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Old 02-09-2006, 10:44 PM
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400 small block idling problems

i just recently rebuilt the top end of my 400 small block chevy. after getting it running and breaking in the cam i started to test drive it and it seemed fine. the idle would drop down enough to sit at a stop. but when i decided to drive it home to my house the car would not idle and i would have to ride the brakes and keep the rpms tached up to keep it running. i have been trying to tune the carb but i have had no luck. i can get the car running but have to keep it way up off idle to run. the setup is rebuilt iron heads-1975 305 60cc castings with port work done around the valves to supposedly equal 64cc's. 2.02/1.60 valves, screw in studs, guideplates, 1.25 inch valve springs. i drilled the steam holes in the heads myself. the compression ratio should be right around 9.5:1 maybe a little less. i realize that these heads have very small intake runners as compared to the 76cc heads that i took off the motor. the cam and lifters i used is a comp extreme energy 268H. i used a summit noisy gear drive in it and an edelbrock performer dual plane intake with a Holley 600 vacuum secondary carburetor. i used fel-pro stainless steel shim head gaskets (forget the exact thickness-somewhere above 30 thousandths). the ignition is a pertronix ignitor II module with flamethrower II coil. the oil pump i put in is a melling m155A which is the higher pressure pump. i used moroso wires and autolite spark plugs. i broke the car in with a mix of 87 and 93 octane gas. I thought this could be a possible problem that i might not have a high enough octane number for the motor. but theres no detonation or pinging as far as i can hear (i could definitely be wrong here because ive only run the motor with open exhaust from headers so far). at first i had a bad ground problem which killed my battery every time we ran it. i fixed this problem and have a fresh die hard battery also with plenty of cranking amps. another thing i thought could be a problem was the carburetor size being to small but since this motor probably makes 80 something percent volumetric efficiency at best, this would seem to be the ideal carburetor (i did the math and this motor would draw right around 600 cfm at 6000 rpm at 80% vol. eff. this cam is rated up to 5500 rpm and 400s are not good high winding motors anyway so this motor will probably rarely see those rpms). to make the car run i turn the idle speed screw almost all the way up to warm it up (so far ive only ran it on cold days and the carb has no choke). the car had a water restrictor in it for the break in but with straight water i could not get the temp above 140 degrees. with anti freeze the car got up to 170. i just put a 180 degree thermostat in and the car now hovers around 200 degrees. i have a weian aluminum water pump which seems to move a lot of water and a aluminum rigid 6 blade fan which draws a lot of air.watch the venturis and i can never get the idle slow enough to stop the fan from coming off the venturis. the fan off the venturis isn't quite smooth either and gas drips from them. i started with a 69 jet in front and 72s in the back (i have a secondary metering block kit). since then i have tried going to a 66 jet in the front as well as going from a 6.5 power valve to a 4.5. i don't think that cam is choppy enough to have a low enough vacuum at idle to be dumping in fuel at idle but i have not got to put a vacuum gage on it. i adjusted float levels and found no results. the only thing i overlooked was the idle mixture screws. these were set while the metering block was in a 450 cfm carburetor i had built so i didn't think they would be out of adjustment because the 450 ran PERFECT on the 305 i had. i started to think that maybe having these set for a carb with a much smaller cfm might be starving this motor at idle. it appears to me though that the motor is loading up at idle. ive had two bad backfires which resulted in stalling and flames on top of the carb but i checked the power valve and it's fine. i can drive the car around but cannot come to a stop because i have to ride the brakes. the car has a 2300 stall speed converter yet still is engaged while keeping the engine at the rpm i need to keep it running. im very confused on what is going on with this motor. another thought i had was that the needle and seats are staying open but i cant see why this would happen. they are the small viton tipped needles and seats but they're fairly new and ive checked them and they are clean. there seems to be way too much fuel getting into the motor but when i checked the plugs they are getting very clean indicating a leaner mixture. they blackened a little when we broke the car in but now all 8 are very clean and some look perfectly discolored. the car seemed rich but the plugs are throwing me off. is there a chance im starving the motor at idle causing it to pop? also the timing is set at 8 degrees initial advance with 12 under vacuum advance. the mechanical advance springs are a little quicker than stock but im not sure of total. i think the total is something around 36 degrees with the springs i have in it? im pretty stumped but i am going to tune tomorrow and im going to start by taking the float bowls off for inspection and adjusting the idle mixture screws. another thought i had was a possible vacuum leak. when i put the car into gear the idle drops way down all of a sudden. also when i stab the throttle and let off the engine stays revved up for a second before coming back down. i know this post is very long but anyone who has bothered to read the whole thing and give me their thoughts would be greatly appreciated.

thanks,
Ian
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