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MG MGB Technical - 80 LE engine power loss

Just acquired a 1980 MGB LE. On drive home at highway speed we experienced a sudden loss of power and RPMs dropped.

Engine starts and idles perfectly. Thought problem was fuel filter. But changing filter did not correct the problem.

Car has Weber DGEV and Lucas CEI electronic ignition.

Double checked mixture setting since car was purchased in Colorado and is no longer in the mountains.

Looking for any advice since I have no experience with the CEI ignition system. I'm planning to replace dizzy cap, rotor and plug wires. I'll check the pick-up gap while I have the cap off.

What else should I look for?

Appreciate the help.

Ken V.
'77 MGB, and now '80 MGB LE
Safety Fast!
Ken Vandruff

Have you tried the old standard? When the engine quits, remove the gas cap and listen for a swoosh.

Dan Robinson

Ken. What happens when you have a power loss? What, exactly, do you mean by that term and when does it happen? What does the tachometer do when you have the loss of power? I know it is hard to describe, and very hard to get everything into a post while you are trying to define the problem in your own mind. But, the more information you can give us, the better answers we can provide you.

As to the Weber. As I remember it, a DGEV is the one with the electric choke. You need, when the loss of power happens, to pull over and pop the top on the air filter to see if the choke is fully open. If it is closing up, it will affect the mixture (the purpose of the choke) making it excessively rich and the engine will not run as well as it would on a proper mixture.

As to "adjusting the mixture", that is only adjustable on the idle circuit. Moving the car from the Colorado area (say 5,000+ feet) down to Kansas (much nearer to sea level) will also mean that you are going to have to install new jets for barrels. This is done by removing the top cover and the jets will be visible inside the fuel reservoir.

The Weber uses a power valve, along with an accelerator pump, to provide additional fuel on acceleration. The power valve is held in the off position by vacuum from the intake manifold and, when vacuum is low, the spring on the plunger forces the plunger downwards, activating the power valve. The diaphragm on the plunger will wear and tear over a period of time, causing the power valve to be on all the time and resulting in an excessively rich mixture.

It might be worth while to pull the carb, clean it throughly, and rebuild it. The rebuild kits by EMPI are the newest ones and they contain the power valve plunger. The older kits, by Weber (on a piece of cardboard with plastic sealed over the parts) do not contain as many parts as the newer kits. It would, also, give you a chance to see what jets are in the carb. I use a 135 in the primary (smaller) venturi and a 140 in the secondary venturi. You could, most likely, go even larger if you wished, but these meet the Arizona emissions testing requirements with no problems.

Watch the tach as you experience a loss of power. If the tach starts dropping quickly, as opposed to winding down more slowly, it indicates a problem in the low tension circuit of the ignition system. If it does not drop suddenly, it is more likely to be either the high tension circuit or fuel related.

What fuel pump are you running? Should be either the factory SU or a low pressure pump with no more than about 3.5 psi pressure to work well with the Weber DGV. David DuBois has some tech articles on checking out the fuel system and includes volume and pressure standards for the SU pumps.

The CEI ignition system is the GM HEI system which is a fairly good model. GM repair parts are supposed to work, but I have not tried this myself. I use the Lucas 45D4 "Euro Spec" points type distributors--easier to trouble shoot.

With more information, we can provide better ideas.

Les Bengtson

Ken - If your problem turns out to be an ignition related problem, go to Paul Hunt's web site at: Click on Spanners then on ignition related articles. If it is a fuel related problem, go to my web site at: Click on SU Fuel Pump Articles, then on Fuel Delivery Troubleshooting Guide. One of the first things to try is removing the fuel tank filler cap as Dan suggests. If the tank vent becomes clogged, you will get the symptoms that you describe. Cheers - Dave
David DuBois

Thanks for the feedback so far.

Les -- the tach drops in direct correlation to the power of the engine. It's gradual as opposed to abrupt. As I push down on the throttle the engine acts as if it is getting no fuel and it loses power. When I release the throttle, the engine returns to a normal sound at idle and you'd think there's nothing wrong -- until I push down on the throttle again. Sometimes feathering the throttle helps build up RPMs and power, but then it hits around 2,500 or 3,000 RPMs and everything drops out again. Sort of makes me wonder if the problem is in the coil or dizzy, but at this point can't rule out carb.

I have definitely considered a carb rebuild, I think I even have a get obtained from Vicky Brit when as a standby for the Weber I had on my '77 B before converting it to a Moss supercharger.

Dave -- Thanks for the lead for info on the ignition system. I've heard the CEI was a good system. I just want to make sure I fully understand it before doing something rash like yanking it off in favor of a Eurospec points-type dizzy.

The real puzzler is that the car ran like a champion when I test drove it, and during the portion of the trip home that took us through 10,000-foot mountain passes. It was 30 miles east of Denver when she decided to act up.

I'm planning to check timing and pull dizzy cap tonight to see if anything looks out of place. Coil had some dirt build up around the LT terminals. I plan to clean that off as well.

The downer of the moment is that we got the LE primarily for my wife who has been in love with LEs ever since she saw a line up of them at the first NAMGBR we attended in St. Louis. She's kind of bummed at the moment.

I'll keep you posted.

Ken V.
'77 MGB, plus '80 MGB LE
Safety Fast!
Ken Vandruff

Ken. Take a good look at the distributor cap. I had one Lucas cap on which the carbon bush self destructed in about 3K miles from new.

When we purchased my daughter's 77B, the hose to the vapor seperator can was detached. Hooked it up properly and began to have problems similar to what you describe. When the fuel cap was removed, there was a "whoosh" sound. Found the line running from the tank vent, to the vapor seperator, then to the charcoal cannister was blocked.

