Missing/Rough under load?
Thread Starter
Drifting
Joined: Mar 1999
Posts: 1,381
Likes: 15
From: Lusby MD
Missing/Rough under load?
I've been battling this issue on my 70 for a while. Car runs smooth as a sewing machine at cruise and light throttle, but as soon as I step into in, even lightly, it breaks up and runs like crap. I recently rebuilt the quadrajet, replaced the points, cap, and rotor, and checked the timing. Car runs fantastic.... until you step into it. Smoother and quieter at cruise but worse under load. Tried a full throttle blast in 3rd gear and the car would barely accelerate and even backfired.
Any idea where I should look next?
Any idea where I should look next?
Safety Car
Joined: Mar 2007
Posts: 3,652
Likes: 49
From: Bridgewater nj
Have you change or checked the plugs? This could be anything from a fouled plug to a wiped cam but start with the basics. If you have headers, you can power brake the car while spraying a little water on each header pipe. That will at least tell you which is the offending cylinder or cylinders. Then you can jokey some plugs and wires around to see if you can duplicate it on another cylinder.
Safety Car
Joined: Mar 2007
Posts: 3,652
Likes: 49
From: Bridgewater nj
I did have a 74 nova that had a bad distributor. As soon as I got past 2000 rpm it would sputter, Buck, pop, fart until I let off. If you pop the cap, grab the rotor and see if it moves side to side. Up and down a little is normal but it shouldn't move at all side to side.
Thread Starter
Drifting
Joined: Mar 1999
Posts: 1,381
Likes: 15
From: Lusby MD
New Plugs. Haven't changed the wires yet because they're in the factory looms and that's a big job. Have new ones so that's next. Checked the distributor shaft when I changed the points and rotor. Nothing crazy in terms of play. Seen way worse.
It's not RPM related. Low speed crack of the throttle is bad. Can roll to high RPM in lower gears at low load and it's fine.
It's not RPM related. Low speed crack of the throttle is bad. Can roll to high RPM in lower gears at low load and it's fine.
Last edited by 69L71; Jul 26, 2015 at 11:27 PM.
Thread Starter
Drifting
Joined: Mar 1999
Posts: 1,381
Likes: 15
From: Lusby MD
Did this before and after the carb rebuild (and yes, new filter was installed).
Last edited by 69L71; Jul 27, 2015 at 10:09 AM.
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Instructor
Joined: Nov 2014
Posts: 161
Likes: 17
From: Georgia
Glad you found the problem. I worked on a 70 Pontiac years ago that would run eratically at times . Finally found that the arm on the vacuum advance was partially broken where it made the 90 degree turn up through the breaker plate. Would allow the points to float around at will.