I mentioned in one of my threads that the guy who tuned my engine (who I'm currently not too impressed with) has had trouble setting the ignition timing. Ill try and explain the scenario as best as possible...

It's currently set up as follows:
The static ignition timing is set at 5 deg ATDC. The vacuum advance is plumbed directly into the inlet manifold. This gives me ignition timing of ~10 deg BTDC at idle. Apparently these were the only timings he could figure out so that it wouldn't ping. It runs fine, bit doughy down low, but revs out great.

Now I'm not happy leaving it like this because it's far from perfect so I had a play today. I plumbed the vac advance back into the carb and set the base timing to 5 deg BTDC as per factory spec. It runs perfectly fine ..right up until you give it a foot-full :(

From what I've gathered a high vacuum load (like WOT at really low speed) doesn't seem to affect it, no pinging. But as soon as the revs start to build it pings like crazy. I tried disconnecting the vac advance to see if I was right about it not being vac related and it helped a little bit but there's still a bigger problem.

My current theory is that the advance curve in my current distributor (from different weights(?)) won't work properly with the auto transmission. I have other things to check (like making sure I lined up the cam and crank sprockets in-line and cc'ing the head to check my C/R) but I'm pretty sure they're fine.

Your suggestions and advice are much appreciated!


ENGINE SPECS:
A14
Manual trans distributor with Pertronix Electronic Ignition module.
Weber 32/36
GT40 Coil
3 speed Automatic transmission

sounds like the ole ported vacuum vs manifold vacuum problem.
WOT would be a low ported vacuum signal and also a low manifold signal, which would make less advance, which is what you want.

ported vacuum is inversely proportional to throttle position and manifold vacuum is more a function of "load"

did you remove those vacuum delay thingies? or make sure they are pointed the right way, and do you have a dizzy with 1 or 2 vacuum hookups?

"vacuum delay thingies?"?

I've kept things as mechanically simple as possible (KISS never fails) so it's just a straight uninterrupted vacuum hose from the vacuum advance unit to either the manifold barb or carb port.

Taken from another thread:
Quote:
Having it hooked up to 'ported' or 'manifold' vacuum won't make any difference when it's WOT (no vacuum)... which is the only time when it pings badly (when set to 5 BTDC and vac advance connected to the carb port)

When I was a young feller doing my apprenticeship [as a mechanic] we were shown how to do the timing.

The deal was to ensure that the bobweights had full & free movement, that the springs were serviceable & connected, then the dwell was set with a dwell angle meter.
The vacuum line was disconected from the distributor & plugged, usually with a No. 2 Phillips screwdriver.

The idle was set with a tach, a vacuum gauge & a timing light so that engine speed, idle mixture & static timing could all be set in the same operation.

Once done, the vacuum line was reconnected & any changes in the timing & the engine speed were noted & re-adjusted if necessary.
We sort of overlooked the fact that any changes would be caused by either a leaking diaphragm allowing bleed air into the carb & thereby changing the mixture, or vacuum signal in the ported vacuum fitting which would advance the timing & showing that something was wrong, needing excessive throttle opening to maintain specified idle speed.

OK, your problem, ... detonation at full throttle. You need to find out just how much advance there should be at full mechanical advance then disconnect [& plug] the Vac line, then bring the engine speed up to about 4,000 rpm while checking the timing.

Bring your variables under control by checking these things so that you KNOW what your engine is seeing.

Also, if you are running on E10, [91 octane] or Lead free [95 octane] then switch to premium 98 octane as a reference test.

I've been reading about exactly how the spring rates/weights affect the mechanical advance and how it behaves.

I grabbed my old distributor and found a marking on the cam assembly (that fits atop the weights) reading '12'. I assume this is the factory marking for the amount of mechanical advance its setup for, which means it will give me a total of 24 degrees mechanical advance.

Now after writing some simple things down on paper, I came up with this:

TUNERS SETUP (CURRENT):
-5 degrees initial advance
24 degrees mechanical advance
= 19 degrees total advance

FACTORY SETUP:
5 degrees initial advance
24 degrees mechanical advance
= 29 degrees total advance

Which means there's a total of 10 degrees of advance difference between the two setups.
Now there's no reason at all it shouldn't work the way the factory intended it too, I just think the advance curve doesn't quite work with my setup which means I need to start and play with the springs...