Duane Spencer - Porsche 356 Performance Guide

from Duane Spencer's Porsche 356 Performance guide, 2001 edition used with permission

Accelerator pump - page 70

The accelerator pump injection quantity is easily set by adjusting the nut on the threaded rod. The manual suggests the following quantities for two strokes; in warm weather, 0.45cc and in cold weather, 0.65cc, both of which are WAY TOO MUCH! With the changes in gasoline technology over the last thirty-five years, and to meet the ever more stringent emission requirements, today’s fuel is entirely different! Here in California, with about five years experience with our new Reformulated fuel, I have found the injection quantity needs to set much lower than the factory setting, otherwise it just floods the engine with too much fuel. I shoot for about 0.1cc per strong for a relatively stock or hot street engine and about half that for an all-out race engine.

If you do not have a vial for measuring the fuel injected quantity, you can use the following technique; unscrew the nut (counter clockwise) on the threaded rod until there is nothing coming out of the nozzle except a bubble when the throttle arm is put through a full stroke. Then, turn the nut in clockwise about 2-½ to 3 turns, install the lock nut and snug it down. It should pee for about one to one and half seconds per stroke; if it last longer than that, cut it back some more. This should provide enough fuel quantity to get it started in the morning, hot or cold weather, without causing flat spots when accelerating. An added benefit, is that it will improve the fuel mileage.

Setting the idle - page 72

For the solid shaft carbs, turn the idle speed screw back (counter clockwise) on each carb until the idle speed drops to around 900-1000 rpm. Check each throat with the Uni-syn to balance the flow through each side. It is a good idea to pop one of the vertical drop rods off of the throttle linkage so that it does not influence the other carb. Then, begin slowly turning in (clockwise) one of the idle mixture screws. The idle speed will generally begin to rise and then begin to drop. As soon as you detect this drop, stop and back the screw out until it is at its best idle speed. Go the next one and do the same. At this point, it may have sped up somewhat, to say 1200 to1300rpm, so it is necessary to back off the idle speed screws some more and then re-balance the carbs. It may be necessary to go through this procedure several times until a smooth steady idle is achieved at around 90-1000rpm. Reconnect the throttle drop rod and adjust it, if necessary, so that it does not influence the idle speed, then rev the engine a couple of times and observe how it comes back to idle. If it comes back and settles into a smooth idle, great! If it drops down too far and almost stops, the idle mixture is a little too weak, so turn each of the idle mixture screws out about a quarter turn and try revving it again until it returns to a smooth, steady idle.

With the split shaft Solexes, the idle speed is primarily controlled with the mixture screws. I start out by turning the idle speed screw out so that the butterflies are completely closed and then bring the speed screw up to the throttle arm so that it just touches the stop. Then I turn each idle mixture screw in as far as it will go, and back it out three complete turns. Fire up the engine, remove one throttle drop rod, and check the flow of air through each of the four throats. Generally, the back throat of each carburetor will be drawing more air than the front throat. This can be rectified by adjusting the little screw located under the float bowl in the center of the carb by turning it in (clockwise), which closes the butterfly. Once the front to back throats are balanced on each carb, adjust the idle speed screws on the throttle arm to balance the carbs on each side. These idle speed screws are very sensitive, so move it just a little. Chances are at this point that the idle speed is higher than desired, possibly 1200-1400rpm. Don’t despair! Begin by turning in each of the idle mixture screws about a third of a turn. The idle speed may go up some, or it may start to come down. Keep turning the mixture screw in slowly until the idle speed comes down. If it starts popping, either through the carb or the exhaust, back the screw out until it stops popping. Go to the next mixture screw and do the same thing until all the screws are adjusted. The idle speed should have come down into the 800-900rpm range and is smooth and steady. If not, keep adjusting the idle mixture screws until the desired idle speed is achieved. Then, reconnect the throttle drop rod and adjust it if it affects the idle. It is not uncommon for split shaft Solexes to come down to a high idle of 1200-1300rpm for a few seconds, then slowly descend to 900rpm or so. Some do it, some don’t and there is not much that can be done to fix it that I know of. I think it is an interaction between the carbs and the distributor advance and once the advance returns it pulls the carbs down to the proper idol speed. Some people are very annoyed by it and others don’t even notice. If it really bothers you, try using the clutch to pull it down to idle - that usually works well.

-------As a new engine breaks in and gets 500-1000 miles on it, it is common for the idle speed to increase somewhat and it may be necessary to adjust the carbs again to achieve the desired idle speed and smoothness. Also, outside air temperature, air density and humidity have an effect on idle speed and some more fiddling may be required, but now you how to do it and its easy!