Need Help Diagnosing Engine Problem
Thread Starter
Pro
Joined: May 2008
Posts: 525
Likes: 95
From: Denver, NC
Need Help Diagnosing Engine Problem
I need some opinions on what may be causing a problem that occurs only under certain conditions.
Engine is a '67 427 - 390 hp with new Holley 4160 (specific carb for the application) and rebuilt stock distributor. Car runs well under normal driving conditions.
The problem occurs following hard acceleration. After backing off the throttle, the engine loses power and sounds like it has a potato in each exhaust pipe when attempting acceleration; however, exhaust is coming out of each side pipe. The engine will idle but falters under any load. After the engine cools down, it runs normally until another hard acceleration attempt. Problem seems to occur when the secondaries of the carb open. The gas cap vent is functioning as expected and the secondary plates of the carb are closing correctly.
Thanks for any suggestions you might offer.
Engine is a '67 427 - 390 hp with new Holley 4160 (specific carb for the application) and rebuilt stock distributor. Car runs well under normal driving conditions.
The problem occurs following hard acceleration. After backing off the throttle, the engine loses power and sounds like it has a potato in each exhaust pipe when attempting acceleration; however, exhaust is coming out of each side pipe. The engine will idle but falters under any load. After the engine cools down, it runs normally until another hard acceleration attempt. Problem seems to occur when the secondaries of the carb open. The gas cap vent is functioning as expected and the secondary plates of the carb are closing correctly.
Thanks for any suggestions you might offer.
Le Mans Master
Joined: Feb 2008
Posts: 6,463
Likes: 1,889
From: Folsom CA
Sounds like fuel starvation, either not enough in the bowl when your floor it or slow refill.What happens if you stay on it with the four barrell open? Does it keep pulling or lay down? ( Scarry thing to do on the street with a 427 )
Instructor
Joined: May 2001
Posts: 143
Likes: 0
From: Church Hill Tennessee
Cruise-In III Veteran
I had a similar problem with my 67 427/390. Ran fine under normal driving conditions but with wide open throttle it ran out of fuel. Turned out to be a weak fuel pump. I replaced the fuel pump and it runs fine now.
Thread Starter
Pro
Joined: May 2008
Posts: 525
Likes: 95
From: Denver, NC
Thanks to all for the suggestions.
Race Director
Joined: Oct 2005
Posts: 12,468
Likes: 1,548
From: Camp Verde AZ
C1 of Year Finalist (performance mods) 2019
2017 C1 of Year Finalist
After you make sure you have adequate fuel (no clogged fuel filter or fuel delivery problem),
I would suggest making sure the centrifugal advance weights are free to move,and don't get sticky when the dizzy is hot. Never had a problem like that on my Chevs, but it was an issue on a Ferrari I have, one cylinder bank would have delayed timing (two distributors on the motor) due to sticking weights.
Doug
I would suggest making sure the centrifugal advance weights are free to move,and don't get sticky when the dizzy is hot. Never had a problem like that on my Chevs, but it was an issue on a Ferrari I have, one cylinder bank would have delayed timing (two distributors on the motor) due to sticking weights.
Doug
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
Race Director
Joined: Oct 2005
Posts: 12,468
Likes: 1,548
From: Camp Verde AZ
C1 of Year Finalist (performance mods) 2019
2017 C1 of Year Finalist
Logic 101:
Car runs fine.
Car runs fine under hard acceleration.
Car doesn't run fine after hard acceleration.
We can deduce that:
this is not a constant problem, that is fixed by an adjustment.
Hard accel causes the problem to be noticed.
Thus, the problem is related to hard acceleration,and possibly heat, as the problem disappears after the car cools down.
I was not there,a nd have no direct time reference to the described problems, but they are either fuel related, as in fuel starvation after acceleration (clogged fuel filter), or ignition related, as in retarded timing, based on the symptoms.
You start with the baseline, does it run, does it run well, and look for things that occurred prior to it not running correctly.
Doug
Car runs fine.
Car runs fine under hard acceleration.
