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TBI To Carburetor Conversion

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dj31499

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Currently have a 1990 K5 (or V1500, not sure what they were identified as during those years...) with the 350 and throttle body injection. It has given me several problems since I bought it, mostly with electronics like the fuel injection, emissions control etc. Along with the rat's nest of wiring from the previous owner putting in his own "alarm system", aftermarket power windows, speakers with crossovers and using telephone wire to run a lot of that stuff, I'd like to strip most of it, start from scratch and wire only what's needed.
I'm hoping by doing this, along with replacing stuff like the distributor, switching over to a mechanical fuel pump, it will make things simpler down the road and diagnosing a problem wont't be as much of a guessing game.
What I'm looking for is some advice on what people think of this swap, is it worth it?
My plan, if I do go through with it, its to replace a lot of the components that were previously EFI compatible, like the distributor and fuel pump. Would I just look for parts for a pre-TBI 350, like for an 85 Blazer for example and just go from there? Or are there physical differences on a 350 TBI block and a pre-TBI block that make the older parts not compatible with the TBI 350.
Another thing I've come across is the transmission compatibility. I checked the RPO codes on the inside of my glove box, and I have MD8 on there. From what I've seen online, that means it's a 700R4, but I wasn't sure if they'd switched over to the 4L60 by 1990. I know people have to get another bracket for the kick down cable if they have a 4L60-E, but I'm not positive if that applies to 4L60, or if a 4L60 is just a re branded 700R4 so it wouldn't matter.
So if anyone has done this before and has an idea of what parts would be necessary to convert it, I could use the help.
 
I would fix what you have and stick with TBI, however, the TBI block would be able to run a carb just fine (need a new intake or adapter). 4l60e didn't come out until '93 - prior to that is 700r4/4l60 (same thing different name). I really do think it's just as much work if not more so to convert to carb. TBI just works, we started @blazinzuk TBI after sitting for 5 years and it ran like a champ (change oil, check trans fluid - check the right dipstick @ashman :haha:, clean plugs). If wiring is really really bad, get an aftermarket harness and run it.

If you run into issues along the way this community is really good about helping out.
 
Concur with others. Realize even your speedometer is run with some of the same wiring as the TBI, sharing some of the components, and not getting into the weeds, you'd even have to swap the transfer case and cluster to "simplify".

You arent going to find many to say the carb swap is a good idea for a truck you want to drive. Spend the time and effort getting the TBI wired right, the reward is there for doing so.

Link to the service manual/wiring diagrams in my signature...overwhelming at first, easy after you really start digging into it.
 
I just did the exact swap on my 1988. You'll have to check your block to see if its machined for a mech fuel pump. If not, you can use the tbi pump with a regulator or bypass the tbi pump and put an aftermarket inline pump in. Although I didn't need to swap cams/springs for the carb install, I went ahead and put in new cam/timing chain/springs/valve stem seals. At the minimum you'll need an intake, carb, hei distributor. Wiring was simple, a hot wire to the dist, oil pressure and temp senders and the alternator. I was able to remove miles of wires! Since the computer was out of the loop, I went with a toggle switch on the dash to operate the 700r4 lock up.

I put about 250 miles on the conversion and then my wife and I drove it 3000 miles from northern Cali to Alaska just last week. Fuel economy was horrible at 8.5 mpg at 65 mph and 11 mpg at 60 mpg put I was pulling a 26' empty boat trailer and climbing hills most of the way up.

For my use (working along the salt water beach launching my commercial fishing skiffs), getting rid of as much of the electronics is mandatory as the salt air/spray has wrecked havoc on the sensors on my other tbi rig.

Good luck with your decision and let me know if you have any questions that I may be able to help you with.
 
One thought here. By converting, aren't you still going to have a hacked up wiring system? It might be wise to fix or replace the wiring first. TBI is pretty forgiving on cold starts and gas mileage. I realize you would remove a lot of wires but the aftermarket power windows, speakers and alarm wiring still need to be dealt with along with.. I'd try to straighten out the wiring first then decide if you really want to change to a carburetor and since the transmission is electronically controlled it might be easier to do than a carburetor conversion. Just my 2 cents worth.

ETA: Sorry for the double post.
 
If you go carb, stay away from the 80s stuff. Make sure it’s a “no-wires” carb. The 80s brought different levels of electronic carbs. Also, you may need stand alone lock up wiring for your trans if you ditch the TBI computer etc.
 
Fix the TBI. As with all things, its complicated at first. Once you spend some time to figure out what all the things do then it is fairly straight forward. After driving both I will not go back to a carb. Also, isn't New York an emissions state? You won't pass inspection swapping to a carb.
 
You want to see a complicated mess? Look at the mid-80's carbed truck vacuum system. When TBI came, there was about the same amount of wiring, but 15% of the hoses.

