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Engine runs rough...

1974vette

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Joined
Jul 8, 2003
Messages
175
Location
myrtle beach, sc
Corvette
1974 Red L-48 Coupe
To start off, its been a while since i've been around. Its good to be back. I've tried searching on this but i couldnt find an exact answer. In the last year and a half, I've had the tired 350 in my vette replaced with a GM crate engine. Its nothing special just a 250 or so hp crate engine. When it was replaced, plugs, wires, distributor cap were also replaced. It has a new holley 650 as well. I havent driven the car in a while. When i went out recently to drive it, it seemed to stumble pretty badly so i figured it was bad gas. i burned the gas out of the car and replaced it with premium. The car seems to run fine but during 45-60 mph i can floor it and it barely does anything. it seems like the car falls on its face. then from about 70-80 mph, the car runs like it should. I know around this past april my dad was messing with the carburetor adjustment screws and he doesnt exactly know what he's doing. I'm wondering if this is attributing to it running rough? I'm planning on checking and replacing the plugs soon. I'm going to run AC delco plugs. Which ones should i get? i have no clue which gap i should get, etc. Also, if any of you have any links that details the process of tuning a carb, i'd appreciate that also! im looking to get this car reliable. Thanks again for the help!
 

wishuwerehere82

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Sep 6, 2003
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2,314
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Rochester, NY
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Red '82 Coupe,Sebring Silver '98 Coupe
To set the mixture screws, first get the engine up to temp, then kill it, then screw the screws all the way in and back them out 3 full turns restart the engine, with it up to temp and @ idle turn one in 1/4 turn then the other one in 1/4 turn go from one to the other 1/4 turn at a time until you hear a RPM drop then back both out 1/8 turn or you can do the same thing with a vacuum guage to get the maximum vacuum reading keeping them even.

That will eliminate one question in the equation.
 

zachh

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Aug 1, 2005
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221
Location
virginia beach, va
Corvette
1979 black 'n' silver L82 1978 Trans Am Y88
im not sure what the gap is for your new engine, but the stock L48 should be 0.035 inches. Check your timing, that may be part of your problem...., and check for any frayed or burnt plug wires.
Good Luck
zachh
 

1974vette

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Joined
Jul 8, 2003
Messages
175
Location
myrtle beach, sc
Corvette
1974 Red L-48 Coupe
thanks guys! wishuwerehere, i recently done that but i couldnt find the screw on the passenger side of the carb. i'll have to give it another shot b/c only the left side has been adjusted. that was the mixture screw my dad was messing with so thats the one i was trying to correct it with. Also, it has new plug wires as i just replaced them so i dont forsee that being a problem. how do i go about checking my timing?
 

wishuwerehere82

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Sep 6, 2003
Messages
2,314
Location
Rochester, NY
Corvette
Red '82 Coupe,Sebring Silver '98 Coupe
how do i go about checking my timing?
Pull the hose off your vacuum advance on the distributor and plug it.
Put a timing light on the #1 plug wire and point it at the timing marks on the tab above the harmonic balancer to see the timing mark. Loosen the bolt that holds down the distributor, then rotate the distributor until the line on the balancer aligns with the right mark on the tab.
Not sure what your timing is for a 74, probably around 12 degrees before TDC.
Tighten the distributor back down, and replace the vacuum hose.
 

wishuwerehere82

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Rochester, NY
Corvette
Red '82 Coupe,Sebring Silver '98 Coupe

Evolution1980

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Cleveland, Ohio
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ZZ4, 700R4, Steeroids rack & pinion, VB&P Brakes
I've had the tired 350 in my vette replaced with a GM crate engine. Its nothing special just a 250 or so hp crate engine. I'm planning on checking and replacing the plugs soon. I'm going to run AC delco plugs. Which ones should i get? i have no clue which gap i should get, etc.
If you still have the OEM paperwork on the engine, it should suggest what plugs to run and with what gap (I believe).
 

1974vette

Well-known member
Joined
Jul 8, 2003
Messages
175
Location
myrtle beach, sc
Corvette
1974 Red L-48 Coupe
again for the help guys. i found the info i needed about the plug gaps on this page that has the same engine as mine. i removed the plugs and gapped them to .035 this evening. all of the plugs were wet when i removed them? im suprised that the car ran before hand. afterwards, it seems to run better but it still "bogs" a little bit throughout the powerband. i suspect it being fuel related because it idled really high after i drove it. im going to try to adjust the timing soon. any other help is appreciated! thanks again!
 
V

vmrod

Guest
I have replaced my engine and puts lots of fun parts on the motor. I had an old holley and it never idled right. I bought a new Edelbrock 750 and it was much better, but still some idling issues and bogging down. I bought a Edelbrock 600, and this helped alot! (I still burns rich).

I checked the fuel pressure and it is way too high for my carb. (It was near 12psi) I ordered an Edelbrock fuel pump and will install it shortly. This should cure the problem. (it better!)
 

fine69

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Joined
Dec 12, 2004
Messages
975
Location
Maryland / D.C.
Corvette
'69 Convertible Vette; '72 Z28 Camaro Rally Sport
I checked the fuel pressure and it is way too high for my carb. (It was near 12psi) I ordered an Edelbrock fuel pump and will install it shortly. This should cure the problem. (it better!)

