Timing / Idle Mixture / Vacuum question

[bswilson80]

Please excuse my novice understandings. I just bought a 1974 C10 with a 350/Th350 in order to learn about maintenance. . Everything seems stock except for the Edelbrock 1406 Carb, and maybe the HEI dizzy..

I have really rough idle and very little response at part-throttle. Once the truck is moving it rides nicely.

Here's where I stand:

-I have to prime the fuel pump by turning the key to ON twice before ignition otherwise the truck will just die.
-Timing is set to 10 degrees.
-Mix screws are 2 full turns from tight.
-Compression is 120-130 on all cylinders.
-No milkshake in oil
-Spark plugs were all dry with a little carbon on each
-Vacuum test shows needle shaking quickly between 14-16
-blew smoke into vacuum system and it only seemed to come out of a small hole right below the carb choke piston (covered hole and no more smoke anywhere else)
-Small backfires (like popcorn) when slowing down, usually at about 10 mph (seems to coincide with auto-downshift)

Anything else I can tell you, let me know. Again apologies for amateur hour, but I'm trying to learn.

[Fairlane514]

Not sure what you mean by priming the fuel pump just by turning the key ON twice. It should be a mechanical fuel pump. Is it electric?  Do you have any holes in the exhaust near the manifolds?

Are you sure about the spark plugs wires being in the correct order? I guess if it rides nicely it must smooth out once its off the idle circuit.

Could be sticking valves if the needle flicks 2-3 inches of vacuum.

[Irish_Alley]

a miss will smooth out once the idle picks up. make sure your firing on all cylinders

[rich weyand]

Check all the spark plug wires.  One way is to clip the timing light to each wire in turn.  The bad wire won't kick the light.

12* BTDC is a better timing setting.  You should be able to set it to at least 16* BTDC.  Earlier is better unless you get knock.

Which metering rods are in the carb?  If you pull one of the rods, the size is stamped on the rod body, like "7052".

[79gmc15]

Also, that popcorn sound could be pinging.

[Boone83K10]

Please excuse my novice understandings. I just bought a 1974 C10 with a 350/Th350 in order to learn about maintenance. . Everything seems stock except for the Edelbrock 1406 Carb, and maybe the HEI dizzy..

I have really rough idle and very little response at part-throttle. Once the truck is moving it rides nicely.

Here's where I stand:

-I have to prime the fuel pump by turning the key to ON twice before ignition otherwise the truck will just die.
-Timing is set to 10 degrees.
-Mix screws are 2 full turns from tight.
-Compression is 120-130 on all cylinders.
-No milkshake in oil
-Spark plugs were all dry with a little carbon on each
-Vacuum test shows needle shaking quickly between 14-16
-blew smoke into vacuum system and it only seemed to come out of a small hole right below the carb choke piston (covered hole and no more smoke anywhere else)
-Small backfires (like popcorn) when slowing down, usually at about 10 mph (seems to coincide with auto-downshift)

Anything else I can tell you, let me know. Again apologies for amateur hour, but I'm trying to learn.

You should have a mechanical fuel pump. You should have to pump the gas pedal a couple of times to get an immediate fire when you crank.
Make sure you are getting fire on all cylinders before you time the engine.
You really should blow smoke all around the engine. The engine sucks in, not blows out. So you will see the smoke get sucked in if it has a vacuum leak.
Set your timing to about 12-14 degrees BTDC (engine fully warmed).
Counterclockwise advances the dizzy, clockwise retards the dizzy.
Obviously you must plug the vacuum advance before timing the engine. Hook it back up once you have timed it.
Small backfire indicates that your carb is out of tune.
Take your vacuum gauge and hook it up to full manifold vacuum. If you have a tach, set your RPMs at idle to 750 in park with the engine at operating temp. Turn the mixture screws in until they seat lightly. Turn them back out 2 turns (your original position). fire the truck up and take note of reading. Turn each mixture screw a half turn in either direction, one at a time the same exact amount.
Take note if the vacuum reading goes up or down. If you turn the screw to the right, you are leaning it out: turning it left enriches the mixture.
While you tune to the highest vacuum reading, go back and check your idle RPM. If it has gotten higher, turn the idle screw back down to 750. The idea is to keep the RPM the same after each turning of the mixture screws so that you are actually tuning the mixture. As the idle goes up it naturally creates more vacuum so you much check this after each turn of the mixture screws.
When you finally get the highest possible vacuum reading, it should hold steady. Most people will say you should have a reading of 18-20 on the vacuum gauge. There are many factors that could change that number such as elevation, what kind of cam you have etc, etc. A hotter cam will lower your number and give you a slight flicking of the needle. Higher elevation means a lower number as well.
After all this, put some new spark plugs in and drive it a while. Try to not idle before shutting the engine off when you go to check them. They should be a slight brown color. If you are still getting dry soot, it's still running rich.

[rich weyand]

Also, that popcorn sound could be pinging.

With timing at 10* BTDC, that's unlikely UNLESS it's a bad load of gas or it's old gas.

You could pump the gas out of the tank and replace with fresh, and use the old gas in your lawn mower.

[74 C-10 Shorty]

Also, that popcorn sound could be pinging.

With timing at 10* BTDC, that's unlikely UNLESS it's a bad load of gas or it's old gas.

Depends on the curve in the distributor??, the total determines if it detonates. But old/bad gas will definitely cause havoc..

[bswilson80]

Fairlane: It's mechanical, I guess I was assuming that turning the key helped as when I don't the truck dies, but when I do it starts up nicely. I've checked and double checked the plug wires, I'm pretty sure they're correct. I can't find any holes in the exhaust.

Rich: I will definitely try that with the timing light, as well as advancing the timing a little more. How old is old gas? I haven't driven the truck that much as I don't want anything to get worse, so the gas is probably about 4 moths old.

79GMC: The sound is definitely coming from near the mufflers, could that still be pinging?

Boone: I'm at sea-level, I will attempt the rest this weekend, thank you.

Thank you all for your replies. I've been lurking this site for awhile now, I'm glad everyone is so willing to share their knowledge.

[74 C-10 Shorty]

Gas today is crap, after 30 days it starts to deteriorate if you let it sit you need to put Sta-Bil in it. Also Edelbrock carbs are notorious for gas going away when they sit for a few days, you either crank the heck out of it or you squirt it to get it going, once the carb fills with gas it's ok, that could be your problem if it's doing this after sitting for a while, I've got one on my truck but I have an electric pump(since my motor don't have provision for a mechanical)so I turn the key on for a few seconds before I start it and it fires right up and runs.

[Boone83K10]

I would challenge that gas is bad after 30 days...

to the OP check this out, it will teach you more about your engine vacuum readings than most can tell you about.

http://www.secondchancegarage.com/public/186.cfm

[74 C-10 Shorty]

I would challenge that gas is bad after 30 days...

Didn't say it was bad after 30 days, it starts to deteriorate after 30 days.

[Fairlane514]

May not have anything todo with your problem but.........do you know if your truck has an oil pressure switch on the drivers side block about midway down near the back?

This switch provides a 12v current to the choke (if its electric) and basically allows the choke to operate when it senses good oil pressure. Dont know if turning the key ON has anything to do with this precess........just a wild ass guess.

[rich weyand]

On the '74, the choke runs off the ignition.

[Irish_Alley]

mess with timing to see if your idle smooths out might be a little retarded. just turn it clock wise a inch or two. then if it smooths out you know that's where to start if it doesn't and your sure your getting fire to all cylinders i.e.( touch the exhaust manifold quickly on each cylinder WHEN THE ENGINE IS COLD and see which one is colder than the rest that will be your dead cylinder).