Timing seems off, vacuum super low

[wasco]

A little background: 1964 383, Edelbrock E-Street heads, Edelbrock Performer intake, Edelbrock 1411-750, Comp Cams CL23-223-4 Xtreme Energy 224/230 camshaft, headers and HEI ignition. Engine was rebuilt and has about 80 miles on it so far. I’ve only used Ethanol-free gas, 89-91 octane. I have the vacuum canister on the distributor turned a couple turns back from all the way to clockwise (least amount of vacuum needed for advance).

I noticed what I thought was the lifters clattering at low RPM, thought maybe I wasn’t giving it enough gas when taking off (four speed), but then thought maybe it was detonation. The car just didn’t seem to have the power I thought it should. So I started down the rabbit hole of getting the timing set right.

I currently have the idle at about 900, the timing set to 18 BTDC and at idle the vacuum is at 12, which seems very low? Could the camshaft kill the vacuum that much? I thought I’d try setting the idle mix screws to maximum vacuum, but they don’t have any effect - unless I have the timing set above 40 BTDC. Currently I have them each two turns out from seated.

The vacuum headlight doors work slowly, but work - I have them connected to the full manifold vacuum port on the carb, using ported vacuum for timing.

Does all this sound about right or should I give the engine more timing? Tuning using the vacuum gauge doesn’t seem to work on this engine.

Any thoughts or insights would be appreciated! Thanks

 

[Don67Satellite]

If the mixture screws aren't doing anything you'll wanna check for vacuum leaks.

Also 12 isn't "low" depends on the cam, and not sure what that particular cam will give you. Others will chime in.

 

[WileERobby]

Is the cam installed advanced, retarded, or straight-up ?

 

[Aron Gleason]

With the mixture screws turned out 2 full turns. I'd check where the throttle blades are in relation to the transfer slots under the carburetor. You shouldn't need more then 1.5 turns at the most.

 

[wasco]

Is the cam installed advanced, retarded, or straight-up ?

I’d guess straight up? The machine shop didn’t mention anything in the paperwork if they did anything otherwise.

 

[wasco]

With the mixture screws turned out 2 full turns. I'd check where the throttle blades are in relation to the transfer slots under the carburetor. You shouldn't need more than 1.5 turns at the most.

I usually set them at 1.5 as default and leave them - not sure why I did 2 turns this time. I’ll correct that. Thanks!

 

[eldubb440]

crank them in and see if it stalls the engine

 

[Aron Gleason]

I usually set them at 1.5 as default and leave them - not sure why I did 2 turns this time. I’ll correct that. Thanks!

You're welcome

 

[Geoff 2]

[1] The mixture screws need to be set where you get the highest/smoothest idle, not some magical number of turns.
[2] The 383 has a high rod/stroke ratio. In layman's terms it means it doesn't like long duration cams at lower rpms......
[3] This another classic example of insufficient idle timing.
[4] When you went to 40* at idle: you would have had a higher idle rpm, which allowed the t/blades to closed more. So NOW less T slot is exposed...& like magic the mixture screws work.
[5] About 12" of vac with low timing is about what I would expect with that cam. With idle timing increased to about 40*, I would expect vac to increase 1-2".

What you need to do:
[a] Engine warmed up, idling, in gear if auto, start with timing at about 20* BTDC. Slowly advance dist until you get the highest rpm/smoothest idle. Now check timing. Say it is 42*. That is what THIS engine wants.
If you have a GM type HEI, that is good. They are best & have a very good adj VA units that can add up to 30* of timing. Crane make an adj limiter plate for these for easy adjustment of the total amount of VA applied. Adj the Allen Key fully CW.
[c] Say in [a] you found 44* of idle timing was best. You need to use a combination of initial + VA [ connected to manifold vac, which is operative at idle ] to provide the reqd 44*. Examples 20+24, 18 +26, 30 +14. If your dist curve is set now with 18*, then adj the VA for 26* [ for this example.
[d] Once the idle timing is dialled in THEN, adj the carb idle speed & mixture screws. Remove carb to see how much Tslot is below the blades 0-0.060" is ok. More than 0.060" requires bypass air, a small hole in each pri t/blade. Start with a 3/32" hole. Once the t slot position is correct, you will find your mixture screws are responsive.

See the link about idle timing.