I doubt that replacing the fuel pump will cure the problem. The stock pump will work fine. Unless yours is broken, I would not spend the money and time on the pump. I replaced mine with the Edelbrock and experienced NO notable improvement, none... so I spent $80 for aesthetics.

You may want to look into getting a fuel pressure regulator that also allows for fuel return. I was told that the fuel pressure should not be more than approx 6 psi for street driving.
 
V

vmrod

Guest
...You may want to look into getting a fuel pressure regulator that also allows for fuel return. I was told that the fuel pressure should not be more than approx 6 psi for street driving.

I was getting nearly twice the fuel. If Edelbrock carbs could efficiently accept 12psi, then they would not stipulate "max 6.5psi" on the carb spec sheet. Rather than install a pressure regulator, I opted for the fuel pump which is designed to produce 6psi. I'll have one less part in the system now, plus a shinier engine!

I've tried lots of things to the engine, and no one thing did the trick. However, every thing makes it run that much better. If I run the proper fuel pressure, then that can't be wrong.
 

fine69

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Joined
Dec 12, 2004
Messages
975
Location
Maryland / D.C.
Corvette
'69 Convertible Vette; '72 Z28 Camaro Rally Sport
I was getting nearly twice the fuel. If Edelbrock carbs could efficiently accept 12psi, then they would not stipulate "max 6.5psi" on the carb spec sheet. Rather than install a pressure regulator, I opted for the fuel pump which is designed to produce 6psi. I'll have one less part in the system now, plus a shinier engine!

I've tried lots of things to the engine, and no one thing did the trick. However, every thing makes it run that much better. If I run the proper fuel pressure, then that can't be wrong.

I purchased the same thing, 6 psi version, I was still getting too much. Unfortunately, I received contractory information from Edelbrock's customer services folks (I left a post in here after that experience).

Hopefully, your setup will work better for you.
 

1974vette

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Joined
Jul 8, 2003
Messages
175
Location
myrtle beach, sc
Corvette
1974 Red L-48 Coupe
okay guys. i tried tuning the carb today. got the fuel level and curb idle at what i thought was right and it ran fine for a few minutes then started to 'bog' again. i pulled the fuel filter and i found a washer in front of it. i removed that. i also found that they (the dealership) had a vaccum line on the passenger front hooked to the wrong spot (there was no vaccum). also while looking, i realized that they removed my kickdown switch for the transmission when they put in the crate engine (im guessing to cover them since the engine had a 3yr/100,000 mile warranty) im going to need to get a new kickdown cable. can anyone give me a link to one that will work? also, is the model number of my carb stamped on it anywhere? im not 100% positive of what it is and i would like to get the correct instructions for it. by the way, its a holley. thanks again for the help!
 
Joined
Jan 1, 2002
Messages
7,246
Location
Washington, Michigan
Corvette
'67 Marina Blue Convertible
also, is the model number of my carb stamped on it anywhere? im not 100% positive of what it is and i would like to get the correct instructions for it. by the way, its a holley. thanks again for the help!

The List number and date code is stamped on the front side of the choke air horn on the primary side - post those and we can decode it. :)
 

1974vette

Well-known member
Joined
Jul 8, 2003
Messages
175
Location
myrtle beach, sc
Corvette
1974 Red L-48 Coupe
The List number and date code is stamped on the front side of the choke air horn on the primary side - post those and we can decode it. :)

picture007jw2.jpg


notice, mine is an aftermarket holley as i posted above. i just dont know the model of it. here is a photo of mine. the numbers are 6210-3 2912
 

wishuwerehere82

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Joined
Sep 6, 2003
Messages
2,314
Location
Rochester, NY
Corvette
Red '82 Coupe,Sebring Silver '98 Coupe
thank you, thank you, thank you!:upthumbs

also, can anyone direct me towards a kickdown cable for the car? its got a TH400.
There is no cable on the TH400 transmission.
The TH400 uses an electrical slide switch, which is controlled by the throttle linkage at WOT.
You might have good luck finding a switch at a local transmission shop. But you may have to modify something to mate up to your Holley carb.
Or you could call Holley and ask if they have a switch for your carb.
I assume the wires in the harness that goes to the carb are still there.

http://www.highperformancepontiac.com/tech/hppp_060200_transmission_kickdown_switch_install/

On Pg. 7 B&M makes a switch kit, but you need a TH350 bracket for it.
http://www.bmracing.com/media/catalog/9.pdf
 

1974vette

Well-known member
Joined
Jul 8, 2003
Messages
175
Location
myrtle beach, sc
Corvette
1974 Red L-48 Coupe
thanks alot yet again! i'll have to check the car. if it came with an electronic kickdown, im sure the solenoid and all is still there and the cable is simply removed. if not, i'll just get another. thanks again!
 

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