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74 Chevy small block loses power at highway speed
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RickDFW
New User
Jan 2, 2013, 1:32 PM
Post #1 of 7
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74 Chevy small block loses power at highway speed
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Hello car junkies. Questions are below, and here is my car's recent history:
HISTORY
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I have a 69 Chevy C10 project. It has a '74 Chevy Small block V-8 (A-body 350) that loses power at highway speeds above 45 mph. Bought it as a non-running project a couple months ago.
I rebuilt the Holly 600 CFM 4-barrell carb with a manual choke. Replaced HEI ignition coil, distributor cap, & rotor. Installed new plugs & wires. I removed and cleaned the Edelbrock intake. I removed and cleaned the heads, valves, springs, rockers, and lifters Installed new gaskets. Installed new inline fuel filter. New tranny filter & gasket. New fluids (oil, tranny, and coolant).
Got the motor running smoothly. It ran and drove well for the past few weeks, and suddenly has began losing power and pulsing (can't think of a better term) at highway speeds. Once I hit 45ish mph, I feel the power drop off, steady sounds of air sucking into the carbs, and the truck lurches to the beat of a fuel pump (possibly?) until I back off the accelerator. After a second or two, it seems to continue running fine again until I try and accelerate above 45 mph. After coming to a stop, it accelerates well with the pedal up to 3/4 of the way down. Any more and it seems to suck too much air into the motor and stalls.
I'vee never been able to smoke the tires from a stop. Engine pulls too much air and stalls. That said, it does accelerate nicely - just no burnouts.
As of recently I can't drive above 45 mph.
I know the tank is dirty as there was rust and crud in the carbs, and I see rust in my newly installed fuel filter. I also noticed the filter's fins are bending inside the clear plastic casing - which makes me think I have a problem with fuel pressure. I don't know how to test for fuel pressure or vacume. I'm confused because it was running fine a few short weeks ago.
I plan to replace the fuel filter, and will probably try and clean out the tank. I'm set on DIY, and having never cleaned a tank; I'll google it and try it myself. If you have any suggestions, please let me know.
QUESTIONS:
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Where is the fuel pressure regulator, and how do I check it?
Is the fuel pump on a 69 Chevy C10 located inside the tank?
Does the THM-350 tranny factor in since the motor stalls around 45 mph?
Any other suggestions?
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Hammer Time
Ultimate Carjunky
/ Moderator
Jan 2, 2013, 1:38 PM
Post #2 of 7
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Re: 74 Chevy small block loses power at highway speed
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What regulator? You have a carburetor.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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RickDFW
New User
Jan 2, 2013, 3:17 PM
Post #3 of 7
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Re: 74 Chevy small block loses power at highway speed
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Thanks for your quick reply.
Correct. I have a carburetor. No pressure regulator to a carbureted engine?
I located the fuel pump mounted to the block. Because the truck pulses and loses power at speed, I suspect the pump is either clogged or faulty.
QUESTION:
How do I test it for pressure and/or vacume?
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Hammer Time
Ultimate Carjunky
/ Moderator
Jan 2, 2013, 4:04 PM
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Re: 74 Chevy small block loses power at highway speed
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Just the fact that you had to ask those questions tells you this is probably something you should be sending to a shop.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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nickwarner
Veteran
/ Moderator
Jan 2, 2013, 8:03 PM
Post #5 of 7
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Re: 74 Chevy small block loses power at highway speed
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You know you have a fuel issue. You know you have a rusty gas tank. Why are you driving this after going through the hassle to get all the junk out of the carb to start with?
Go to www.por-15.com and order a fuel tank sealer kit. You need to follow the instructions to the letter but it will fix the rust issue and seal up your tank. The fuel pump is mechanical and is bolted into the passenger side of the timing cover. Its driven by an eccentric mounted on the cam gear. Two bolts pulls it out. Seal up the fuel tank and replace the fuel filter first to see if you even need to mess with the pump. A mechanical vacuum/ pressure gauge is cheap and you should have one as you'll need it for working on carbed vehicles. You install a tee into the fuel line between the pump and the carb after the filter. You're looking to have about 6psi while running.
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RickDFW
New User
Jan 3, 2013, 10:24 AM
Post #6 of 7
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Re: 74 Chevy small block loses power at highway speed
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Thanks guys. I'm determined to learn, and I enjoy doing the work myself.
I've ordered the POR15 and will give it a shot. Once I get the tank cleaned, sealed & reinstalled, I'll check the vacume pressure in the fuel line. I'll follow up with you in about a week.
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nickwarner
Veteran
/ Moderator
Jan 3, 2013, 5:56 PM
Post #7 of 7
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Re: 74 Chevy small block loses power at highway speed
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The sealer alone takes 72 hours to cure, but it does work. Don't some rusty gas tanks with the kit and it saved me from buying a new tank. The gauge I'm talking about reads about vacuum and low pressure. You shouldn't have vacuum on your fuel feed, you should have pressure.
The POR-15 kit comes with 3 bottles. The first is a degreaser to remove any oil and fuel residue. The second is an etching compound that stops the rust in the tank. The third is the sealer. It would help to try to get some of the rust out of there first. Drop your tank, empty out the gas and if you can use a pressure water hooked up to hot water that helps. Then put maybe a gallon or two of water and some sand. It will help if you have someone help you, as this can get to wearing on your arms fast. Next shake it around as hard as you can for a few minutes. You're using the sand to break loose as much of the loose rust as you can. Then rinse and pressure wash again to remove the sand and rust scale. From there, follow the directions that come with the kit.
You probably should get a new fuel sending unit lock ring ahead of time. They tend to get wrecked when you take them out. If the outside of the tank is rusty, POR-15 also has a kit for that. Same degreaser and etching compound, but instead of sealer its the POR-15 paint that will make sure it never rusts again. If your fuel tank straps are rough, toss them too. Lot of them just snap or have to be cut to take them off when they get old. It's going to take a few days for the sealer to cure anyway so you'll have time for the parts to arrive if they aren't in stock.
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