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Discussion Starter #1
running 1 step hotter plug
have small 100 RPM miss "ONLY" between 1800 and 2100 RPM

pretty much ruled everything else out.

can a hotter plug cause this
 

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no the heat range of a plug wont cause a miss. at least i have never known them to, and i have used a wide variety of plugs over the years in street and race engines. what may be happening is that you have a slight lean condition off idle, and that is causing the miss.
 

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Discussion Starter #3
its on cruise.
off idle is fine. high RPM is fine. just in that band. does not do it in neutral
only other thing I can think of is the throttle shaft or bushing is worn in the spot and its sucking air

issue is that its such a PITA to change plugs on this car( thank you to Mr Shock towers) i was hoping to rule it out first
 

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Maybe you are a bit to lean at cruise? I know when my car is lean I can feel a miss at cruise rpms.
 

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its on cruise.
off idle is fine. high RPM is fine. just in that band. does not do it in neutral
only other thing I can think of is the throttle shaft or bushing is worn in the spot and its sucking air

issue is that its such a PITA to change plugs on this car( thank you to Mr Shock towers) i was hoping to rule it out first
1800 rpm is still off idle as the throttle plate is still hovering around the transition holes. it is possible that the carb shaft bushing are worn, or you dont have enough fuel misture moving through the transition holes/slot. try back the idle mixture needle out about a 1/8 to 1/4 turn to get a bit more fuel out.
 

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^ Yup. EXACTLY what he said. That'll tell you if the miss is mixture related.
 

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Discussion Starter #7
I have the mixtures out 2 1/2 now to obtain smoothest idle and highest vacuum
not sure i want to go out any further on them
 

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You may want to check distributor phasing... Are you using vacuum advance and from what vacuum source?
 

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I have the mixtures out 2 1/2 now to obtain smoothest idle and highest vacuum
not sure i want to go out any further on them
Well, you can rule out a surge or lean miss ;)
 

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Discussion Starter #10
using ported vacuum to single canister
vacuum on idle 20 and steady
in drive 16 and steady

original factory cam 122K miles
 

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I have the mixtures out 2 1/2 now to obtain smoothest idle and highest vacuum
not sure i want to go out any further on them
I nave noticed numerous times on several different cars that if I adjust the mixture screws for the highest vacuum I will have a slight stumble. After adjusting them for the highest vacuum I always turn them out 1/4 turn.
 

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Sounds like a lean issue, needing 2 1/2 turn out on your idle mixture might indicate you have jetting issues that are not within the realm of a mixture screw. Also the rpm range you are dealing with is likely a transition area of the carburetor.
Also you should probably be using manifold vacuum rather than ported.
 
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