Author Topic: TBI to Carburator Conversion  (Read 233 times)

Offline Jon87V20

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TBI to Carburator Conversion
« on: November 09, 2018, 09:16:09 PM »
Hi, everyone:

My '87 Suburban has been parked since mid summer because it started rod-knocking on me. Before that, I was having what I suspect to be TBI related issues. It was throwing a code 42 intermittently if I remember correctly.

Anyway, I was planning on buying a 350 long-block to put in there. I was originally planning on just bolting all my original TBI stuff on to the new motor. However, I don't want to put my brand new engine in the truck and then keep struggling trying to chase down these issues. I would like to just have the new engine and enjoy driving it again.

If I were to go the carburetor route with this new engine, I would need to change some things from what I understand. I'm assuming I'd need to change out the following:

1) Distributor since it's computer controlled. Can I just drop in a standard aftermarket HEI and run power to it?
2) Intake manifold
3) Transmission kickdown. For this situation, can I rig up some kind of wide open throttle switch to send 12 volts down to the kickdown solenoid when appropriate, or is it not that simple?
4) Fuel system. From what I've seen during searching around, people have said the fuel pressure for the TBI is very excessive for what the carburetor requires. Can I just add some kind of pressure regulator, or do I need to change out the pump etc.?

What else do I need to do? Is there anything I should watch out for in particular?

Also, will any Chevrolet small block 350 crate motor bolt up to this truck, or is there nuances that make it so I should try and make sure it's for this exact year?

Thanks in advance!
1987 Chevrolet Suburban V20 Silverado
350ci TBI TH400 NP208

Offline VileZambonie

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Re: TBI to Carburator Conversion
« Reply #1 on: November 10, 2018, 06:42:56 AM »
I don't agree with ditching the TBI system because you are getting a DTC42 especially since 90% of the time it's just a faulty ignition module. If your engine has been knocking the knock sensor has probably been overexercising the system as well. Giving up the convenience and drivability improvements from carburetion just doesn't make good sense to me. You can also eliminate the KS and EGR and replace the prom in the ECM for about $200 bucks.

If you decide to regress back to the old ways then yes you will need a non-ECM carburetor, electric choke, HEI, return fuel pressure regulator, intake manifold, throttle brackets, there is no solenoid unless it's a TH400 otherwise it would use a TV cable.
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Offline hatzie

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Re: TBI to Carburator Conversion
« Reply #2 on: November 10, 2018, 09:54:41 AM »
The stock L05 350 TBI makes more power than the older carbureted 350 and uses less fuel to do it.  It's not more or less reliable than a carburetor.

It may take some time to get yourself up to speed on the system but it's really quite simple.  The GM emissions and driveability manuals are essential tools.  Thankfully someone ;) scanned them and posted PDF files.
SVC & wiring mans --> Here http://tinyurl.com/7387BRD-SVCMAN or My Bucket @ http://tinyurl.com/SQ-SVCMAN
Parts & Illustr Books -->http://tinyurl.com/SqParts
GMSTG Textbooks-->http://tinyurl.com/STG-TEXTBK
Radio Manuals-->http://tinyurl.com/DELCORADSVC

Offline csdineley

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Re: TBI to Carburator Conversion
« Reply #3 on: November 10, 2018, 11:48:45 AM »
I'm with you. Ditch that POS TBI. I did it a few years ago and couldn't be happier.

1. Yes
2. Yes
3. Don't know I have a manual
4. Just need a fuel pressure regulator. TBI pump puts out around 13 psi. Carb need around 5-7 psi.
5. For the most part. It'd make it easier if you get one with a 1 piece rear main seal.

I used a crate Vortec 350 (12681431) from GM performance.
So I had to use a vortec specific intake
A cheap chinese HEI (Replaced that when got around to rebuilding a stock hei)
A Quadrajet from Cliff High Performance and Mallory Fuel Pressure Regulators 29387

As for the results.
-It starts right up even on the coldest day, maybe even better then the tbi setup. Now I do let it warm up a little bit now with the carb. Don't think I have to but I do it anyways.
-Fuel mileage went from 12 mpg to 16 mpg. That was back in NC on flat land. Now in WV and after a 2wd to 4wd swap I'm back down to 10-12 mpg.
-Power definitely increased ( more due to the vortec heads then the carb). 
« Last Edit: November 10, 2018, 11:50:36 AM by csdineley »
1987 V20 Custom Deluxe, Vortec 350, SM465,Np205 14Bff 3.42

Offline Jon87V20

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Re: TBI to Carburator Conversion
« Reply #4 on: November 10, 2018, 08:01:28 PM »
Thanks for the feedback, guys. After seeing what you had to say, I'm a bit on the fence now. I did really like the TBI setup when it was working properly.

Oddly enough, when it first started throwing the 42 at me, I replaced the ignition module and the problem went away. However, a few weeks later it was back again. It was starting to do weird stuff the last time I drove it where I would feel it sort of stumble, then give itself some extra throttle just for a moment then go back to normal (this has only happened on the highway). One night not long after, when it was cold, it was idling fine, then suddenly shut down. It refused to start. I kept cranking on it and eventually it starting kicking like it wanted to start but still wouldn't. I let it sit for a few minutes then tried again and it still kicked a few times and then started shortly after as if the waiting helped for whatever reason.

So if the ignition module has been replaced, are there any other common culprits for this behavior? I should also mention I did download one of the PDFs for troubleshooting this, but haven't gone through all the steps. Just wondering if you guys have any ideas from experience I could glean to save time.

I'd like to have her all back together and happy again for some road tripping next summer!

Thanks.
1987 Chevrolet Suburban V20 Silverado
350ci TBI TH400 NP208

Offline VileZambonie

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Re: TBI to Carburator Conversion
« Reply #5 on: November 11, 2018, 07:15:23 AM »
Sounds like the pickup coil in the distributor is bad. It's probably cheaper to just buy a new distributor. If you do stay with TBI get a custom chip and enjoy the best of both worlds.
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Offline Jon87V20

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Re: TBI to Carburator Conversion
« Reply #6 on: November 12, 2018, 07:54:37 PM »
Custom chip you say? I've never heard of that regarding these trucks. What's a good one to pick up? Can you throw up a link for a Vile approved product I can check out?
1987 Chevrolet Suburban V20 Silverado
350ci TBI TH400 NP208

Offline VileZambonie

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Re: TBI to Carburator Conversion
« Reply #7 on: November 13, 2018, 03:27:20 PM »
tbichips is pretty reputable or you can find other guys who custom burn chips as well. The one I did for my orange truck is a custom burn.
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