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MG MGB Technical - Loss of Power at 3000 RPM

We recently acquired a 71 MGB. It has a newly rebuilt engine and we have just put on a new fuel pump BUT still we are experiencing loss of power around 3000 RPM. The fuel filter and lines appear to be working as it squirts out fuel liberally when the line is removed from the carb. Does anyone have any ideas? PS - the exhaust system is in rough shape and needs to be replaced (not sure if it related or not)
K Olohan

When you checked the pump's output, did you route the line into a container and measure the pints per minute and check for any air in ght flow? The pump should give a flow of about 2 pints per minute and there should be no air bubbles in it. If you are using a SU fuel pump, the pressure should be around 2.7 psi. If you are getting air in the flow of fuel, you could have a leak in the pump, in the connections to the pump or at the connection at the fuel tank or you could even have a hole in the pickup line inside the tank. If you are using a pump other than a SU pump, you need to check the flow and pressure. Many of the after market pumps put out way too much pressure for the MGs and need a regulator used with them. Finally, the old truism is still true - if you are having problems with the SU carburetors, check the Lucas ingition first. Good luck - Dave
David DuBois

Agree pump is the first area to probe - an error in the timing or a partial coil or plug leads failure can also have the same effect - I always throw away the plug leads, dizzi cap and rotor with a 'new' car - people tend to hang on to them much too long.

When all else fails - check the spec of the carbs needles.

good luck
RMW

Assuming you've got good fuel flow, start with a good inspection of the ignition, as suggested. Check the points for wear and proper setup, check the plug and coil wires, check the connections at the coil to make sure they are clean and tight, and give the timing (and advance) a test. Then you can turn to the carb side of the motor if the issues persist. May as well start with the simplest, cheapest to fix, and most common problems first.

The ragged exhaust system is not desirable. If it's not supplying enough back pressure, then you may have an overall loss of power. However, I wouldn't think it would be something you could pinpoint to a specific RPM range.

Brad Batchelor

Thank you for all of your advice. We checked the fuel pump's output - can fill half of a beer bottle ;) (about 200 mls) in about 20 seconds. So it seems like the output is okay. The main symptom continues to be on 'acceleration' - a definite lag around 2500 to 3000 rpm. The timing is good. Will check the points. Any other ideas? (the exhaust is totally rusted out from the header to the tail)
K Olohan

You might check for problems with the fuel level in the float bowls or debris in the bowls. If one or both of the floats are set too low or are partially blocked you can have fuel starvation on acceleration. While you are at it you should check to see if each needle is rising uniformly when you flip the throttle. They make a set of SU tools for this but in a pinch a pair of large paper clips can be bent to shape to do the trick.
Best regards
Jim
Jim H

I would assume it could possibly be in the advance mechanism in the dizzy. If you have another dizzy w/ good advance springs & vacume diaphram try it on.
Luigi

Check the gap on the points (or the dwell angle), or better yet, replace the points and the condenser. If the points gap has closed up, it will cause probmlems at higher rpm, as will a weak spring ont he points. The condenser can get flakey causing all kinds of symptoms. Check the oil level in the carburetor dash pots.
Good luck - Dave
David DuBois

Thanks again. Also, along with totally rusted out exhaust, the gas tank has sprung a leak!! (thankfully near the top) Could that be playing into the equation at all. We will check the points tonight!
K Olohan

Odly enough I just went through a similar situation with my 77 B. You didn't say what type of carb you had, but my problem was with a Weber DGV style carb. I fought the problem through out a 2000 mile trip, never getting above 4000 rpm. I discovered after returning home that the problem was mostly due to the carb float being set too low allowing the jets to become uncovered during a hard level or just about any type of uphill pull.


R Hill

The gas tanks always seem to go from the top. That shouldn't play into it all. Although, it should be a top priority to repair or replace.
Brad Batchelor

A rusted through gas tank definitely could play into the problem. If the top is rusted through, bit and pieces of rust have fallen into the tank and the inside of the tank is undoubtably also rusted. All of this could be providing debris in the tank to clog gas lines, etc. That said, I would suspect that if this was the problem it would appear over the whole rev range rather than just 3000 rpm, but never discount the problems that rust particles can cause. Good luck - Dave
David DuBois

This thread was discussed on 13/07/2004

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