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Car Stuttering at High RPM

Started by september2021, August 06, 2021, 02:19:52 PM

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Our '77 MGB with a coil, 25D distributor, and professionally rebuilt Zenith Stromberg carb can rev up to 5K just fine in neutral.  But, when it is in 2nd or 3rd it will start to stutter at around 3500 and 4000 rpm.   

I am thinking this is a vacuum advance issue.  Should I check other things like timing?

Patrick Lewis


Check your fuel delivery. It takes more fuel to drive a car to 5000 rpms as opposed to it revving to 5000 sitting in your driveway.

You could also have spark issues as spark can change under a load.

Could be timing.

Unlikely vacuum leak as that would be noticeable at idle and other rpms.

Did you make any changes right before this started?

Time to start checking basics.

Patrick Dockerty
American MGB Association
phone/text: 773-769-7084

North America's oldest and largest club for all MGBs, MGB-GTs & Midgets.  A nonprofit organization founded in 1975 and offers color magazines, emagazines, technical advice, member recommendations on service shops, registration of your MG, a tradition of service & FREE member classified ads that appear in the magazine & on the website.


Hi Patrick,

Frank says I'm a broken record about this, but with a 25D distributor, they do have a history of issues with the vacuum and mechanical advance systems.

Generally, the most common fail is that the vacuum advance pot (the large silver disk outside the cap) can come internally detached or loose from the actuator arm that advances the timing, so fails under hard acceleration causing the car to bog, stumble and, in the extreme, stall. Feels like the fuel is leaning-out while you're driving.

Your car would have originally had one of the factory electronic systems, like the Opus system, which had unique distributors that often failed and were then replaced with older Lucas point and coil distributors (often with PerTronix electronic conversions in them) or aftermarket units from like Crane or Mallory. 

The 25D is the oldest style distributor and was replaced by the 45D by about 1973.  Not as many parts available for the 25D not shared with the later units, but the vacuum advance pot is. 

You can also still get a complete new 45D distributor. Different cap and primary lead, but otherwise it is a bolt-in replacement for the 25D. A PerTronix kit is about $100 for either the 25D or 45D (they are unit specific only for the primary lead) and a great investment.  All available from the usual catalogs and some tractor supply outlets (Massey-Ferguson, I-H and other tractors used the same distributors).

Based on what you describe, I'd suggest to look at the distributor right after you check the fuel feed.

Hope that helps you.

Safety Fast!


Art Isaacs
AMGBA Tech Staff

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