alhiggs

Ok guys, I have read over 300 threads and gotten some ideas to check, but I'll ask anyway.

I had a 305 TBI w/200k miles in my '91 RS that's finally been swapped to:
(all new parts except block which was machined/decked/etc...)
350 / Vortec heads; all new internals; roller cam (w/1.6 rockers I think)
flat top pistons; eagle rods
Comp cam (not sure but my builder says "middle-of-the-road" streetable)
MSD ignition; RPM Performer pro air gap intake
600CFM holly w/electric choke
very free-flowing eXstream K&N filter
Flowmaster American Thundercat 3" exhaust from Shorty headers back...
Stock TBI fuel pump in tank with mallory return-style regulator set to 5 PSI with gauge before carb--reads 5 LBS steady with key on
Fuel pump relay has been bypassed to be on with ignition...
Fuel level/floats supposedly set ok; have not verified
5 speed
Timing as I am told is about 12 BTDC with vacuum port disconnected and plugged @ 500 RPM
Idle mixture screws about 1.5 turns from full-in

Symptoms / conditions:

Idles smoothly
Runs perfect if I "feather" the gas very carefully (booooo)
Runs perfectly from about 3000-5000 RPM Lotsa power!!!
PROBLEM: Say I'm running 3rd gear at about 1500 RPM and I press the pedal anything but *very gradually* I get backfires (carb) / stumbling / blah-blah-blah then when the RPM gets up to about 3000 it "catches" (maybe vacuum secondaries opening?) and takes off like a ROCKET and runs GREAT!!!
Mileage is fine and actually better than the tired old TBI 305 I took out.
I do see gas in the carb when I work the throttle with the air cleaner off and the engine stopped (accel pump SEEMS to be working, but I know very little about "tuning" the accel pump)

I have the following gauges / equipment to help diagnose:
Autometer Fuel mixture gauge (with NEW oxygen sensor)
Timing light
Vacuum gauge

The mixture gauge reads slightly rich at IDLE, goes completely off-scale to LEAN when I get on it until the RPM gets up then shows stoic-to-rich, so I think I have a fuel starvation problem

HELP!!!! I have over $3500 in new engine and even though my tired 305 TBI only developed 125 HP at the rear wheels (on a chassis dyno the day before I changed themotor) It sure was smooth and never misfired a day in the 15 years I had it....

From the posts I have read; things I'll try tomorrow and post the results:

Check for possible timing setting problem because of the harmonic balancer issue (my guy has been building SBC's for years; I shouldn't second-guess him but I'll check)

Vacuum IS connected to the ported vacuum as recommended in the holley carb manual. This car has no smog control whatsover; air pump removed, etc...

Vacuum is about 20 at idle
I did notice that when cold (1st 5 minutes) the motor does run better (maybe choke is helping with mixture?)

Right now the engine is probably getting some hot air from the engine compartment as I have not boxed in the air cleaner to get cold air from the 5" hood scoop I installed (with big hole in the hood) but I think it's at least getting SOME cool air as I can see the air cleaner poking thru the hood into the scoop :-)

ANY IDEAS?

Thanks!

Austin H

1320_Guy

iTrader: (3)

Is it a vac secondary carb?

Jer82Z28

sounds like accel pump tuning. try adjusting your pump to deliver more fuel

alhiggs

Yes it has vacuum secondaries
----------
I will give that a try. The carb is "almost new"; someone else bought it new 2 months ago, and for whatever reason didn't like it and went with a double-pumper. My engine builder got it at a good price, bought a kit and "went through it", so I don't have the different colored accel pump cams y'all mention, but I could try the "2nd hole"? and check the clearance as mentioned in other threads?

Thanks and I'll update later today
All suggestions welcome.

Austin H

alhiggs

Well it pays to read all these threads and decide for yourself. I went over to the shop and had the timing set to 12 deg with the vac adv pulled and the carb port plugged, THIS TIME at 500 RPM. It was at least 20 degrees too retarded. My engine builder races, and he had set the timing at about 2500-3000 rpm, on purpose. He had forgotten about "street" settings.

It does not backfire or diesel on shutoff anymore. Power is much smoother from 1000 and up. It's like a whole new beast!
My only complaint now is minor hesitation instead of undriveability. I will tinker with the settings and if I can't get it right, I know where to ask.

THANKS for all the HELP...

Austin
'91 RS 350 vortec/RPM Air Gap/Shorty headers/3" Catback