Hit the search button and you'll find a dozen threads about TBI-->Carb downgrades. Here's one example: https://ck5.com/forums/threads/tbi-to-carb-swap.330445/
 
Not to pile on, but stick with the TBI. Prune the previous owners hack wiring. Get the basic tune up parts on it, cap/rotor plugs and wires as you would be doing this anyway.

Part of the good side of TBI is the basic diagnostics it had. It's no where near as in depth as later systems but it is better than nothing. With a carb, you are old school, zero data from the truck except for what you can see, smell or feel.

You loose the self adjusting ability for timing and fuel. That leads to the crappy cold or hot starting we all know and grew up with.

Personally we are spoiled with EFI. It's so nice to hop in and twist the key and the truck starts. Going backwards to pumping the gas pedal and dealing with chokes that hang up open or closed to make it a bitch to start sucks. You may understand it and be used to it, but handing it off to a kid, wife or buddy to use it for an errand or emergency might have it flooded out not going anywhere.

Keep this in mind, many are going the other way with these trucks that came with carbs and putting similar aftermarket TBI systems on because they are tired of dealing with the bs carbs come with. They get better fuel mileage and driveability than carbs for a reason. I for one would rather push through the wiring issues and keep the TBI vs going back to a carb.
 
ZooMad75 said:
Not to pile on, but stick with the TBI. Prune the previous owners hack wiring. Get the basic tune up parts on it, cap/rotor plugs and wires as you would be doing this anyway.

Part of the good side of TBI is the basic diagnostics it had. It's no where near as in depth as later systems but it is better than nothing. With a carb, you are old school, zero data from the truck except for what you can see, smell or feel.

You loose the self adjusting ability for timing and fuel. That leads to the crappy cold or hot starting we all know and grew up with.

Personally we are spoiled with EFI. It's so nice to hop in and twist the key and the truck starts. Going backwards to pumping the gas pedal and dealing with chokes that hang up open or closed to make it a bitch to start sucks. You may understand it and be used to it, but handing it off to a kid, wife or buddy to use it for an errand or emergency might have it flooded out not going anywhere.

Keep this in mind, many are going the other way with these trucks that came with carbs and putting similar aftermarket TBI systems on because they are tired of dealing with the bs carbs come with. They get better fuel mileage and driveability than carbs for a reason. I for one would rather push through the wiring issues and keep the TBI vs going back to a carb.
Click to expand...

This, I've had quite a few problematic TBI issues that I just wanted to convert to a "simpler" carb. Once I got a carbed rig, it got converted to EFI as soon as budget would allow.
 
I've pulled more of my buddy's "electronic fueled" trucks out of the woods than I ever did carb'd trucks..

One of my friends dunked his rig in a hole crossing a river,,and he lost signal to his TBI injectors. He managed to get to the bank of the river, but had to abandon the truck for the night until he got a hold of me. When we got back out to the truck, it was about two thirds underwater due to bad rain that night. Pulled him out, and trailered it home, after much drying and time and parts he got it running again and sold it.

My 97 Yukon was about 6k miles out of warranty when the in-tank fuel pump took a dump in the middle of downtown Houston,,
That was an expensive tow home..plus draining fuel, dropping the tank, blah,blah,blah...

I guess my point is, if you plan on doing any serious off roading, you have to weigh the advantages of the simplicity of a carb setup and a HEI dizzy..
worst case, carry a spare fuel pump/dizzy and a carb kit in the glovebox...
 
I can't agree with you on the off roading being better with a carb. All of my carbs have been horrible on side hills and steep climbs. All systems can and will break from time to time. I will still take the driveability of TBI any day. It just works better. Swamping a truck does not prove a carbs superiority at all and mechanical fuel pumps fail too.
 
Yup...and they told me I couldn't run my crawler with a 8-71 blower and twin 750 Holley's...

But if you set them up properly...side hills and angles are less of an issue...

it's all subjective, for sure...but for the OP, and for many people that don't trust electronic parts in the middle of nowhere, I was just pointing out some of the cons of fuel injection...I definitely live by the KISS principal..
 
Didn't think this thread would get so many responses, so thank you guys for giving your input! To respond to HankScorpio, as far as I know the last time it inspected it was only a safety inspection. Cars older than 25 years don't require an emissions inspection in New York. But based on what you guys have said, I'm leaning more towards sticking with the TBI. In that case, I'd like to go in and replace as many of the sensors/electronics as possible to to ensure everything is working properly, not sure where I'd find a list of stuff to replace. Haynes or Chilton manual? Another question I have is the emissions controls on these engines. I'm probably not right about this but I feel like it could be hindering the performance of the engine, bogging it down. Maybe I'm wrong, I just can't help thinking that all of that crap is choking the engine. I've already gotten rid of my cat, when I took it off the honeycomb was rattling around, wasn't secured in. Gave it to my friend to scrap, turns out the thing was a Chinese aftermarket part, got 3$ in scrap for it :haha: . But I'd like any input on that, if someone knows how much the EGR/Charcoal canister and all that stuff affects performance.
 
wasted wages said:
Yup...and they told me I couldn't run my crawler with a 8-71 blower and twin 750 Holley's...

