[C2] Dropped cylinder
Thread Starter
Racer
Joined: Nov 2020
Posts: 322
Likes: 110
Dropped cylinder
66 327 built to l79 spec with roller cam. Bought as not run in 3-5 years.. motor would run but not at idle, 2500 rpm just to get on trailer. Did timing according to lairs to the letter , I KNOW it’s right. Motor starts right up and dies, need full choke and pumping of gas to run. Did compression test 1-160 3-160 5-160 7-160 2 155 4-155 6-155 8-160 IR on cylinders 1-230 3-550 5-550 7-440 2-400 -443 6-445 8-420 Plug on 1 wet , new plug, no change. New wire, no change. Vacuum at startup 2500 rpm 15”, after warmup can get 1000 rpm ,vacuum at 11. I don’t want to pull intake but thinking I may have internal vacuum leak. Opinions?
Racer
Joined: Jan 2026
Posts: 319
Likes: 56
From: Tacoma, WA
People also ask
What if wet compression test is higher than dry?
If compression rises significantly (often 40 PSI or more) during the wet test compared to the dry test, it indicates worn or damaged piston rings as the source of compression loss. If there's little or no change in readings, the problem likely lies with the valves, valve seats, or cylinder head gasket.
What if wet compression test is higher than dry?
If compression rises significantly (often 40 PSI or more) during the wet test compared to the dry test, it indicates worn or damaged piston rings as the source of compression loss. If there's little or no change in readings, the problem likely lies with the valves, valve seats, or cylinder head gasket.
Racer
Joined: Dec 2025
Posts: 277
Likes: 95
From: Pachuta, MS
This isn't a compression problem as the variation isn't significant. My first thought is bad fuel. How long has this thing been sitting? If you didn't flush and clean the fuel system, you're starting it may have been the worst thing you could have done. How does the fuel smell? If it's bad you won't even have to ask. It could be something like a badly leaking brake booster or stuck PCV valve or missing (large) vacuum hose but before doing anything if that fuel is bad get it out of there, replace the fuel pump, rebuild the carburetor, flush the fuel lines, and replace the filter. The fuel sock may be completely gummed up so dropping the tank is probably wise and hope that it hasn't coated the valve stems. You may have a separate ignition problem and my guess with the parts you have replaced is distributor cap.
Race Director
Joined: Jul 2018
Posts: 10,590
Likes: 8,414
From: San Antonio, TX/Mahopac, NY
2026 Restomod of the Year Winner
2024 Corvette of the Year Finalist - Modified
2024 Corvette of the Year Finalist - Unmodified
2023 C7 of the Year Winner - Modified
You could have a wiped cam lobe on #1 cylinder.
Le Mans Master
Joined: Nov 2015
Posts: 7,466
Likes: 2,644
From: Really Central IL Illinois
If the plug is wet on #1, you are getting fuel. You have confirmed compression. If you have an induction timing light or in-line tester, check the wire with the engine running. I would test all 8. Then pull the cap and inspect for gray smoke trails indicating a crack or look at the contacts inside and outside of the cap. You can pull the valve cover and check the rocker / valve action, but if you are getting fuel on the plug, it is electrical. Bad gas would effect all cylinders IMHO. What ignition system? Points or electronic?
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
Le Mans Master
Joined: Feb 2008
Posts: 6,487
Likes: 1,918
From: Folsom CA
The fastest check would be to pull the valve cover and measure movement on the #1 intake and exhaust valves as they open and close. Compare to cylinder #3 next to it. A flat lobe cam will lose a lot of lift fast. .200 or more is not uncommon but in roller applications it us usually a flat or damaged wheel on that lifter. Best of luck, hope it is a minor set back.
Tech Contributor
Joined: Jan 2007
Posts: 19,458
Likes: 1,165
From: Dyer, IN
66 327 built to l79 spec with roller cam. Bought as not run in 3-5 years.. motor would run but not at idle, 2500 rpm just to get on trailer. Did timing according to lairs to the letter , I KNOW it’s right. Motor starts right up and dies, need full choke and pumping of gas to run. Did compression test 1-160 3-160 5-160 7-160 2 155 4-155 6-155 8-160 IR on cylinders 1-230 3-550 5-550 7-440 2-400 -443 6-445 8-420 Plug on 1 wet , new plug, no change. New wire, no change. Vacuum at startup 2500 rpm 15”, after warmup can get 1000 rpm ,vacuum at 11. I don’t want to pull intake but thinking I may have internal vacuum leak. Opinions?