You are here:

MG Car Repair/loses power on hills


I have a 1979 MGB when i take the car out and its driven for 15-20 min on the freeway it will lose power going up hills, its like the motor is starving popping and sputtering. This doesn't happen when its cold. Only after the car is warm, and it struggles up all hills.Could this be a fuel pump or maybe ignition problem?

thanks much for your help

Hi Wyn,

Loss of power under a load can be a lot of things so it is necessary to do some testing to pin point the problem.

You will need a few tools besides a normal set of hand tools. You will need a compression gauge which is not expensive.

It only takes 3 items to make an engine run well and that is "Compression", "Fire" and "Fuel" with conditions on each. They MUST be tested in that order.

"Compression" -- Remove all four spark plugs and have someone hold the throttle open or wedge it open. Spin the engine over at least 5 or 6 revolutions with the compression gauge in a spark plug hole. You must see from 135 PSI to 170 PSI on each cylinder and there should not be a lot of difference between each cylinder. If you have less then that on any cylinder it is no use looking further until that is corrected.

"Fire" (Ignition) if the compression test was ok then and only then can you look at Ignition.
You already have the spark plugs out so attach each spark plug to it's wire and lay each plug on a metal part of the engine. First spin the engine over and watch the plugs fire and note the color of the spark and the thickness of the spark. (don't do this in bright sunlight).
The spark should be blue in color and as thick as a lead in a pensil. If it is then put a timing light on #1 plug wire and have someone spin the engine over and chaeck the ignition timing. If there is still a sticker under the hood it will have the specs. If the sticker is gone set it at 10 deg BTDC and later set it at 10 deg at 1500 RPM when you get it back running again.

"Fuel" (Carburetor) The stock 79 MGB had a single Stromberg carburetor and a large air cleaner. First confirm that the air filter is inside the canister and is fairly clean. Then unscrew the top cap on top of the carburetor and lift the cap up a little and push it back down. You should feel hydraulic resistance when pushing it down. If not remove the cap and long pin with the little piston on it's end and put a little engine oil in the top and take a long 3mm Allen wrench and put it down into the top of the carburetor and see if it engages a Allen screw. If it does don't turn it just note if it is there. (Some carburetors had the Allen screw blocked off.) it is the mixture adjustment screw. Replace the piston and cap.

If everything was correect so far, you need to purchase a fuel pressure gauge and a long piece of fuel hose (About 3 ft.) and hose clamps and a "T" connection. These are not expensive tools.
Remove the fuel hose off of the carburetor and cut a short piece of your new fuel hose and put clamps on it and the "T" fitting and your long hose and the fuel pressure gauge. (Be sure to put clamps at each joint)

Run the hose out from under the hood being careful not to pinch or cut the hose and place the gauge under a wiper arm so the gauge faces inside so you can monitor fuel pressure as you drive. Note the fuel pressure when you start the car. It should be from 1.5 PSI to 3. PSI and take the car down the road and get it to run poorly as before and at that time note the fuel pressure. If the fuel pressure dropped low when it was running poorly you either have a fuel pump problem or a tank, filter or fuel line problem.

If the prssure was still good when it started running poorly, quickly open the hood and put your hand on the coil to see if it is too hot to touch and quickly remove the plugs while it is hot and lay all the plugs out like you did earlier with their wires attached and spin the engine again to see if the color is still blue. If it is now yellow or orange color and thin as a hair then you have an ignition problem.

let me know the results of these tests and I will give you the next procedure.


MG Car Repair

All Answers

Answers by Expert:

Ask Experts


Howard M. Fitzcharles III


MG from 1956 (USA versions only) up and Engine theory.


Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

Associate member SAE EAA member

Import Car magazine

ASE Master Auto with L-1 certification up to 2000

©2017 All rights reserved.