OK, here is my combo is an much detail as possible. 498 w/ around 10.3/10.5 comp. 049 ovalports portmatched,bowl blended 2.19/1.88 valves roller rockers, a very non-impressive crane cam( shoulda stayed w/lunati) 296/306 238/248 @.50 and .553/571 lift 114LSA. I run a Holley High rise dual plane intake portmatched to the heads. has about 11 degrees initial. At idle the vacuum gauge jumps between 10-12in. First is this normal of the gauge to jump around like that?

Now for the real problem: Car idles rich, I have opened up the secondaries, drilled holes in the primaries changed the powervalves to 4.5 both front and back and still have no control over idle-mixture screws. The carb I believe is a 750(chokehorn milled off) has a secondary metering block w/ no idle-mixture screws, it's not machined for them. I changed the mainbody to my old 3310-2 750 still idled rich then took the secondary metering block off and put the block off plate back on like original and still idled rich. The only thing I can think of is that either BOTH the main bodies are warped or a vacuum leak, but I can't find one, tried by spraying carb cleaner around intake and baseplate no changed in engine idle. When I seat the idle-screws and lean it way out the idle picks up as if that helps it. If I have fuel leaking down somewhere, what would be causing it? No fuel is coming from the above the throttle plates. Another thing could the wrong baseplate gasket be causeing this problem? Thanks for your help in advance.

1BadSS,you stated you have no contorl over your idle mixture screws which very offten is a result of your intial timing being too retarded.

This will cause the need to crank up the idle speed/throttle plates past the idle circut in the carb thus rendering the idle mixutre srews/circut usless/inoperative.

Maybe this is whyyou think your crane came is not very impressive and your idle vacuum is a little jumpy,it looks to be a decent perf cam for your setup so check the intial timing. I have used crane cams in the past and they are decent performers and your grind is not that mild so i would think it should give your motor some punch.

Most perf cams need approx 16-18 deg btdc intial timing not counting the mech & vacuum advance so if it's not there crank it up.

It looks like you have the correct power valves installed and the carb fairly well squared away from what you said in your post so i hope it's just a retarded timing issue at this point.

Get back to us with how you make out.

Scott

When I first built the motor, we had the initial timing set at 18-19 degrees and it kicked back hard on the starter and would "run-on" pretty badly, since i've pushed it back it starts easier and shuts off now. I've never heard of a big block chevy with that much initial timing before, doesn't mean that's not the problem though?????? Thanks for the info Scott

Rassan

Rassan,what intial timing are you currently running ,i would opt for at least 14-16ish with that cam so try it there.

Maybe if your intial timing is advanced enough it will allow you to drop the idle speed enough to get the throttle plates back down to activate the idle circut. This will enable you to adjust the idle mixture scerws and get the idle & low speed tuning back under better control.

If your car runs better with the timing advanced but starts hard hot youv'e got to get the stock starting system componenets all working to 100% to eliminate this known problem for GM SBC & BBC motors. Or install an ignition cutoff switch to allow for better hot starting by turning off the ign cutoff,spin the motor over then turn the ign cutoff back on a soon as the motor spins over decently. Dont let it spin too long with no ign becasue it could load up and backfire when you hit the ign power on.
If you motor does not spin over any better with the igniton power shut off then that shows you have got a really tired starter,battery or alt along with any number of the items i listed below as possible starting componenet issues.

The battery must be fresh and at least 800+CCA or more,fresh 60 amp + alt,properly functioning volt reg,fresh high torque starter,H/D high temp solenoid with H/D spring,starter heat shield,good clean gorunds between the body,motor,frame,and good clean battery cables. If any 1 or more of these componenets are not operating @ 100% good reliable hot starts on high comp high performance chevywill not be your friend.

Scott

1badSS, I agree with Scott that this could really be a bad combination of a carb misadjusted to compensate for too low initial timing. I have spent the better part of 2 weeks going through similar issues, trying to get a consistent idle in gear. I bumped it up from 10 initial to around 18 initial and readjusted the carb (turned idle way down, adj mixture screws) and it made a world of difference... the car now idles @ 950 rpm out of gear, 900 in gear at 10 inches vacuum.

Here is some recent reference material:
http://www.chevelles.com/forum/ultimatebb.php?ubb=get_topic;f=4;t=017484

Good luck,

Chris

1bad70SS
I to have a healthy cam combo I had to go to manafold vac. so I could set my timming at 12 then the vacuum advance would pull it to 16 I have posted several articals try them if no help email me.We need to check to see what vacuum advance you have if like mine at idle it was floating not stable so we might start there.


19_chevelle_71 Thanks for passing on the info
I think it's great
:D

First off, thank you guys for all of the info, I am gonna try to bump up my timing to aroung 16*-18* initial. What do you guys think about locking out the vacuum advance? If i set it at 16* then I could put the advance stop bushing in to only put in 18* so the total would be 34* another thing I was thinking if I have to open the throttle plates so far could that be causing the engine to run-on? Sat I'm gonna recurve the distributor and play with the timing. Thanks guys!!! graemlins/thumbsup.gif