I have same problem as 76Cutlass 442 had in 2015. My 69 Cutlass S has an HEI with a Mallory 6A ignition control box. Car starts and idles no problem, but shudders when I try to accelerate. I changed the fuel pump and distributor pickup coil with no luck. I pulled the vacuum hose off the distributor, plugged the hose and rev'd the engine and it runs fine. WTF? Could my vacuum advance canister on the distributor be bad? What else could be causing the problem?

 

I moved your post to its own thread, your issue is different. Was your advance hooked to ported or manifold vacuum, did you have to raise the idle speed when you pulled the hose off the advance? What is your timing set to? Is this a new install, or has it been running good up till this point?

 

Sounds reasonable the problem might be the vacuum advance. Hook up a hose to it and blow or suck into the hose. If it's bad you shouldn't be able to get any air through it.

 

Try driving it with the vacuum advance hose plugged and see how it runs. I've read that you replaced pickup coil; but, no issues with the vacuum advance plugged is usually a bad pickup coil/pole piece in the distributor. It could be a "notching or binding" vacuum advance.

Good luck!!!

 

The entire car shudders or the engine shudders, or both?

 

Thanks for so many responses so quickly. Here's some additional info:
- I can't remember when last time car ran properly, been down for a few years (3 or 4) due to other priorities and other issues - exhaust, ignition, etc. that I finally fixed and am trying to make it road worthy again.
- Current timing set to 20 degrees BTDC (had it set at 10 degrees BTDC but had same shuddering issue)
- Vacuum advance hooked to manifold port at first, but moved to carb port (same problem either way)
- Engine idles slower with vacuum advance plugged
- I drove with vacuum advance plugged and car ran fine, but sluggish. It didn't have good pickup and shifted gears early (lower rpm)
- I watch the engine while I advance the throttle on the carb by hand and it will starts shuddering/shaking as I try to increase throttle

 

You do have both ends of the ATM (Automatic Transmission Modulator) vacuum hose connected with no apparent vacuum leaks in the ATM hose?

 

Suspecting it’s one of three issues:
(1) Electrical - Coil windings not delivering required output as RPM advances (assuming timing, plugs, wiring, are correct);
(2) Fuel delivery - Carburetor A/F mixture screws are set for highest achievable vacuum and carburetor operation is nominal;
(3) Vacuum - Leak(s) in one, several hoses. Should be able to pull ~17.5” Hg.

 

If it has old gas in it from sitting that could cause a lot of issues. If it runs better with the vacuum disconnected its probably because it needs to be limited to 10* with a stop. You need to put a timing light on it to see if your mechanical advance is working properly.

 

Ok, I determined the issue is most likely electrical/timing so I decided to take it back to a more simple set up to better isolate the problem. I completely disconnected the mallory ignition box and swapped the HEI distributor with my original distributor that has a pertronix kit to replace the points and condensor. Now, I can't adjust the timing below 30 degrees BTDC due to my vacuum canister on the distributor hits my intake manifold. Does this mean I might have jumped a gear on my chain?

 

No it means you installed the distributor wrong. You can pull it and rotate the rotor CW 1 tooth and readjust timing, or move the wires to the right (CW) 1 hole and readjust timing.