Restoring my '75, I have taken the original L48 out and am using a ZZ4 Crate Engine. I have also swapped the original QJet Carburetor for a 600 CFS Holley with electric choke and vacuum secondaries. Right now, my gas mileage is about 10-11mpg city and 12-13 highway and I am skeptical that the carburetor is the problem. My spark plugs are not fouled, however, my Vette is due for a tune-up. The exhaust is heavy and sooty, and a surge of black exhaust is emitted when I first step on the pedal. Should I be using a greater CFS Carb on this engine? I would greatly appreciate anyone's advice or comments on how I might troubleshoot this problem.
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Re: '75 Carburetor Cause of Poor Gas Mileage?
Vol Vette-----
First of all, given the "black smoke and soot" symptoms that you are experiencing, I'm surprised that you are getting as good of mileage as you are. The mileage figures that you mention are not all that far off from what I would expect for your car and engine. Perhaps a few MPG low, but that's about all.
Also, I don't think that your 600 CFM carb is really too small for your application. I'd say that it's about right, especially if you're using a Holley carb. Q-Jets, which usually run in the 750-850 CFM range can get by with such a high rating due to their small primary-large secondary configuration. Most street engines are WAY over-carbureted anyway, and I'd say that you're just about right for under 5,500 rpm operation (which is, basically, where a ZZ-4 is at). A "too small" CFM carburetor will provide better street operation than a "too large" one.
In any event, I see no way that carburetor CFM rating, high or low, could have anything to do with your "black smoke and soot" problem. However, there's very litle doubt that, experiencing the symptoms that you are, there is some sort of carburetion problem.In Appreciation of John Hinckley -
Re: '75 Carburetor Cause of Poor Gas Mileage?
My '74 L-48 with 4 speed and stock Rochester 4MV (rebuilt by holley) gets about the same gas mileage as you are seeing. I thought it was low when I first got the car, but my research several years ago satisfied me that it is normal.
Todd RobidaComment
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Re: '75 Carburetor Cause of Poor Gas Mileage?
VolVette,
My 75 with an L48/4spd/3.36 consistently gets 14-15mpg around town. On the road on long drives it once got 18mpg. Everything is stock.
John 75coupeComment
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Re: '75 Carburetor Cause of Poor Gas Mileage?
VolVet--
As usual,listen to Joe. The cfm is just right for a Holley on a ZZ4. I have installed many ZZ4 motors, and customer feedback tells me you should be getting mileage in the same range as John. This assumes that you have 3.08 or 3.36 rear gears. If you are running lower gears, forget good mileage. You will have your foot much closer to the floor.
Also make sure you are set for 10 deg base timing, and 32 total with vacuum advance plugged.
Fix your carb and you will love the engine . Good luck!
Wild BillComment
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Re: '75 Carburetor Cause of Poor Gas Mileage?
Your power valves in your Holley could be open letting the gas flow because of the low engine vacuum. I don't know what the power valve rating is in your Holley, but you need to find out. I have a ZZ1 in my 70 with a Holley 650 double pumper. I removed the primary power valve that came in my Holley and installed a Holley 124-45 power valve. I removed a power valve that opened at 6.5 inches of vacuum or less. The valve I installed opens at 4.5 inches of vacuum. With my car being an automatic with the air on and in drive stopped, the engine vacuum at idle was low enough to permit the original power valve to open. I run 14 degrees BTDC and run vacuum to the vacuum advance on the distributor full time. Highway milage is 17.5-18 at 70-73 MPH. I don't do any city driving, so no information on that. You power valves could have been ruptured during a engine backfire--this does happen. When your engine is idling, put a finger over the front fuel bowl vent. If your engine dies after several seconds, the power valve is bad. This test is not written in any book I have read, but it has worked for me and I have been running Holley's since 73. Good luck with your tune-up.Comment
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