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I have just installed a 6-71 blower with 6 pounds of boost on a 350 chevy steel crank, 2.02 heads dish pistons for 8 to 1 compression and a blower cam with 2 Holley 650 carbs with .058 primary and .068 secondary on both and cannot get it from running to rich and fouling the plugs. Do you have any ideas? The power valves are 65 also.
Colorado Springs, CO
The first thing I would ask is, are you sure the power valves aren't bad? It is possible that one or more is ruptured if you ever had a backfire through the carb(s) for any reason. Also, what heat range of spark plugs do you use? Is it possible that you have a "cold" plug installed? If so, try using 1 heat range hotter and see if it helps. Have you actually checked to see how much vacuum the engine has at your idle speed? And lastly, the main jets (.058 and .068) don't have anything to do with idle mixture unless the throttle blades are opened enough to open the transfer slot in the baseplate. Have you tried simply adjusting the idle mixture screws on the sides of the metering blocks? As a baseline, try gently bottoming the screws and then back them out 1 1/2 turns. See how that works and if possible have a vacuum gauge connected to the engine while adjusting the screws. Adjust for highest vacuum. Hope this helps. Let me know what you find.
I appreciate your help. I did use a vacuum gauge on it and the best I could get was 6lbs of vacuum so that is where it is at now. I have tried both AP 24 and AP 26 Autolite plugs and found that it does respond to the AP 26 the best, but maybe I'll try and go up to AP 28 and see what happens. I checked the power valves on both carbs, but I am installing new one's just incase today. Do you know what the opening should be on the throttle plates because they appear to have a bit more opening than another old 750 I have laying around. Lastly these carbs are not boost referenced but I have the instructions on how to do it, so I thought I would do that also.
Again Thanks for the advice!
Ahah! If you are seeing 6 in. of vacuum at idle and have 6.5 power valves they are, and should, probably be opening which would create your rich condition and then some. Power valves open at anything below what they are rated for. Try using 5.5 or smaller valves and see what happens.
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