The Stovebolt Site Forums The Shop Area Engine Shop bog off idle

I have a bog off idle on my 283 with a rochester 2G carb. Maybe this will narrow it down. It doesn't bog with the manual choke partially closed. I tried everything I've read on this subject. I haven't had any success. Thank you in advance. When I adjust the idle mixture screws the engine doesn't respond quite like how I have read it should. Even adjusted all the way in the truck still runs. Shoudn't the engine die?

Not necessarily stall when all the way in, is the engine warmed up, is the choke in, is the idle correct? Bogging could be acc. pump, have you replaced that?

Ed

Engine warm. The bog feels almost the same as when it's not warmed up but not as bad. I have good throttle after the flat spot. Timing, dwell, idle all good. When I pull the choke about half closed it doesn't bog. The carb has been rebuilt and the accelerator pump if fine. I didn't know if there was a circuit for off idle that might be clogged. I guess I need to take the carb off again to see if a circuit is clogged up.

Advance the spark a few degrees to test.
Possible: float level?
What air cleaner?

Do you know anything about the atmospheric valve? It sits on the top of the carb above accelerator pump area. A little flap that opens when you depress the gas pedal.

I'm at 4 degrees BTDC now. I'll try a few more. That is if the harmonic balancer hasn't slipped. I've tried several different settings. I'll keep going until I find something. It ran great when I first put the carb on. Then I decided it might need the dwell and timing reset since it had been a while. The dwell was at 20. I reset it to 30 and readjusted the timing. I don't know where the timing was before I reset the dwell. That is when the bog began. Maybe I should have left well enough alone. He he

I haven't checked the float level but will try. I've never done it. I have confidence that the carb was rebuilt well. I just put on a new air cleaner. A 12" with new filter. It was doing it with the original air cleaner.
Thanks

My bet is that you have developed a vacuum leak somewhere. I base this on the fact that you are not getting much of a response from adjusting the mixture screw. It should absolutely not run with it all the way in.

Another possibility is the following.
What is your RPM when you adjust the mixture screw? If the Idle is set too high, the throttle plate is open past the idle circuit hole, creating a scenario where adjusting the mixture screw causes little to no change in how the engine runs.

A had a friend say I had a vacuum leak somewhere also. He said to spray some brake cleaner around where the intake contacts the head, where the carb contacts the manifold and around where the distributor goes into the manifold. His dad found a leak there once but wasn't sure if that would be an issue on my engine. I thought I found the leak at the bottom of the carb area but it turned out to be a hole in the carburetor on the passenger side toward the bottom and towards the firewall. This hole leads into the carb below the throttle plate. Do you know what that hole is for? Other than that I don't sense any difference in RPM when I do this. I can research other ways to find the leak. But what ways would you suggest? I have about 12lbs of vacuum at idle. I know that is low. It goes up to 20 or so around 2000 rpm. The truck runs perfect in every other sense.

My idle speed is around 600 when adjusting the screws. I'll make sure the throttle blades are closed and try adjusting again.
Thank you for your suggestions. I'll keep reading, learning and trying everyone's suggestions one thing at a time until I get it. I'll post my results.

Get your idle down to about 450rpm. Take an unlit I repeat unlit propane torch and turn the valve allowing gas to flow. Move the torch around the carburetor and base and the intake runners. If the idle speeds up you’ve found your vacuum leak. Good luck.🛠

Thanks. I'll give that torch idea a try.

That hole on the back of the carb is probably a Manifold Vacuum port.

It's too small for that. Pencil lead or smaller. It's located in the throttle body right above the the nut that attaches the carb to the manifold. I do have an actual stemmed port on that side too but it has no vacuum at any throttle position. I suspect the gasket may have that closed off but I've never checked. I'll have the carb off again so I will look. It's curious to me why it would be non-functioning. The only other port is the one I use for distributor advance. It is on the same side and up by the float bowl and has vacuum all the time.

Thanks for your response.

Its a 283. It may not stay running at 450 RPM.

Well give it an extra 50 if it don’t.🛠

It runs at 450. I got a little more response. I turned the left one in a couple of times between test driving. I got the bog off idle but now is just off idle. ?