# 68 GTO 400 valve causing rough idle?



## musclecar65 (Mar 17, 2017)

Have narrowed down the rough idle to this: Idles smooth when cold but as it warms up a metallic clacking sound develops in the left valve cover and as it does, the idle gets uneven, like a plug going out. The miss in idle corresponds to the clacking noise. It is growing louder over time and is like something hitting the valve cover although I can't feel it. Ideas???


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## Sick467 (Oct 29, 2019)

Rocker arm may have come loose.


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## musclecar65 (Mar 17, 2017)

Sick467 said:


> Rocker arm may have come loose.


Thanks! 
Can I adjust it with the motor running?


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## musclecar65 (Mar 17, 2017)

musclecar65 said:


> Thanks!
> Can I adjust it with the motor running?


They all feel tight by hand with 4 just loose enough to move back and forth


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## RMTZ67 (Mar 12, 2011)

musclecar65 said:


> They all feel tight by hand with 4 just loose enough to move back and forth


Do you have a pic of what you have? Original just get torqued down and if it has poly locks, they need to be adjusted idling and warm. Is the nut deeper in the rocker or higher up on top?


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## Sick467 (Oct 29, 2019)

Some rockers will feel tighter than others, with the engine not running, due to the orientation of the cam and the valve spring pressure. You can adjust them with it running...back them off, one at a time, until that rocker starts to clatter, then tighten them down until the clatter stops and, down and dirty...give them another 1/4 to 3/4 turn. Given a rather stock motor, you should be good enough. I've done stock rocker nuts and polylocks this way with good results. Old stock nuts may be worn out and the clatter could come back if they are backing off.


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## Jared (Apr 19, 2013)

Need more info here. If the engine is stock, you probably torque all to spec. If it's a later stock build, or has had some work done, need to adjust as said above. Could be other issues as well. If the engine is a very old build, or has never been rebuilt, could be losing compression on one cylinder or have a cam lobe (or more) that's going away. If nothing else, pull the valve covers and look at what's going on in there.


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## ponchonlefty (8 mo ago)

this is just a suggestion but could it be a valve trying to stick? have the heads been worked on?could be a valve guide too tight.


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## musclecar65 (Mar 17, 2017)

Ran it up to operating temp with cardboard surround and only got the usual low clatter, sounded normal, and all were oiling. Backed off the idle mix to increase vac to 17 and bypassed the thermostatic switch. Drove it 25 miles in traffic with good idle and no clatter noise or breakdown. I am sure now that the new TVS was kicking in and increasing the vac on warm up so timing was way off. The engine is stock, 94,000 miles and has never been opened. All I can do is scratch my head... Many thanks for all of the suggestions!


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