Engine tuning mystery

[K70]

Hi,
This is a mystery that has baffled dyno operaters and engine builders alike.
This engine has a fault improved by an over rich condition.
But what is it???
It will idle all day.
It will accelerate flat out.
It will rev to 6000rpm.
Between them it will fall flat and violently jerk and backfire.
??????????????

[John S.]

What induction and ignition does it have?

[K70]

Hi,
the type 4 has csp manifolds and linkage with pair of
DRLA 36.
Ignition is factory hall sensor with one side of vacuum in use now.
All the parts bolted to the outside of the engine have been swapped or tested.
Suspicion of internal parts more now.
Cam timing gear damage?
Loose timing gear wheel ?
Cylinder head problems?

[Bruce.m]

Backfire / spitting up the carbs is a symptom of lean.

If you mash the throttle the accelerator pump jet will add fuel and when that runs out you are on the main jet only.
What jets are fitted and roughly what the position of the mixture screws?

Does it spit back on both sides or just one side?

[Bruce.m]

Also, if the cam is not correct aligned with the crank, you could get some weird problems. That would affect all cylinders.

Check the opening & close of the intake valve to see it matches the cam spec.

[K70]

Hi and thanks for your help.
The main culprit is the distributor (bosch 007/vw#071-905-205).
This has excessive wear causing shaft play.
The hall sender is aftermarket as no other available. This can be moved on the base plate with the slightest touch. Wires barely connected.
All of this moving around allowed the shutter rotor to contact the wires and only visible when the rotor stopped in a particular position.
The question now is how to rebuild the distributor to the best spec possible.
Drag racers with a LOT of power use the vanagon distributor.
What to do with it please while out and apart???
Does the vacuum need to be improved with the DRLA carbs?

[K70]

Any one have a spare NOS Distributor please,
Or Mallory pre 2006...

[Bruce2]

I had a similar experience many years ago. The engine would run perfectly in the driveway, but once there was a load on it, it would sputter to death.
It turned out to be the stupid distributor clamp that the owner decided to get powdercoated. It resulted in the distributor not being able to ground itself properly. At idle, it grounded well enough through the drive dog.