MGB & GT Forum

1973 MGB with a fresh engine and carb rebuild. Accelerates well but stumbles at cruise at anythng over 3000rpm. Have checked for air leaks....nothing visible.
Rebuild included early no-smog head; converted to 1.5in HS4. Bored .20-thou over. Otherwise a stock motor.
Played with carb fuel mix but it does not change the performance.


Any suggestions are welcome. Frustrated that I cannot enjoy the fruits of my labor and cheque book.

Barry

Check the oil in your dashpots. I use Su oil but I think the manual says light mineral oil.

will add to the list...doing that now

Sounds lean.

X2 on lean. What do the plugs look like? Lean it until they are sooty and then go back to lean. When I put my carbs on, it took numerous adjustments to get them rich enough.

Plugs look great, you might say text book, and all four look the same. Dash pots are full

Did you adjust the valves recently ?
Did you adjust the timing recently ?
Did you check the air filter ?
Then adjust the carburator.

Higher than 3000 rpm could be a bad coil too, not enough voltage in the battery, bad condensor (if you have still points).

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1977 MGB Red 'Victoria' and 2018 Jaguar XE 'Meghan'
All Classics:
Married, 2 children, MG MGB May to December, Jaguar the rest of the year.

I added a slight amount of timing, it seemed worst. The coil and is brand new, battery is a year old, but I will check into them. The valves were adjusted at 700 miles after the engine refesh, I'm just over 2000 miles now.

Great tips thanks

What kind of shape is your distributor in? New points, etc? Points set right? Sounds like an ignition problem.

Try replacing the fuel filter. My Toyota Corona wagon stranded me for the first time while on vacation. Nosed off hard under acceleration until it finally just gave up before we figured out what it was. It's a cheap thing to do (also replace rubber fuel lines with ethanol-barrier hose while you're there) and it certainly isn't going to hurt anything if it doesn't fix you problem.

New coil huh. Check the primary resistance it should be 3 ohms if it is 1.2 then you have the wrong coil. The coil will also get real hot.

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Dan Wood
70BGT driver, OD, Pertronix, HS4's, Peco, .060 over, Elgin cam, Superlite wheels, poly bushings, panhard rod, rear tube shocks, 1" lowered front end, HD shock valves, etc, etc.
68 MGB rebuilt engine D9 cam balanced, still a work in process but runs real nice.
88 Saab SPG Turbo
86 Vanagon Westy (South African conversion engine 2.0 OHC 135HP) This car burnt to the ground. Bought another, same year and color but with a 2.4 Waterboxer.
2004 Dodge Ram 6sp with HO Cummins diesel
Lucas= Loose
Unsoldered
Connections
And
Splices

A long time ago I experienced something like this. Assuming your still running points, check the lead to your condenser. I had routed it incorrectly and as the vacuum advance disk thingy moved it would dig into the condenser wire, eventually wearing through the insulation and grounding it out. Then the timing would retard and the engine would run smooth again. It drove me CRAZY!

Visibly verify the movement of your jet.

Although the screw is moving...raise the piston and LOOK inside the bore to ensure you can see it travel up and down as it's being adjusted in relation to the bridge. After you've verified it's movement, start with 2 turns down from level with the bridge on both and adjust from there.

Also...remember you need to ensure equal airflow through both via Uni-syn/ear/bic lighter etc...You must check the balance after any adjustment to the mixture is made.

I actually had the same scenario where I was turning and turning but no change. Lo and behold...the jet was stuck fully down and was not coming up at all.

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"One test is worth a thousand expert opinions"--Alvin "Tex" Johnston...Boeing test pilot.

"Who do you think you are? I am."...Pete Weber

73 MGB. Tires: Round, black, hold air. Oil: Sometimes old, sometimes new...always slippery. Oil filter: Yellow, usually full of oil. Carbs: 2 SU HIF. Distributor: Yes. Headlights: Not that bright but bright enough. A bunch of other stuff most cars have but not really important enough to itemize. Oh, wait...it has a cool sounding exhaust with stickers on the chrome tips. Really slays the ladies...

All good ideas but just to make me happy, pull your alternator out and have it tested. That sounds more electrical than fuel :-)

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Basil C. Adams
1956 MGA Coupe (Show Car)
1957 MGA Roadster (Driver)
1958 MGA Coupe (Racecar)
1959 MGA Coupe (unrestored)
1960 MGA Coupe (unrestored)
1960 MGA Roadster (Driver)
MKIII Elva Courier (E1056)
1967 427 Cobra
1972 Alfa Romeo Montreal
A coupla late MGBs
1960 Austin Healy BN7
More Cars than Brains

You can easily test the alternator without pulling it.
Although I suspect the issue is you are running an engine /head combination that yields a very poor running 7.5:1 compression engine.

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