Distributor Woes - Bosch JGFU4

[Nicholas M Walker]

I purchased a Robin Hood S7 a couple of years ago as a running project and have been chasing engine gremlins pretty much from day one.  The engine is a 1.6l Pinto from a 1983 Sierra - not massively potent but plenty fast enough from behind the wheel!  Initially there were issues with the dirty fuel affecting fuelling, so I rebuilt the fuel system with an electric pump, fitted a new 32/36DGAV carb and had it setup by a good friend of mine.  Unfortunately the issues remained and persisted even when I replaced coil, rotor arm, distributor cap, leads and plugs - the car would start and run, accelerate OK but would always 'stutter' and misfire in the mid-range when trying to hold a given speed.

Given that there wasn't much else to try, I bought a Powerspark Electronic Distributor kit - unfortunately this turned out to be useless with my engine.  After a bit more invstigation, I have finally pieced together the story - so if you're sitting comfortably I will begin!

My engine was fitted with a Bosch JGFU4 distributor - a factory fit item with inductive discharge system and vacuum advance.  I won't go into the full detail of exactly how the inductive discharge system works but basically there is a metal 4-pointed 'star' mounted on the distributor shaft, a static disc with four posts within the body and wire coil to generate the magnetic field.  When correctly setup, the four points of the rotating 'star align with the four posts in the body when the rotor-arm is aligned with the No. 1 mark on the edge of the body. The 'star' itself is made up of two parts, an inner steel collar that goes over the rotating shaft and which is located with a small steel pin, and the outer 'star' which is an aluminium pressing.  These two parts are then pressed together.  Unfortunately, this distributor also has a bit of a design flaw - with the Distributor Cap removed the clips used to secure the Cap onto the body have a tendency to drop inside and catch on the edge of rotating star.  This means that if you bar the engine over with the Cap removed, it is posible to damage the rotating 'star' so that it goes out of alignment. 

Having taken my distributor apart, this had happened - the 'star' was out of position by about 10 degrees!  I have attached some pics which show the spider in the actual and correct positions - there is a locating pin which sits in a groove in the side of the shaft and the inner part of the star.  Another telltale sign of this issue was that the leading edge of the rotor arm tip was clearly burnt, a sign of serious arcing occuring because of the offset between the inductive discharge system and the actual firing position of the engine.  Again, I have attached some pics showing the damage to the rotor arm. 

Following discussions with Jonny Best @ Bestek Crawley, I have been able to remove the original 'star' from the rotating shaft (carefully) and am now awaiting a replacement item to complete the rebuild.  I hope that this is of interest to some of you - I will post more pics once this is completed and hopefully wrap-up this issue once and for all.....!

[Bob Tucker]

Well done Sherlock. That took some tracking down. Hopefully you will be able to enjoy using it in the good weather at the moment.