need help with BBC - The 1947 - Present Chevrolet & GMC Truck Message Board Network

sinister72 · 06-11-2006, 10:32 PM

I have my fresh 402 in finally.Its a .030 over 402 10:1 hyper. pistons , voodoo cam(272),performer manifold, new Holley 3310 750 cfm carb,new dizzy,wires ,etc,etc. having problem getting it to idle?? Have checked for vacumn leaks ,seems ok, starts right away ,but have to keep the gas to it to get it to idle . throttle runs up without stumble ,but comes down then stumbles and wants to die. Have adjusted curb idle up and down and will run at about 1200 rpms. Timing is set at 12 degrees. any lower and it wants to backfire back through carb. Any ideas??

Oldtruckfanatic · 06-11-2006, 11:15 PM

Double check for a vacuum leak especially if there are ports in the carb for pcv or brake booster fittings etc. (I'm not familiar with your particular carb. Another thing that comes to mind is the firing order. I've been a mechanic for a long time and I know the chevy and ford firing orders by heart but I still cross two wires now and then. Hope this helps.

kelsfine69 · 06-11-2006, 11:24 PM

Id say make sure your dizzy is on TDC no 1. line up the balancer to 0 deg. then pull the cap to make sure your not out 1 tooth. it can happen so easy.

RSRXION · 06-12-2006, 01:43 AM

I'm betting on a vac leak. The vac advance diaphragm in the distributor is an often overlooked source . If it's ruptured or torn it will leak like crazy and you'll easily miss it on a quick check. Just a thought, Dean

70rs/ss · 06-12-2006, 02:38 PM

Start it up and keep it running while spaying carb cleaner around the carb, if it dies or the rpm's go up you have a leak. It sounds like a vac leak to me as well.

dwaite72lnghrn · 06-12-2006, 05:15 PM

On a fresh rebuild and new carb and dizzy. Alot of possibilities, Mine came back from a rebuilder and had a really rough idle, after carb idle adjustments(mixture) and timing set to 12* with no vacuum I went in and adjusted the valves that took what was left of the roughness out. With the 10-1 and the 272 cam it will be lopy at idle but a smooth lope. I was running stock 8.5-1 and a crane 272/284 cam idle was 750rpm to keep it from shaking to bad in drive, Neutral/park idle speed was about 850rpm.

jhow66 · 06-12-2006, 05:34 PM

You have a lot of cam there with the "272" (what is it at .050 ?). Run as much time as you starter will crank and run your vac. advance from manifold vacumn.

sinister72 · 06-12-2006, 09:10 PM

Hey guys thanks for all the replies. This weekend I'm going to check everything once again and see what comes up.

Nolowrider · 06-13-2006, 08:44 AM

Put a vacuum gage on it and see what you have. May need to swap out the power valve. Take you vacuum reading, cut it in half. That will tell you what PV to run. Do you have the vacuum advance hooked up? If so, disconnect it, plug it off and then check your timming/idle. It is better to set your air/fuel mixtures with a vacuum gage and not by ear. Plug check. You should have no problem getting 12-15 inches of vacuum. If the gage jumps around, you have a vacuum leak. Do you have a auto or standard tranny? If you have a auto, and you drop it into drive, what happens? Does it die in park and drive?

sinister72 · 06-13-2006, 03:03 PM

Hey steve, I have an automatic 700r4 , and i do have a vac gauge. When i do put it in drive it does die unless you give it some gas. I check the readings this weekend.

Nolowrider · 06-13-2006, 03:29 PM

Try disconnecting the vacuum advance, plugging it off and then set timing and idle. I don't run the vacuum advance at all. I just run a mechanical in the dizzy. Try going to about 16*, no vacuum advance. Do you have a stall?

sinister72 · 06-13-2006, 05:36 PM

not at present ,ordering a 2800-3000,,,,,next project

Nolowrider · 06-13-2006, 05:52 PM

That will help for when you drop her in to drive. Try disconnecting the vacuum advance and set her up and she how she runs. It could be trying to advance too much at idle.

06-11-2006, 10:32 PM	#1
sinister72 Registered User Join Date: Apr 2002 Location: Bowling Green, kentucky Posts: 182	need help with BBC I have my fresh 402 in finally.Its a .030 over 402 10:1 hyper. pistons , voodoo cam(272),performer manifold, new Holley 3310 750 cfm carb,new dizzy,wires ,etc,etc. having problem getting it to idle?? Have checked for vacumn leaks ,seems ok, starts right away ,but have to keep the gas to it to get it to idle . throttle runs up without stumble ,but comes down then stumbles and wants to die. Have adjusted curb idle up and down and will run at about 1200 rpms. Timing is set at 12 degrees. any lower and it wants to backfire back through carb. Any ideas?? __________________ 72 chevy C-10 swb fleetside,402 BB.700r4,buckets,console.etc. 72 Blazer CST 350,AT, new project 1971 Chevy Nova ,355 SB blow through turbo in works.

06-11-2006, 11:15 PM	#2
Oldtruckfanatic Truck junkie Join Date: May 2006 Location: Reno NV Posts: 697	Re: need help with BBC Double check for a vacuum leak especially if there are ports in the carb for pcv or brake booster fittings etc. (I'm not familiar with your particular carb. Another thing that comes to mind is the firing order. I've been a mechanic for a long time and I know the chevy and ford firing orders by heart but I still cross two wires now and then. Hope this helps. Last edited by Oldtruckfanatic; 06-11-2006 at 11:25 PM.

