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MG MGB Technical - High Rev Missing

At higher revs (3500 - 4000), my MG loses power and the car seems to misfire. This only seems to happen on the highway in 4th gear. Sometimes at this speed, the car will backfire. Through the gears, everything is as it should be and the car feels very powerful.

The car is a 1976 MGB with a twin (rebuilt) SU conversion. Everything is new and adjusted properly (I think).
Carbs are balanced and set a tad rich. fuel pump is a new Lucas pump and is pumping the proper amount of fuel. I have a fresh tank of "high test" Sunoco fuel and a new fuel filter.

The distributor was advanced to best engine sound then retarded slightly. I am using a pertronix electronic system, so there are no points to adjust. The engine doesn't "ping". Distributor cap and rotor are new.

Plugs and wires are new. Engine was completely rebuilt about 6000 miles ago.

Does anyone have any suggestions on what could be causing this problem?

Jim Webb

I'm no expert tuner, but I think I'd advance the timing 2 or 3 degrees and see if that helps.

Does a '76 have a TCSA switch and if so, is it connected? If all of this is true, then your vacuum advance will kick in (only) in fourth gear. That may be affecting your performance as well, although what it would be doing is increasing spark advance in fourth gear, so it's not realy likely this is a problem.

Matt Kulka

Jim-
Backfiring in an MGB is usually caused by a leak in the exhaust system. Such a leak will often cause symptoms that seem like misfiring at higher RPM. Go under the car with some soapy water and a sponge, squeeze some onto the joints of the exhaust system and look for bubbles. Tighten it up and take the car for a drive. As they say, "Sometimes its the little things."
Steve S.

Jim,
Your problem lies with the fact that your car has a blue coloured oil fiter - the car looks too good, so it backfires.
It says - "I want to be me!" - change the filter to a white coloured one and it'll settle down.


Bob K ;-)
Robert Klymciw

Steve...

How does an exhaust leak mess with the engine in a big way? I don't know much about exhausts so I'm intrigued :o)

ttfn,
--
Olly S
Oliver Stephenson

Oliver-
If you stop and think about it, it's just a matter of simple physics. A leaking exhaust system will provide insufficient backpressure, allowing some of the fresh fuel/air charge to escape into the exhaust system as the exhaust valve is still closing. When the combustion-temperature exhaust charge hits it, it goes bang! Can cause rough running, too.
Steve S.

Here is a bit of an update.
I installed a new vacuum advance (even though the old one held pressure) and I advanced the distributor about another 8 degrees. The engine idled a bit "lumpy", so I leaned out the carbs. I adjusted them with a colour tune.
At about 20 miles an hour, I put the car in 4th gear and throttled the car fairly hard. I couldn't hear any "ping". After this test I took the car on the highway. At 100km (60 miles) the car was powerful - at 120km the car was powerful and there was no missing - at 130km I got scared and slowed down.
Bottom line, I don't know if the problem was the vacuum advance or the timing (or both). I am leaving the settings where they are.
By the way, the muffler is new and tight - no leaks.
Jim Webb

This thread was discussed between 22/04/2002 and 24/04/2002

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