78 C3 misses at mid-throttle
Thread Starter
Instructor
Joined: Jan 2008
Posts: 144
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From: Marco Island Florida
78 C3 misses at mid-throttle
I've been working on my wife's 1978 Vette. 29,000 miles, all original except the A/C Compressor. When the car is warm it seems to miss when driving at highway speeds. If I put my foot in it, the miss goes away and accelerates fine but will begin stumbling at mid throttle. It idles fine and when the engine is cool the miss isn't there - only after it is up to running temp. I've already installed a new dizzy cap and rotor. I've tried plugging vacuum ports (to see if something down the line might be leaking) but there seems to be no change.
Any ideas?
Thanks.
Any ideas?
Thanks.
Race Director
Joined: Apr 2009
Posts: 19,294
Likes: 2,754
From: Charlotte NC
What vacuum ports did you plug????.
AND if you can...post a photo of the engine with NO air leaner on it from each side ...if possible.
How old is the fuel????
Are you sure the timing is correct????
Knowing this is it low miles...have you looked at your spark plugs???
DUB
AND if you can...post a photo of the engine with NO air leaner on it from each side ...if possible.
How old is the fuel????
Are you sure the timing is correct????
Knowing this is it low miles...have you looked at your spark plugs???
DUB
Thread Starter
Instructor
Joined: Jan 2008
Posts: 144
Likes: 0
From: Marco Island Florida
Thanks for the response!
We've been driving the car quite a bit over the last few months. Fuel is fresh, timing has not been changed since we've had the car. The problem "suddenly" appeared about three weeks ago. As far as the vacuum lines, I've disconnected and plugged each line from the carb and intake, one at a time, then drove the car a bit to see if the problem got better or worse. There was no change. Since the car is 36 years old, I did go ahead and order new plugs and wires today. I'll get them sometime next week and put them in.
Unless someone tells me something differently, I suspect that if plugs and wires don't solve the problems (after the dizzy and rotor swap also) that I'm looking at carb issues.
We've been driving the car quite a bit over the last few months. Fuel is fresh, timing has not been changed since we've had the car. The problem "suddenly" appeared about three weeks ago. As far as the vacuum lines, I've disconnected and plugged each line from the carb and intake, one at a time, then drove the car a bit to see if the problem got better or worse. There was no change. Since the car is 36 years old, I did go ahead and order new plugs and wires today. I'll get them sometime next week and put them in.
Unless someone tells me something differently, I suspect that if plugs and wires don't solve the problems (after the dizzy and rotor swap also) that I'm looking at carb issues.
Melting Slicks
Joined: Jan 2014
Posts: 2,478
Likes: 254
From: Downers Grove Illinois
Thanks for the response!
We've been driving the car quite a bit over the last few months. Fuel is fresh, timing has not been changed since we've had the car. The problem "suddenly" appeared about three weeks ago. As far as the vacuum lines, I've disconnected and plugged each line from the carb and intake, one at a time, then drove the car a bit to see if the problem got better or worse. There was no change. Since the car is 36 years old, I did go ahead and order new plugs and wires today. I'll get them sometime next week and put them in.
Unless someone tells me something differently, I suspect that if plugs and wires don't solve the problems (after the dizzy and rotor swap also) that I'm looking at carb issues.
We've been driving the car quite a bit over the last few months. Fuel is fresh, timing has not been changed since we've had the car. The problem "suddenly" appeared about three weeks ago. As far as the vacuum lines, I've disconnected and plugged each line from the carb and intake, one at a time, then drove the car a bit to see if the problem got better or worse. There was no change. Since the car is 36 years old, I did go ahead and order new plugs and wires today. I'll get them sometime next week and put them in.
Unless someone tells me something differently, I suspect that if plugs and wires don't solve the problems (after the dizzy and rotor swap also) that I'm looking at carb issues.
Tech Contributor
Joined: Aug 1999
Posts: 14,382
Likes: 6,401
From: At my Bar drinking and wrenching in Lafayette Colorado
Lars
"90% of all carb problems are ignition problems"
(Quote from my "Tuning for Beer World Tour" Seminar Series)
Last edited by lars; Jan 1, 2015 at 11:17 PM.
