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I'm working on a 66 GTO with tri-power, fresh rebuilt 400, 9.5 to 1 comp, round 490 lift roller cam. My first problem is that is runs on. I've tried multiple carbs, changed timing, colder plugs. And all with block off plates on the secondaries. Any advice?
 

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Do a search on this website on tuning a 400 with tri-power. It will tell you what carburetors that you are suppose to use. The center carburetor has the idle circuit and choke. The other two do not have an idle circuit and choke. The outer 2 carbs are on a progressive linkage so they are only used during 3/4 to wide open throttle.
 

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Yep, need to know what heads and pistons you are running to get 9.5:1. The differences in heads that will fit a 389/400 can make it ping or run on easily. Can you tell us what the number is cast into the center exhaust ports, if there is one there? Need to start somewhere, might as well be with the combination you have.
 

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Run on is caused by too much fuel in the chamber after shut down.

Big cams require lots of timing. I would use at least 20 degrees at idle.

In most cases this is what happens: Low timing brings down the idle speed, so the idle speed screw is used to increase the rpms, this will put the throttle blades past the idle circuit and into the main circuit which causes a very rich idle. Making it run on after the engine is shut down.

Advancing the timing and lowering the idle speed screw will cure run on.

If the carbs aren't letting enough air in at idle speed to keep the throttle blades in the idle circuit then you will need to drill a few small holes in the throttle blades or crack the front and rear carb open a bit.
 
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