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A Systematic Approach to Full Throttle

September 26, 2010 | Troubleshooting & Repair

By Ab Tiedemann

The ball sockets should be checked for wear before the task is started and all replaced if any clearance or binding is felt. There are 9-10 sockets to consider and .05-.1 mm clearance in all those places could amount to about 12-15 mm of arc at the accelerator pedal. They also need some lubrication. Graphite grease works well.

There is a fundamental design dimension that should serve as the starting point and it should be preserved when you are finished.

The center of the ball on the little bell crank that attaches to the fan housing and connects to the pull rod coming up from the bell crank at the transmission should be 50 mm from the vertical surface of the fan housing. This is the starting point. The geometry is illustrated in the 356 C Workshop Manual, p SF 26, Figure 8.

With the pull rod free from the transmission bell crank and the push rods free of the carburetor connection, adjust the near vertical push rod that connects the aforementioned little bell crank with another bell crank on the cross shaft to obtain approximately a 15 degree angle with respect to horizontal of the lever that connects the cross shaft to the carburetor lever arm via the carburetor push rods [still free at this time]. Secure length. This rod has right hand threads both ends so it will be necessary to take one end off to adjust. Next adjust the idle advance screw [not the idle mixture control screw] until it is completely off the stop. Turn the screw until it just touches the stop and advance it 1/2 turn more, same on each carburetor. This will assure that the butterflies are not resting against the throttle bores.

Now adjust the length of the carburetor push rods until the ball sockets can be engaged without compression of the rod and secure this length. Due to the geometry of cylinder offset and attachment points at the carburetor, the push rods are necessarily different lengths. This is not relevant for the initial adjustment or for the final adjustment [actually a check for end travel and carburetor stop clearance].* These rods have left and right hand threads and the left hand thread is always installed at the carburetor. This is an easy pick-up for the engine judge. There will be a nut permanently attached to the right hand thread. This is used to turn for adjustment and hold for arrestment.

Disconnect the pull rod that runs up through the tunnel from the bell crank at the transmission. Install the pull rod [with formed end] into the groove on the transmission bell crank and connect it to the little ball on the bell crank that is installed on the fan housing. This is illustrated in the 356 C Workshop Manual p SF 26, Figure 9 [bottom end of pull rod] and in Figure 8 for the top end of the rod. Note the angle of the bell crank with respect to a reference line on the transmission case. Actuate bell crank at transmission for full travel and note angle with respect to the previously chosen reference line. Adjust the pull rod to the fan housing crank until these angles are equal [referred to in original post as..." perpendicular to the linkages"?] Comparative geometry of the former and later designs are shown in the 356 C Workshop Manual, page SF25 Figure 6. In addition there should be a little clearance—say 1-1.5 mm between the connection point of the pull rod [item 2 page SF 25, op cit] at the transmission bell crank and the formed end at the initial position of the pull rod that runs up to the back of the fan housing. Note position of "loop" in formed end is up. This point should have a little grease applied as well. If you do not have this little clearance the engine is likely to idle high when warm due to the expansion of parts "shortening" the rod and resulting in pressure on the linkage however slight.

With the pull rod connected to the pendulum crank near the accelerator pedal, adjust the length of the pull rod [socket adjustment] until it can be engaged at the transmission bell crank without compression.

Now you can make the first test of "full throttle".

Depress accelerator pedal until it touches the stop on floor while an assistant looks down the carburetor bores. If near vertical, you are done. The stop at the carburetor should have clearance.

If not, there are two options. You can lower the stop or you can change the angle of the pedal. If you have an original rubber covering on your accelerator pedal, you may find that it has a small "notch" cut into the underside of the pedal. This is to assure that the metal of the pedal contacts the stop screw. Sometimes this thickness of rubber prevents full travel if not removed.

There is another push rod that connects the accelerator pedal and the pendulum crank under the floor boards. This rod has right hand threads on both ends. To make the latter adjustment, you will need to get under the mats and the boards, etc. It all depends on the angle of the pedal that feels right throughout the motion. It cannot be leaning back too far or require the foot to push too much forward—a balance of feel so to speak.

It is a fiddling task, but the above has worked for me for more than 45 years. Starting from scratch it will take 2 hours minimum.

*After the initial verification of "full throttle" it will be necessary to balance the air flow of the carburetors at the idle position and at some intermediate position associated with a steady, comfortable for the pilot, cruising speed. This might correspond to 2500-3000 rpm. When the air flow is balanced at this intermediate rpm range, the air flow balance is disturbed at idle, but easily corrected. The latter has the benefit of more equalizing the butterfly openings side to side over the entire spectrum from a maximum of 3 degrees to 1 degree, for example. This can be demonstrated, but a topic for another dissertation. When all balances are achieved, it is only necessary to verify that the throttle lever is not hitting the stop at the maximum excursion of the accelerator pedal. If this is observed, the accelerator pedal stop would need to be raised until clearance is achieved at the carburetor stop.

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