This was the result of my first attempt at tuning the 403 in Project Impossible. I have never tuned a carburetor before. It seems to still have a slight twitch in the idle, I'm not sure if this is because it still has a few gallons of really old gas in the tank or if it needs its timing adjusted or maybe both. I haven't noticed any pinging on acceleration. Here's some short videos I took, thoughts or feedback?

 

Just some simple tech thoughts to begin with:

Are you starting with known-quantity / known-quality tuning-related parts (plugs, wires, cap/rotor, clean/proper pcv) ? No vacuum leaks coming off the intake or carb/carb base?

A decent dial-back timing light would be good to have so you can actually see/set your timing. Older non-digital (analog) units seem to be the preferred type for many but a newer one is still better than none.

The vacuum gauge will for sure help with tuning your carb fuel/air mix screws but the other stuff is probably best to affirm first.

Others will likely offer more detailed suggestions

 

The spark plugs are all brand new, they came gapped according to the stock specifications so I didn't mess with it. The cap and ignition module are new, the old one had burnt out. The wires are still there from before but they looked like they were in good shape with no breaks. The PCV was a problem and has been replaced, which fixed a stalling issue it was having. The carb gasket is brand new and torqued to the correct specification. I have a question about vacuum leaks, if there was one then wouldn't the vacuum gauge read much lower?

I have a question about that, given that a shop did an EGR delete (without my permission) I take it factory timing specs aren't going to help that much, so if timing needs to be adjusted then wouldn't it be better under the circumstances to do it with a vacuum gauge and by ear?

For context what had happened is while a friend and I were working on getting it running consistently we had discovered a thread on the block for the thermostat housing had been stripped causing it to leak gobs and gobs of coolant. I had it towed to a shop who told me they could fix that and get it running. That shop told me I had a blown head gasket (it didn't) but the guy they have who does classics was going to be out for several more weeks on medical leave from a heart attack, they suggested a different shop who has a reputation for being able to fix classics. Since I still wasn't sure what was wrong with it and not having much confidence in myself (big mistake) I had it towed there where they found the bad ignition coil and a bad EGR valve. Rather than just replace the valve or ask me what I wanted to do they just went and did an EGR delete and retuned it for that. It was running great at that point for the most part, but it started running poorly again after I put fresh gas in it. For context when the project got to the point where I could start running it I figure there was around 12 or 13 gallons of 3 or 4 year old gas, most likely non-ethenol. Before it got towed to the shop I put 5 gallons of fresh 87 regular into it. I suspect that whatever was in there before was significantly different from what I put in there because after running it down to 1/4 of a tank (25 gallon capacity) I refilled it with more regular. Then it started rough idling again, leading me to try and retune it again with the vacuum gauge using the idle air screws, I'm just not sure if that was enough. So that's pretty much the story of how it got to this point if this is of any help.

 

So as it turns out there was a vacuum leak, in the distributor vacuum advance. I wasn't putting the gauge in the right spot, I had thought that putting the vacuum gauge on the vacuum source for the cruise control transducer it would see what the engine is doing. Turns out that's not entirely right. Replacing the transmission vacuum modulator with the vacuum gauge, which is on a T line with the distributor the vacuum reading was a perfect 10, no movement at all. Recalling the gauge diagram I realized there was a leak. Spraying some brake clean I found it was coming from the distributor. At first I thought it was the hose, so that was replaced with a better fitting one. Still didn't fix it. Tried putting a clamp on there to tighten it down further, still didn't fix it. Somehow the canister itself sprung a leak.

 

Glad you solved the mystery. I'll just add this: don't rely on the manufacturer to properly gap your plugs. Always check before installing.