I have had a few similar experiences.
One was a "constant RPM" situation -- it only happened in one RPM range, in
any gear, and when held steady for a certain length of time (not in
transitory situations). The symptom was loss of power, accompanied by
sputtering, loss of speed, then spontaneous recovery.
This turned out to be due to the combination of Grose-Jet needle valve
replacements, and the stock SU HS4 floats (with the "bendable" brass arms).
Unlikely as it may sound, the "resonant frequency" of the float arms
corresponded to the engine vibration at a particular RPM, which happened to
be very near to cruising RPM (as I recall, around 3000 or 3100). As the
floats vibrated sympathetically, they "dribbled" the balls in the
Grose-Jets, allowing extra gas to literally dribble into the bowls, causing
flooding within a very short period. As the car slowed down, the vibration
ceased, the fuel level reverted to normal, and it appeared to "cure" itself.
My fix was to replace the floats with the all-plastic, non-adjustable
replacements.
On another occasion, a more intermittent problem turned out to be the
Grose-Jet body unscrewing itself, again allowing flooding. If you have ever
worked with these, you will know that it takes a very precise sort of wrench
to tighten them adequately. The fix this time was to use LockTite.
If you are getting the impression that the Grose-Jets are in themselves
troublesome, you may not be far off the mark. But the reason I adopted them
in the first place was to solve a very similar intermittent situation, which
turned out to be the gas line disintegrating from the inside out, and
clogging the needle valve. Replacing the original braided style gas lines
seemed too expensive at the time. I still haven't replaced them 15 years on,
for that matter.
Anyway, you might take heed of the fact that ALL of these are fuel system
problems, not electrical.
--
Max Heim
'66 MGB GHN3L76149
If you're near Mountain View, CA,
it's the primer red one with chrome wires
on 1/25/06 3:27 PM, Phil Bates at jello@ida.net wrote:
> This morning as I drove to work, I had odd losses of power (again, I wrote
> into the list about it a couple weeks ago thinking it could be PCV
> related). Anyhow, I now have an MGA-like PCV system, have checked that
> none of my charging system or ignition system wires touch anything else,
> and I've had a hard pipe water heater fitting made (so I know it's not a
> leaking valbe dripping on the distributor. I should preface this with the
> fact that I've installed some extra gauges - a voltmeter, ammeter, and
> manifold vacuum gauge. The car is a '67 MGB fitted with an internally
> regulated alternator. Here are the symptoms - I'm driving along at
> freeway speeds (80mph) and I can feel the car lose power. So I look at
> the gauges - which tell me nothing. I give the car some gas, and can hear
> it stress, but it maintains speed. The manifold vacuum goes from 5 inches
> to 1 (as would be consistant with flooring the car). The power loss lasts
> 0.1 to 0.3 miles. Then all goes back to normal.
>
> This happened 5 times this morning during my 30 mile commute. It happened
> aobut 3 times one morning a couple weeks ago (and BTW, I'm in Idaho - it
> was about 15 this morning). Between those, I've had no similar incidents.
>
> It seems that this must be an electrical/ignition issue, or the power
> wouldn't come back 100%.
>
> Anyone got any suggestions??
>
> Phil Bates
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