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Old November 2nd, 2006, 07:44 PM   #1
kevinr839
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Question Help with TBI converting to carb

I want to get rid of my throttle body and go carburator and hei ignition on my 90 chevy fullsize with a 5.7 and th400. When I convert will my full pump still run ok or do I have to run a power wire to the pump, and will I have to regulate the pressure down and run a return line or should I just be able to hook it up to the carb? Sick and tired of messing with the tbi for now and I need to get it running good for winter.
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Old November 2nd, 2006, 08:54 PM   #2
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Keep the TBI. It is easier to work on than the carb. You will need to change your fuel pump...
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Old November 2nd, 2006, 09:41 PM   #3
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Default Won't idle

The truck will not idle and is low on power. When it tries to idle it smells very rich. If driving at light throttle it coughs and stumbles. I have tried everything I know to fix it. Replaced the distributor, map, o2, tps,plugs, rotor, cap. wires. I took it to a shop that said he could fix. $300.00 later he also changed all the above items and it ran worse than when I took it in. I believe he has the timing off on it (advanced). The truck is very very sensitive to its timing, too low by a couple degrees and it will not run hardly at all, too high and it does the same and is hard to crank. I am not saying 10-15 degree swings, I am talking 2-4 degrees either way. The shop I took it to said the map sensor is losing voltage/vacuum at idle and losing control of the fuel. I did a compression check and 7 cylinders were at 120 and 1 was 150. I know the motor is not perfect but it should run. I would just like to get it through the winter until I can rebuild my 454 and put it in it come spring.
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Old November 3rd, 2006, 12:04 AM   #4
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if the bolts on you intake are straight up and down they are different than a 4barrel intake. the best way to make that work would be different heads. i think it is different than a vortec 4barrel too. your fuel pressure will need to be regulated. really it would probly be more work than getting the tbi working right. check you fuel pressure the regulator might make it run like that.
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Old November 3rd, 2006, 12:31 AM   #5
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to change to carb need to change intake and distributor use a fuel regulator or go back to a mechanical pump which is cheaper change the clip on the coil just did a camaro we also changed to an older style alt and completeely removed the computer most the time the electric pumps wont work right unless you rewire them
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Old November 3rd, 2006, 08:01 AM   #6
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have you tried a different computer? they are fairly inexpensive for that vehicle. might be worth a shot.
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Old November 6th, 2006, 10:39 AM   #7
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Default Tried it

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Originally Posted by jeepfreak81 View Post
have you tried a different computer? they are fairly inexpensive for that vehicle. might be worth a shot.
The other guy who worked on it said he also tried a couple computers with no change. Did not really want to pull the motor and rebuild this close to winter, but I might have to.
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Old November 30th, 2006, 08:36 PM   #8
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Check the play in the timing chain. I'll betcha it is loosey goosey.
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Old November 30th, 2006, 08:41 PM   #9
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Coolant temp sensor?
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Old December 4th, 2006, 04:51 PM   #10
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Quote:
Originally Posted by special ed View Post
Check the play in the timing chain. I'll betcha it is loosey goosey.
I was thinking this too, and changing to a carb aint going to help...
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Old December 4th, 2006, 07:51 PM   #11
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Timing is easy to check, should be 0* with the timing wire disconnected.

Check fuel pressure? Should be ~10-14PSI

Checked trouble codes?

TBI really is a good simple way to run that engine and their isnt much to it. I'd take chevy TBI over carb on a DD any day.

Im also not sure I understand "MAP is losing vacuum?".
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Old December 4th, 2006, 07:55 PM   #12
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anyone ever clean out the idle air control? something ont he side of the TBI you unscrew clean off with carb clean or brak clean and put back in.. has helped in the past.. just a thought?
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Old December 4th, 2006, 08:29 PM   #13
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edelbroc does make an intake to fit the 88 newer TBI heads, but from what i've seen the intake is like 300-400, is your egr hung open?

most blocks cast after 88 do not have the option to run a mech pump,
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Old December 4th, 2006, 10:06 PM   #14
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Sounds like your mechanic isn't worth a damn. Assuming all sensors are ok I would be suspicious of the catylitic converter or a stuck open EGR valve. The low vacuum that the MAP sensor is seeing is being caused the engine running overly rich, cat plugged, or EGR open when it shouldn't.
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Old December 5th, 2006, 01:23 AM   #15
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I vote EGR hung open also.
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