[Bruce Edwards]

No you do not have to do that. What you have found is that the system is lean at coast. Since it runs ok under power and idles smoothly you can not be that far off on the jet size.
All you need to do is pull the front bowl and check the jet size. It will show a number say like 63. Go to your local speed shop and get a new front gasket and try to up the size by 5 so you would look for a 68.
When you pull the front bowl take a bottom bolt out first. Place a cup or a BIG rag under it as a lot of fuel will leak out. Make sure you clean it up before you try to start the car back up.
The whole thing should take less than 15 min start to finish.
Test drive the car again and see if it is better. Should be. If it now loads up, runs rich, get a small jet.
Since you say it runs well when you floor it I would not mess with the power valve setting or the rear jet size.
The whole thing should cost you less than $20 and a little of your time. Being a Cobra owner is all about working on your car! Just be sure to get the lever for the accel pump under the arm when you install the front bowl.
You can do it!! If you get in to trouble I can bail you out from here.