Old School 911 RSR Twin Spark Ignition Issue – Help Please?

[drhildr]

Old School 911 RSR Twin Spark Ignition Issue – Help Please?

Long story short…

The RSR was running well so it was time for a final fettle on the rolling road to get the fuelling spot-on. That was going well and we were looking at about 295bhp so all good. But then it developed a bit of a “fluffy” spot at about 5000rpm and was struggling to get the numbers back. After much testing and faffing around we concluded that one of the CDI units was playing up. Ended up sending off the pair for a refresh to John Speake who found a duff ground track and replaced the output capacitors. Fully expected to plug everything back in (nothing else was changed) and the motor to be back on song.

Sadly not though! Car would not start at all with both new units fitted. You could get it going with just one CDI unit on the top set of plugs but even then it was not healthy. More testing showed that the newly invigorated CDI units seem to be sending such a powerful charge to the distributor that the cap / rotor seems to be overwhelmed and the spark is being pushed through all the pins regardless of the ones being hit by the rotor. Best guess seems to be a duff (but virtually new!) distributor cap / rotor. This is a J B Racing unit from the USA who are not (yet) responding to emails.
Has anyone seen anything like this before? Any guidance would be welcome as we are otherwise baffled!

Likewise – does anyone know of a UK / European supplier of J B Racing parts?

Thanks!

[BILLY BEAN]

Don"t JB Racing use the Jaguar V12 distributor cap and a modified Jaguar rotor ? If I am correct then the V12 cap is readily available in the UK.

If your rotor is mounted on a plastic disc then you definitely have the modified V12 Jaguar set up. I have a new and unused rotor for this should you want one.

[drhildr]

Thanks Billy - I have the newer, bespoke version of the J B Racing distributor.

[Jonny Hart]

If a JB distributor sensor (made by MSD) is wired with the wrong polarity, the rotor will be rotationally out of phase with the spark event. In layman’s speak, the spark will fire when the rotor is not pointing at the post. Performing ignition timing on the out of phase spark also results in the distributor having to be rotated by about 30 engine degrees from its normal position to bring it to TDC.

At low cylinder pressure, the spark often can jump the gap back to the post it was meant to be firing but as the cylinder pressure rises, it would sooner go someplace else than all the way down the lead to the plug. In my experience on a twin plug this happens at around 5000 RPM. Whether the spark can jump the gap is dependent on the voltage generated by the CDI unit.

In a nutshell, engines can often idle and rev under light load with the rotor out of phase. As soon as load is applied, the spark will not reach the plug. Another symptom of a mis wired sensor is that the timing appears to ‘wander off’ as the RPM rises.

The sensor polarity confusion is caused by MSD boxes firing on a positive rising edge so they label their sensors in the following way:

Orange (+)
Purple(-)

Bosch and almost all other ignition suppliers use the negative edge. This means that on a Bosch CDI, the Purple(-) wire should be used as the signal and the Orange(+) is the GND. If you used a premade harness designed for an MSD unit, it will be wrong for Bosch.

Do a continuity check between the pin labelled 7 on the CDI unit and the distributor sensor. It should be connected to Purple. If it isn’t, reverse the wires at the sensor. At this point you will find that the distributor will have to be turned back 30 degrees to get the timing back to correct.

[drhildr]

Jonny - many thanks for this and your time on the phone yesterday.

[drhildr]

Clean forgot to update this - Jonny was bang on and the issue is now sorted. Running sweet as a nut - 280BHP and 220 torrques

[hot66]

always good to hear what the solution was

[drhildr]

Basically the CDI units had been serviced many times in their lifetime and, unbeknown to me, had been set up differently each with the opposite polarity. Once that penny had dropped it was plain sailing. Could have tuned for more pwoer but now the car is running sweet from tickover right to max chat - no more fouled plugs from slow running etc. Thing is mental!

[dragonfly]

Great to hear Jonny could help you sort it, but this thread is lacking photos of what sounds a very interesting 911...

[drhildr]

Will post some pics later but this is it in words:
ENGINE
3.2 CARRERA CRANK CASES CRANKSHAFT AND RODS
WOSNER FORGED RSR PISTONS C/R 10.5 to 1
CYLINDER HEADS MODIFIED TO RSR SPEC INLET AND EXHAUST PORTS 43MM TWIN PLUGS
INLET VALVES 49MM
EXHAUST VALVES 41.4MM
CAMSHAFTS GE100
PERIOD PORSCHE RACE MFI HIGH BUTTERFLY INTAKE SYSTEM
ITG FILTERS
MFI PUMP MODIFIED TO RSR SPEC
PERIOD AMBER LIGHTWEIGHT ENGINE SHROUD
MODIFIED BOSCH TWIN SPARK DISTRIBUTOR
STAINLESS STEEL RACE EXHAUST MANIFOLDS AND SILENCER
RSR REAR ENGINE MOUNT
GENUINE MFI PUMP

GEARBOX
3.2 CARRERA 915 OIL COOLED GEARBOX WITH QUAIFE ATB LIMITED
SHORT RATIO BOX
SLIP DIFF
WEVO SHIFTER


SUSPENSION AND BRAKES
930 TURBO REAR SUSPENSION ARMS DISCS AND CALIPERS
26MM REAR TORSION BARS
REAR BILSTIEN TURBO SPORT SHOCKS
930 TURBO FRONT SUSPENSION DISCS AND CALIPERS
FRONT BILSTIEN TURBO STRUTS WITH RAISED SPINDLES
ADJUSTABLE TWIN MASTER CYLINDER BRAKE PEDAL BOX
BRAIDED FLEXIBLE HOSES
FRONT AND REAR DELRIN BUSHES
STEERING RACK FITTED WITH BALL JOINTS


WHEELS
FRONT 9 X 15 FUCHS BY BRAID
REAR 11 X 15 FUCHS BY BRAID
NEW MICHELIN TB15 TYRES

SHELL
GENUINE 1971 “T” SHELL WITH PORSCHE COA
RSR STYLE FLARED WINGS IN STEEL
GRP BONNET WITH CENTRE FILL
GRP DUCKTAIL


INTERIOR
CORBEAU RACE SEATS
BOLT-IN CAGE
HARNESSES
PLUMBED-IN LIFELINE EXTINGUISHER
KILL SWITCH

[drhildr]

Struggling to load images - need to read the "how to"....

[Max911]

Congrats!! Waiting for pictures and revving video
Ps if helps... Picture uploading is easier through tapatalk app on smarthphone.
Max


Inviato dal mio iPhone utilizzando Tapatalk

[Jonny Hart]

Clean forgot to update this - Jonny was bang on and the issue is now sorted. Running sweet as a nut - 280BHP and 220 torrques

Good to know. Enjoy.