Andy,
We would all like to think that internal combustion engines are a science, but
problems like this show us that they are really art.
My guess would be fuel. My experience is that weak compression at cranking
speed or with a leak down tester gets better at high RPM, due to dynamic
compression. I found two bad exhaust valves in my 427 during a routine check.
They were reading 90% leak (Which is terrible), yet the car was able to pull
140 mph on the front straight at Willow the day before. It showed me how
sensitive the leak down test is.
One missing cylinder would not slow a 3000 AH to 50 on a hill. More than one is
miss firing. If it idles well and mid throttle is good, but misses on hills and
long accelerations, it sounds like fuel. Bad pump, clogged filter, swollen
kinked or blocked fuel line, etc.
Craig Wright
PS: The compression won't get any better. It may not be related to your present
problem, but it will be a problem soon.
Andy Walker wrote:
> Hey, guys:
>
> I know this may sound like a dumb question but, the truth is I just flat
> don't know. I'm going to be helping a friend of mine try to chase down a
> high rpm miss tomorrow in his '60 Austin Healey 3000 Mk1. He has already
> changed the obvious things, such as plugs, wires, rotor, cap, points,
> condenser, coil, fuel filter, etc. We are going to check out the entire
> ignition circuit tomorrow for loose or possible broken connections in an
> attempt to find something amiss.
>
> He has stated to me however that, even though the engine was rebuilt only
> approximately 4000 miles ago, #2 cylinder shows significantly lower
> compression than the other five. A squirt or two of oil down the plug hole
> will raise the compression back up to the level of the other cylinders, so
> a possible broken ring is suspected. My question is this...could this
> situation be the cause of his high rpm miss? His miss only happens when
> the car is hot (in hot weather) and running at highway speeds. He reports
> that, when pulling a mild upgrade on the highway at 70 mph, his engine will
> begin to miss and his speed could, depending on the length of the grade,
> drop off to 50 mph.
>
> I have offered to help him but I will be there more for moral support than
> technical, as I am just as much at a loss as he is for an explanation. If
> some of our mechanically minded listers out there could give me any insight
> on whether or not his low compression problem in one cylinder could be
> causing this, please let me know. My friend and I will be forever
> grateful.
>
> Thanks,
> Andy Walker
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