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MG MGB Technical - Quits making power above 5k rpm

Gentlemen: My MGB powered car has the following equipment: stock su fuel pump with points, mallory dual point distributor, 1.75" su's with slightly richer jetting and hotter coil. The car at the race track ran beautifully and strong until it got to 5000 revs and then just didn't pull any higher. Where would you begin your diagnosis? I think it might be a fuel pump issue but wonder what the consensus might be prior to removing perfectly good equipment.
CD Chris

Hi
I would like to be able to help you out here so-
A question or two
Will it rev over 5000 in neutral
Is the problem at exactly the same revs in every gear
Does the tacho play up when it happens
What exhaust and type of muffler are fitted
Have you checked fuel supply(pressure and flowrate) at the carburettors
Which needles and piston springs have you fitted to the 1 3/4 carbs
Have you checked the total advance of your dizzy,a lot of mallory dist. have adjustable adv.
And just out of interest what sort of car is the engine in.
Cheers Willy
WilliamRevit

What type of vehicle is it that you have MGB power in? Is this only happening in top gear or all gears? Without any other info I would guess points first. But if its more in top gear fuel could certainly be an issue.

-Bill
Bill Mertz

The engine quits making power in all gears at 5k rpm. The old engine with 1.5" carbs and the su fuel pump would twist up to 6 easily. Have the points in the mallory set at .016". Haven't checked to see if it will rev in neutral above 5k rpm. Sounds like the timing, advance and points might be the starting point.
CD Chris

Check the instructions for the Mallory dizzy. I believe that for a 4 cylinder the point gap should be .026. The .016 is for the Lucas dizzy.
John H

.026 would be for the dual-point, if Mallory do a single point it would be the same or similar to the Lucas.

Does the revving 'hit a brick wall' like Diesels i.e. rev fine up to the limit and then suddenly stop? Or does acceleration gradually fade away as it approaches 5k?
Paul Hunt

The car feels like it has a rev limiter on it. I will modify the clearance on the points but doubt that is the complete remedy. I think I had the correct point gap at another race and it still suffered at 5k rpm. Time to get out the Mallory directions.
CD Chris

Is it possible you are getting early valve float due to weak valve springs?
John H

It does abruptly quit making power like a diesel. Weak valve springs? Acts almost like it has a rev limiter on it. Just quits making more power. At 5000 I was going 90 and could have used the additional 1000 rpms to get over 100 to catch up and pass a leading car.
CD Chris

Points bounce, valve bounce or fuel starvation, either or all! The fuel problem could be a) The pump, b) the bore of the fuel line, c) restriction of the fuel filter, or d) I have known it caused by vibration putting air in the fuel.
Allan

As it's a race car is it new to you? Or has it only just started doing this? If the former I don't suppose someone else has fitted a rev-limiter?...

Fuel starvation problems due to the pump almost certainly wouldn't consistently be at 5k, but related to demand. But you can check delivery, direct the feed pipe into a container and turn on the ignition. Should deliver *at least* one Imperial pint per minute, and in practice closer to two, in a series of regular pulses and with negligible bubbles.

Is the tach steady when this happens or jiggling around? Points problems should cause jiggling around.

The big question is does it rev over 5k at a standstill?
Paul Hunt

sounds like you need a bigger capacity fuel pump to me.
P J KELLY

CD
Have you checked yet to see if it will flick over 5 grand cleanly out of gear. Did you have the malory in it when on the smaller carbs. Have you fitted a different muffler. Are the bigger carbs the only change made to the engine or have there been other changes as well from when it used to rev ok. Also what needles and piston springs have you got hiding in there.
Really just guessing here it sounds a bit like the fuel supply isn't up to the job. specially if everything everything else is still the same as when on the little carbs. If your needles are a tiddle on the rich side the the demand for fuel could be greater than the supply. What colour are the exhaust and plugs straight after a race. Cheers Willy
WilliamRevit

This thread was discussed between 07/10/2008 and 11/10/2008

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