MGB & GT Forum

1972 MGB. Revs and drives fine up to 2500 rpm. when driving in 4th gear it won't go above 40mpg. It just starts chugging. When stationery (like in the video) it will only stay at 3k if i keep applying more throttle

I have attempted to set the valve timing. But i may not have got it quite right

I bought the car like this. It had a new engine in 2019. I don't know how long it hasn't been used for or what it was like when last used

Sounds like the carburetor pistons are stuck or won't rise above a certain point. Might be time to do a comprehensive tune up of everything, valves, ignition, fuel supply, carbs, plugs etc.


GMc

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Wait for me at the gate Tuffy, because without you they're never gonna let me in.

SDCH WTCH-X Twin Oaks Tuff Nutt
2008 - 2022

Good idea to start with the engine tuned top to bottom.

The library has a good flowchart of what to check.

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Wayne Sanders
Rose Lodge, OR
(TD 4288)

"I don't care to belong to any club that accepts people like me"-joined Willamette Valley Club in 2011

79/65 MGB - Carmine- V-6 - T-5

This car is now very nearly completely done. Sure to find something else, but not now.........And 12 years later, I'm starting to fix things that were new when we built the car. It's pretty well de-bugged!

As others have said do a full tune up.

Start with the valves, then the timing, then points (if used) and finally the carbs. An oil change is in order as well if you don't know when it was done last.

BH

Check to be sure air cleaners and their gaskets are not obstructing the air holes in the faces of the carburetors...

Dick

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Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

You’re sure the throttle cable is properly adjusted, so that the throttle plate is fully open when the gas pedal is fully pushed to the floor?

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Pogo is right.
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The power of reasons is an illusion. The belief will not change when the reasons are defeated. The causality is reversed. People believe the reasons because they believe in the conclusion.

I have two guesses: either the carburetors don't fully open, or the distributor does not advance sufficiently at higher rpm.
It would be best to systematically tune the car from beginning to end, especially as the car is new to you and you don't know its current state.

Back in 75-76 I had a man come to me & he was a good mechanic, with a 72-73 MGB & it would not rev more than 3,000 RPM, that had a fully new tune up, everything gone through & he had worked & re tuned for 1 & 1/2 days to make it run right & told me he would gladly pay me to fix it. I told HIM to take my timing light out & hook it up & start the engine & see if the timing advances. He came back & said that it does not move at all! I told him he owed me BIG TIME!!!! James, a very ease test & I would start with checking the advancing of the timing with the increases of RPM.

James,

I think the most likely culprits for your issue have already been mentioned. I'll try to provide a bit more info. As Dick Moritz suggested, ensure that those air holes on the faces of the carbs are not obstructed. You can pull off the air cleaners and just bolt the air horns in place for the testing - again ensuring those air holes are not blocked. See if that helps.

I've had personal experience with the carburetor pistons not being able to rise, preventing air from getting into the engine at higher rpm. In my case, a friend really overfilled the carb dashpots with oil. If for any reason those pistons can't rise (as a few have already mentioned), your rpms will be limited. You can observe the rise and fall of the pistons while you have the air cleaners off by revving the engine.

Perhaps, as rocannon mentioned, the throttle cable is not adjusted to allow the throttle plates in the carbs to open enough to get air to the engine for higher rpm. With the engine not running, pin the gas pedal to the floor, and lift each carburetor piston and view the throttle plate through the carburetor opening. At wide open throttle, the plate should be horizontal.

If none of the carb issues are present, the issue may be with the distributor's centrifugal advance as Glenn and others have mentioned. You'll need to hook up a timing light - with the engine running, and the vacuum advance hose disconnected from the distributor, rev the engine and see if the timing mark advances. If not, you have a frozen centrifugal advance mechanism in your distributor. With a dial-back timing light, you can check to see exactly how much timing advance you're getting. If the centrifugal timing is not advancing in line with the specifications for your car's year, that can definitely limit your engine's ability to rev.

I hope the above proves helpful. If so, let us know. If not, post back and smarter folks than me will likely be glad to assist.

Best regards, George

I want to know how you get even close to 40 MPG ????

My impression was that it was a typo, where mph (speed) was meant instead of mpg (fuel consumption).

LOL I knew that but was just having fun. My MGB would not get 40 MPG going downhill with the engine off.

One other possibility is that the exhaust is very restricted. If you hit something and impinged the pipe, it will not allow the engine to rev. It will start, idle, and run well up until a point, then it will act just as your engine does. As well as the other good suggestions mentioned, take a look under the car.



Edited 1 time(s). Last edit at 2024-04-15 09:34 AM by Mustangsix.

As Jack mentioned, I helped a friend get an MGB that had sat for 20 years started and we had to use a shop vac on the suction mode at the tail pipe to get enough flow though the exhaust system to get it to start. All kinds of mouse nest stuff and acorns came out, but once cleaned out it ran great.
I also had a Subaru that was coughing and spitting out the carb and it was due to a clogged catalytic converter. Once I dropped the exhaust at the headers to ran fine. Ended up removing the cat and it ran great after that.