High Idle

The most likely cause of this on a carburetor vehicle is the carburetor itself.

When this was rebuilt did you have it tuned or did you tune it so that you can set the idle and fuel mixture?

However, if it was then we can simply adjust it a little more the drop the idle until it is running slower and smooth.

First and foremost, you need to check and set the timing.

Based on the chart attached, you need to be around 4 degrees before TDC. If not, let's start with setting the timing correctly and then use the info attached below for adjusting the carb to see if you can lower the idle.

Please let us know what questions you have on this, and we can go from there.

Thanks

Yep. Roy is correct that most of these older vehicles like a little more timing advance because there are other parts that are worn. So, the timing compensates for that.

However, anytime you rebuild or refresh anything to do with the carb or ignition, you want to start back at factory settings and then adjust it from there to where this specific vehicle likes. You will know where this is by the way it is running.

Basically, you are going to be chasing this down a rabbit hole if you try to leave things set where they were and then you change something.

You may get away with it but if you think about it, those screw settings and timing were all set with a carburetor that was not rebuilt. Now that you have everything inside sealing like it should and clean, those settings are most likely not going to be okay.

So anytime I change something like this, you start back at the factory settings and make new adjustments. So, I would set the timing back to 4 degrees, then tune the carb so that you bring the idle down to where it should be. Then if it is not running smoothly, add a little more advance back to it so that it is running smoothly which will raise your idle again and you can then back that down again until it is idling where you want it.

It is a dance so just stay patient and if you get out of sorts, start over by setting it back to the factory setting and start over.

Based on all this, I suspect that you may back the timing down and it brings the idle right down to where you want, and you may not need to adjust the carburetor at all.

The Quadrajet was used for many years. I think that cut out was started in the early 1970s.

It is to cut down heat-soaked components that will cause fuel boiling.

Plus, I am not positive, but it also helps with airflow balance between the bores.

Your carb should look like the first picture below and use the one with the cut out in the mounting gasket. They provide both just in case you have the older style carburetors.

They look like they are composite gaskets, meaning they are not paper or cork, so they are reusable as long as there is no damage on the mating surfaces.

That sound is normal, and it is air being pulled in and pulling in fuel. It is dampened by the air cleaner, but I suspect if you have not heard that before that you had a vacuum leak.

Yes, when the line is reinstalled, the RPM will adjust slightly based on engine vacuum. If it is unstable then we need "t" in a vacuum gauge to the line so that we can monitor the engine vacuum.

However, if it is running properly and smooth then I would not worry about it. If you want to check your vacuum, then hook in the gauge and look at the page attached.

You should have a steady needle when the engine is idling and then when you spike the throttle, it will drop and then start to level back out when it comes back to idle.

Yes. Technically the total timing on spark advance vehicles is just a check and adjust as needed. Basically, you set the base timing and the distributor will advance itself to where it is needed.

Race applications set total timing because they are looking for specific power at the higher RPMs, so they need to set the base timing so that the total timing is what they want achieved.

Basically, you are looking for best overall operation. So set the base timing at the best spot and then make sure you are not stumbling or other issues at higher RPMs. As long as you are not having issues at higher RPM then you do not need to adjust the base timing.

Yes. That is by design. When the engine completely cools down that cam will reset.

Then when you tap the throttle, it will drop the idle speed, but it holds it higher like this because we want to warm the engine as fast as possible. We need to warm the engine ASAP so that everything runs efficiently, and the metal components turn better when they are at the higher temperature. There is more damage done to the engine before it is warmed up than any other time.

So, keeping the idle higher until it warms up gets the engine hotter quicker. So that cam will keep the engine at a higher idle until you get in it and start driving it.

They actually do this with modern computer-controlled engines as well, but it is not as noticeable.

Great question. Thanks

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