06-11-2006, 11:24 PM	#3
kelsfine69 18 Till I Die Join Date: Mar 2005 Location: Alberta, Canada Posts: 1,522	Re: need help with BBC Id say make sure your dizzy is on TDC no 1. line up the balancer to 0 deg. then pull the cap to make sure your not out 1 tooth. it can happen so easy. __________________ 2011 GMC Acadia 1969 GMC BBC LS7 2007 GMC Crew Duramax 1987 Camaro Iroc 350 Tuned Port Injection 42000Km 1987 Chevrolet Silverado 350 TBI 1987 Chevrolet Silverado 350 TBI(yes 2) Take me drunk, I'm too home...

06-12-2006, 01:43 AM	#4
RSRXION Senior Member Join Date: Jun 2006 Location: Arkansas Posts: 227	Re: need help with BBC I'm betting on a vac leak. The vac advance diaphragm in the distributor is an often overlooked source . If it's ruptured or torn it will leak like crazy and you'll easily miss it on a quick check. Just a thought, Dean

06-12-2006, 02:38 PM	#5
70rs/ss Registered User Join Date: Dec 2004 Location: Northern AZ Posts: 7,271	Re: need help with BBC Start it up and keep it running while spaying carb cleaner around the carb, if it dies or the rpm's go up you have a leak. It sounds like a vac leak to me as well.

06-12-2006, 05:15 PM	#6
dwaite72lnghrn Registered User Join Date: Jun 2002 Location: central pa Posts: 1,525	Re: need help with BBC On a fresh rebuild and new carb and dizzy. Alot of possibilities, Mine came back from a rebuilder and had a really rough idle, after carb idle adjustments(mixture) and timing set to 12* with no vacuum I went in and adjusted the valves that took what was left of the roughness out. With the 10-1 and the 272 cam it will be lopy at idle but a smooth lope. I was running stock 8.5-1 and a crane 272/284 cam idle was 750rpm to keep it from shaking to bad in drive, Neutral/park idle speed was about 850rpm. __________________ 72 longhorn c30 502BB/th400/fact air/4.11rear/ custom camper 72 bug(the better half's) under reconstruction 2009 HHR aqua blue 71 GMC k20 350/sm465 2009 Chevy hd3500 6.6 Victory Red crew cab dually Dave & Jeanne

06-12-2006, 05:34 PM	#7
jhow66 Registered User Join Date: Jun 2002 Location: Knoxville Tenn. Posts: 3,058	Re: need help with BBC You have a lot of cam there with the "272" (what is it at .050 ?). Run as much time as you starter will crank and run your vac. advance from manifold vacumn. __________________ 56 Chevy Bel-Air 2dr. HT (purchased new) 71 Chevy Cheyenne SWB PU (502HO) 65 GMC short bed step--work in progress and my gofer

06-12-2006, 09:10 PM	#8
sinister72 Registered User Join Date: Apr 2002 Location: Bowling Green, kentucky Posts: 182	Re: need help with BBC Hey guys thanks for all the replies. This weekend I'm going to check everything once again and see what comes up. __________________ 72 chevy C-10 swb fleetside,402 BB.700r4,buckets,console.etc. 72 Blazer CST 350,AT, new project 1971 Chevy Nova ,355 SB blow through turbo in works.

06-13-2006, 08:44 AM	#9
Nolowrider Senior Member Join Date: Jan 2005 Location: New Orleans Posts: 1,365	Re: need help with BBC Put a vacuum gage on it and see what you have. May need to swap out the power valve. Take you vacuum reading, cut it in half. That will tell you what PV to run. Do you have the vacuum advance hooked up? If so, disconnect it, plug it off and then check your timming/idle. It is better to set your air/fuel mixtures with a vacuum gage and not by ear. Plug check. You should have no problem getting 12-15 inches of vacuum. If the gage jumps around, you have a vacuum leak. Do you have a auto or standard tranny? If you have a auto, and you drop it into drive, what happens? Does it die in park and drive?

06-13-2006, 03:03 PM	#10
sinister72 Registered User Join Date: Apr 2002 Location: Bowling Green, kentucky Posts: 182	Re: need help with BBC Hey steve, I have an automatic 700r4 , and i do have a vac gauge. When i do put it in drive it does die unless you give it some gas. I check the readings this weekend. __________________ 72 chevy C-10 swb fleetside,402 BB.700r4,buckets,console.etc. 72 Blazer CST 350,AT, new project 1971 Chevy Nova ,355 SB blow through turbo in works.

06-13-2006, 03:29 PM	#11
Nolowrider Senior Member Join Date: Jan 2005 Location: New Orleans Posts: 1,365	Re: need help with BBC Try disconnecting the vacuum advance, plugging it off and then set timing and idle. I don't run the vacuum advance at all. I just run a mechanical in the dizzy. Try going to about 16*, no vacuum advance. Do you have a stall?

06-13-2006, 05:36 PM	#12
sinister72 Registered User Join Date: Apr 2002 Location: Bowling Green, kentucky Posts: 182	Re: need help with BBC not at present ,ordering a 2800-3000,,,,,next project __________________ 72 chevy C-10 swb fleetside,402 BB.700r4,buckets,console.etc. 72 Blazer CST 350,AT, new project 1971 Chevy Nova ,355 SB blow through turbo in works.

06-13-2006, 05:52 PM	#13
Nolowrider Senior Member Join Date: Jan 2005 Location: New Orleans Posts: 1,365	Re: need help with BBC That will help for when you drop her in to drive. Try disconnecting the vacuum advance and set her up and she how she runs. It could be trying to advance too much at idle.



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