Car doesn't run fine after hard acceleration.
We can deduce that:
this is not a constant problem, that is fixed by an adjustment.
Hard accel causes the problem to be noticed.
Thus, the problem is related to hard acceleration,and possibly heat, as the problem disappears after the car cools down.
I was not there,a nd have no direct time reference to the described problems, but they are either fuel related, as in fuel starvation after acceleration (clogged fuel filter), or ignition related, as in retarded timing, based on the symptoms.
You start with the baseline, does it run, does it run well, and look for things that occurred prior to it not running correctly.
Doug
Thread Starter
Pro
Joined: May 2008
Posts: 525
Likes: 95
From: Denver, NC
Logic 101:
Car runs fine.
Car runs fine under hard acceleration.
Car doesn't run fine after hard acceleration.
We can deduce that:
this is not a constant problem, that is fixed by an adjustment.
Hard accel causes the problem to be noticed.
Thus, the problem is related to hard acceleration,and possibly heat, as the problem disappears after the car cools down.
I was not there,a nd have no direct time reference to the described problems, but they are either fuel related, as in fuel starvation after acceleration (clogged fuel filter), or ignition related, as in retarded timing, based on the symptoms.
You start with the baseline, does it run, does it run well, and look for things that occurred prior to it not running correctly.
Doug
Car runs fine.
Car runs fine under hard acceleration.
Car doesn't run fine after hard acceleration.
We can deduce that:
this is not a constant problem, that is fixed by an adjustment.
Hard accel causes the problem to be noticed.
Thus, the problem is related to hard acceleration,and possibly heat, as the problem disappears after the car cools down.
I was not there,a nd have no direct time reference to the described problems, but they are either fuel related, as in fuel starvation after acceleration (clogged fuel filter), or ignition related, as in retarded timing, based on the symptoms.
You start with the baseline, does it run, does it run well, and look for things that occurred prior to it not running correctly.
Doug
Race Director
Joined: Oct 2005
Posts: 12,468
Likes: 1,548
From: Camp Verde AZ
C1 of Year Finalist (performance mods) 2019
2017 C1 of Year Finalist
OK, in your first post you said the car runs fine again after it cools down, so so maybe cool down isn't the correct term.
Perhaps car runs fine again after it sits a while is a better description.
That points more towards fuel delivery, than anything else.
Doug
Perhaps car runs fine again after it sits a while is a better description.
That points more towards fuel delivery, than anything else.
Doug
Le Mans Master
Joined: Feb 2008
Posts: 6,463
Likes: 1,889
From: Folsom CA
Melting Slicks
Joined: Jan 2002
Posts: 2,132
Likes: 71
From: SOUTH SHORE Massachusetts
Have you checked your spark plugs to see what they look like??? I had something like you had when I put the correct carb back on..ran good under normal conditions...on the highway..cruising along at 2500 RPM..I could feel a surge..almost bucking...check of plugs showed the car way to lean...so I fattened up the jets a few steps and the car runs great now.
Safety Car
Joined: May 2003
Posts: 4,797
Likes: 239
From: Ocean Township NJ
It sounds like vapor lock.
Insulating the fuel line and routing it further away from hot metal parts and hoses will help. Try raising the float levels slightly. More fuel in the bowls should forestall it.
Keep the hood closed but unlatched to let heat escape and try a road test before you do any changes. See if it has any effect.
Insulating the fuel line and routing it further away from hot metal parts and hoses will help. Try raising the float levels slightly. More fuel in the bowls should forestall it.
Keep the hood closed but unlatched to let heat escape and try a road test before you do any changes. See if it has any effect.
Thread Starter
Pro
Joined: May 2008
Posts: 525
Likes: 95
From: Denver, NC
To report on the outcome, the set of NOS piston return springs did the job - NOT!!
I replaced the coil and distributor with an HEI unit and the problem is gone. Thanks for all the replies, even the humorous ones.
I replaced the coil and distributor with an HEI unit and the problem is gone. Thanks for all the replies, even the humorous ones.