vacuum leak? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

short69 · 03-19-2007, 06:05 PM

ok, so I put in a crate 350 last october with an Edelbrock intake/carb and the only way I can get it to idle well is if I richen it up, otherwise it idles like crap. Its like the more rich it is , the smother and lower the idle can go , but if I try to lean it up, it has to idle at like 1k just to stop shaking. any ideas?

dwaite72lnghrn · 03-19-2007, 08:17 PM

What is your vacuum gauge doing? Have you checked the plug in the bottom back and the manifold port for loose connections. Have you had any backfires lately that might have knocked off a hose somewhere.

GREASEMONKEY72 · 03-19-2007, 08:22 PM

also check the timming to, and spray some carb cleaner around the base of the carb and if the idle changes you have a leak there if not keep checking around, pcv if you have it, power brake booster also and just a thought but make sure youre not leaning it out to much

short69 · 03-19-2007, 08:25 PM

well, i dont have a vacuum guage and i think all the hoses are secure but i can double check. I noticed that the intake gaskets looked a little greasy and was thinking that it was an indicator that the leak might be there.

JHART · 03-19-2007, 08:54 PM

I'm no expert but how many miles do you have on this motor? Also is it a mechanical choke or a electric?

I assume you have a aftermarket cam? I'm wondering about the cam when you mentioned the shaking.

fastwillie 696969 · 03-19-2007, 09:34 PM

Quote:

Originally Posted by GREASEMONKEY72

also check the timming to, and spray some carb cleaner around the base of the carb and if the idle changes you have a leak there if not keep checking around, pcv if you have it, power brake booster also and just a thought but make sure youre not leaning it out to much

NEVER use carb cleaner to check for leaks use propane if the c/c get to the distr. and spark hits the fumes
kaboom car on fire !!!!!

short69 · 03-19-2007, 10:03 PM

yea Ive heard of people using carb cleaner but I've always been kinda leary about using it because of the afore mentioned "ka-boom". BTW this is a brand new GM crate 350/ 260hp, its got the stock cam. ALSO how would I use propane? The only kind I have is hooked up to my grill.

wheels67 · 03-19-2007, 11:06 PM

was the carb recommended for the motor? Sometimes guys get to many cfm's and have to run rich......600 cfm is plenty for a 260hp motor.

Do not go thru life without a vacumn gage....go pick one up at parts store, itll save you years of wrenching time.

fastwillie 696969 · 03-19-2007, 11:23 PM

Quote:

Originally Posted by short69

yea Ive heard of people using carb cleaner but I've always been kinda leary about using it because of the afore mentioned "ka-boom". BTW this is a brand new GM crate 350/ 260hp, its got the stock cam. ALSO how would I use propane? The only kind I have is hooked up to my grill.

get a bottle of propane for camping and get a rubber hose on it then find your leak

short69 · 03-20-2007, 01:04 AM

thanx for the help everyone. I was just out listening to the engine and I can hear it hiss, and when I have the hood up I can hear the hissing sound gradually fade away when I turn off the engine, is that normal?. It sounds like its coming from the drivers side but all I could see is the booster hose and I'm pretty sure it's not leaking.

Farmstore34 · 03-20-2007, 01:16 AM

Short69, its real common to have a leak in that little plastic elbow that plugs into the booster, or in the grommet around it. Seen it many times.
CLT

mnunn454 · 03-20-2007, 06:24 PM

Quote:

Originally Posted by short69

thanx for the help everyone. I was just out listening to the engine and I can hear it hiss, and when I have the hood up I can hear the hissing sound gradually fade away when I turn off the engine, is that normal?. It sounds like its coming from the drivers side but all I could see is the booster hose and I'm pretty sure it's not leaking.

Sure sounds like a vacuum leak to me.

dwaite72lnghrn · 03-20-2007, 06:38 PM

Carfull with the plastic connector it is a check valve that is suposed to keep vacuum in the booster after the engine stops. When it is finished hissing push the brake pedal and if it is that the brake pedal will travel before you feel pressure. But it looks like you found your problem just narrow it down to the booster, check valve, rubber line or the carb connection. but it really is heading to checkvalve or booster or the grommet that goes into the booster.

Huck · 03-20-2007, 08:51 PM

Take the hose off the booster and plug it---that will tell right away if there is a vacuum leak there. Your engine should now run smooth--you can do the same process with each vac line till you find the leak.

jhow66 · 03-20-2007, 10:41 PM

Did you use RTV on your intake gaskets around the ports? If you did-rut-o. RTV and gas don't mix.

