MGB starts easily and runs on fast idle, then stalls when dropping to slow idle?

1978 MG MGB
72,000 MILES • AUTOMATIC
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JD78MGB
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This MGB started and would run properly throughout the spring and summer. After a month without starting and driving, started it up to take it for a drive and it started quickly and idled fine but as soon as it dropped from fast idle at start-up, it stalled. I could restart it, but now had to hold the gas on or it would immediately stall. Have checked everything I know and don't see any reason for this behavior.
Would love to have some ideas to try to sort out this problem.
Sep 30, 2026 at 2:31 PM
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STRAILER
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Sure, 2 things might be happening. You may have a vacuum leak which this video can help you find.

https://www.youtube.com/watch?v=xN5pcM-5bso

Or you have a clogged idle jet inside the carb. Which carburetor does this engine have, single Zenith-Stromberg, twin SUs, or a Weber conversion? A picture would help, because the checks differ? If it has the Zenith-Stromberg, inspect its piston diaphragm and movement. A damaged diaphragm, sticking piston, or obstructed float-chamber vent can cause poor running. Does it run smoothly when you hold it around 1,500–2,000 RPM after warming up, and is there black smoke or a strong fuel smell? Those details, along with the carburetor type, will help narrow the next check.
Oct 1, 2026 at 10:24 AM
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JD78MGB
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Thank you for your reply. I attached a picture of the carburetor as you requested.
Answering your questions:
1. No black smoke or strong fuel smell when start and running.
2. When holding between 1,500 to 2,000 RPM's it runs pretty smooth, but not perfectly smooth. There is an occasional very short "sputter". This sputter also occurs during fast idle.

I will watch the video now but wanted to thank you for your reply and send this information back to you promptly.

Jim
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Oct 1, 2026 at 11:06 AM
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STRAILER
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That is perfect, yes, so it can be a vacuum leak, or the idle jets are clogged and you need to remove the carburetor and rebuild it. So I would start with the vacuum leak first. Please go over this video, also check the brake power booster, it might have an internal leak and check the CPV valve it might be stuck full open. Can I ask if this engine has an EGR valve?

https://www.youtube.com/watch?v=LRKWV4Are5c&t
Oct 1, 2026 at 5:58 PM
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JD78MGB
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Thank you. I will follow suggestions in the video to look for vacuum leaks.
The engine does not appear to have an EGR valve - based on pictures I find that show where it would have been if it was installed.
Oct 2, 2026 at 7:26 AM
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