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Replacing a TBI with a Carb on a 350 SBC 1987

 
trcummingscraft
I love my Chevy Chevette!
Posts: 3
Joined: 10/10
Posted: 10/21/10 07:37 PM

Good Day, I could use a little help I have a 1987 GMC 1/2 ton PU 4X4 5.7 350 and am going to take the TBI off and set it up with a Carb and HEI Dist. what I need to know is what am I to to about the in tank fuel pump and that return fuel system ?? will that pump work with my New Carb ??  will I need to replace it with a out side type Pump??? any Ideas??? Thank for any input  

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waynep7122
Everything in my driveway’s a Chevy
Posts: 556
Joined: 08/09
Posted: 10/21/10 08:29 PM

its easier to install a return type fuel pressure regulator..  to drop the TBIs  13 to 17 PSI to 4 or 5..  

you can also swap pumps in the tank...  there are 4 to 6 psi versions available that will slide right into the same location and use a strainer..

why take the TBI off...  having problems???  they are not hard to fix.. and there are ways to improve power from the TBI systems...    

just think... no float level problems when going up steep hills... or down..  or leaning to the side.. it just continues spraying.. right where you want it..    

with a carb...   go up a steep hill... the primaries run rich... until the float bowl empties.. then the float angle is such.. that the bowl never refills...  why do you think that hill climbers run out of power at the top of the hill.. and the guys who win..  run fuel injection .. and small fuel cells that are full...

the one thing that you really need to work on these is a scan tool...   an OBD1 scan tool..  like a actron CP9110... a snap on MT 2500 with the proper adaptors..  a OTC monitor 2000 , 4000, 4000E...   some of these are available on CL for cheep..  

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trcummingscraft
I love my Chevy Chevette!
Posts: 3
Joined: 10/10
Posted: 10/22/10 03:03 AM

Thank you for the ideas, as for changing the TBI out , this truck has all ways been a big pain, when it works it's grate but that is not most of the time.it's my work truck and I don't get off road much other than a bad driveway now and then . Thank you, I will look in to a return line pressure regulator ,the last time I had the Pump checked out they told me  the pressure was between 6 and 10 PSI. the Carb I am putting on is a Carter 625 AFB type I have one of these on my old 76Jeep Wagoner with a 360 it's great.  

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waynep7122
Everything in my driveway’s a Chevy
Posts: 556
Joined: 08/09
Posted: 10/22/10 08:05 AM

TBI motors that have intermittent problems can usually be fixed fairly easily...

bad grounds...   the ECM is only grounded to the intake manifold behind the thermostat usually..

worn throttle shaft bores ... allow the throttle to return to different positions and throws off the throttle position sensor idle voltage...  so the computer thinks you have your foot on the gas still so it does not revert to idle programmed fuel trim..

engine coolant temp sensor and pigtails that go bad... when they go bad..  they tell the computer its -20F and it just dumps fuel into the engine..  once in a while the white and yellow twisted pair of wires looses its insulation back in the harness.. causing problems...  but it can be tested with the trouble shooting charts

map sensor hose cracked or loose...   the map sensor tells the ECM how much manifold vacuum there is.. it has to be accurate for the ECM to adjust the fuel mixture..

leaking diverter valves into the upstream air injection tubes.. (the tubes screwed into the exhaust manifolds..) when this happens during closed loop operation the uncommanded air dilutes the exhaust stream and fools the ECM into changing the fuel mixture..  exhaust leaks can do the same.. as can leaking or corroded air injection check valves..

low fuel pressure... TBI motors were not equipped with a fuel pressure test port...  so adaptors have to be installed where the fuel filter is..  clogged fuel filters...  noisy fuel pumps are probably going bad..

having the ignition coil leak high voltage out the sides... this will really make a huge difference when changed out...    the small cap HEI distributers have a problem with cracked magnets in the reluctor on the distributer shaft.. with push on rotors.. you pull the rotor off.. shine a bright flash light at the shaft and the exposed magnet that is the second layer of the reluctor.. if its cracked.. replace the Small cap HEI with a reman unit from autozone... they have a far superior design ...  cracked magnets cause phantom signals to the module.. this effects the electronic spark control.. causing it to throw random sparks out of phase...  

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redneckjoe69
Everything in my driveway’s a Chevy
Posts: 612
Joined: 02/10
Posted: 10/22/10 02:07 PM

fuel injection is nice when it works right. and t.b.i. is not the worst to work on. but i can understand people wanting to go carb. im a carb man myself. i must say i like the reliabilty, & simplicity.  if your going to carb, id dump the electric fuel pump. loose the computer, & put an h.e.i. on it.  

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trcummingscraft
I love my Chevy Chevette!
Posts: 3
Joined: 10/10
Posted: 10/22/10 05:19 PM

Thanks guys for the imput I will save all the parts in case I ever wish to go back but I am at that point of no return. I have a carb Carter AFB625, and a hi output Dist.HEI and have found an intake that will fit my heads so soon I hope to switch over. You all have been a big help Thank you very much Tim www.trcummingscraft.com  

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