Running poorly under load *sometimes*
Thread Starter
Racer
Joined: Feb 2018
Posts: 254
Likes: 73
From: Chicago, Illinois
Running poorly under load *sometimes*
Hi guys, I have a problem that I just cant seem to figure out. I put new mufflers on my car (76' L-48 stock specs )and removed its cat which was plugged, and after a few days of driving it it developed a new issue. over maybe 45 mph if you try to accelerate at anything more than a snails pace it bucks and loses power. (buck being it goes from 2700rpm to 1500rpm and repeats until I lift off the gas). After it bucks once it just runs poorly for the rest of the drive. in park you can rev it to the moon and its fine. There are times it doesn't do it, there are times when it does, there's no set pattern. I pulled all 8 plugs and they appear to be lean. I ohm'ed out all the wires and they are within spec. I put a new fuel filter on it, the pump was brand new 3 years ago, and the carb was rebuilt 2 years ago. the car doesn't sit much at all. New ignition coil. Spark plugs and wires have somewhere around 4500 miles on them and they were good replacements from Acdelco and MSD. cap and rotor are clean-ish, around 7000 miles on them. anyone have any ideas? all i can think of now is a fuel pump (Which I thought either worked or didnt) tank pickup (Which might cause this issue all the time?) or some other electrical issue IE; the new coil is bad. any input is appreciated.
Last edited by MGC/D_7601; Apr 29, 2020 at 01:29 PM.
Race Director
Joined: May 2006
Posts: 18,734
Likes: 2,582
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
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
Thread Starter
Racer
Joined: Feb 2018
Posts: 254
Likes: 73
From: Chicago, Illinois
its sort of a hit and miss kinda deal. It always runs fine cold. But it doesn’t always do it hot either. I changed the cap and rotor 3 years ago. HEI module is at least from 1997. Ill test the pickup and module tonight.
Last edited by MGC/D_7601; May 3, 2020 at 06:01 PM.
Thread Starter
Racer
Joined: Feb 2018
Posts: 254
Likes: 73
From: Chicago, Illinois
Thread Starter
Racer
Joined: Feb 2018
Posts: 254
Likes: 73
From: Chicago, Illinois
Thank you all for your help, sorry it was a few days before i can respond, I have a lot on my plate with my senior year. Apparently at some point a previous owner has cut out the stock fuel lines and replaced them both with rubber lines that look like garbage. I also found that my return line was loose on the fuel pump. ill do the tests mentioned before, and problem or not id like to replace these lines too. ill check back in when I can, thanks again.
Race Director
Joined: Nov 2006
Posts: 13,227
Likes: 7,825
From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
2024 C3 of the Year Finalist- Modified
2022 C3 of the Year Finalist - Modified
Cruise-In VIII Veteran
A few thoughts, intermittent issues are the hardest to solve, for any mechanic. When you test things everything checks fine. That said, if it never does it cold. Not likely it's the fuel pickup sock. And no, you cant get it out threw the filler hole on a 76 with the tank in the car. I have a 77, same tank setup. I recently had my tank out. Sock looked very clean after 43 years. Tank bladder was also in excellent condition.
if the issue only come's on warm. I'd be trying a replacement module and the pickup winding in the distributor. Act's up hot does sound like a coil issue. You have already tried the main ignition coil with no success.
if the issue only come's on warm. I'd be trying a replacement module and the pickup winding in the distributor. Act's up hot does sound like a coil issue. You have already tried the main ignition coil with no success.
Thread Starter
Racer
Joined: Feb 2018
Posts: 254
Likes: 73
From: Chicago, Illinois
I just wanted to make a quick post to say I solved the issue, I tightened every clamp on the fuel system and replaced a bunch of them. I suspect there was air getting in the lines. All the ignition stuff tested good and the EGR is free. I waited to post because I wanted to make sure it was good. 600 miles later it hasn't stumbled once and the plugs are no longer lean. Thank you to all who helped.