xflow experts - help with weber 3d

[7]

I have aquired an engine with weber 3d ignition system. But I have conflicting information whether the rotor arm should be fixed (e.g. not advancing)

 

Weber say no - not fixed. Original engine builders notes say yes - fixed.

 

So what is the considered opinion?

 

Background:

Earlier type weber 3d iginition system that uses the Dizzy to distribute the spark, but shouldn't be using the Dizzy to time the ignition.

My Rotor Arm is not fixed, hence I am confused, as it would appear to included in the timing equation.

BTW - The engine works with current igntition setup!

 

🤔

[Jorgen]

I dont know the Weber ECU so could be wrong. But I cant at all se how the ecu should work if the distributor advances the ignition by it self. The stupid thing is that you can not see what the ECU thing it is dooing as you can not acess it by your self. I am surprised that the engine runs well with current set up.

 

Cheers

Jorgen

[alextangent]

What I know about 3D ignition you could write on the head of a pin.

 

But... there are a limited number of options. There has to be something that gives the cam position at a fixed # of degrees from TDC, and something that generates a timed spark without using the normal coil/rotor.

 

1. Using a fixed dizzy; something must generating the spark timing and the spark separately from the dizzy, as it's not possible to vary the timing on a fixed dizzy.

 

2. The dizzy is not fixed; the ECU is mapped to the dizzy, so it knows the mechanical advance for a given RPM. There's still the issue of generating a timed spark though; it still can't use the coil/cap/rotor mechanism to do it.

 

What about calling Webcon? They may know.

 

 

Alex McDonald

A loud 1700 SS in bits

[oilyhands]

Normally with 3D using a dizzy, the dizzy triggers *the ECU* well before the spark is needed (typically 66 degrees BTDC), the ECU then calculates when to fire the spark according to the current load/speed site and waits for the appropriate length of time.

 

The ECU in this case may not be providing any 3D timing, it may simply be amplifying the signal direct from the distributor trigger rather than applying map values. To give an idea use a timing light to establish what the timing is a idle and at various RPM points.

 

For further reading see http://members.aol.com/DVAndrews/ems.htm .

 

Oily

[Roger King]

I assume that you have a toothed wheel and a position sensor on the front of the engine? If so, that will pick up the crank position and engine speed while the ECU will control the ignition timing.

 

If the above is true, the fact that the rotor arm is able to advance will have no bearing on the control of the timing, but it will allow the rotor arm to move position as revs rise and therefore stay approximately aligned with the relevant contact in the dizzy cap as the timing changes. This will not be perfect, particularly as the timing will also change with respect to throttle opening, but it will be better than nothing. With a fixed rotor arm it is possible that the arm won't align with the cap at certain points in the map and you can get a poor spark at the plug.