Special getting closer...

[Gary71]

hmmm....

You'd have thought if all three were banging then removing one of the leads would have reduced it by about a third...

Thinking about what has remained common after the carb swap, do you have a cracked inlet manifold?

I'm off to have a think...

[chris68]

So ....
Valve clearances good
Removed no.4 plug lead and engine runs ok
Leads replaced and engine still bangs
Plugs 4 & 5 cold after inital engine run (6 is hot)
Distributor cap and rotor arm replaced and engine still bangs

It seems like ignition is erratic so I'd try exploring this area more. Maybe go back to a basics i.e. conventional coil, points and condenser with standard plug gaps. Note, coils for conventional and CDI systems are different so make sure you're using the right one(s).

Not sure if you're running CDI, but I've heard of cross-fire issues with these, where the fast rise time and high energy HT pulse can couple to adjacent leads and cause unwanted sparks in other cylinders. This can particularly cause problems when leads are running close and parallel. Shielded leads can ease the problem.

Cheers,
Chris

[911hillclimber]

That's interesting Chris.

The leads from the dizzy to the 4.5.6 bank are all tied together and follow in contact with each other.
The good side are all free fall and not tied (with cable ties)

The ignition is a normal Aldon Automotive system to replace points wuth a hall sensor type inside the dist on the points plate. It also uses a special coil to compliment it.
As an experiment, i wonder if i can use the Aldon coil with the Porsche points?

There are no Porsche parts to it except the Bosch dist. at the moment.

I have the 4 pages of carb set-up sheets now Mike, very comprehensive. Love the comments about the shape of your knees after a day tuning the carbs!

After practice at Shelsley I'll do a runner and try some more things like separating the leads by as far as i can etc.

I very much appreciate everyone's help on this. Need all the encouragement I can get!

There is a randomness to the banging to say the least.

Gary: The manifolds are huge PMO units as was the manifold for the webers, just different bore sizes.

[Gary71]

My leads are also wrapped together over to the 456 bank...

[chris68]

Maybe try changing one thing at a time, checking methodically for improvement.

I'm not saying any of my ideas is the cause, just that they each need to be ticked off the list. You'll find it by a process of elimination .

If you're stuck for a coil, I've probably got one you can try.

Good luck and happy hunting ....

Regards,
Chris

[chris68]

To try and answer your question ....

It seems that the Aldon unit is just a hall effect transducer and associated electronics to act as a trigger mechanism replacing the points. It probably also ensures constant spark energy at higher revs and interrupts coil current faster than a points/condenser system. The Aldon coil probably just provides more energy for a hotter spark, possibly allowing you to open up the plug gaps a bit.

However, it's still as a conventional system whereby the plug spark is produced when coil current is interrupted. With this type of system, the coil stores energy while the points are closed and current is flowing, then transfers it to the HT side when the points open and current is interrupted.

Hence, a traditonal coil should be fine, just be sure it's for ballasted or non-ballast ignition, whichever is appropriate.

Note for CDI systems the coil is designed like a voltage transformer, instantaneously transfering energy to the HT side when the CDI capacitor energy is dumped into the coil primary.

Cheers,
Chris

[911hillclimber]

Thanks Chris, and for the coil offer too.

I notice in the Demon Tweeks book they do a 'universal' coil with balast resistor.

My plan now is to get the whole system back to basics as it ran faultlessly before for Terry in enduro races hours on end.

I will pull the dist tonight and do the points/condenser after 40 years of fooling with cars I don't have a coil! Strange.

I will perhaps try the car with the leads separated first just in case...

After this lot it will be down to the rollers and hope Mick still has his magic wand.
After that and it still does it there is always the skip for the whole car.

The 911 ran faultlessly, was fast (leading the class in practice) and is fab to drive.

As before, why did i bother with this thing!!

[911hillclimber]

'Electronic device' out of the dist now.
The coil is 1.5 ohmes and says it can be used with contact breaker points, so I will.

Points now in and soon the condenser: but time for tea!

Thinking though:

If this hall effect system was'randomly' not sparking it would surely not spark on the other carb bank?

Never has the other side been a problem.

It is always # 4 and 5 cylinders...so must be the leads or the guts of the engine/heads on 4 and 5?

I need to make a leak-down tube from tube and an old spark plug. The PMO's were miles off the basic set-up settings so they have been 'balanced'.

Hope to try to fire it tomorrow evening when back from the races.

The fault
must reveal itself soon?

