Technical 750 FIRE Preignition

[kurpelaci]

Hello,
Got a 750 a while ago, and just had time to sort some things out, it's been knocking or pingin (don't know the exact english term for it) for a while, but other than that it runs and starts fine.
Checked the igniton timing, it was way off, like at 23-25 degrees with the vacuum connected, but the distributor is already at the limit, I can't turn it more.
Tried to offset the valve timing by 2 degrees, this way I could set the ignition to the correct value and it made the knocking noise go away, but made the car weak at low rpm, and it was hard to start.
So I realigned the valve timing, now I'm 99% sure it's at the correct spot, but the ignition is still off by 10-15 degrees, and the noise is back.
Do anyone have some idea what can cause this?
Thanks!

 

[Dragon Man]

does the vacuum advance system work? ive found this only happens when that has failed.

 

[kurpelaci]

does the vacuum advance system work? ive found this only happens when that has failed.

It works, if I unplug the vacuum tube, the ignition is just a bit over the 15° mark.

 

[Dragon Man]

hmm may be a bit strange to say but put a bit of pipe on the vacuum solenoid and try sucking on it. you should be able to suck a bit but not keep doing it. the diaphragm may have a pin hole in it.

 

[portland_bill]

This takes me back a bit, long time since I had my 750.
Ignition timing is usually set with the vac pipe disconnected, and plugged, to prevent an air leak into the engine. 2 deg.
With vac pipe connected, at tickover, timing is 15 deg. So vac adds 13 deg.

As above, suck on the vac pipe to the distributor. You should be able to hold a vacuum. With the cap removed, you should see the baseplate move as you suck and release.

There could be a hole in the diaphragm. If there's an air leak, it will cause weak mixture, and likely pinging, but wouldn't move the ignition advance.
The baseplate might be worn and loose. There must be a spring to return the baseplate when vacuum is released, I can't remember if this is in the vac unit, or in the distributor. That might be detached or broken, allowing the baseplate to advance too far.
It is easy to get the cam timing one tooth out.

 

[kurpelaci]

The vacuum unit is brand new so that is working, while it's unplugged the ignition is at 15 deg. I will check this return spring, maybe that is the problem.

 

[portland_bill]

The vacuum unit is brand new so that is working, while it's unplugged the ignition is at 15 deg. I will check this return spring, maybe that is the problem.

Do you still have the old vac unit to compare? Timing should be at 2 degrees with vac disconnected. Both old and new unit should be same length operating rod when at rest. Is the vac unit correct, or just one that will fit this distributor. When fitting, the operating rod should attach to the baseplate without needing to move it.