But if you set them up properly...side hills and angles are less of an issue...

it's all subjective, for sure...but for the OP, and for many people that don't trust electronic parts in the middle of nowhere, I was just pointing out some of the cons of fuel injection...I definitely live by the KISS principal..
Click to expand...
Nobody is saying it can't be done with a carb. Hell it had been done for decades before EFI ever came out. I totally get the trust factor too. But the GM setup has been pretty well vetted out and proven, provided the system is in good working order. Fuel pumps fail if they are electric or mechanical. They are not that hard to change if the fuel level is low enough. Plus some have added access panels into the floor to make removal a 15 minute operation if need be.

Stuff fails. Given the fact that Carbs have been out of production since the late 80's chances of picking up needed carb parts is getting slim. Even less so in the boonies unless you bring spares. The TBI and later EFI parts, by comparison, are more available for sure.

My dad thought I was nuts to swap in a LS engine into mine for the same reasons of possible failure in the sticks. This is the same guy that carries spare points/condenser and a float for the carb in the glovebox of his Nomad. But I assured him the swap was set up like stock for reliability and the parts can be had at any parts store in the country. If and when something did break I'm usually out with other rigs so if I needed something I didn't have I would at least have a ride to town to get what was needed. As it sits, I've put close to 14,000 miles on the swap and most of that being well off pavement in Colorado, Utah, Arizona, California and Nevada with no issues what so ever. It's just as reliable as a newer truck as far as that goes.

I'm not saying you or anybody for that matter has to go LS to get the reliability (although it is nice), but a properly set up stock TBI system will work with the same level of reliability. Hot or cold, elevation changes, side slopes, nose up or down the engine will run the same way.
 
Will never be an LS guy either....:whistle:

It's kinda like going camping...
some guys gotta have the biggest motorhome they can find, just to have all the comforts of home... me? I'm the guy in the bed of my truck in a sleeping bag. :D
 
dj31499 said:
Didn't think this thread would get so many responses, so thank you guys for giving your input! To respond to HankScorpio, as far as I know the last time it inspected it was only a safety inspection. Cars older than 25 years don't require an emissions inspection in New York. But based on what you guys have said, I'm leaning more towards sticking with the TBI. In that case, I'd like to go in and replace as many of the sensors/electronics as possible to to ensure everything is working properly, not sure where I'd find a list of stuff to replace. Haynes or Chilton manual? Another question I have is the emissions controls on these engines. I'm probably not right about this but I feel like it could be hindering the performance of the engine, bogging it down. Maybe I'm wrong, I just can't help thinking that all of that crap is choking the engine. I've already gotten rid of my cat, when I took it off the honeycomb was rattling around, wasn't secured in. Gave it to my friend to scrap, turns out the thing was a Chinese aftermarket part, got 3$ in scrap for it :haha: . But I'd like any input on that, if someone knows how much the EGR/Charcoal canister and all that stuff affects performance.
Click to expand...

Buy factory GM manuals. Haynes and Chiltons are way too generic to be helpful. Ebay or on the web, they are out there. They will cost a little more but the detail is much more worth it.

What to replace? Full tune up parts for a starting point. MAP sensor, TPS sensor and O2 sensor are the short list as far as sensors would go. Fuel filter too.

For the emissions, the plugged cat would be the biggest thing bogging it down. From there the EGR could be a giant vacuum leak if the valve is stuck open at the wrong time and would cause a driveability issue. Other than that if it's stuck closed it won't effect performance at all. The charcoal canister is only collecting fuel vapor from the tank. Again, if the purge valve is stuck open it's going to act like a vacuum leak and run crappy. But properly working it will eliminate the fuel smell from the tank. I'm running without it on mine and I do get a raw fuel smell after long stints on the road. But either system properly working won't choke or bog down the engine power.

I wouldn't disable the EGR and EVAP systems without evaluating if they work correctly or not.
 
Don't forget to check the rod that runs through the TBI unit. If the bushings are warn and the shaft has play, that will cause air to pull through past the sensors as well.
I have had a charcoal canister on my truck for the past 8 years. It's only hooked up to the gas tank and vented to air. No purge stuff or vent to the TB.
 
You should look I to keeping the TBI, mine gave me no issues for nearly 10 years literally not one issue. I've recently changed over to EFI but the TBI was never an issue I just got a better engine.
 
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