Re: TBI Fuel Pressure

["Walter Sherwin" <wsherwin@home.com>]

9-13psig fuel pressure is spec for most low pressure GM TBI's.  9psig would
typically indicate a bad pumping circuit (pump, filter, sock, voltage,
etc...), and 11psig is a typical observed factory setting.  13-15psig is
"better" if you have mild mods to feed.  More than 15psig is problematic
unless you change the cal to suit (particularly if your TBI cal happens to
idle open loop).  15+psig (and upwards to 50+psig) is often essential if
feeding significant engine mods when you either can't, or don't want to,
move up to larger flow injectors.

Within the regulator/meter cover assembly  some TBI's have a constant bleed
orifice which will cause your residual fuel pressure to bleed away quite
quickly after keying-off.  This is normal.

You might want to monitor the fuel pressure and O2 value while WOT.  This
will usually give you a pretty good indication of whether your WOT power
enrichment A/F mixture is close.  Look for constant and adequate fuel
pressure while WOT, and O2 in the high 800's.  If these appear okay, then
you'll have to look at other things within the system (ie: closely
scrutinize the distributor & secondary ignition in the case of a "small" cap
external coil HEI setup).

If lean, and if the pressure holds at WOT, and if out of cal headroom, then
either try larger injectors (not possible in your case since you said you
have the 75's already) or a different regulator.  Aftermarket companies
offer bolt-on adjustable TBI regulators that are typically adjustable to a
max of 14-15 fixed psig.  The GM P/N 17113079 regulator will give you 30
fixed psig.  The GM P/N 17113186 regulator will give you a manifold
referenced pressure range of approximately 11-18psig, or, a fixed pressure
of approximately 18psig with the vac line disconnected.  All will bolt in
place on a GM 2bbl TBI unit.  Another option is to graft an external
regulator into the return fuel line, downstream of the OE TBI regulator, as
Bruce advocates in his tune tips.

Lastly, if you are using the '75 #/hr BBC TBI injectors typically engraved
"17084304" (service P/N 17112560) then take a look at your squirt patterns
with a timing light and/or have them flow checked at an injector shop.  I've
had two of these injectors (at different times) go bad for unexplained
reasons.  Also,  with the 2bbl TBI manifolds,  runners 2/3/5/8 and 1/4/6/7
are coupled to a given TBI injector.  You should be able to see signs of a
bad injector on the plugs of cylinders coupled to that injector.  The
likelihood of two simultaneaously bad injectors is slim, but I suppose it
could also happen.  In my case, the give-away was the fact that the HPC
coating burned off three of the four header pipes.  The "bad" injectors
passed an ohms check, but failed to flow as anticipated when compared to a
new injector.


Walt.




>Okay, First as a sanity check: 9-11 Lbs/sq in is bad right?
>13-15 would be good?  Second, should the fuel pressure drop
>to zero within 10 sec of the engine shutting off? Engine
>acts likes it's running out fuel steady throttle up hill,
>and higher rpm's (sorry didn't record actual range) WOT.
>Thinking of adjusting reg (for 13-15) and possibly replace
>pump. Any suggestions, comments?
>
>Specifics: GM tower and regulator, 92 Roadmaster pump, 2/
>75lb injectors, 7747, Summit Racing 30lb fuel pressure gauge
>(input side).
>-Brent
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