Bosch Distributors

I finished the BR18 you sent to me for a rebuild and I examined the BR18 you sent due to "hanging up." I will be mailing them back to you today. I checked on the Sun 404, of course, and my second 404. I put it in my '57 356 coupe and ran it around the freeways here for over 20 miles so it go nice and hot. It always came back to idle. The findings are written on the build sheet for that distributor. (Yes, I keep all the records for everyone that goes out.) It is attached along with the tester curve. More on that later.

In regards to the "hanging up" BR18 you should know how I go about determining the start of advance, and the curve, of one of my distributors. The first test for the finding the start of advance and also the curve is done using a Pertronix electronic ignition. I do this instead of points for speed. It only takes three screws to install and remove the Pertronix already mounted, of course, on a contact plate. After the curve is established I remove the Pertronix and install the points and insulators. I then document the curve with the points set up. I use a digital tachometer to read the rpm of the Sun 404 distributor tester. It measures the movement of a reflective strip on the degree wheel on the tester. It is accurate to four places. E.g., a reading below 1000 rpm would be, say, 864.3 rpm which I round off to 865. Above 1000 rpm I would get a reading of 1467 which is rounded off to 1465 rpm. So, I actually test the distributor twice. Once with the Pertronix and once with points. After completing assembly by adding the clips and securing the drive dog I put it on the tester once more to set the dwell and check the starting rpm of the advance. If there is any doubts for anything I run it on a second Sun 404 that was fully restored a couple of years ago. The digital tach is used with it as well. With several tests for the curve and the digital tach I feel very confident of what I send my customers.

All said, I believe what may customers say and always invite them to send it back so I can check it. If I am missing something I want to know right away so I can avoid future mistakes or change my process. I am never upset if someone sends one back. In the ten years I have been doing this only one has come back with a starting advance of less than 1000 rpm. It was 950.

There are two main reasons that a engine doesn't come back to idle. The first is the carburetor adjustments (or lack thereof) and the linkage. I recommend first in either case to remove the throttle rods from the carburetor's throttle levers. Check to see if there enough slack in the linkage. You should have to move the throttle rod up about three sixteenths of an inch before you can push it onto the throttle arm ball fitting. This slack allows for the expansion of the throttle linkage. If there is no space for all the throttle linkage parts to expand when the engine warms up then the throttle rods will push against the throttle arms and keep the butterflies from closing to the stops you have set for your desired idle speed. This is truly "hanging up." Next make sure the carbs are sync'd. With the throttle rods still disconnected assure that both carbs are pulling air at the same exact flow. I highly recommend the synchrometer sold by most of our vendors. It does not change the air flow when you are using it and the red flow indicator lets you see very small movements in air flow. The old "bouncing ball" flow meters work but are not as accurate and cannot be left on the carb but for a few seconds at a time. OK, once they are in synch adjust each idle mixture screw so the highest idle is achieved with each screw. When the idle goes over 1000 reset the idle stop screws. Go around at least twice to make sure the idle mixture is the best you can get. I.e., screwing in or out makes the idle speed decrease. You should be out about 1 1/4 turns. If it is under one turn the idle jets are too large. If it over two turns the idle jet is too large. Change if necessary or it will never be right. If you do all these things and you cannot get the idle to be steady under about 950 rpm then you have an air leak. Either from the worn out carbs or elsewhere. Always exceptions, of course.

It is the centrifugal force from two swing weights that move outward and allow the distributor cam to turn thus advancing the firing time. It is the fuel/air mixture and the timing that dictates the speed of the engine. If the mixture is correct the engine will fall back to your idle speed at the timing you set it, e.g., five degrees BTDC. If the springs in the distributor have enough tension to keep the weights from moving until some rpm below 1000 then all be well. If the springs do not have enough tension to hold the weights in rpms under 1000 then all will be so well. You can test this for yourself. With the engine at idle and the distributor clamp loosen (only enough so you can move it by hand) move the distributor slightly counter clockwise which will advance the timing. The engine speed will increase. This is why you need to check the idle mixture when you change distributors. Who knows what distributer or conditions were set before you arrived? Worst offender is the VW 009 that most people have to set at a ten degree advance to get enough total advance for our 356 engine. That ten degree advance will require a different fuel air mixture to keep the idle down. Richening or leaning the mixture can be used, too, to get an engine to idle. Not recommended.

This is a note that John Jenkins sent to a customer after troubleshooting his distributor. It is posted here as it gives some excellent information regarding how John sets up / tests his distributors as well as some useful information about how to set the linkage arms and the carburetor's idle.