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Rough running at highway speeds

 
Old 05-12-2019, 07:56 PM
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Bhebert449
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Default 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. Iíve 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.
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Old 05-12-2019, 08:56 PM
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Distributor cap?
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Old 05-12-2019, 09:02 PM
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Originally Posted by mortgageguy View Post
Distributor cap?
How do I know if itís bad? Problem only presents at highway speeds. Seems to run fine in city driving.
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Old 05-13-2019, 12:06 AM
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Vacuum leak.
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Old 05-13-2019, 06:47 AM
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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.
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Old 05-13-2019, 08:21 AM
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Wondering why it would only happen when cruising at 3k rpms? Something in the distributor?
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Old 05-13-2019, 01:21 PM
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Originally Posted by Bhebert449 View Post
Wondering why it would only happen when cruising at 3k rpms? Something in the distributor?
vacuum leak causing timing to be erratic?
Just a guess
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Old 05-13-2019, 01:48 PM
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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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Old 05-13-2019, 06:34 PM
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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; 05-13-2019 at 06:35 PM. Reason: spelling
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Old 05-13-2019, 07:25 PM
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Remove the dist. Cap. Look for burned or worn contacts. Also carbon tracking. Anything that doesn't look normal. Check the tip of the rotor while in there.
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Old 05-13-2019, 11:02 PM
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So most likely a vacuum leak or something in the ignition system?
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Old 05-14-2019, 06:47 PM
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Thats the general consensus so far. Its hard to be a mechanic long distance.
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Old 05-14-2019, 09:03 PM
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Wouldn't a vacuum leak affect all rpm ranges and cold or hot engine?
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Old 05-14-2019, 09:58 PM
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Originally Posted by mortgageguy View Post
Wouldn't a vacuum leak affect all rpm ranges and cold or hot engine?
Only happens when engine is warmed up and when cruising at highway speeds. 3000-3500 rpms.
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Old 05-14-2019, 09:59 PM
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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.
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Old 05-14-2019, 10:19 PM
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Originally Posted by kossuth View Post
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.
I will replace coil next. How do I monitor fuel pressure while driving?
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