Carburetor Selection & Rebuilding

Deciding to build a unique and custom engine is both exciting and daunting at the same time. During the early stages of planning you need to figure out which components you need an how they will work together. It can be a tough process, especially when you get to the more complicated parts like the carburetor. Not that a carburetor is a very complicated part, but figuring out which one will work best for your custom engine with an undetermined amount of power is definitely a challenge.

Before we go any further, lets consider the options. Turbo Camaro was obviously a factory carbureted vehicle, but it doesn't need to stay that way. Several companies have released fancy throttle body fuel injection systems that are apparently easier to tune and obtain solid performance gains. A carburetor sized part and a few wires and you just need to pull out your laptop to tune it. You can research for hours and hours but their are a few reasons to stick with carburetion:
  • It's about 10 times cheaper to locate and rebuild a carburetor than to purchase a new EFI system. 
  • With proper maintenance very little can go wrong; no wires or modules to fry.
  • Slightly easier initial installation and doesn't require a computer to setup.
The downside of running a carburetor is tuning. They are a pain in the ass to get the tuning right as you have overpriced cams, pumps, valves and jets to configure. That being said, education is the key. If you have some idea what you're doing, it's not difficult to get the tune stable. If you're not an experienced tuner, you may need to take your ride to someone who is, or a shop equipped with a dynamometer to get it right. If you're like Turbo Camaro and going from bone stock Chevy 250 to a custom built turbocharged performance machine, you'll probably want to visit a dynamometer for tuning and to see exactly how much power you've made.

What carburetor will work best on a Chevy 250 inline 6, mild cam, 0.070" over pistons, 9:1 compression ratio, with a big turbo and alcohol injection? It's not an easy choice but it came down to two options. The Holley 350 CFM 2 barrel or the Holley 600 CFM 4 barrel double pumper with mechanical secondaries. After extensive research and deliberating on forums, it was obvious both carbs would work but with the expectation of more than double the stock horsepower, the 600 was chosen. The 600 is no stranger to performance applications, and with the mechanical secondaries, it will allow precise tuning as to when the third and forth barrel open to feed the turbo. Obviously it's easier said than done, but the logic is sound.

A complete "was working when removed" Holley 600 was picked up on eBay for $100US. A BPT performance Holley 600 rebuild kit was picked up for approximately $50US. A can of carb cleaner, a jug of aluminum safe Simple Green and a few wire brushes and you've got yourself a weekend project.

Unless your an experienced mechanic or carb builder, take a bunch of "before" pictures of your carb. The pictures can help you put things back where they belong if it ends up taking you more than the weekend to complete. Dismantle the carburetor, that means every screw, check ball and linkage comes off. Taking it apart in sections can help simplify the process. Perhaps do one fuel bowl on the right side of the table and the other on the left. Take the main body apart in the center of the table so it has a "exploded" look. Take pictures as you go to ensure a smooth rebuild.

Once you've gotten it all apart, drop a portion into a vat of aluminum safe Simple Green. Note some versions of Simple Green aren't safe on all metals so be sure to use the right one. Leave the parts in for about 20 minutes. Pull out the cleanest part and give it some attention with the wire brush. Do the same with the remaining parts. Phase 2 of the cleaning is to blast it all with the carb cleaner. Spray it, brush it, use lint free cloths to remove any residues. Keep re-spraying until their's no residue. Keep in mind this method doesn't use overnight harsh chemical submersion so you might not get rid of every little stain but results will be good if your diligent with the wire brushes.

With a table full of clean parts it's time to crack out the rebuild kit. Keep the original parts aside and replace them on the table with the new ones in the kit. Rebuild the carb with the old/new parts. It's at this time you'll want to research which size pumps and jets you want to start with. Your always best to use the jet/pump/valve sizes that were originally shipped with the carb, but feel free to pre-tune if you have that ability. Some common rebuild mistakes are forgetting to install the pump check balls or squirter/nozzle discharge check needles. The base plate has 8 mounting screws, it's common for these carbs to be run with only 6 as people fear the 2 middle screws will fall out into their engine. While this is possible, if they are installed correctly with Loc-Tite this will not happen. Running your carb with only 6 screws may have no obvious side effects, but could cause fuel leakage or premature gasket failure. Once it's all assembled you can temporarily set the idle mixture screws by turning them all the way in, then backing them out 1.5 turns. This adjustment should allow the vehicle to start, and then the real fun begins.

A Summit chrome adjustable dual feed line with built in filter and fuel pressure gauge was purchased to supply the 600. The cheesy NPT hose adapter was swapped out for a -AN fuel line fitting.

In order to mount the Holley 4776 to the Offenhauser 5416, you'll need to ensure your manifold has the required adapter. The factory holes on the manifold are for a smaller "old style" carburetor and the Holley has a wider non-square bolt pattern. The adapter used in this build was technically the Trans Dapt 2064 but that was not the part ordered. The 2064 is a similar adapter but required significant modification to work. The correct adapter (supposedly) is the Trans Dapt 2090. You can also hunt down a Offenhauser adapter that bolts directly to the manifold but they are harder to find and typically double the price of the Trans Dapt adapter.

Notice the adapter and spacer below the carb. Also,
that throttle is still very close to the manifold. 
Once you have the adapter, you will need at least a 1/2" spacer before you mount the carburetor. The spacer is required because the Holley throttle linkage will hit the middle flange of the intake manifold if it isn't raised up. You could modify the linkage to miss the manifold but we used the linkage in that spot to hookup the throttle to the gas pedal. The spacer was $5 and has been known to increase fuel efficiency and power so don't cut up your Holley, get the spacer. Lastly, do not forget about hood clearance. You might be tempted to buy a bunch of spacers to increase flow but realize the sky is not the limit. We wanted to add a 1/2" spacer above the carb but the wing nut securing the plenum poked the underside of the hood. This isn't carburetor specific but is important when adding spacers above and below.

With the carb mounted on a running engine, you'll probably need to open it up at least a few times for tuning. Even with extensive pre-mount configuration, our Holley needed a good reboot to factory settings. The process was filmed and posted to YouTube below:

This is not a complete carburetor tutorial but gives you an idea the process Turbo Camaro's Holley 600 went through.

In November 2017 the Holley 600 Double Pumper was replaced by a Holley Sniper Electronic Fuel Injection System. As an interesting comparison, they are technically both over-sized for our Inline 6 but the Sniper is able to electronically minimize and compensate when necessary and allows for a much easier fuel curve compared to tuning the carburetor. By no means do I want to imply the Holley 600 wasn't up to the task of running this engine well, but for the amount of time required to tune and re-tune with the amount of driving done, it was too much of a hassle. With the Sniper EFI setup, it's a set it and forget it component that also allows us to use the eBay GM HEI Large Cap distributor for electronic timing control. Not to mention the other benefits of an ECU like control over cooling accessories and other safety functions. The carburetor had a good run and will stay in the hall of fame as the most competent all mechanical device ever installed on Turbo Camaro.

Turbo | Camaro