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Tractor Talk Discussion Forum

O/T TBI to carburetor change

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frankiee

10-29-2006 04:45:08




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I am going to put a carburetor in place of the throttle body.
Its on a 1993 Chev 350 cid
I am changing carb and center head
I know I have to drill a couple holes straight down the center head for it to fit.
My question is can I use the same electric fuel pump in the gas tank.
Or will I have to put a mechanical pump on it.

This will only be a temporary thing




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buickanddeere

10-29-2006 18:40:08




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 Re: O/T TBI to carburetor change in reply to frankiee, 10-29-2006 04:45:08  
What is going wrong that new parts only last 2-3 days. Sounds like the symptoms are being repaired the fault hasn't been found yet. As previously stated this sounds like more work and more $$$ to have less when finished.



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RusselAZ

10-29-2006 09:56:52




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 Re: O/T TBI to carburetor change in reply to frankiee, 10-29-2006 04:45:08  
The fuel pressure spec is a regulated pressure at idle. Carburators don't have a regulated return to the tank.

Dead head pressure is much more. You can put an electric pump on the frame but you have to remove the pump in the tank as you can't suck gas thru the existing pump.



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Bob

10-29-2006 09:44:11




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 Re: O/T TBI to carburetor change in reply to frankiee, 10-29-2006 04:45:08  
There's NOTHING involved in that TBI system that can't be cheaply fixed with new or salvage parts. You must have a buddy or someone who well help you at a resonable cost with a little knowledge of the system that can tell you what's wrong with it!

It's a GIANT step backwards to put a carb on it.

BUT, if you INSIST....

The "center head" is called the intake manifold. In about 1986, they changed the angle of the 4 center bolts to nearly vertical, so older carburetor manifolds wont fit. As you know, the carburetor won't bolt directly to the manifold you have. There should be SOME used factory "in-between" manifolds around that fit the new bolt angle heads, AND have a carb mount. The aftermarket guys (Edelbrock, etc.) make $$$ manifolds just for this application.

Eliminating the TBI will leave the 'puter dazed and confused, and it will probably revert to "base timing", meaning the engine will run like crap, and be a worse gas hog. That can be eliminated by installing an older distributor with vacuum and mechanical advance, or a $$$$ performance distibutor with built-in adjustable electronic advance curves. Also, if it's got a slushbox, even in 1993, the now-confused 'puter will at least control TC lockup (OR USED TO)! Potential BIG problem there. (There are $$$ aftermarket "workarounds" for that, too.)

You could use a fuel pressure regulator ahead of the carburetor, the type with a return line.

The fuel pump is NOT designed to operate against a closed inlet needle valve. In the TBI system, there is a fuel pressure regulator that holds the pressure in the 10-13 PSI range, and bypasses the excess to the tank. If connected to a carburetor, the pressure will go WAY above that, since there's no return, overloading the pump, and, likely forcing the carb's inlet valve open, causing flooding. Also, with this type of pump, the constant flow of excess fuel helps to keep the pump cool.

Unless you have a LOT of time and money to do this right, why not just repair the TBI?????

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Jon Hagen

10-29-2006 10:17:01




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 Re: O/T TBI to carburetor change in reply to Bob, 10-29-2006 09:44:11  
Bob is right,that old low pressure TBI system is cheap and easy to repair,the fuel pump will die quickly from the restriction and near 0 fuel flow of an idling or stopped engine with a carb. The pump will make considerably more pressure than a carb can handle without the pressure regulator(actually a relief/dump valve) to control the pressure buildup to 13 psi and allow a constant flow of fuel through the pump for cooling flow that returns to the fuel tank. Add to that the computer problems with the ignition advance and transmission shifting caused by the computer reverting to the "limp home" mode when it senses that the injectors are not working.

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Allan In NE

10-29-2006 04:52:54




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 Re: O/T TBI to carburetor change in reply to frankiee, 10-29-2006 04:45:08  
Frank,

The mating surface between the intake manifold and the heads are different depending on model year.

If you take the intake manifold off of an old CCI system, which used a carb, they will match.

Yes, you can leave the fuel pump.