Your symptoms indicate a fuel system problem of some sort. Removing the fuel cap and changing out the fuel filter are the two easiest things to do first.

Les Bengtson


Dizzy cap and rotor looked OK. Turns out the vent pipe to fuel tank is open. It also was disconnected since the rubber hose connections in the trunk had gotten brittle and broke loose. Disconnected the vent line from carbon cannister and blew compressed air through it to verify that it was open. The vent pipe connections are now correct.

Checked timing. It was 20BTDC @ 1500 RPM. Reset to book spec of 10BTDC @ 1500.

I can at least now coax, cajole and threaten the engine to sputter up to 4000 or a quick touch to 5000 RPM, but still not a condition that I'd want to drive very far.

My next idea is to replace the coil. I have a spare that I removed from my '77B when I converted that car to a Lucas Sportcoil. Can I use the former '77 coil on the '80 LE with CEI? I think I read somewhere that I can.

If changing the coil doesn't do it, I guess my next step is to pull the Weber for a rebuild.

Thanks for the help so far. I am determined to find the remedy.

Ken V.
'77 MGB, plus '80 MGB LE
Safety Fast!
Ken Vandruff

Just remember one other test that I conducted.

The LE has an aftermarket fuel pump - it's a cube-shaped pump that was bolted to the inside of the trunk and the hoses routed though the round hole that normally holds an SU fuel pump.

Anyway, it pumped a pint (16 oz.) of fuel in 30 seconds. I talked with John Twist today and he said it should be at least one pint in one minute, so I am assuming I'm getting enough fuel. Unless this is an indication that I've got too much fuel pressure for the Weber to handle.

Will appreciate hearing anyone's thought on that theory.


Ken V.
'77 MGB plus '80 MGB LE
Safety Fast!
Ken Vandruff

Ken. You may want to give John T. another call about the coil. One Brit engineer told me that the CEI system needed "a CEI" coil and that the coil for the Opus system would not work. But, since I do not use the CEI system, I am not sure what is the truth of the matter. John will know.

As to the fuel pump, the combination vacuum gauge/fuel pressure gauge is relatively inexpensive (about $25 when I last priced one) and would allow you to determine what your pump pressure is. Pat Braden's book on the Weber carbs states that you need about 3.5 psi pressure. If your pump is putting out much more than that, you need to replace the pump or install a pressure regulator.

Dave DuBois has posted his website address. He has a tech article on installing a back up fuel pump which may be of interest to you. I used it as an outline on how to install a second pump on a car that I thought the fuel pump was going dodgy. When the primary fuel pump quit, crossing a line of traffic, a flip of the switch brought the second pump into use and got me home. Might be a very good idea on the car for your wife.

Les Bengtson

Ken - That Facet pump that you have could be acting up even though it was pumping 1 pint per minute into an open container. It could be bogging down when it is loaded (I had one of them on a Mazda pickup that I had and it would periodically not pump enough fuel to the carburetor when I was under load with a lot of throttle. As soon as I got off the throttle, it would be fine. Those pumps are only around $40 at NAPA. go to my Back Up Fuel Pump under the Other Tech Articles on my web site for the part number of the correct model (you want a low pressure pump (I believe that they are 2.5 - 4psi). Cheers - Dave
David DuBois

Ken: I recently ran into a similar problem. I discovered the coil wire had worked its way "loose" thus creating the spark to jump from the coil to the coil wire lead. Once I went over 2000 RPM the engine would lose power, "flatten out" and start to back fire through the carb. The electrical connections to the fuel pump had also worked loose over time as well.

So the zero cost option is check and clean all the electrical connections.

Gary :>{D
79 MGB


Thanks for the input since the coil is the next item I plan to check for gremlins.

The terminals on the coil that's in the car are covered with dirt. A good cleaning is in order. I also have a coil that powered a points-type distributor sitting on the shelf. I may make a swap just to see if that helps.

Ken V.
'77 MGB, plus '80 MGB LE
Safety Fast!
Ken Vandruff

The CEI system is *very* reliable, unlike the earlier Opus system, and the units are still available as they were used by a huge number of manufacturers.

If the tach *drops* as you push the throttle but the engine doesn't rev any more, then it is almost certainly an ignition LT problem. The 'points' plate twists under changing vacuum i.e. changing throttle, and this bends the wires that come from the trigger. It's possible that the conductors have fractured internally and are making and breaking as the wires are bent. Points and pertronix distributors also need a ground wire between the points plate and the distributor body and this can also be a cause, but neither of the factory electronic systems need it as the electronics that fire the coil are separate from the trigger.

You need to be careful swapping coils, there were different types for different years. A rubber bumper car would originally have had a 6v coil as it had a ballast resistance in the harness, a chrome bumper car had a 12v coil and no ballast. Get the wrong coil in the car, or the wrong ballast, and you will get either weak sparking or coil overheating and premature failure. See my post on the 'pertronix distributor' thread to see how to determine which is which.
Paul Hunt

AH-HA! Found the problem!

Believe it or not, a tiny piece of foam from a deteorating air filter fell into the fuel bowl of the Weber carb and got sucked into the primary jet. Corked it right up. No fuel passing through no matter how hard you pressed on the accelerator.

Removed the foam, reassembled and reinstalled the carb, fired right up! A new rectangular air filter assembly is now in place on the carb.

Wife did happy dance in the garage. Says I'm her hero. Good day all along.

Thanks to everyone with your advice and comments. They kept me from getting too discouraged along the way. That's what I love about this BBS, there's always great people offering great advice and encouragement.

See you on the road!

Ken V.
'77 MGB, plus '80 MGB LE (now running)
Safety Fast!
Ken Vandruff

This thread was discussed between 16/07/2008 and 19/07/2008

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