Here's a linkage setting procedure, which I believe was put together by George Davis although I am not sure. If someone will correct me on the author's name, I will change it:
W113 Basic Fuel Injection Linkage Adjustment and Idle Adjustment Procedures
Part 1. Fuel Injection Linkage Basic Adjustment
Correctly adjusted linkage is an important and basic part of tuning the engine. It’s up to the fuel injection system to inject the right amount of fuel to match the amount of air going into the engine. The injection pump is driven directly by the engine, and the pump internals (a three-dimensional cam, a set of fly weights and some springs) automatically take rpm into account. The other piece of information that the injection pump needs is the throttle position. This information is provided by the linkage, and we want that information to be accurate. If the throttle is open 25 degrees, we don’t want the pump to think it’s open 20 degrees, or 30 degrees, because it will then inject the wrong amount of fuel, either too little or too much. When we adjust the linkage, we synchronize the pump to the throttle. Proper adjustment of the linkage is important and fundamental to the proper operation of the engine.
The procedure described below is for the Basic Linkage Adjustment. There are more advanced adjustments that can be performed, but the BBB says these advanced adjustments are normally not needed. The more advanced procedures are described in the BBB and require additional equipment.
Caveats:
This procedure generally follows the procedure set out in the BBB for the 280 SL. As far as I know, it is also applicable to 230 and 250 SLs, but I can’t guarantee it. Further, I have tried to set this procedure down correctly, but I can and do make mistakes, so follow this procedure at your own risk.
If your car was tuned up by someone else, the linkage may be misadjusted for two reasons: (a) it was done incorrectly; or (b) it was deliberately misadjusted to try to make the air/fuel mixture richer or leaner; for example, to improve fuel mileage. In the case of (a), you are about to correct things by following this procedure. In the case of (b), you are about to undo the work you previously paid for, and you might be making things worse. Be sure you want to do this before you start.
Terminology:
· Left and right are determined by standing at the rear of the car and facing forward
· Front or forward mean toward the forward end of the car, rear or rearward means toward the rear
· Control shaft refers to the main linkage shaft that runs across the engine
· Throttle rod refers to the linkage rod from the right arm on the control shaft to the throttle arm
· Pump rod refers to the linkage rod from the left arm on the control shaft to the injection pump arm
· Middle rod refers to the short rod that connects to the middle arm of the control shaft
· Progressive linkage refers to the cam-and-roller assembly that the other end of the middle rod is connected to on manual transmission cars
· Right control shaft bracket refers to the bracket that supports the right end of the control shaft, it is mounted on the intake manifold.
A final cautionary note before you start:
At idle the throttle plate is supposed to be completely shut, and when the engine is warmed up all the air needed for idling goes through the idle air valve (big round screw) on the front of the intake manifold, close to the thermostat housing. Part of adjusting the linkage involves making sure the throttle is closed. If you find that your throttle is NOT closed at idle, it means that part of the idle air is coming through the throttle. When the throttle is properly adjusted, this supply of idle air will be gone and you will have to adjust the idle air valve to compensate, or the engine may not idle. In this case, you should read and understand the Idle Fuel/Air Adjustment procedure (Part 2, below) before going any further.
Tools and supplies needed:
· Open-end wrenches that fit the nuts on the linkage rods
· Flat-tip screwdriver
· Grease
· A 2 inch long piece of 3/8 inch (10 mm) diameter round aluminum tubing *, or
A ruler that can measure 233 mm (9+11/64 inches) *
To decide which you need, look at the right control shaft bracket. If it has a round hole under the control shaft bearing, you need the piece of tubing. If no hole, you need the ruler.
Adjusting the linkage
1. Engine should be cold. Disconnect both ends of the throttle rod and set it aside (the ball and socket joints pop apart, use the screwdriver to lever them).
2. Disconnect both ends of the pump rod and set it aside (don’t mix them up!)
3. Disconnect the middle rod and set it aside.
4. Clean all of the balls and the sockets, and lightly grease each ball. Check for looseness and replace any sockets that are noticeably worn.
Checking and adjusting the throttle:
Using the throttle arm, press the it against the idle stop, and then open the throttle gently. The throttle should stick slightly in the closed position. Close the throttle completely and open it several times to confirm. If there is slight sticking, the throttle is properly adjusted. If there is no sticking, or strong sticking, adjust by loosening the idle stop lock nut and turning the adjusting screw in or out as needed. Tighten the lock nut and recheck for slight sticking, readjust if necessary.
Note: if you had to make a significant adjustment of the throttle, remember that you will have to adjust the idle air screw or the engine may not idle when warm.
Adjusting the pump rod:
1. Check the arm on the injection pump to be sure it moves freely and seats fully against the idle stop.
2. If you have an automatic transmission, make sure the idle compensating solenoid does not interfere with these adjustments. The idle compensating solenoid may need to be adjusted after this procedure.
If you have the hole in the right control shaft bracket, use this procedure:
1. Insert the piece of 3/8 inch (10 mm) tubing through the hole in the right control shaft bracket and align the ball on the linkage arm with the end of the tubing. This positions the control shaft correctly for the pump rod adjustment.
2. Make sure the arm on the injection pump is fully seated against the idle stop.
3. Adjust the length of the pump rod as necessary to achieve a perfect fit over the balls without moving the pump arm or control shaft.
