At 08:34 AM 5/13/2008 -0500, Robert J. Guinness wrote:
>After rebuilding the .... carbs on my 1961 MGA .... the engine fired
>right up . . . to 3000 RPM! .... I assume I have issues with the
>jet because the adjusting nut will not tighten.
Adjusting nut will only affect fuel mixture, not cause idle speed that high.
Throttle shaft would never be worn so badly to pass enough air to
make it idle that high.
>Here are my questions for the group's wisdom. What is/are the
>likely cause(s)?
3000 rpm idle requires quite a lot of excess air. An air leak that
large down stream from the carbs would lean out the mixture so much
as to require substantial amout of choke to keep it running, If it
"idles" at 3000 rpm without choke it is probably not a leak down
strean. If you must hold at least half choke to keep it running,
then it may be a LARGE air leak at a down stream gasket or an open
hole in the in take manifold.
The more likely source of air flow for the high idle speed is a
partially open throttle valve. If the throttle plate is off center
it will bind on the wall of the throat preventing closure. Solution
is to loosen the screws, close the throttle plate fully to touch all
around the edge, and tighten the screws.
Perhaps the most likely cause of the high idle is carb air flow
grossly out of synch (also easiest to fix). The MGA has flexible
"accordian" clamps linking the shaft(s) between the carbs. If these
are tightened while the carbs are grossly out of synch they will stay
that way regardless of what you do with the idle screws. Solution is
to loosen one of of the throttle shaft clamps to allow teh two
throttle valves to move indendently while you synchromize air flow,
then tighten the shaft clamp(s).
>Can I fix the carb without removing it ....
If teh carbs ar jsut out of synch, it's an wasy fix in
situ. Otherwise, probably not. Down strean vacuum leak would
require R&R of carbs to fix a gasket leak (unless ittorned out to be
a hole in the intake manifold). Misaligned throttle plate requires
acess to the small screws in the throttle shaft, which would be quite
tricky with the carb mounted (but may be possible if you're a
contortionist with a mirror).
> Can I just remove the rear carb? ....
Yes, if you disconnect on of the throttle shaft clamps. But you have
to remove both carbs for access to gaskets inboard from the heat shield.
Barney Gaylord
1958 MGA with an attitude
http://MGAguru.com
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