TBI to Carburetor Conversion on a Chevroletby Richard Rowe
While it works fine for stock applications, the fact is that General Motors' Throttle Body Injection (TBI) system is a nightmare for modified engines. Though it is possible to replace the injectors and throttle body, and to reprogram the computer, many find it simpler to "down-grade" to a carburetor setup.
The most basic things you'll need to convert from TBI to a carburetor is a carb and a matching manifold. You could try to track down a used stock manifold, but aftermarket aluminum manifolds like the Performer RPM make more power, are lighter and will only run you about $60. Simply remove the old manifold, drop the new one in its place and install the carburetor.
One ancilliary you'll need to replace is the distributor. The stock distributor is computer controlled, which is a bad thing considering the fact that you'll be removing the computer. A stock High Energy Ignition (HEI) unit is an affordable, direct bolt-in for most applications. If you're using a roller-lifter camshaft, then you're going to have to swap out the distributor's drive gear for a softer bronze unit, or risk damaging the cam gear. Keep in mind that the HEI distributor is likely wider and taller than your stock unit, which will be a problem if your engine is close to the fire-wall.
The TBI system uses fuel pressures far in excess of what a carb requires, so you'll need to install an adjustable fuel pressure regulator into the existing line. The stock fuel system does use a return line, so the fuel pressure regulator will need provisions for this. Of course, the simpler solution is to simply replace the fuel pump, which will serve the dual purposes of simplifying installation and replacing the worn fuel pump with a new unit.
Though the swap is fairly straightforward, be forewarned that vehicles with the computer controlled 4L60-E automatic overdrive transmission will need additional work. You could either purchase a stand-alone transmission control computer (at about $975 from Chevrolet dealers), install a different transmission (like the non-electric TH700 from earlier Chevrolets), or purchase a manual valve-body that does away with all electronic intervention. Of these, the simplest and cheapest solution is to install a TH700, as it can often be purchased used for less than $300. The transmission is a bolt-in replacement for the 4L60E, but your going to need a stock replacement or adjustable aftermarket throttle kick-down cable. Some TH400s differ in length enough from the 4L60E that you may need to change the driveshaft length while you're installing the new TH400-specific yoke. In all, a TH400 swap will run somewhere in the vicinity of $600.
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