[chris68]

Primary resistance of 1.5 ohms suggests the coil is for use with a series ballast resistor (normally also around 1.5 ohms). The separate ballast resistor is shorted out by a relay during cranking to compensate for voltage drop caused by heavy starter current.

Cheers,
Chris

[911hillclimber]

Aldon say for 6 cylinder engines you need a 3 ohm coil, but for motorsport/high rpm a 1.5 with no resistor.

I went to Aldon (local to me) and gave them my dist and asked for the conversion/kit to hall effect.
I've just removed the lot!

The papers with the coil state for use with points or their Flamethrower system.

[911hillclimber]

Back from racing the 911 at Shelsley where I came second in class behind a huge 9 Liter Dodge viper. i was infront in the damp this morning, but despite rain dacing, it stayed dry, and he slipped ahead.

Dashed home to focus on this Lola.

Some important info.details for the techies:
1
Set the PMO's to the set-up settings on both sides. Some of the 'adjustables' were very uneven, so normalised them all.
2
Put the points back in, check the continuitity of the leads on the 456 bank.
I checked the firing order too.
3
Fired it up. It ran terribly. Put the Aldon system back in with the air gap at 30 thou and not the 50 it was...
4
Fired it up. It ran perfectly. Not a pop in the air.
The instructions say to wind the mixture screw in until the engine revs drop, then open the screw about +1/4 turn.
I tried this, and to my surprise the engine speed did not change. I left it tight and did the same to the other two. Nothing changed except the exhaust ran cold! (hot as you would expect on the 123 side.)
5
Opened the screws up to get a mixture going (no pops unsurprisingly) and with a few pops the 456 side is a bit warmer.

Increased the revs to about 1200 and pops/bangs galore!
The 123 side is perfect. Bangs on the 456 side only
6
The PMO sheets say if the cylinder you screw the mixture screw closed on does not change speed then
- There is no spark
- There is no fuel getting there.(so remove the idle jets and look for blockages. They are all clear)
7
From this i presume there is nowhere enough fuel getting to the 456 BANK (not just one choke).
I cannot see why one half of the ignition should not work and the other side runs perfectly!

From all this i am now going to pull the plugs to see if they have been running, set the tick-over to 1200 ish and wind the mixture screws in/out to see if the engine will drop speed.
In cracking the engine to 1200 the butterflies come off the throttle chokeand pass a lot more air.

Any thoughts PLEASE!

[hot66]

I know this sounds obvious, but have you checked the fuel lines / fuel distribution point ? Is there fuel getting through to the 456 PMO ?

[MikeB]

Graham

At least the PMOs have the little window to each float chamber, so you can see how the levels are going. See where they are at tickover and then what happens as you gradually increase the revs. If the fuel level drops then you have a blockage in the pipe (or tee piece) feeding that bank of cylinders.

It the tee piece for the fuel plastic? If so you can sometimes get a bit of cast flashing in one of the tee pipes

[911hillclimber]

Flippin' thing has just run out of fuel!

1
Took the plugs out on 456 bank.

4 looks wet'ish but has done some work, 5&6 sooty so have been doing something!
I conclude from this that 4 is not getting much of a spark supply, so will change the lead AND the plug top incase there is a crack in it(?) Looks perfect though...
2
I thought that maybe the fuel pressure was low, but the car screamed to red line many times at Prescott without starving.
The window levels are exactly where PMO say they should be and all 4 are the same.
Mike: havn't checked if they remain constant, but know where you are coming from.
3
With no fuel around and getting late with the kids next door I've called it a day.
I will change the lead to #4, re-fuel and try again on thursday night.
Sadly, I have to fly to Stockholm tomorrow to an International Standards meeting no less, all a bit of a jolly with a few serious hours in between.
4
Observatons;

The 456 bank seems to flow far more 'exhaust/air' than 123 at 1000 rpm.(tested by holding a hand in front of the exhaust outlet)

The 911 on webers flow the fuel into one carb and in series to the other. I have a common feed from the red-top pump to the Filterking regulator which feeds to a brass tee (Mike) and then equal pipes to each carb.

I would have thought this the better flow route to Porsche's way?

Just WHY does the 456 carb not change engine speed like the 123 side when you tighten the mixture screws?

Stay with me please, I need help!

[pncarrerars]

Might be a bit late in the day with this suggestion but have you tried using a colourtune. I have always found them very helpfull as you can see exactly what is happening in each cylinder.

Pete