4. When adjusted, install the pump rod on both balls.
5. Remove the piece of tubing from the right control shaft bracket.
If you do not have the hole in the right control shaft bracket, use this procedure:
1. Adjust the length of the pump rod to exactly 233 mm from center of socket to center of socket.
2. Install the pump rod on both balls.
Adjusting the throttle rod:
1. Make sure the fuel pump arm is fully seated against the idle stop.
2. Make sure the throttle arm is fully seated against the idle stop.
3. Adjust the throttle rod so it fits perfectly over the two balls without moving anything.
4. Install the throttle rod on both balls.
Checking throttle-to-pump synchronization:
1. Move the control shaft using the middle arm on the shaft while carefully watching the throttle and pump arms. The throttle arm and pump arm should begin moving at exactly the same time. If they don’t start moving at exactly the same time, adjust the throttle rod until they do. The pump arm and throttle arm should both be on their idle stops when the shaft is in the idle position. Both arms should reach the full-open position together, with about 1 mm extra movement possible on the throttle arm.
2. If you do not have progressive linkage, adjust the middle rod for a perfect fit over the balls.
3. If you have progressive linkage, adjust the middle rod so the roller is fully seated in the rear/upper corner of the triangular slot. The BBB says there should be no tension on the rod, but I find that slight tension is best. If the roller is not fully to the upper/rear of the triangular slot, the throttle pedal will be difficult to move.
Basic linkage adjustment is complete. Check pedal travel and ease of operation.
Part 2. Idle Fuel/Air Adjustment
The idle fuel and idle air are controlled by two separate adjustments. As you learned above, the injection pump arm and throttle arm should both be against their idle stops; this means the linkage plays no role in synchronizing idle fuel and idle air when the engine is idling in neutral. Therefore, it is necessary to synchronize the idle fuel and idle air manually. It is a repetitive, trial and error procedure.
The procedure basically involves adjusting the idle fuel setting, then adjusting the idle air valve for best idle, then checking idle speed to see if it’s correct. If it’s not correct, you try another idle fuel setting, readjust the idle air for best idle, and recheck idle speed. This procedure is repeated until best idle and correct idle speed are achieved.
Note: Before adjusting the idle, all other aspects of engine operation should be as perfect as possible: points and timing properly set, good plugs, valves properly adjusted, no leakage at the Cold Start Valve or thermostatic warm up system, linkage properly adjusted and the throttle valve must be fully closed (see Basic Fuel Injection Linkage Adjustment Procedure, above).
Equipment needed:
1. A vacuum gauge and a tachometer (the tach on the dash will do in a pinch, but a more accurate tune-up tach is better).
2. A straight-tip screwdriver.
3. You must also have a pretty well-charged battery, because the engine gets stopped and started a lot in this procedure.
IMPORTANT NOTE: Do NOT attempt to adjust the idle fuel setting on the injection pump with the engine running! You will hurt yourself and the pump if you do. The engine MUST be stopped for this adjustment.
This procedure is based on the BBB for the 280 SL; as far as I know, it is valid for the 250 and 230 SL, but I cannot guarantee it.
The engine must be fully warmed up. This procedure is done with the engine idling in neutral or park.
1. Stop the engine and connect the vacuum gauge some place where it will read manifold vacuum at idle. There is (allegedly) a vacuum port on the throttle body, but a special fitting may be needed to use it. With some distributors you can tee into the distributor vacuum line, with others you can’t, so check before you do this. You may have to be creative. The air supply pipe from the injection pump to the Cold Start Valve may be a good choice. Remove the rubber sleeve and attach the vacuum gauge there, making sure there are no leaks.
2. The idle air valve is on the front of the intake manifold, near the thermostat housing; it’s the big, round slotted screw. DO NOT use the throttle idle stop for these adjustments.
3. ITERATIVE PROCEDURE BEGINS HERE
4. Start the engine.
While watching the vacuum gauge, screw the idle air valve out until the vacuum begins to fall (idle gets rough). Now screw the idle air valve in; the vacuum reading should increase for a while then begin to fall again (idle gets smoother/faster, then gets rough again). Now slowly screw the idle air valve out again until you achieve the highest vacuum reading and smoothest idle.
Note: I found that the highest vacuum reading occurred over a small range of adjustment; staying within that range, adjust for smoothest/fastest/best idle.
5. Now check the tachometer. If idle speed is correct (approximately 700-750 rpm for 280 SL, check your specs to be sure), you are done. Restore your vacuum gauge connection.
Note: My engine doesn’t seem to like 700-750 rpm, it’s much happier at 800 so that’s where I set it.
6. If idle speed is not correct, stop the engine.
7. If idle speed is too high, the idle fuel setting is too high (too much fuel).
8. If idle speed is too low, the idle fuel setting is too low (not enough fuel).
9. Find the idle fuel adjuster on the injection pump. It is located on the rear of the injection pump, near the bottom. On my car it is a flat brass “knob” with a serrated edge, about an inch or so in diameter; the solenoid(s) may hide it.
IMPORTANT NOTE: Do NOT attempt to adjust the idle fuel setting with the engine running! You will hurt yourself and the pump if you do. The engine MUST be stopped for this adjustment.
The idle fuel adjuster engages a slot inside the pump when it’s pushed in; when it’s not engaged in the slot, it simply spins free. To increase the idle fuel, turn the adjuster clockwise, when facing forward. To decrease the idle fuel, turn the adjuster counter-clockwise.
Push the idle fuel adjuster in (forward) and rotate it slowly until you feel it engage the slot.
Keep pressing the adjuster in and turn in the appropriate direction 2 clicks at first (or 1 click if you think you’re very close). Adjust in complete clicks, don’t try to stop half-way between. If you don’t feel clicks, you haven’t engaged the slot.
Now go back to ITERATIVE PROCEDURE BEGINS HERE, start the engine and repeat the idle air adjustment. Keep repeating the entire procedure until you achieve best idle and correct idle speed.
Restore your vacuum gauge connection.
Cees ("Case") Klumper in Amsterdam
'69 white 280 SL automatic
