On ported vacuum, you should not have vacuum at idle.
I would recommend reading the writeup on installing a distributor, and finding TDC that I wrote. Once you are guaranteed the distributor is set properly on TDC Compression stroke , then you can time the engine with out vacuum hooked up.
After you time it, tighten down the distributor, and rev the engine with the timing light still on. The mark will advance if you have a distributor with mechanical advance built into it. There are weights with springs inside. The faster the distributor spins, the more centrifugal force wants to push the weights outwards. The spring tension holds them in. When they are allowed to fly outwards, it advances your timing.
So you have initial timing, then with added RPM's you get mechanical advance. Then when you hook up the vacuum line, you get vacuum advance.
There are two kinds of vacuum Ported and manifold.
Ported is coming off a ported source on carburator. For ported vacuum, at idle there will be no vacuum when the engine is warm, thus no advance.
It can get a bit tricky, when you introduce a CTO switch (vacuum switch) into the picture.
The CTO srews into the manifold next to the thermostat housing (on a 304)
It has receives input from both ported and manifold vacuum. It senses the coolant temperature, and when the engine is cold, it sends the distributor manifold vac. When it get above the temp built into the switch, it sends ported vac to the dist. A lot of people simply hook their dist. up to manifold vacuum only.
I don't know what HEI distributor you have installed. but with the vacuum disconnected, you can increase engine rpm's and watch the advance and write it down. You can plot it on graph paper and get a curve. That's what is called advance curve. You can change the springs from light to heavy tension. This changes the curve, making it so that more RPM is required befor the advance mechanical advance kicks in.
Vacuum advance can be adjusted on most distributors. There is a screw on the inside of the vacuum housing on the dist.
Shine a flashlight inside where the vacuum hose hooks up, and should see a screw. This is how you set vacuum advance.
Sounds to me like your new HEI is set with too much advance.
This assumes your initial timing is correct, and you have the distributor installed correct with the correct firing order set.
Here are some diagrams showing the CTO hookup.
79 CJ7 "CJ Express" 4.2L, NV4500, D44 rear
69 800 with SBC, SM465, NP205, 14BT
Last edited by BioTex; 11-29-2009 at 06:03 AM.
Reason: added pictures