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MG MGB Technical - MGB Flat spot

Have 76 B roadster in mint condition totally original apart from unleaded head. Engine rebuild by Rolls Royce specialist. Has passed all uk legislation tests with flying colours, Problem.... Only when engine hot developes(bad)flat spot on acceleration till revs approx 1500. Starts fine when engine cold - will not start when hot. Fuel filter/pump all ok - heat shield good and cooling fan fine. Emissions fine. Help - please!!!!
M Western

M-
As per the Owner's Manual, do not allow the engine speed to drop below 2.000 RPM. It could be a slightly lean (weak) fuel-air mixture. Try adjusting the carburetors a bit richer. On the SU HIF Series carburetors, this is accomplished by rotating the idle fuel-air mixture adjuster screws on the bodies of the carburettors a quarter-turn at a time. On both SU HS and SU HIF carburetors initial testing of the fuel-air mixture strength can be ascertained by lifting the vacuum piston of the rear carburettor by means of the vacuum piston lifting pin. If the engine speed increases only slightly, then the fuel-air mixture is correct. If the engine speed increases, then the fuel-air mixture is too rich, so on SU HS Series Carburetors you will need to rotate the adjuster nuts of the fuel jets of both carburettors upwards in order to lean it out. On the SU HIF Series carburetors you will need to unscrew the adjuster screw outwards. If the engine speed decreases, then the fuel-air mixture is too lean (weak), so you will need to rotate the adjuster nuts of the fuel jets of both carburetors downwards in order to enrichen or to lean (weaken) it. Be aware that the difference on SU HIF carburetors is almost subliminal, whereas with SU HS carburetors it is more marked. If you adjust the fuel-air mixture in one direction just enough to barely detect a weak fuel-air mixture, and just enough the other direction to barely detect a rich fuel-air mixture, then the correct position is halfway between the two. With practice, you should be able to get these two points closer and closer together. Once this balance is achieved, the front carburetor can then be adjusted using the same technique. However, the rear carburettor should be rechecked, as the functioning of the carburetors are interdependent.
Steve S.

On SU's acceleration enrichment is via the damping of the piston, brought about by the oil in the dashpot and the damper. The viscosity of the oil is a moot point! SU recommended SEA 30, BMC suggested multigrade engine oil. I use 20/50 in summer and dilute it, in winter, with Red X. If you are having difficulty starting it when warm, it might suggest heat soak, vapourising the fuel in the fuel line or float chamber. You say the heat shield is good, are the insulaters in place between the manifold and carbs? Also check that the diaphram in the vacuum capsule, is in tact.
Allan

Hi,
Personally I would book the car in for a session on a rolling road. Can save a lot of guese work, and get the car really running at its best right through the rev range.

Charlie.

Charles Goozee

Flat-spots usually indicate a weak mixture during acceleration, failure to start when hot a rich mixture. Lack of damping together with a rich mixture could therefore cause both problems. To check the damping unscrew the damper cap, lift it up, then press it back down. As long as you can feel the effect of the damper *before* the cap reaches the neck of the cover then you have enough damper oil, how much before indicates how much of a 'reserve' you have. The books for some years show the level of the oil *above* the top of the cylinder, which is incorrect as the oil level will drop from there very rapidly to the top of the cylinder and you will be forever topping-up.

What *are* the emissions? Haynes indicates 3% CO for a UK 76, whereas the MOT limit is 4.5%. Some stick rigidly to the 3% figure but personally I use what is best as long as it is inside the MOT limit (for the test at least ...). Mind you Haynes also says a North American 75/76 is 6.5% whereas in 74 it was only 2.5%, so I have difficulty in believing those Haynes figures at all!

When using the lifting pins if the revs drop away immediately it is too weak. If they rise and stay up as you hold the piston up it is too rich. The revs should momentarily rise as you raise the piston then settle back *even while the piston is held raised* for the correct mixture. Note that the lifting pin itself will rise a lot further than the piston, you need to raise the *piston* by about 1/32", which can be judged by unscrewing the damper cap and peering in there, removing the air cleaners and peering in the intake, or simply feeling the additional 'weight' of the piston on the pin.
Paul Hunt

For best power & smoothest running, these engines need 5% CO throughout the rev range.

Lifting the pin distance is not as critical as it sounds. Just lift the pin slowly until the engine responds as Paul notes above.
Chris at Octarine Services

This thread was discussed between 03/08/2008 and 11/08/2008

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