Allan



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frankiee

11-01-2006 05:21:48




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 Re: O/T TBI to carburetor change in reply to Allan In NE, 10-29-2006 04:52:54  
Hey Allan
Funny thing is that my father had given away a bunch of scrap to a neighbour about 9 months ago and the intake manifold we needed was part of what was given away. We bought it back and a carburetor. No drilling necessary.
The conversion went well and my father says that the carburetor gets better fuel economy then the fuel injection.
We went down south with it about150 miles and back again to buy another vehicle to replace it.
It ran well except for the fact that it slams from 1st gear to second. The rest of the shifts were smooth as silk.
I have the manifold absolute pressure hooked to manifold vacuum.
The rest of the computer stuff I have left unconnected (coolant sensor, tps)
You seem to have knowledge of GM's so I ask you , why do you think the harsh shift.

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mjbrown

10-29-2006 05:25:54




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 Re: O/T TBI to carburetor change in reply to Allan In NE, 10-29-2006 04:52:54  
Are you sure about the fuel pump? I thought TBI used the tank pump for injection pressure, something like 60psi which is way more than a carb.wants.



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jdemaris

10-29-2006 08:27:26




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 Re: O/T TBI to carburetor change in reply to mjbrown, 10-29-2006 05:25:54  
The 60 PSI pumps (actually 59 PSI) were used with the Vortec engines and CPI injection - I think it started to be used mid-94. The CPI system got upgraded around 1996 to CSFI. I know with my 94 AWD van with a 4.3 - was available - early 94 with a low-pressure fuel system and TBI and late 94 with high-pressure fuel system and CPI. The jump to high-pressure fuel makes me nervous. My van is getting rusty and has a fuel line runing just above the muffler. I wonder what happens when that fuel line with over 50 PSI rusts out and springs a leak? Yeah - I could replace the line - but this Chevy van probably the worst piece of junk I've ever owned - and we only keep it as spare 4WD vehicle for winter use. I bought it from a local church and suspect it is cursed by God.

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Allan In NE

10-29-2006 06:13:02




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 Re: O/T TBI to carburetor change in reply to mjbrown, 10-29-2006 05:25:54  
Hi MJ,

The carb N/Seat will hold it and shut it off.

Lots of the old computer controlled fuel systems used that electic pump with the carbs. Not a problem.

Allan



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frankiee

10-29-2006 05:52:48




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 Re: O/T TBI to carburetor change in reply to mjbrown, 10-29-2006 05:25:54  
The carb we will be puting on is off a old 81 Chev truck we have here.
The spec for fuel pressure is 7.5 to 9psi as listed below. The spec for the fuel injected 93 is 9 to 13 as listed below.

A person on the other end of town did it with no problem so I figure it will work for me.
I just cant get a hold of the guy right now.

I think the fuel pressure difference can be handled by the inlet needle. We will see.

We are getting another truck in the new year and its more or less just a yard truck right now.

The darn thing keeps acting up and I am getting tired of replacing parts just to have it go bad again in 2 to 3 days. Its actually my dads truck so I have to go there to get it going again.
So I figure to experiment with the TBI to carb changeover to make a quick gauranteed fix once and for all till we replace the truck in a couple months.

For your 1981 GMC Truck C1500 1/2ton P/U 2WD: Fuel System Pressure 7.5-9.0 PSI(Pounds per Square Inch For your 1993 Chevrolet Truck C1500 1/2ton Sub WD: Fuel System Pressure 9-13 PSI(Pounds per Square Inch)

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frankiee

10-29-2006 05:07:50




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 Re: O/T TBI to carburetor change in reply to Allan In NE, 10-29-2006 04:52:54  
Thank you Allan
Allan,..... what is a CCI system???



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Allan In NE

10-29-2006 05:16:06




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 Re: O/T TBI to carburetor change in reply to frankiee, 10-29-2006 05:07:50  
Those were the ones where the computer controlled the carb. Somewhere around '85 thru '87s?

The angle between the intake manifold and the heads is different than on a straight TBI engine, so ya have to be careful in this area.

Look for a manifold with the mounting bolts standing straight up and down instead of at an angle.

Allan



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