Possible vacuum issues

[PECO]

'70 383 magnum w/ AFB carb.
Here's the issue.
The engine starts and runs great when cold [ summer cold ] and as it warms up. Then as the temp. rises within the normal range the RPM's
begin to increase. Within the normal zone as per the guage , the higher the temp. goes the faster the engine idles until the RPM's are up at around 1500 - 1700
which becomes difficult to hold at a light or the engine stalls. I've been told that the prob. could be a vacuum leak. Has anyone had this prob. and did they find a solution.

 

[pschlosser]

Usually, a vacuum leak causes a drop in idle, or a stumble from idle to open throttle.

Most of us know, as the combustion engine gets warmer (from a cold state), it becomes slightly more efficient and internal parts swell and tolerances tighten.

My first instinct is to check for a rich fuel mixture when the engine is warm or hot.

Is your choke fully disengaging once the engine is warmed up? And by fully disengaging, I also refer to the fast idle cam that may force the engine to idle higher. So too, may a partially closed choke make the mixture richer, and thus, the engine idles higher.

Those "idle mixture screws" at the bottom of the carburetor impact fuel mixture at idle. If you keep track of the number of turns you make, you can put them back without any heartache. Once the other issues, mentioned above, are inspected and checked, turning the mixture screws to the highest RPM (and resetting the idle RPM back to the factory speed using the idle speed screw), 1/2 turn at a time, may correct things. The mixture screws are used to tune for the highest possible engine RPM at idle.

 

[Xcudame]

Hook up a vacuum gauge when it's cold and see how it reads. Then drive it to warm it up and check the vacuum gauge again. Use the attached guide to diagnose the problem. You can efficiently tune your car with nothing more than a vacuum gauge and rpm gauge!

 

[moparleo]

Before you do anything, can you give us some details ?
When did this start to happen ? Was it after you made any parts changes or adjustnments to the engine.
Things generally just don't happen on their own so we need to back up to when it started to happen to properly diagnose your issue. Otherwise we are just throwing darts at it hoping one sticks.

 

[PECO]

Usually, a vacuum leak causes a drop in idle, or a stumble from idle to open throttle.

Most of us know, as the combustion engine gets warmer (from a cold state), it becomes slightly more efficient and internal parts swell and tolerances tighten.

My first instinct is to check for a rich fuel mixture when the engine is warm or hot.

Is your choke fully disengaging once the engine is warmed up? And by fully disengaging, I also refer to the fast idle cam that may force the engine to idle higher. So too, may a partially closed choke make the mixture richer, and thus, the engine idles higher.

Those "idle mixture screws" at the bottom of the carburetor impact fuel mixture at idle. If you keep track of the number of turns you make, you can put them back without any heartache. Once the other issues, mentioned above, are inspected and checked, turning the mixture screws to the highest RPM (and resetting the idle RPM back to the factory speed using the idle speed screw), 1/2 turn at a time, may correct things. The mixture screws are used to tune for the highest possible engine RPM at idle.

Thanks for your response, I think I'll go back to the guy that rebuilt and restored the carb.

 

[PECO]

Hook up a vacuum gauge when it's cold and see how it reads. Then drive it to warm it up and check the vacuum gauge again. Use the attached guide to diagnose the problem. You can efficiently tune your car with nothing more than a vacuum gauge and rpm gauge!

View attachment 128686

Thanks for your reply and the readings info. Will try. My first thoughts were that as then engine got
warmer that it was losing vacuum and the carb was closing.