03-19-2007, 06:05 PM	#1
short69 Registered User Join Date: Oct 2006 Location: las vegas Posts: 88	vacuum leak? ok, so I put in a crate 350 last october with an Edelbrock intake/carb and the only way I can get it to idle well is if I richen it up, otherwise it idles like crap. Its like the more rich it is , the smother and lower the idle can go , but if I try to lean it up, it has to idle at like 1k just to stop shaking. any ideas? __________________ 69 shorty w/ 67 front clip

03-19-2007, 08:17 PM	#2
dwaite72lnghrn Registered User Join Date: Jun 2002 Location: central pa Posts: 1,525	Re: vacuum leak? What is your vacuum gauge doing? Have you checked the plug in the bottom back and the manifold port for loose connections. Have you had any backfires lately that might have knocked off a hose somewhere. __________________ 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

03-19-2007, 08:22 PM	#3
GREASEMONKEY72 Registered User Join Date: Aug 2006 Location: Bakersfield,CA Posts: 7,893	Re: vacuum leak? also check the timming to, and spray some carb cleaner around the base of the carb and if the idle changes you have a leak there if not keep checking around, pcv if you have it, power brake booster also and just a thought but make sure youre not leaning it out to much __________________ -steven Alte Seele S.S. "Allison" '72 C/10 LWB 350SB/TH350 - DD PITBULL IS NOT A CRIME

03-19-2007, 08:25 PM	#4
short69 Registered User Join Date: Oct 2006 Location: las vegas Posts: 88	Re: vacuum leak? well, i dont have a vacuum guage and i think all the hoses are secure but i can double check. I noticed that the intake gaskets looked a little greasy and was thinking that it was an indicator that the leak might be there. __________________ 69 shorty w/ 67 front clip

03-19-2007, 08:54 PM	#5
JHART 1972 Shorty Owner Join Date: Feb 2007 Location: Stationed in Cali , born and raised in PHX Arizona. Posts: 796	Re: vacuum leak? I'm no expert but how many miles do you have on this motor? Also is it a mechanical choke or a electric? I assume you have a aftermarket cam? I'm wondering about the cam when you mentioned the shaking. __________________ 1972 Chevy Shorty C-10- Project In Work http://67-72chevytrucks.com/gallery/...ATT1721941.jpg- New Project

03-19-2007, 10:03 PM	#7
short69 Registered User Join Date: Oct 2006 Location: las vegas Posts: 88	Re: vacuum leak? yea Ive heard of people using carb cleaner but I've always been kinda leary about using it because of the afore mentioned "ka-boom". BTW this is a brand new GM crate 350/ 260hp, its got the stock cam. ALSO how would I use propane? The only kind I have is hooked up to my grill. __________________ 69 shorty w/ 67 front clip

03-19-2007, 11:06 PM	#8
wheels67 Registered User Join Date: Dec 2006 Location: southern Utah Posts: 103	Re: vacuum leak? was the carb recommended for the motor? Sometimes guys get to many cfm's and have to run rich......600 cfm is plenty for a 260hp motor. Do not go thru life without a vacumn gage....go pick one up at parts store, itll save you years of wrenching time.

03-20-2007, 01:04 AM	#10
short69 Registered User Join Date: Oct 2006 Location: las vegas Posts: 88	Re: vacuum leak? thanx for the help everyone. I was just out listening to the engine and I can hear it hiss, and when I have the hood up I can hear the hissing sound gradually fade away when I turn off the engine, is that normal?. It sounds like its coming from the drivers side but all I could see is the booster hose and I'm pretty sure it's not leaking. __________________ 69 shorty w/ 67 front clip

03-20-2007, 01:16 AM	#11
Farmstore34 Registered User Join Date: Mar 2007 Location: Oregon Posts: 109	Re: vacuum leak? Short69, its real common to have a leak in that little plastic elbow that plugs into the booster, or in the grommet around it. Seen it many times. CLT

03-20-2007, 06:38 PM	#13
dwaite72lnghrn Registered User Join Date: Jun 2002 Location: central pa Posts: 1,525	Re: vacuum leak? Carfull with the plastic connector it is a check valve that is suposed to keep vacuum in the booster after the engine stops. When it is finished hissing push the brake pedal and if it is that the brake pedal will travel before you feel pressure. But it looks like you found your problem just narrow it down to the booster, check valve, rubber line or the carb connection. but it really is heading to checkvalve or booster or the grommet that goes into the booster. __________________ 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

03-20-2007, 08:51 PM	#14
Huck Senior Member Join Date: Jun 2001 Location: Shelbyville, KY Posts: 3,269	Re: vacuum leak? Take the hose off the booster and plug it---that will tell right away if there is a vacuum leak there. Your engine should now run smooth--you can do the same process with each vac line till you find the leak.

03-20-2007, 10:41 PM	#15
jhow66 Registered User Join Date: Jun 2002 Location: Knoxville Tenn. Posts: 3,058	Re: vacuum leak? Did you use RTV on your intake gaskets around the ports? If you did-rut-o. RTV and gas don't mix. __________________ 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



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