Hot Rods BBC Rough idle with new carb, plugs, wires, cap

I replaced the plugs, wires, cap and carb. The BBC runs smooth all but when idling. It sounds like it's missing. I held my hand over carb when the engine was warm and while running and the idle speed increased quite a bit. Am I too advanced on timing?

 

Sounds like vacuum leak

 

Why did you replace the carb? Cutting off air to the carb shows that it's lean.

 

What is the timing set at? I'll second the vacuum leak though. Spray some carb cleaner around the carburetor and see if the idle changes

 

did you play with the idle mixture screws?

 

It would be helpful to provide more details on the engine like displacement, stock or modified, current timing, carb size/type and Jetting.

 

454 V8 with stock intake, brand new Quadrajet. I don't know the timing or jetting. Just bought the car. I did notice that there was oil on top of the intake manifold and the bolt next to it was loose.

 

I haven't touched the carb yet. It was brand new when I bought it. I bolted it on and it fired right up.

 

I don't know what the timing is yet. Just got a timing light. Haven't ever used on before.

 

Thanks. I changed the carb because the one on the engine was a 600 CFM and the 454 V8 came with a 750 CFM.

 

Y

Maybe the intake manifold.

 

Replace the intake manifold gasket. Then get back to us for carb tuning instructions.

 

Maybe start with making sure all the intake manifold bolts are tight and make sure the carb to intake gasket is sealing and try your test again. As Lloyd suggested maybe spray a little carb cleaner around the gaskets and listen for a change in rpm.

 

I found oil by one of the intake manifold bolts which was loose.

 

How did it run with the old carb? Shoot some carb cleaner around the mounting surface, then go after the intake. Of course look for open vacuum ports around the back and side of the carb.

 

As mentioned, you have a a vacuum leak. When you put your hand over the carburetor, it should want to start killing the engine.
First just snug down the intake bolts, look on line for the proper pattern, but basically you start inside on one side, go the the other side, and keep doing that until you get to the ends. “Inside out”. Google will yield the proper pattern to do it.
Use a box wrench. Some you can’t get a socket/ratchet on. So be consistent, keep in mind you’re not tightening the lug nuts on the car.
I say start with the intake bolts first, since you found one loose one.

 

https://www.summitracing.com/parts/...L5YSwd8jHC5zRaytzcpkVGq-kwSnnFLEaAglAEALw_wcB

 

Which carb? And did it use an base adapter to bolt to the stock spreadbore intake pattern? I'd be looking at a possibility of a vacuum leak at the base plate gasket.

 

Which Quadrajet? Identification number?

Is it a new old stock original Rochester or Carter (yes, Virginia, Carter made Q-Jets under license from GM)?

Or is it a new imitation?

Jon

 

What @carbking said ^^^^^

We also need a picture of the intake manifold and carburetor.

 

Thanks to everyone helping. It's awesome of you guys. So check this out. I have a electric fuel pump that is a cheap Chinese junker and so I am replacing it with a mechanical fuel pump. The electric fuel pump is on a switch. Today, I started the car cold with the electric fuel pump on, no pumps to the QJet, and it fired right up with a slightly high idle and no missing. Then I jabbed the gas and it started to miss. Then I turned off the fuel pump and the idle decreased and it really started missing. Would it be reasonable to suspect that perhaps the fuel pump is failing; meaning it's losing pressure?

 

If the bowl was full of fuel, you should have a few minutes of “fun time” under no load. I/e idle and a few romps on the pedal.

 

So what do you think my symptoms mean?

 

I don’t know. I was just giving my experience based on your “could it be a fuel issue” after what you mentioned.
Stumbles, etc are super tough to figure out on the internet for most of us. Some may know immediately what the likely 3 causes are. I’m not one of them. I have to have it in front of me and fiddle with it over a course of time.

 

We also need a picture of the intake manifold and carburetor.

 

Ok I'll take some pics

 

Did you do two things as suggested before, cinch down the intake and spray looking for a vacuum leak? If you did, I missed the results.