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Motor question

Garagetoyz

Well-known member
Joined
Sep 20, 2005
Messages
53
Location
New Jersey
Corvette
78 Silver Anniversary 4 spd Crate engine
My C3 has a crate motor in it and when I got the car 2 years ago it was a rocketship. Recently however when I press the gas I get a lot of motor stumbling until the RPM'S reach about 3000 and then it takes off . It's pretty severe and happens all the time now. Do anyone have an idea what could cause this? I have recently installed headers with an X pipe could this be related?;help I know this is kind of vague but any ideas would be a big help!
 

minifridge1138

Well-known member
Joined
Feb 2, 2007
Messages
908
Location
USA
Corvette
1982 Black Fastback
Personally, I find it hard to believe that installing an X-pipe would cause that problem. Did it start as soon as you installed the X-pipe?

More information would help. Does it idle correctly? Does it only happen in first gear, 2nd gear, etc?

Hesitation/rough driving below 3,000 rpm's sounds more like a carburetor problem.

Post more information and you'll get more accurate opinions.

Good luck, and let us know more.
 

Garagetoyz

Well-known member
Joined
Sep 20, 2005
Messages
53
Location
New Jersey
Corvette
78 Silver Anniversary 4 spd Crate engine
It idle's well at 800 rpm. I noticed today that it is running very rich, after converting from stock exhaust to duals should I have the carb/timing reset? Like most of the older cars here it sits a lot but I am wondering if it should be resetup. The car came with Bosch plugs so maybe I'll yank em later tonite...
 

minifridge1138

Well-known member
Joined
Feb 2, 2007
Messages
908
Location
USA
Corvette
1982 Black Fastback
Personally, I would take baby steps and fix what you know is wrong.
If it is too ruch, then the first thing I would do is adjust the mix settings.

My guess is that your aftermarket exhaust is so much less restrictive, that you're engine is now moving more air through the motor. So it makes sense that you'd need to adjust the carb.

After the carb, i'd check the timing. If it is off, then fix it.

After you KNOW that those 2 things are correct, drive it and see if the problem has been solved. If it still runs rough, then you'll know 2 things that it is NOT and we can work from there.

Good luck!!
 
Joined
Apr 29, 2001
Messages
2,141
Location
Rio Rancho, NM
Corvette
1981 HD Suspension; ZN1 Option
Garagetoyz,

What year model of C3 you have? [well duh, your details says 78 Vette]

What engine carburetor, ignition distributor?
 

Garagetoyz

Well-known member
Joined
Sep 20, 2005
Messages
53
Location
New Jersey
Corvette
78 Silver Anniversary 4 spd Crate engine
Sorry I didn't include this information previously. It has a zzz crate motor ( I was told from 1992) with a 750 holley carb, hei ignition. I will try the baby steps tmmrw also thx. I checked the plugs today and they seem ok and installed new wires while I was in there. I'll keep reading anything here and trying any suggestions out. I know it's hard to say what could be wrong but any input is appreciated, been waiting all winter for this weather, must drive!
 
Joined
Apr 29, 2001
Messages
2,141
Location
Rio Rancho, NM
Corvette
1981 HD Suspension; ZN1 Option
Sorry I didn't include this information previously. It has a zzz crate motor ( I was told from 1992) with a 750 holley carb, hei ignition. I will try the baby steps tmmrw also thx. I checked the plugs today and they seem ok and installed new wires while I was in there. I'll keep reading anything here and trying any suggestions out. I know it's hard to say what could be wrong but any input is appreciated, been waiting all winter for this weather, must drive!

One simple check would be to remove the distributor cap, and check the advance weights, Sometimes they hang-up at lower RPM's, and then at higher rpm's they free themselves, and that could explain your engine running better at 3000 rpm's.

At the risk of re-igniting the manifold vs. ported advance argument, If you cannister is stock, and the diaphragm is shot, then it is requiring a higher vacuum signal to get your advance timing to kick-in, so you can apply vacuum with a pump and check for a leak of the diaphragm.
 

shark454

Well-known member
Joined
Mar 30, 2007
Messages
177
Location
Illinois
Corvette
2002 Torch Red Coupe 1972 454 auto red
When you step on the gas, the carb dumps more gas into the engine via the accelerator pump. If you have too much gas going into the engine, it will stumble on acceleration. However, if you do not have enough gas, it will stumble as well. I'd suggest looking at the accelerator pump mechanism. See if it squirts gas into the engine when the throttle is pressed. You can do this without the engine running. Look into the carb as you move the linkage.
You might want to look for a vacuum leak, as well.

