351C timing/vacuum adv

[Tataocb]

I bought a 73 convertible last week and had time to start inspecting and messing around with it today. I noticed, since the day I bought it, that there was no vacuum line going from the carb to the distributor. The distributor is not stock, it looks like an electronic ignition type distributor from the 80s.

So I marked the timing marks on the balancer and checked the timing. It was set at 6. This was with no vacuum line going to the dist. and the vacuum port plugged in the 2 bbl carb. I then set the timing to 12, and hooked up a line going from the carb to the dist. (ported vacuum, not manifold). The timing went up to 30° at idle and the car started stumbling. It also felt as if it was "skipping a beat" as the timing light seemed to "skip" a flash every now and then.

I tried to drive it, but it started backfiring and went back under the hood, set the timing to 10° BTDC, disconnected the vacuum line from the carb and plugged the port on the carb. It drove fine, without the vacuum advance, but I feel like I am missing out on some power driving it like that.

What should I check? Or do electronic ignition type distributors not need/use vacuum advance?

It also seems the carb is missing the wire going to the electric choke housing. The choke plate closed and opened just fine though. Here are some pics of the carb and distributor when I connected the vacuum hose.

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[Don C]

I don't believe that carburetor has ported vacuum, only manifold vacuum. And, you are right, that is too much advance at idle. That would be adding 18° with the vacuum advance, which is way more than the normal 10°± you should be getting from the vacuum advance. Have you checked what your idle RPM is with and without the vacuum line connected? Adding advance usually increases the idle speed and your idle speed may be getting high enough to cause the mechanical centrifugal advance to enter into play. If you are truly getting 18° vacuum advance I would replace the vacuum canister.

It looks like you may have a Duraspark distributor. Do the wires from it run to an ignition control box?

 

[Tataocb]

I believe it is ported vacuum because it increases when I move the throttle on the carb. The vacuum ports really affect idle when I unplug them, this one does not affect it much, if at all.

I have not checked the actual RPM, but they do go up when I connect the vacuum line. I adjusted the idle speed and lowered the rpms (by ear) but the total timing is still close to 30.

I will look into replacing the vacuum canister on the distributor, which is hooked up to a box right below the starter relay. So I believe it is a duraspark distributor.

How does driving without vacuum advance affect performance?

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[Don C]

Ported vacuum should have no vacuum at idle, which means you should not have 30° of advance just by connecting the vacuum line to a ported vacuum source.

Manifold vacuum will vary when you open and close the throttle.

Vacuum advance helps with driveability and provides more advance when driving for improved economy. When driving and then accelerating the vacuum reduces, retarding timing, helping to reduce the potential for pinging.

 

[Tataocb]

Turns out that port was manifold vacuum. I did find the ported vacuum port on the carb and connected the line there, the RPMs went up. I hooked up a tach and RPMs were at 1500 so I reduced it to 1000 rpm (900 in Drive).

Timing went up to 14° at idle from 10° when disconnected. When I revved the engine, total timing went up to about 45-50 (timing marks only go up to 30). Total timing was around 35-40 with the vacuum line disconnected.

I took it for a short test drive and there was detonation. It seems I need to change the springs, or it might even have too much mechanical advance.

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