My 1966 327 300hp Troubleshooting Endeavor
Thread Starter
2nd Gear
Joined: Mar 2025
Posts: 2
Likes: 13
My 1966 327 300hp Troubleshooting Endeavor
There’s hope for us weekend mechanics and a lot to learn.
Last April I bought a ’66 327/300hp, numbers-matching, Trophy Blue convertible. It ran “pretty good”… or so I thought. After a few days of driving, I could tell it wasn’t quite right. I can turn a wrench, but I hadn’t seriously worked on cars in about 30 years.
First issue was the exhaust smell. My wife noticed it immediately. Forum research suggested it might be running rich, so I bought a vacuum gauge and adjusted the idle mixture screws, floats, and idle speed. It smoothed out slightly, but the smell didn’t change much.
Then I noticed the starter often struggled while cranking, like the battery was weak — even on cold starts. The forums pointed to heat soak or excessive timing advance. I wrapped the starter and checked timing with a basic light. It appeared to be at 12° BTDC, seemed high, but still at the top end of what some say cane be normal (more on that later).
I also had a significant stumble off the line and hesitation in higher gears at low RPM. I replaced plugs, wires, and the coil (Pertronix conversion had been done years ago). It ran smoother, but the hesitation was still there. So I rebuilt the Holley carb.
After the rebuild, it improved but still wasn’t right. There was occasional popping through the exhaust and a burble under mid-to-high RPM with heavier throttle. More forum digging led me to the vacuum secondaries. They weren’t opening — the passage was clogged with gasket material. After clearing it and experimenting with secondary springs, most of the hesitation went away. Still not perfect.
Then I found Lars’ timing and tuning papers. That’s when I realized the timing marks were in 2° increments, not 1°. My “12°” was actually 24°. I reset it to 6°, convinced I’d solved it — and the car ran worse. Rough, hesitation, popping.
I bought a dial-back timing light and followed the instructions properly. Set total timing to 36°, adjusted the mechanical advance springs, which brought the idle timing around 5°. I reconnected the vacuum advance and noticed the timing didn’t change. Took it out for a drive and it was still horrible. I read some more and realized he timing should have changed when I connected the vacuum advance.
Tested the vacuum advance canister — it wouldn’t hold vacuum. Replaced it, and the car finally ran the way it should.
At some point, someone had increased the base timing to compensate for the failed vacuum advance instead of fixing it. That explains the hard starting (starter fighting too much advance), the drivability issues, and the vacuum leak symptoms.
It took eight months of reading, trial and error, and learning, but I finally got it solved! Posting this in case it helps the next person chasing similar issues and may need some encouragement.
Darin
Last April I bought a ’66 327/300hp, numbers-matching, Trophy Blue convertible. It ran “pretty good”… or so I thought. After a few days of driving, I could tell it wasn’t quite right. I can turn a wrench, but I hadn’t seriously worked on cars in about 30 years.
First issue was the exhaust smell. My wife noticed it immediately. Forum research suggested it might be running rich, so I bought a vacuum gauge and adjusted the idle mixture screws, floats, and idle speed. It smoothed out slightly, but the smell didn’t change much.
Then I noticed the starter often struggled while cranking, like the battery was weak — even on cold starts. The forums pointed to heat soak or excessive timing advance. I wrapped the starter and checked timing with a basic light. It appeared to be at 12° BTDC, seemed high, but still at the top end of what some say cane be normal (more on that later).
I also had a significant stumble off the line and hesitation in higher gears at low RPM. I replaced plugs, wires, and the coil (Pertronix conversion had been done years ago). It ran smoother, but the hesitation was still there. So I rebuilt the Holley carb.
After the rebuild, it improved but still wasn’t right. There was occasional popping through the exhaust and a burble under mid-to-high RPM with heavier throttle. More forum digging led me to the vacuum secondaries. They weren’t opening — the passage was clogged with gasket material. After clearing it and experimenting with secondary springs, most of the hesitation went away. Still not perfect.
Then I found Lars’ timing and tuning papers. That’s when I realized the timing marks were in 2° increments, not 1°. My “12°” was actually 24°. I reset it to 6°, convinced I’d solved it — and the car ran worse. Rough, hesitation, popping.
