My 73 cutlass 350 is using ported vacuum for timing advance. (With that thermo doohickey that will switch it to mani if overheating). It has 12btdc degrees timing at idle per specs.
If I were to switch advance source to manifold vacuum, what should I set my timing to?

 

Set timing to what you normally set it to. Switching to manifold vac at idle only effects your idle timing, once your out of the idle circuit it works as it always had.

 

Agreed, simply switch the vacuum advance canister hose to a manifold source, then maybe adjust the idle speed afterwards.

 

I check timing at idle though. So it's going to be wildly different. .with the hose connected factory way, to ported vac(so no vac applied to distributor at idle), spark is at 12 deg btdc factory spec. When I moved the hose over to mani vacuum (which is around 20" at idle) and the idle re adjusted, timing was sucked up to around 20 or more deg btdc? (way off the tab.. could barely see the line). I guess that's fine but car had a grueling hesitation off idle like that. Undriveable. ...

Am I missing something? I put the hoses back to original for now. It runs fine. I just went down a rabbit hole of ported vs mani and many say mani best but maybe not for this 350. The hesitation with mani vac advance is so pronounced it's dangerous

 

When setting the initial timing, the vacuum advance is disconnected and the port plugged. This removes the vacuum advance completely.

Your experience with stumbling is why adjustable vacuum advance canisters and advance limiters are available. Some factory vacuum canisters have 25 or more degrees of advance, which may be too much when used on a manifold vacuum source. You have demonstrated the need for tuning after making a change from the factory setup.

 

It certainly sounds like you aren't disconnecting the vac canister when establishing initial timing - which is a must. My 350 is set to 12° BTDC, initial timing established w/ vac canister disconnected and plugged and I use ported vacuum - no issues. I obtain ~17.5" Hg vacuum.

 

cool.
your setup is basically the same as my factory setup. I didn't see any reason to disconnect the vac canister because I checked timing when it was hooked to ported vac. And at idle, the vacuum there is zero.(I checked)

 

If you didn't see any reason to disconnect the vac canister then you aren't correctly setting the timing.

??? I'm not following what you just stated. You need to disconnect the vac canister to establish initial timing otherwise you're adding initial advance during idle which is not what you want to do to establish initial timing. I'm sorry, but you must disconnect vac canister to establish initial timing regardless if you're using ported or manifold vacuum (or a turkey baster). And, let's be clear by measuring vacuum, the vacuum you measure is after dwell & timing has been established. You measure vacuum when establishing your A/F mixture. You establish optimal A/F mixture ratio at the highest achievable vacuum - I obtain ~17.5" Hg @ idle.

 

The manufacturer’s specs for initial timing for my car is not at idle, but at at 1100 RPM. At that engine apeed there is some vacuum at the “ported” source, which is why GM says to disconnect the hose and cap the port.

 

My initial timing is set with engine @ 1100 RPM, as well. After I establish A/F mixture, I then establish (set) slow idle with AC.

 

 

This vacuum routing is from the 1971 CSM. You have a 1973 Cutlass 350; therefore, your routing may be dissimilar - do not use the one I am providing as a reference for the correct routing. But I am providing it to you as a check & balance for you to establish and review in your own 1973 CSM whether you have the vacuum hoses routed appropriately for your so-called "thermo doohickey". Double-check your vacuum routing specifications.

 

Why did you change it? As said, disconnect the vacuum advance and plug, then bring the idle to 1100 rpm. You might have 5 extra degrees of timing if your base idle is 750 rpm. Any low compression, 8 to 1 Olds 350 I have run, liked close to 50 degrees at idle and nearly 60 at cruise. Even my 9 to 1 350 ran fine with those numbers except for low speed bucking. A 20 degree vacuum advance canister fixed the bucking. The factory HEI canister was 30. Look for a number on your vacuum advance, a number like 30 will be the degrees of advance. The HEI is usually around 20 at 1100 rpm for the spec. Confusing since 3 of my motors used the same #8 heads🤷‍♂️.

 

Was it hesitating with gentle acceleration or wide open throttle?

 

I would have thought so too, but nope. Even at 1190 rpm (pic), there is no vacuum at the ported location. Around 1300 rpm, it climbs to 5 or 10 inches. And if you snap the throttle it jumps to what seems to probablybe full manifold. So disconnecting the vacuum can is going to yield no different result. But i did it anyways this morning. No change. And yes I'm saying idle, but I'm dialing up a fast 1100 idle with the stop screw.

I just put the hoses back to factory and it runs pretty good like that. Idle is a bit rough but if it had stock mufflers I'm not sure id notice. What i did today, was rewire that idle stop solenoid to the AC clutch circuit so my idle is the same with air on. I don't seem to have any dieseling so it's better this way.
Best of all, no hesitation. For a stock engine, it does a pretty good 1 wheel peel on a hot day with it's highway gears and it pulls smooth through all rpms. So I'm just going to avoid the rabbit holes for a bit and just drive it 🙃

 

FWIW, many of us have deleted the DVCS completely - removing it from the engine altogether. Just a consideration. Some want it some don’t.

 

I probably will eventually because I want to steal the port for a temp sender.

 

I stuck around 16 initial because my idle is high even with the screw all the way out. Might have to go back to ported vacuum

 

Your motor is the first low compression, iron head Olds that doesn't like aggressive timing. I would also get a dial back timing light and check the whole advance curve.

 

vacuum leak maybe ? carb linkage hung up ? choke still on ?

you should be able to adjust the idle down

 

Disconnect the throttle linkage from the carb and see if the idle drops.