Hello Listers,
This evening, I took my 74 MGB out for a spin to view the open
floodgates at the Chickamauga Dam. All seemed to be running well as I
rarely got above 2500 rpm's between shifts. Part of the drive involved
some time at over 50 mph. As my car is not od equipped, this meant the
engine had to turn at a minimum of 3000 rpm. Upon reaching that point,
the car began to hesitate and sputter a bit. It seemed like there was a
bit of noise from the muffler underneath, but I'm not sure. Backing down
the speed solved the problem, but that limited the speed to around 45 mph.
I've tried to think through in my head what could be causing this and
tried to make the correct changes, but so far to no avail. Here's some
of what I've done:
- The engine and carbs are from a 72 B, so I adjusted mixture and idle
screws to make the mixture richer, as the Haynes manual said that lean
mixture can cause backfiring and sputtering at high speed. This made no
change. Less than two weeks ago, I used a Carbalancer to adjust and
balance the carbs and all seemed well then. Also, I had the carbs
reshafted and bushed a little over a year ago and the shafts seem tight.
I also cleaned the float chambers at the time I made the adjustments.
The dashpots are filled with 20 weight oil.
- I've recently set the gap and checked the points, so I doubt that this
would be a problem. However, anything's possible.
- I removed the spark plugs and checked their gap. Two seemed too
narrow, so I adjusted them all to .032. The Haynes manual also says that
some should be gapped to .024 or .026, but that too small a gap can
cause uneven running at high speed. The plugs are Champion N9Y or their
modern equivalent. Can't remember exactly. They also seemed black like
the car had been running rich.
- The plug wires are a little over two years old. A new set might not hurt.
- The fuel filter might need replacement, but I thought that a dirty
filter would effect overall operation rather than causing trouble at
high speed.
-The fuel pump's an SU and is about two years old and has never caused
any trouble.
This is all I can think of at this time. Any suggestions would be
appreciated as British Car Week is coming closer and I would like to get
mine out without being hazardous to its health.
Thank you,
-William Killeffer
1974 MGB
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