I'm very puzzled.  I've got a 1986 GMC 305 CID 3/4 ton van on which the
Rochester 4GC carb with about 140,000 miles on it went out for all but idle
and in-town driving.  I've rebuilt lots of $GC's but never one with the
electric mixture control idle solenoid.  So I ventured for a rebuilt carb
for this job.  UPS brought it in.  Before I put it in I'd done a LOT of
work chasing out all the vaccuum leak old hose stuff.  Actually had been
watching and replacing hoses for years at this.

The old carb had fine idle speed adjust range.  At bottom of butterfly
range on the primary side the screw adjust could be set so it wouldn't
idle.  But when I got the new carb on, suddenly the idle speed was a little
high even down to the bottom of the idle speed screw range!  Beautiful
idle, but seemed a tad too fast.  This is a new rebuilt distributor on it
as well, was put in before the carb job and was working fine with the old
carb.

Suddenly I noticed something.  This is a van, so with the bonnet off I can
actually drive it, push on stuff, visegrip vaccuum hoses shut and all that
to chase stuff.  I happened to notice that if I touched the primary lever
plate where the throttle and transmission hooks up to the carb, there was a
TINY little glitch in the primary!  I could put a little pressure on it
with my finger and it would settle down to what I could actually use as an
idle setting, but was still strange.  To me, the idle needed is still at
the bottom of the butterfly position in the throttle bores.  But then.

I tried to chase this down carefully.  I cannot find ANYTHING on the
outside of the carb that is causing this little glitch bind.  More
interesting, I also have discovered that you can feel the tiny glitch, but
ONLY with the motor running and apparently vaccuum on the whole carb!  Now
come the more interesting stuff.  I have absolutely confirmed to my
inspection that there is NO relation to this tiny glitch to ANY hardware
OUTSIDE of the carb!

Further, there is NO evidence of any burr or anything that I could see with
the carb back off the engine.  And if the motor is stopped there is NO
glitch at all you can feel or observe in any way.

Duhh .. I sent the carb back to the rebuilder.  They absolutely can't find
this at all either!  So back it came with the same error.

I thought maybe I could just reduce the timing a tiny bit and get the
glitch into a range where I could drive it anyway.  You can lightly tap the
throttle with your foot and it will fall to the bottom at any time.  Nope! 
Then I noticed something else.  I put a vaccuum guage on the distributor
vaccuum line.  Sure .. plenty of vaccuum.  some 20 inches hard high. 
Normally about 15 inches at operational levels for advance purposes.  And
then I noticed that there actually is NO change toward retard until the
vaccumm goes down below 5 inches, actually to about 2-3 inches or so!  Duhh
.. I never realized that this was the way things work.  The whole idle
speed up from the ditributor advance deal really romps up the idle and the
in-town advance work!  For test purposes I even shut off the distributor
vaccuum and tried it with only hand adjustment on the distribtor.  With
that way I can get the idle away from the little glitch spot.  But the
driving operational characteristics are not that smooth.

Duhhh .. What can I do?  I have *NO* idea what is causing this tiny glitch.
There is no way to see it at all unless the carb is on the vehicle and the
engine is actually running with vaccuum on the carb base.  There is *NO*
mechanical interference with the throttle butterfly assembly and anything
else either the primary or the secondary stuff.

Any suggestions?

Thanks!

Mike Luther NC117 for FidoNet Net 117.  My first post ever here!


---