I bought a dial-back timing light and followed the instructions properly. Set total timing to 36°, adjusted the mechanical advance springs, which brought the idle timing around 5°. I reconnected the vacuum advance and noticed the timing didn’t change. Took it out for a drive and it was still horrible. I read some more and realized he timing should have changed when I connected the vacuum advance.
Tested the vacuum advance canister — it wouldn’t hold vacuum. Replaced it, and the car finally ran the way it should.
At some point, someone had increased the base timing to compensate for the failed vacuum advance instead of fixing it. That explains the hard starting (starter fighting too much advance), the drivability issues, and the vacuum leak symptoms.
It took eight months of reading, trial and error, and learning, but I finally got it solved! Posting this in case it helps the next person chasing similar issues and may need some encouragement.
Darin
Le Mans Master
Joined: Jan 2007
Posts: 5,996
Likes: 1,502
From: Prescott Arizona
Darin - Welcome to the Corvette Forum. Sounds like you're getting your Corvette into roadworthy shape, good job. You do know that new members are required to post pics of their Corvettes, right? ;-).
Mike T - Prescott AZ
Mike T - Prescott AZ
Instructor
Joined: Jul 2019
Posts: 179
Likes: 20
From: Rehoboth, Beach DE
Darin,
Welcome to the forum. There's lots of experts here that are happy to help and advise on your car. Glad you were able to do searches to troubleshoot your issues. Life is so good when the car is running well.
Oh yeah, please post pictures of your ride. Sounds like a nice car.
John
Welcome to the forum. There's lots of experts here that are happy to help and advise on your car. Glad you were able to do searches to troubleshoot your issues. Life is so good when the car is running well.
Oh yeah, please post pictures of your ride. Sounds like a nice car.
John
Le Mans Master
Joined: Nov 2015
Posts: 7,444
Likes: 2,625
From: Really Central IL Illinois
Welcome to the MONEY PIT.
R66 is also a 300hp / 327, love it. I have only had to rebuild the engine, Holley, headlight motor gears, and transmission so far. Worth the $$$ as it is much more fun to drive than the boss' Supidroo Outback.
Hope you are in a warm state as right now here in IL it is only starting to warm up above freezing.
Ron
R66 is also a 300hp / 327, love it. I have only had to rebuild the engine, Holley, headlight motor gears, and transmission so far. Worth the $$$ as it is much more fun to drive than the boss' Supidroo Outback.
Hope you are in a warm state as right now here in IL it is only starting to warm up above freezing.
Ron
Instructor
Joined: Jan 2012
Posts: 214
Likes: 44
From: Peoria Az, Oceanside Ca
Welcome Darin,
A very nice 66 L75...mine is as well only in Nassau Blue. I went thru a similar timing education and I found the same issue with the vacuum advance not working...only my solution was to remove the BB in the rubber vacuum advance line between the canister and manifold vacuum...hehe. That one had me going for a bit.
I am still fighting a slight stumble but it is such a joy to work on these machines I dont mind the current trouble shooting. I also fought the vacuum secondary operation only to find my throttle linkage was so mis adjusted and also was hitting a wire bundle at the firewall, I never got to WOT. Learned a lot about the vacuum secondary operation.
take care and enjoy it out there in the wild where these awesome machines were meant to be...drive it like ya stole it!
cheers,
willie
A very nice 66 L75...mine is as well only in Nassau Blue. I went thru a similar timing education and I found the same issue with the vacuum advance not working...only my solution was to remove the BB in the rubber vacuum advance line between the canister and manifold vacuum...hehe. That one had me going for a bit.
I am still fighting a slight stumble but it is such a joy to work on these machines I dont mind the current trouble shooting. I also fought the vacuum secondary operation only to find my throttle linkage was so mis adjusted and also was hitting a wire bundle at the firewall, I never got to WOT. Learned a lot about the vacuum secondary operation.
take care and enjoy it out there in the wild where these awesome machines were meant to be...drive it like ya stole it!
cheers,
willie
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
Instructor
Joined: Sep 2018
Posts: 237
Likes: 116
From: Western suburbs of Minneapolis
Similar issues with our 66 327 300hp and finally tracked it down to excessive fuel pressure. Your vacuum gauge may have the ability to check it. Just disconnect the line at the carburetor and connect a gauge to the line. No need to tee it in. Ours was around 9 if I remember correctly an no I think it is at 4.5 and it runs much better.