Worthwhile to convert from TBI to Quadrajet?

[Brad H]

Recently purchased a 1988 R20 Suburban with 350 tbi for 1300 bucks. Drove well coming home, but never came down to idle speed. Oh, the truck only has 43,000 miles on it! It sat a lot, was only used for taking dogs to the river and pulling a car to the racetrack.
I'm very familiar with quadratoilets and love them. I'm not familiar with tbi. Is it worth converting to carb, and aside from the intake and carb what else is involved in the conversion? I have an extra Edelbrock performer and quadrajet on the shelf, so that expense is covered. I've heard rumors that I'd have to swap the distributor, but can't find any info confirming that. I'd consider just making it run perfect using the tbi, but was hoping to avoid any computer control altogether. If anyone has input on making the tbi work its best, please let me know.
I'm in process of replacing all the power window motors, power lock actuators, door seals and window seals/tracks, and a few other things to make it "like new" again. It's loaded with comfort options including rear A/C. Must have been quite the rig in its time. I'll post some pics a little later into the project.
Thanks all!

 

[legopnuematic]

Yes you will need to swap to a traditional hei distributor (4pin) if you go carb.

 

[Ricko1966]

Recently purchased a 1988 R20 Suburban with 350 tbi for 1300 bucks. Drove well coming home, but never came down to idle speed. Oh, the truck only has 43,000 miles on it! It sat a lot, was only used for taking dogs to the river and pulling a car to the racetrack.
I'm very familiar with quadratoilets and love them. I'm not familiar with tbi. Is it worth converting to carb, and aside from the intake and carb what else is involved in the conversion? I have an extra Edelbrock performer and quadrajet on the shelf, so that expense is covered. I've heard rumors that I'd have to swap the distributor, but can't find any info confirming that. I'd consider just making it run perfect using the tbi, but was hoping to avoid any computer control altogether. If anyone has input on making the tbi work its best, please let me know.
I'm in process of replacing all the power window motors, power lock actuators, door seals and window seals/tracks, and a few other things to make it "like new" again. It's loaded with comfort options including rear A/C. Must have been quite the rig in its time. I'll post some pics a little later into the project.
Thanks all!

If you swap from TBI to carbureator ,you also have to go to Conventional HEI. I wouldn't do it,you lose the computer controlled timing which is way more sophisticated than a standard HEI. And you lose a lot of the mixture control that TBI can do. TBI works well for a driver that has minimal mods. And is very reliable,and not that hard to work on.

 

[AaronW]

FWIW, Brad, this is what we did to my '89. Went to a quadrajet, with an HEI where you can build firing maps you can select between. I was getting about 6 mpg before (on a 454), where I'm getting about 10 now. There are of course lots of other variables (put a new cam in it, too), and I'm not technically knowledgeable enough to say what causes what and to what extent. But so far, I've been happy enough with it. I've driven it for pushing two years now, with no issues. My old TBI system wasn't working before, though, so I'm not sure if I'd spend the $$ on it, if my old fuel system hadn't needed work anyway.

Aaron

 

[Vbb199]

Why not do an injector clean, TBI freshen up (gaskets and fpr), and see how it does ? Sounds like a vacuum leak or something.

 

[Ricko1966]

My 88 350 tbi burb got 17 mpg on the hiway coincidently my friends got 17 also.

 

[Vbb199]

Ive said it on this forum and got laughed at, but I'll say it again

My stock 1990 burb with 3.08, tbi 350 and a 700r4 got 22 or so on the highway

 

[gmbellew]

I’d keep the TBI. Guessing something simple is causing the high idle. Probably a vacuum leak as was suggested previously.

 

[YakkoWarner]

I have contemplated this multiple times while staring at an unsolvable error code blinking at me. I also have a higher-than-desired idle (last time I measured it around 950RPM) but live with it because theres no logical answer for it.

Right now the TBI is not throwing constant errors at me (still get an occasional "lean fuel" error for about 5-8 seconds when going up a hill and a bit of a rattle noise under load that I'd swear is timing related but supposedly the computer knows best). I like the TBI system when it works, and if you happen to have one of the rare golden-unicorn devices that can extract actual data from the primative computer it is probably not hard to work on. Without that you are just mostly guessing at what is causing the issues - the computer will give you an error code but not much in the way of clues to go on.

If your TBI is working and not giving you grief, I wouldn't go through all the headache of switching over. The HEI swap out is pretty simple, but you also have to change your fuel delivery system because no carb is going to be happy with that 12-15 PSI output from the TBI fuel pump. You'll have to rig up some kind of pressure reduction system or change out the fuel pump for one that delivers 3-5 PSI for carb. applications. The fuel system problem is what makes me just live with the quirks for now, when it colossally eats itself again I may reconsider.

 

[xm20k]

I've done this a few times, but they were FUBAR by previous owners that screwed up the TBI and/or computer and they didn't want to spend the money to fix them. I'd buy them cheap and swap them over but most of the time also did other mods to make more power and often ended up with cams that the computer would have thrown a fit over.

 

[77Dmax]

I'm a q-jet guy. That being said I would not recommend this. TBI is no powerhouse but it dead nuts reliable. Figure out what the issue is and repair it. A high idle is probably a vacuum leak.

 

[AbleDanger]

If the idle is the only worry, Google GM TBI idle adjustment. The TBI on these trucks are super simple and the idle could be due to the idle screw being out of adjustment. There is a simple process you can do when the engine is warm that involves a short wire and a screw driver. Or could be vanish and needs a cleaning, like a carb. Gasket kits are easy to find. If you can understand quadrajets you can figure these things out. Very simple, less parts, and the computer just uses a few sensors and makes only a couple of adjustments. If you want to run it stock, a good running TBI will be easy to live with.

 

[JT58]

I have TBI on my 87 with 5.7. It is running on a 74 block. It was not running good when I got it and it is now much improved. I had a bad kink in the main fuel line, right next to the TBI inlet. It also needed new spark plug wires and I just replaced the plugs. I have to say the TBI for sure is reliable. Just do the basics first- look for vacuum leaks or something not connected and give it a tune up. And I'm surprised at the MPG I get with the TBI- and I have the 4 speed manual transmission. It's in the high teens- can't complain.

 

[Brad H]

Thank you all for your input on this. I'll most likely just fix the issues with the tbi and see how it does. I'll mostly only be towing a car trailer with another truck on it, so the 350 should do fine. And yes, the fuel economy would be nice. This will likely replace my '04 Silverado as my daily driver. It may actually use less fuel!
I'll start with the idle control valve and search for vacuum leaks. Having sat so much, it's likely I'll be replacing a few vacuum lines anyway. The previous owner had replaced the fuel pump and brake master and vacuum booster.

 

[Ricko1966]

Thank you all for your input on this. I'll most likely just fix the issues with the tbi and see how it does. I'll mostly only be towing a car trailer with another truck on it, so the 350 should do fine. And yes, the fuel economy would be nice. This will likely replace my '04 Silverado as my daily driver. It may actually use less fuel!
I'll start with the idle control valve and search for vacuum leaks. Having sat so much, it's likely I'll be replacing a few vacuum lines anyway. The previous owner had replaced the fuel pump and brake master and vacuum booster.

Have you just tried resetting the IAC ? And making sure it's timed correctly?