Idling engine dies below 950 rpm

[dkraven]

Rebuilt the stock carb on a 1977 GMC C25 I was gifted last fall with a basic Oreilly’s kit. That was before I found this forum and other better sources of rebuild kits. It’s a divorced choke 17057213. Got it back on the truck and truck fired up, which was a victory in itself since this is my first carburetor project. Got my timing set to 12 degrees adv. Before I set to fiddling with the air/fuel screws I got stuck because I can’t lower the idle past 950, really below 1000 rpm. If I advance the timing more I’m able to get it a bit lower, but it still chugs and finally dies generally below 1000.

Should I just get to tuning the air/fuel mixture with the higher idle? Or is there another issue I need to address here? The truck drives ok, but I need to get it past emissions and want to get it as set up for that as possible.

[Cliff Ruggles]

Go back to basics.  That is a unique carb and requires special gaskets.  The cheap kit may not have had them in there or you used the wrong ones.  The N/S assembly and accl pump aren't going to be correct or make the grade in this new fuel anyhow.

I'd suggest getting one of my kits and starting over.

Also check side play at the primary throttle shaft.  Likely it needs bronze bushings there as well......

[dkraven]

That checks out and I'll likely do that, but I also want to understand why this is happening. Right now I have the idle at about 1100 and the truck runs fine, though I'm wondering if I'm not entirely on the idle circuit because of the idle speed screw being farther in and opening the throttle blades.

Anyways, before I do that, what's the nuts and bolts of what's happening do you think?

[Cliff Ruggles]

Is this the original stock engine?

Anything been done to it that you know of?

950-1000rpms indicates a vacuum leak, or too much timing at idle speed.  Those engine don't like, want, or need much initial timing.  Hopefully you aren't adding any timing with the VA at idle, those engines didn't use it.

Weak distributor springs will cause a high idle and stalling if you try to idle it down.  So will one of those POS advance curve kits.

LOTS of unknowns with this one, but if it didn't do this before the carb build I'd stay with the carb and things related to removing it, like a big vacuum hose leaking or left off someplace......

[dkraven]

Stock engine. But I just realized something that is changed, I replaced the cap and rotor on the hei distributor using a distributor tune up kit and I did change out the springs and weights on the mechanical advance with the ones the kit says are for my sbc 350. The weights seem a little lighter so I could put the originals back on and could use one of the heavier spring sets. When I set the timing I did plug up the va on the distributor.

[Cliff Ruggles]

Get that junk out of your distributor and put the stock parts back in it. 

Hopefully that will improve things some....

[dkraven]

I appreciate your response and understand in your experience a lot of the stuff out there is cheap junk, but I also want to understand why. This stuff doesn’t come easy to me, but it’s interesting.

The distributor tune up kit came with a new module, vacuum advance, weights, and springs in addition to the rotor and cap that I actually needed to replace. I do still have the old va, module, weights, and springs, though the old weights and springs were starting to rust a good deal. But why would those parts affect the base idle?

[quadrajam]

The weights and springs will sling outward when you rev up the engine advancing the timing.
Then when you let off the gas the springs must pull it all back but if rusty and gunked up
this wont happen. It wont return to initial timing.  At least not consistently. Also the
upper shaft is lubricated by grease from the factory. It can get sticky as well. If so, you
will have to totally disassemble it to put grease where it needs to be on the shaft.
Might be a good idea to get a shaft shim kit and new roll pin too.

Like Cliff said clean up and lubricate the old stuff and if its not too worn out you can
get by with it. And If your original module is a GM it may be more reliable than what you bought.

QJ

[Cliff Ruggles]

There are fundamental flaws in those aftermarket spring/weight kits.  They not only change the weights and center cam, they change the geometry between those parts.  The idea is to get the timing "all-in" much sooner.  What happens with every single one of those kits I've messed with is that they are certainly good and getting the timing in, and in quickly.  However, they don't have enough spring tension so pull the timing back to the base setting at idle speed.  By the time you put enough spring tension on them to keep some of the timing out at idle speed, the advance curve has a "delay" and doesn't come in early or smooth.

The factory knew what they were doing and the springs, weights and center cam will provide an ideal timing curve starting just above idle speed and advance smooth and steady with RPM increase.

The problem, as mentioned above is rust, dried up grease and wear on the parts.  Once completely/correctly rebuilt your factory HEI with the stock parts will work flawlessly.  I'd go that direction and ditch the aftermarket kit.........FWIW....