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Lackluster midrange response -- Overhaul

With the Corvette now on the West side of the country at my house, I was taking it out for a spin and noticed a real drag of a problem: at sustained 1/4 to 1/3 throttle acceleration, the car started sputtering and just didn't put power down until I got all the way on the throttle. Sure, if I jummed the pedal to the floor it would power through,but the more I had driven the Corvette, the more I came to realize I wanted it as a comfortable street cruiser. Sure, it's a powerful brute of an engine, but good for the street none-the-less due to it's wide torque curve. This giant issue was not going to stand, I needed this car to be more streetable.

This lead to a dizzying pursuit of attempting to troubleshoot this issue. I investigated options such as not having the vacuum advance hooked up, fuel delivery issues, carb not setup right, getting an A/F gauge, going straight to fuel injected... the amount of possiblities was dizzying.

After a lot of confusing research, a few dead ends, and many countless hours of reading articles, cruising forums, and watching videos... I developed a plan.

Fuel injection on a budget was out as GM components were just not going to work with my cam. The Holley 650 would work, but was reputed for bad mileage, and per my own research, dizzyingly complex to troubleshoot. Also add in all the parts for the possible areas needing service... and it was just too much.

The new plan was to go with the Summit Racing 750 cfm carb, with vacuum secondaries. Vacuum secondaries are reputed for their streetability, and are also noted for being "difficult to overcarb with". All equations for engines said I needed 660cfm, but everyone pretty much runs 750cfm carbs and raves about it. The entire kit needed to tune this carb was also very affordable, as I picked up a summit jet kit, and a collection of secondary springs for under $45 total, giving me full control over tuning the carb. The old holley 650 double pumper also had a manual choke, that was basically just not setup to be used at all. This carb has an electric choke, and I was very much looking forward to the easier starting procedure.

I tackled some of the basic tasks myself. The carb needed to be swapped out, which in turn meant needing longer carb studs. As with all things on the Corvette, the more I touched, the more I discovered needed work. While removing two of the studs, they took the remaining threads with them, so a helicoil kit was needed. While checking into the ignition setup, I discovered my dad had made a temporary precarious install of the module... 30 years ago. This required relocating, re-routing wiring... the list goes on.

I got the basics in place: the new carb was on, the ignition was re-routed and in a safe location. All seemed well. I fired up the Engine, and it idled! Hell, it even reved a bit to give me some hopes for the future. Below are some pictures of this initial step of getting everything going.

The car roared to life after a few cranks, giving me high hopes for it all. Unfortunately, there were issues. The carb wouldn't drop off of the choke/high idle, and when I turned it off the engine backfired, almost sounding like as if it was running backwards. Dieseling it turns out. There was much work remaining to be done, but at least I was on my way to completion!

A few short days later, I had a Mechanic stop by. He's not only an experienced, but he also writes tech articles online and really knows his way around a car. I was a bit out of my league, so his help was much appreciated. We quickly sorted out the idle using a vacuum gauge, tackled the choke settings, and all seemed well. We checked into the timing, hooked up the vacuum advance, and got the whole car dialed. It was way off to start with, but things were looking great once it was dialed.

We took the car for a spin and troubleshot some little issues. The return springs were shot, the idle was a bit high... little things that just needed some attention. We hit an auto parts store to get the few things we needed, along with some wire loom to clean up the engine bay. After a bit more futzing about, it was wrapepd up. The Corvette is VERY streetable now. It's throttle response is crisp, it hesitates only mildly when you get on it, and then it just RIPS! It breaks the tires free on command... it's amazing!

Pulls like a freight train when you get on it, purrs like a civic at idle!

All in all, the car runs great now! I highly recommend the Summit Racing Vacuum Secondary 750cfm carb. It's amazing on this car. It needed some basic tuning to get everything right... but it was impressive how quickly we got it to work and how close it was to dialed right out of the box!