Hi all. I am new to the forum and to Corvette ownership as well. I purchased a 75 that has had a frame off resto and complete drive line rebuild. The motor has disappointed me in response and pull given the 300hp claim by the engine builder. The block is now .010 over with Speed Pro 9:1 pistons, a Speed Pro 442/288 cam and the heads have been completely rebuilt with new rockers, springs, valves and seats. After all the engine work the builder rebuilt and reused the original Rochester #7045223 carb. Would an upgrade to another carb without the emission features of the Rochester help the performance of my engine? My transmission is the Borg Warner 4 speed and tires are 245/60 Goodrich TA radials. Thanks for any suggestions you have.
Jerry

 

Was the engine run on a Dyno to come up with the 300HP claim and is that number rear wheel or net engine?
Was anything done with the distributor as far as adjusting initial and total timing?
Are you still running the stock exhaust system?
The carburetor is a 750 CFM so unless the builder did something wrong it is more than sufficient for the engine.
Those numbers you listed for the cam are probably the lift and duration of the cam, what is the part number of the cam?
Do you have an automatic or manual transmission?
What is the rear gear ratio?

 

The Rochester is a superb carburetor. If you are disappointed with performance, look elsewhere.

 

If the Rochester is in good working order not much to be gained by switching.

HP numbers get thrown all over the place. My 427BBC with big cam, headers only made 330 REAR wheel horse power and 399 TQ.

Unless you strap it down its all guessing.

My guess is 300 HP at the crank and even he just guessed. At the wheels it may only be 240RWHP or so which is pretty weak compared to today's cars.

Start with the basics, tune up, verify timing and adjust the idle mixture.
Also help to have the distributor setup for correct advance.

 

I bought the car from a third party and do not know the engine builder or how he came up with the hp numbers. I do have all the package end panels he used for the build. The Speed Pro cam number is CS-1014R and the lift and duration I 442/288 was for the exhaust lobes, the intake is 420/278 and it is a hydraulic cam. Rocker ratio is 1.5-1. My dual exhaust is 2 1/2 inch from manifold to Summit/Corvette #630125 mufflers with the Catalyst removed. I am waiting for my timing light and dwell meter to arrive so I can't comment on the timing setup yet. The transmission is a Borg 4 speed and I do not know the differential ratio. This performance might be all I will ever get from my car but If my memory from 45 years ago is correct, my 290 hp Dart 340 4 speed would leave this Vette in the dust.

 

204/214 .420/.443 112

"If you'd like to add more power to your ride, but are concerned about meeting emissions standards"

Basic Operating RPM Range: 1,500-4,500

This seems like a pretty small cam.

 

As stated above, that is not a performance cam, it is more concerned with emissions than anything else. Just google the part number.

 

Yes, The man I bought it from said it was a light cam and the car has more top end than I will probably ever use. When I get my tune up equipment I will do a complete work up and see if the performance improves. I am not looking for high speed just a snappy response.

 

Look to the distributor for extra power. Read the many posts on ignition timing (or better yet, read Lars' papers on the subject). Make sure you have as much timing advance as the engine will tolerate without pre-ignition-usually a total of 36* initial and centrifugal at 3000 RPM. Also be sure the vacuum advance is operating properly and is connected to a manifold vacuum source.

 

You're correct. The Dart would be about 2 seconds faster in the 1/4.

 

I also have a '75 with 4-speed. In my opinion you have two big problems, cam and heads. The original cylinder heads on the '75 are awful. Do some research here on the forum about modern technology heads and get a bigger cam, around .500 lift and 270 duration. The Q-jet is not your problem, it's a very good carb. when tuned properly.

 

Thanks to everyone for their help. I installed new cap and rotor, new plugs with the correct gap, new plug wires. Set the total advance timing to 36*and used a Manifold vacuum for the advance can. There is a big improvement in response and quickness. My car is actually fun to drive now.

The problem I have now is the idle is way too fast. With the engine fully warmed up and off of the fast idle choke cam my RPM's are at 1200. Without the vacuum to the advance can the idle is 850. The carb does not have a throttle solenoid and there is a gap between the curb idle screw and throttle linkage. I have disconnected the throttle cable to eliminate a short or binding cable. Al my vacuum lines seem to be okay and the secondaries close fully. If you guys can help me through this one I will be good for the rest of the season.

 

Glad you were able to get her going better. But my experience has not been great with Quadrajet Carbs. With that said I agree if the 40 year old Quadrajet is in perfect condition and also "Properly Tuned" you really can't improve on the performance.
In the case of my 69 I was disappointed in the performance but really did not have a good comparison for what a stock 350/300 hp with AT should do. I knew the 40ish year old carb wasn't working at peak but it would stall until warmed up (which is typical), I would also see fuel setting on the intake occasionally, and she would barely brake the tires free.
I considered sending it into one of the 2 guys I know of that can rebuild them and also purchasing a "Holley Remanufactured" unit which is what I decided to try. I tried two and both had similar issues and ran much worse that the original. In fact they would both backfire occasionally when running 70 down the highway.
I returned them to the vendor, put the original carb back on, and ran it for another year before running across a very lightly used complete Edelbrock ProFlow EFI that included the matching Mallory SSI. I purchased and installed it in the 69 and the difference was unbelievable. I wish I had the before and after 0-60 and 1/4 mile numbers but I have never taken to the strip or dyno, but it will certainly squall the tires now and chirp them when it shifts into 2nd from a WOT start.
A couple of other things to note is she starts and runs without wearing out the accelerator when cold and does not stall once started and the fuel mileage jumped up from 11 to 15 on road trips.
Like I said I have nothing to compare this too and note mine was/is a bone stock car. Just stating what myself and the few others who have driven my car have noted before and after the EFI installation.