Good luck!
 

Evolution1980

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Administrator
Joined
Feb 25, 2002
Messages
4,302
Location
Cleveland, Ohio
Corvette
ZZ4, 700R4, Steeroids rack & pinion, VB&P Brakes
When you step on the gas, the carb dumps more gas into the engine via the accelerator pump. If you have too much gas going into the engine, it will stumble on acceleration. However, if you do not have enough gas, it will stumble as well. I'd suggest looking at the accelerator pump mechanism.
Accelerator pump is what I was thinking as well.

Garagetoyz, when you are rolling at about 2000 rpm or so and you stomp the throttle, does the car hesitate? Like, it bogs down and then catches up? If so, replace the accelerator pump. It's a relatively cheap replacement and can make all the difference in the world. My car ran like crap for years and then I finally replaced it. Wish I had do it years sooner! :(

However, if the car is stumbling at light acceleration all the way through the RPM range up to 3000 rpm, then you likely have a different problem.

(Changing your exhaust should not cause your car to exhibit any of the problems you are noticing.)
 

Garagetoyz

Well-known member
Joined
Sep 20, 2005
Messages
53
Location
New Jersey
Corvette
78 Silver Anniversary 4 spd Crate engine
Thank you for all the replies guys , I will follow all these leads and I appreciate the help. Btw did the 78's come with an O2 sensor?
 

Evolution1980

Well-known member
Administrator
Joined
Feb 25, 2002
Messages
4,302
Location
Cleveland, Ohio
Corvette
ZZ4, 700R4, Steeroids rack & pinion, VB&P Brakes
Thank you for all the replies guys , I will follow all these leads and I appreciate the help. Btw did the 78's come with an O2 sensor?
Nope! No sensors/computer monitoring until '81
:upthumbs
 

AKRAY4PLAY

Well-known member
Joined
Oct 14, 2001
Messages
584
Location
Wasilla, Alaska, U.S.A.
Corvette
'77 L-82, black on black. Full mod 406 small block
you said the plugs look good, what color are they and what was your gap?? did you inspect the distributor cap and rotor?? i have run into similar problems in the past, they ended up being cracked insulators on the plugs. i've noticed that the expensive plugs usually have poor insulators. they hairline crack and short out causing a stumble, motor chugs until 3000 rpm or so. when you get on the throttle the mixture gets richer and raises the resistance of the air that the spark must jump, with hairline cracks this can be enough to cause the plug to side arc and misfire the cylinder. also check that your cap and rotor has good clean terminals, preferably brass instead of aluminum. try a cheap set of autolite plugs, they run about $10 for the set
 
C

cooleyenterprises

Guest
Definitely adjust the carb...since the exhaust was changed....also HEI's don't like low voltage....we do a plug read at every engine tune....light brown color on the plugs ...good luck ...
 

Garagetoyz

Well-known member
Joined
Sep 20, 2005
Messages
53
Location
New Jersey
Corvette
78 Silver Anniversary 4 spd Crate engine
Great tips guys much appreciated. Going to buy the new plugs tmmrw. Also looking into the accelerator pump with the help of a few local friends and while their under the hood we are going to check the distributor and weights.
On that note does anyone have any tips on the best way/method to tune a holley carb? ;help
 
Joined
Apr 29, 2001
Messages
2,141
Location
Rio Rancho, NM
Corvette
1981 HD Suspension; ZN1 Option
A chassis dyno is best when tuning the carburetor. That's the short answer.
 
C

cooleyenterprises

Guest
Stumbling under hard acceleration (full...stomping on the throttle?) or moderate (1/2 throtlle?) Manual secondaries or vaccum? ....You said it is running rich.....loading up at idle? (eye burn...visible smoke)...any black smoke when stumbling?.......Before you change the plugs, set the 4 corner idle mixtures and the floats.....you can use a vaccum guage or even just observe when the engine stumbles....I would just systematically go one step at a time....for starters check the accelerator pump to make sure it's working....next address the jetting...change the power valve while you're in there....We always tune the primaries ........then go to the secondaries separately......

If you don't have access to a chassis dyno...The spark plugs will tell you everything you need to know....( mixture and timing)....
 

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