After that, any time you need to adjust idle speed you should turn both idle screws exactly the same amount to keep them in synch. Idle speed around 1000 RPM is good for throttle response from dead idle.

    If you are picky and want to fine tune the mixture, reach under the side of a carburetor with one finger and press up on the lifting pin to raise the dashpot air float very slightly (1/32 inch after the pin touches the piston is enough). If the engine stumbles, the mixture is too lean. If the engine speeds up a little and keeps running faster, the mixture is too rich. When mixture is correctly adjusted, lifting the pin will result in a momentary speed up and then settling back to the original idle speed. This technique is very accurate. The difference between being noticeably too rich or noticeably too lean is plus or minus about one flat on the nut.

    For cold weather driving you may adjust the carbs 2 flats (1/3 turn) more rich. For best fuel economy and clean running you want the mixture to be as lean as possible consistent with good running. For best power you want the fuel mixture to be somewhere between slightly rich to flooded, but never lean, as too-lean mixture can burn valves and aluminum pistons. You should also check the manual choke linkage and adjust if necessary for proper cold start fuel enrichment and fast idle.

    Spark Timing Adjustment:

    If you do not have a timing light you can sill find a good setting for the spark timing. With engine running at idle speed turn the distributor body anti-clockwise to retard spark timing. Engine speed will slow down when you do this. Rotate the distributor clockwise to advance spark timing. Engine will speed up when you do this. Reset idle speed as necessary to keep engine speed under 1000 rpm. When you go too far advanced the engine will start to shake a bit. Turn slowly back anti-clockwise to retard timing just enough to make the shake go away. Then turn slightly more anti-clockwise to retard timing until you notice a slight drop in engine speed, not much, just 50 rpm drop is enough. Lock it down there and be prepared for a nice test drive. You might also do the lift-the-pin test on the carburetors again just to verify the carb setting is still valid.

    You will want to do a test drive to be sure the engine does not ping (pink) noticeably under load. Some engines will not ping under any circumstances, so do not use audible ping to set spark timing to maximum. An uphill incline can be helpful. Drive at moderate speed using 3rd or 4th gear. Starting around 1500 to 2000 rpm, floor the throttle for maximum acceleration. As it accelerates through the 2000 to 3000 rpm range, listen for audible pinking. If it pings you may consider using premium grade fuel to suppress the pinking. If it pings on premium fuel, or you don't like the cost of premium fuel, you can retard the timing a few degrees to eliminate the pinking. A high compression engine may produce slightly more power with premium fuel and more spark advance. There is otherwise no advantage to using premium grade fuel if your engine does not ping.

    Once you have the timing set where you like it you should use a timing light for a final check, even if you have to borrow one just once. You can check the dynamic spark timing at idle and record this setting for future reference. With a setback timing light you can also check timing at higher engine speed with full cumulative spark advance. Without the setback feature on the timing light you would need to extend the row of timing marks out to 35 or 40 degrees BTDC. You want to verify and assure two things with the timing light. First do not set idle timing to more than 20d BTDC. Also never allow high speed timing (at 3500 rpm or higher) to exceed 34 to 36 degrees BTDC with vacuum disconnected and plugged. Too much spark advance at high speed can cause reduction of power, poor fuel economy, overheating, burned valves and pistons, or broken ceramic insulators on spark plugs. With vacuum line connected you may see more spark advance at high speed and low throttle setting, which is good for fuel economy. But when you mash the throttle for max acceleration the vacuum signal should go away, and the spark should drop back to 34-36d BTDC max.

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