Minor Surge at Idle

[minifridge1138]

Hello everyone,

Yesterday I adjusted my carburetor and changed the vacuum source for the distributor's advance (thank you JohnZ for the explanation of how advance SHOULD work).

The car is more responsive than ever before. But when i come to a complete stop, something happens: about every 10 seconds, the rpm surges about 150-200 RPM (just a guess since my Tach doesn't work)for 1 second and then returns to normal. I can definitely live with it, but since it is new it is obviously the result of something i did.

Any ideas on what is causing it? Improper timing, improper mixture, wrong idle RPM?

 

[JohnZ]

What kind of distributor do you have - original computer-controlled HEI? Replacement?

 

[minifridge1138]

It is an HEI, but I don't think it is the original (doesn't same GM anywhere on it).

And I don't think it is computer controlled.

 

[JohnZ]

To optimize the setup, the vacuum advance unit needs to be matched to the engine's idle vacuum characteristics, so the unit is fully-deployed at normal idle. Post what your vacuum gauge reads at normal idle when connected to a full manifold vacuum source and I'll tell you which vacuum advance unit you need - they're about ten bucks at NAPA.

 

[minifridge1138]

I was getting 19.5" to 20" on my vacuum gauge when i tuned my carburetor.

I guess I could hook my vacuum pump up to the advance, take of the distributor cap, and see at what level the advance is fully deployed. If not, then I will take your advice and buy a new advance from NAPA.

Thanks JohnZ!
(when i grow up, i want to be just like you)

 

[JohnZ]

The selection of vacuum advance units for HEI distributors is limited compared to those for point distributors (and they're not interchangeable due to the length of the mounting bracket). What you're after is to have the vacuum advance fully-deployed at a vacuum level at least 2" Hg. below your engine's normal idle vacuum so it's pulled solid against its stop for a stable idle.

The NAPA/Echlin/Standard brand #VC-1853, stamped "AR23", will work fine for you - it starts moving at 5"-7" Hg., and is fully deployed at 12.5" Hg., providing 7.5 (distributor) and 15 (crankshaft) degrees of advance.

 

[minifridge1138]

I pulled out my vacuum advance last night and played with my vacuum pump. It starts to pull in at about 5" and is fully deployed at about 12".

The only thing i'm not sure about is the amount of advance it provides. The only way i can think of to do this is to hook up the timing light, and see how the timing mark moves as use the vacuum pump on the distributor.

Any other ideas?

Thanks for all of your help!

 

[JohnZ]

I pulled out my vacuum advance last night and played with my vacuum pump. It starts to pull in at about 5" and is fully deployed at about 12".

The only thing i'm not sure about is the amount of advance it provides. The only way i can think of to do this is to hook up the timing light, and see how the timing mark moves as use the vacuum pump on the distributor.

Any other ideas?

Thanks for all of your help!

Sounds like it's on the money for what you need in terms of vacuum specs - your check with the Mityvac and timing light will work fine. If it adds 13*-16* advance, you're good to go. Re-check your idle speed - if it's a little higher than it was before with "ported" vacuum, you may be into the beginning of the centrifugal advance, which can cause some idle speed variation.

 

[petes74ttop]

With the advance disconnected and pluged check the initial timing,
then reconnect the vacuum hose and check the timing, that's how much advance you unit is adding.
Ex: 10* initial with hose pluged.
23* with unit connected, so the can is giving you 13*. Pete.