1228746 TBI 305V8 stumble

[Jay Vessels]

Hi there!

Still working on the '85 Caprice, though we're much closer now.

'85 Caprice 305V8 / 700R4, retrofitted 1228746 ECM APFN code, TBI, etc.
from '89 Caprice 305V8/700R4.  This conversion has been at least as much
trouble as the TPI swap we did on Dad's '86 Caprice.  My S-10's TBI
conversion was cake.

First, thanks for the help dealing with TBI fuel pressure.  After
disassembling and reassembling the TBI we got the return port on the TBI to
flow fuel.  Gauge in series with the supply line shows 12psi.  Fuel flow
test gave at least 8 ounces in about 5 seconds, much faster than the 8 oz.
in 15 seconds minimum mentioned earlier.

Drove the car on its first shakedown run, and generally went well.  Car
starts fine, no codes.  I don't have a scantool for this car so I can't say
for sure what the ECM thinks is going on.

Problems:  Car idles fine, but off-idle is rough enough to cause a stumble.
 General lack of power.  If I didn't know better I'd suspect a fuel
pressure problem.  Could a restriction in the return line cause these
problems?

Timing seems to be set right (emmissions label for '85 calls for 0 degrees,
which seems to make the car start and idle fine, no strain on the starter
at all).  Set minimum air wihtout problems.

TPS on this throttle body isn't adjustable, and closed-throttle voltage is
0.75V.  Is this right?

The '89 used the small-cap HEI which we didn't want to use.  We re-used the
(known good) CCC large-cap HEI.  I swapped connectors on the harness so
that the '89 harness would plug into the '85 distributor.  Timing light
shows about 12 degrees at idle with the ECM controlling the timing.  Could
I have a timing-related problem?  Surely if I wired it wrong I would have
thrown a code.

Any help is greatly appreciated,
Jay Vessels
1982 Chevrolet S-10 Sport, 2.8V6 TBI
1994 Chevrolet Cavalier Z24 Conv., 3.1V6 MPI


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