On 4 Mar 2005 at 12:10, Michael Marr wrote:
> You should make sure your carb dashpots both have oil in them. I have
> had similar symptoms in the past and this was the cause.
Good point. I had a similar but opposite experience before I finally
dialed in the MGB SU's a DPO had put on my GT6. In cold weather it
would run moderately okay for half an hour or so. But after that
when on the highway it would occasionally stumble and even backfire
whenever I'd try to accelerate a little. If I simply feathered the
throttle or let the speed fall away slowly it would seem okay. If I
floored the pedal it would jump forward with enthusiasm.
This behavior was the result of several factors. The needles were
too thick so it was actually running lean at partial throttle. But
the springs were also too weak, being the original MGB springs. They
weren't stiff enough for the higher flow rate of the GT6 2000, so the
plungers were topping out too early. This also compensated partially
for the needles because they were running too far toward the thin
end. As the flow rate increases in a fixed-size venturi, the mixture
goes up. So once the plungers had hit their stops the mixture would
get richer than it "should have been" and the car would run just
great. Stiffer springs helped the early-topping-out problem but the
lean mixture remained until I went to thinner needles.
The real puzzle was why soft acceleration became worse after half an
hour or more in cold weather. The key was the dashpot oil. It takes
a long time for the carbs to warm up. Before they do the plungers
move sluggishly. Any increase in throttle is accompanied by a
momentary increase in mixture until the plungers finally move into
their final position. When the engine was cold this took so long
that my typical minor acceleration would apparently be over by then.
When the oil finally warmed up so that the plungers moved quickly,
the lean mixture phase could happen and the car would stumble. In
fact, the dashpot oil was doing exactly what it was supposed to do,
but the rest of the calibration was so far wrong that it ran well
only when it shouldn't have and didn't run well when it should.
Firmer springs and better needles fixed everything. Don't know if
they are fully optimized but I don't have many options. It is sooooo
much better now that I consider it done, period.
--
Jim Muller
jimmuller@rcn.com
'80 Spitfire, '70 GT6+
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