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19nova72 02-09-2006 10:44 PM

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 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.


57 Chevy 02-09-2006 11:09 PM

That almost sounds like a vacuum leak. Do you have a vacuum gauge? Some guys spray carb cleaner or WD-40 around the base plate and other areas to see if there is a change in RPM.

19nova72 02-09-2006 11:11 PM

yes ive used that method before, i will probably try that tomorrow. i do have a vacuum gage but have not got to use it on this motor yet. but thanks for helping to steer me towards the problem.

GoneNova/406 02-10-2006 12:56 AM

yes,i also think it sounds like a vac. leak.sounds like you're going to have a good light to light motor with those heads.i'd be curious to here about it when you get the problem solved.did this carb work before on the old motor?one thing you never ever want to do is start a new motor with a defective carb. but i know how yuo feel at this point.we've all been there.try spraying the wd-40 and see what happens and come back if need be.good luck

68ss 02-10-2006 07:12 AM

just a thought do you have power brakes?

StevenC 02-10-2006 07:48 AM

why is the timing initial set so low?

19nova72 02-10-2006 08:59 AM

the car has manual brakes so i know that isnt the problem. but what would be a better initial timing to start with?

19nova72 02-10-2006 09:06 AM

also i did make the mistake of totally rebuilding a carb to start this motor just cause the carb is fresh and i was confident with my carb tuning skills. the only reason i tore apart the 450 was because i thought it would be too small but it prob would have been fine for break in and tuning. the 600 is set back to just about stock and it is clean and nothing leaks. i will try the carb cleaner/wd 40 method and wil play with idle mixture screws. its definitely possible that its a carb base gasket leak. my friend says that he has 20-25 initial timing in his 357 with the same cam and he says the motor loves it. i thought this was high but i guess i could be wrong. any other suggestions?

StevenC 02-10-2006 03:04 PM

Try this.

Unplug your vac advance and plug it at the carb.
Set your timing to 12 degress advance
Hookup your vac advance to the "MANIFOLD" port on the carb.

Reset your idle and mixture screws

454C10 02-10-2006 03:17 PM

If you have the idle speed screw in that far then the carb is idling the main circuit (blades open past the transition slots). You either need to get a bigger carb (bigger throttle blades) or you need to drill some holes in the front throttle blades to let air in without opening up the throttle blades past the transition slots. A 0.100" hole in each front blade should do it.

Also a 268 cam should have about 15 to 20 degree initial ignition timing.

Try using paragraphs next time. Your post is very hard to read.

StevenC 02-11-2006 08:56 AM


If his timing is too low, then the required throttle opening would be affected no? meaning if he bumps up his timing his carb is fine

19nova72 02-11-2006 05:47 PM

the vacuum advance is hooked up to manifold vacuum and the other port on the metering block is plugged. as for the timing i will prob try bumping it ahead to 15-20 as you guys recommend. i thought about modifying the carb or getting a bigger one because i thought the same thing that the carb is idling on the main metering circuit and i need to let more air by.
i dont know the affect changing the timing will have though. i only have the idle speed screw up this high because it needs it to run because the motor will not idle which still leads me to believe i have a vacuum leak. i still havent got any time to start up the car and run it again though so i havent gotten any further. by drilling the throttle blades i will be able to keep the idle speed screw turned in farther but not draw as much fuel correct?

i will get back to you guys as soon as i get to run the car again.
thanks for all the help so far :thumbup:

57 Chevy 02-11-2006 06:23 PM

Don't drill anything unless your real sure you know what your doing. :nono:
This is an area for pro-tuners or at least their guidance.

StevenC 02-11-2006 06:41 PM

I wouldnt change carb or drill yet. You shouldnt have these problems with your size carb

hotrodf1 04-13-2006 06:48 AM

You mention that your vac. adv. is already connected to the manifold port, which is good. Do you know the specs on the can you are using? In order to make the most of the manifold vacuum setup, I had to use Crane Cams' adjustable vacuum advance kit (which also includes a vac. adv. limiting plate that is VERY useful).

The goal here is to get the vac. can set to peg out below the least amount of vacuum (which is at idle in gear of course). I set my 406 to 18 initial and with the vac. advance it idles with about 33 degrees total. The motor runs great like that (and the throttle blades are almost entirely shut). My 355 was the same - Crane Cams adjustable kit, 15 initial, for a total of 32 at idle.

The adjusting plate that comes with the kit is nice because you can limit the amount of vac. adv. that you get, and then you use the screw in the vac. port to adjust the "Hg that allow it to open.

All this will help your fuel economy too. I've found a lot better mileage after setting the vac. adv. up well. Although both motors still run a little rich at idle- but never load up or foul plugs.

Another thing: I drilled holes in the 670 HOlley for the 355 - it actually didn't seem to do that much for me. I had better luck getting the idle correct with the vac. adv., but the holes don't seem to have hurt me. The 406 is much hotter, only has a 750 holley, and I cracked the secondaries just a little. Have the primaries almost totally shut here, too, made possible by vac. adv.

Good luck!

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