Engine hesitant under acceleration - Coil?

[fcd911]

I have an electrical issue with my 911 2.7 MFI rally car – in the dark under load (climbing a hill) or accelerating from mid-range there is a pronounced hesitancy. This happens initially on main beam with spot lights on before not even liking just main beam (dip beam it is OK). My initial thought was alternator and or voltage regulator or battery. However I have pretty well eliminated these (in my head) due to the fact that the lights do not dim when the engine is hesitating or when ticking over and the car starts OK even 60 mins after switching off so the battery is charged.
The car has standard electrics – points, black coil etc. with a Classic Retrofit CDI Unit. The problem only occurs when the car has been running for a while and is hot. All was fine until about 15 minutes into a night section (after a long run out).

I have not tested anything yet but I suspect the coil. I have a couple of spare used black coils but as the problem can take a long time occur I thought it may be sensible just to buy a new coil. Not so straightforward it appears as there are a number of options and prices (!), ranging from about £63 to £223 (Design 911) the later claims to be a match with standard CDI and Classic Retrofit versions and is referred to as a Transformer (1 to 100 step up) - so if this particular coil is claimed to be for CDI what are specs for the others coils (eg – BERU) which are also supposedly also suitable for CDI?

Any thoughts on my coil theory and also recommendations about which coil to go for - if it is that at fault.

Thank in advance

John

[Mick Cliff]

Check points gap.

[fcd911]

Yes, in fact I checked and reset the points gap before the event.

[911hillclimber]

Has this occurred after the pints check or was it there before John?
Can you swop the coil from the Targa as an experiment?

[fcd911]

Returning home in the dark from the previous event very near home, on main beam for an extended period, there was just a slight hesitation once, however the rev counter had been a bit unstable and I put it down to points gap. Hence the reason for the adjustment. Needless to say in a day light test everything was perfect.

I am very reluctant to try the coil off the Targa, it is (probably) 50 years old and I would not want to stress it in any way. The headlights get switched on only for the MOT and the biggest current draw the electrics experience normally is from the electric aerial when the radio get switched on!!

If the symptoms point to the coil being the culprit (also probably very old) I think a new one would be the answer. It is a question of which one. As I said I have spares but they will be old and the last thig I want is another failure on an event.

I should say that although I describe the fault as hesitancy it actually feels like the engine very momentarily cuts in and out

[jwhillracer]

Yes, in fact I checked and reset the points gap before the event.

Did you check the ignition timing and re-set it if necessary after adjusting the points?
JW

[BILLY BEAN]

Jonny Hart used to sell black coils suitable for his CDI boxes.

[fcd911]

Yes, in fact I checked and reset the points gap before the event.

Did you check the ignition timing and re-set it if necessary after adjusting the points?
JW
[/quote]

Caught me out John I did not at the time - I was just readjusted them back to spec - I have now and all OK.

Jonny Hart used to sell black coils suitable for his CDI boxes.

My first thought was to look on his website but there was nothing showing.

John

[911hillclimber]

John, I have the black Aldon Automotive hi po coil off my original Lola engine you are welcome to try or have if it works.
I have also the black simple coil off the Lola engine which ran points but not CDi, again, you are welcome to it if it works.
Let me know and I'll pop them over to you.
Could do with using the Boxster next week!

[jwhillracer]

For early Porsche CDI you need the proper Bosch coil.

JW

[911hillclimber]

Ah..
Yes, now you say it, the coil is special, you have jogged my memory.
Been about 30 years since my 911 ran CDI.
But is one needed for one of Jonnie's CDI's that John has on this car?

[Nine One One]

The Black CDI coil was 0 221 121 001 a superseded one had a part number of 0 221 121 010

[fcd911]

Thank you for confirmation of the Bosch part number. The well known specialist suppliers list a number of coils (none are Bosch) for early cars but only a couple are quoted as being Transformers (1:100) or specifically for CDI

I suspect that pretty well any coil will work OK with CDI until you put a demand on it. My existing coil is the correct type but I believe is past its best!

I have found the Bosch Coil on a number of sites. If I recall correctly, at one time these Bosch coils were manufacture in Brazil (?) and had a reputation for poor reliability. Could this still be the case?

[jwhillracer]

I have had one of the silver Bosch coils on our CDI equipped 914/6 for more than 10 years and it’s been OK - so far!
JW

[Nine One One]

Have you got a multimeter to test your current coil?