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I'm trying to chase down a tuning problem I have with my 355. This is my first build and I have very little knowledge when it comes to carbs. My car is running super rich and I've tried adjusting the mixture screws but its doesn't seem to change. After talking to a few different people a common answer is i need to change my power valve. Will a stock power valve with that cam cause my running rich. I'm pulling about 10 to 12 lbs of vacuum. So I'm thinking a 5.5 or 6.5 valve. I know alot of people run that cam so I was hoping I could find out what other people are running. thanks.
 

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Popwer valve

I dont think your power valve is making you run rich i had that cam in my 357 sml blk it took a 3.5 power valve you have to go half of the the reading so a 6.5 or 7 would b a 3 or 3.5.try jeting down some and when you adjust mixture make sure trottle blades are not open much, general rule for mixture screw all the way in till engine almost cuts off then out 1 and a half to two turns outward till you get your best idle when you check vac have car in gear. I dont think its your power valve, a power valve is designed to prevent a lean condition, the cam you have needs a lower power valve try this if it dont help pm me and we can go from there, ive rebuilt and sold lots of holleys on craigs list and no complaints good luck...
 

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I don't think this is a power valve issue either. You mentioned that turning the mixture screws isn't making any difference in how the engine is running, and this should not be the case. You should be able to get the engine to stall out by turning those screws in all the way.

It seems likely that your throttle plates are open too far and exposing too much transfer slot, thus dumping way more fuel at idle than is needed. This relationship between the throttle plate and transfer slot needs to be correct before you do any further tuning. Make sure that no more than .020-.040" of the slot is exposed (it should look like a tiny square, you can find pictures of this if you do a search on Google). Once this is correct, you will be able to regain control of your idle mixture with the mixture screws, and you can begin to tune accurately.

As for the power valve, most Holleys come stock with a 6.5. Your current vacuum readings will probably change some once you get the T-slot/idle mixture situation resolved, so I would deal with that first before using those readings to help select a power valve.

PS. Selecting a power valve number that is "half of idle vacuum" has not worked well for me at all with street cams that have 12"+ of vacuum. I do not use this method. Run a vacuum gauge into the car (doesn't have to be permanent) and drive around, see what the vacuum reading is when you feel a flat spot during acceleration of part/half throttle. Then select a power valve that opens about .5 to 1" above that reading.
 

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drill a 3/32 hole in each throttle plate , toward the outer part of the plate , then reset your idle , new carbs have this adjustment put in , the ultra HP's ,,, it is the easiest way to get rid of this ,,, any other way , is going to pull fuel out of the idle circuit , so then your going to have a lean spot in the throttle

try it with a 3/32 hole , then go to 1/8 in the front 2 , try that , then do the back 2 1/8 , see if its better

the timing of course needs to be right ,, what is your initial timing and total ?

take your carb off , and locate the transfer slot ,,, remove the carb without disturbing the idle setting , so you can see how much is exposed at idle ,, like was posted, .020 - .040 is all you want ,,, i like to have the carb to idle off the holes drilled , so then you can get the most out of the transfers when they open,,, but thats me ,

open the throttle plates , you'll see a vertical slot above the area on the outside of the barrels in the base plate , that is the transfer slot ,, its most likely pulling too much fuel from there so the mixture screws have no effect , its not pulling fuel from them ,

before you crank up the drill ,, make sure there is no vacume leaks , and the timing is right ,,

where do you have the valves set ? if you have a bunch of preload in them , it will make for a lazy idle, and it will need more throttle to idle, and cause this problem too
 
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