Rough running at highway speeds
Thread Starter
Pro
Joined: May 2013
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Rough running at highway speeds
Please help diagnose what is causing my 69 427 tri power to run rough. I typically only notice after warm up and a few miles of highway driving. I can feel the engine to start to run a bit rough and it is most noticeable after I slow down and reaccelerate after cruising on the highway. Also on deceleration at high speed the exhaust is very rough sounding.
Carb has been rebuilt by Lars. I’ve replaced wires and plugs and set timing according to Lars paper at 36 degrees all in. I feel like the issues is electrical but need to know what I should consider next.
Carb has been rebuilt by Lars. I’ve replaced wires and plugs and set timing according to Lars paper at 36 degrees all in. I feel like the issues is electrical but need to know what I should consider next.
Is my vette stock?? HAHA
Joined: Oct 2015
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From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
Check your vacuum lines, choke (the pull off maybe bad), fuel pressure and the ignition system. I was chasing a problem all weak and some of it was due to a bad distributer.
Is my vette stock?? HAHA
Joined: Oct 2015
Posts: 20,235
Likes: 9,376
From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
Could be too much vacuum advance. Last summer my cars distributor was fine. It had two issues this spring. The upper bushing broke and was allow the plate to move that has the points on it and the stock plastic sleeve that limits the vacuum advance deteriorated so i was getting about 15* to 20* of advance if i remember correctly. I put in the brass replacement limiter then noticed the shaft bushing
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Safety Car
Joined: Dec 2012
Posts: 3,822
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From: Summerville SC
I had a C2 SB and I had a similar issue, Went through all the known issues over time and could not figure it out. Out of desperation I put in a new distributor and the problem was gone. Back then I wouldn't have known exactly what the problem was, bad bearing, bent shaft, who knows but I had gone through the points, weights,the cap etc and it was the dizzy itself. Not knowing how old yours is or what kind of shape it might be in, I just thought I would throw my experience out there. 
Last edited by kanvasman; May 13, 2019 at 06:35 PM. Reason: spelling
Is my vette stock?? HAHA
Joined: Oct 2015
Posts: 20,235
Likes: 9,376
From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
Thats the general consensus so far. Its hard to be a mechanic long distance.
Burning Brakes
Joined: Jan 2003
Posts: 961
Likes: 275
From: Frederick MD
May also be a coil. Spark might be cutting out/getting erratic if the windings in the coil are beginning to break down. Like others said kinda hard to diagnose online. Make sure it's also maintaining proper fuel pressure too and the tank sock isn't plugged up or something. These are both things that can happen from a long run condition.
Thread Starter
Pro
Joined: May 2013
Posts: 570
Likes: 22
May also be a coil. Spark might be cutting out/getting erratic if the windings in the coil are beginning to break down. Like others said kinda hard to diagnose online. Make sure it's also maintaining proper fuel pressure too and the tank sock isn't plugged up or something. These are both things that can happen from a long run condition.