only 12 in hg - The 1947 - Present Chevrolet & GMC Truck Message Board Network

ckenwright · 01-30-2004, 11:50 PM

Today, when I got home from work, I tried to tune the truck. I used a vacuum guage, timing light, and external tack. No matter how hard I tried, I could not get the vacuum below 12 in hg unless I went way advance. I finally settled on 6 or 8 BTC and test drove it to find that it knocked under full throttle. Could all this be happening as a result of having put a 4 barrell carb on a motor that had a one barrell on it with out changing the cam also? Any input would be appreciated.

stelth2002 · 01-31-2004, 12:09 AM

The more vacum at idle the better. I assume that this must be an inline if it had a monojet on it before. Back the timeing off if its knocking under full load. Your vacumm goes to zero when your at full throttle. That is if you have enough carburation

*rage'nrat638* · 01-31-2004, 12:11 AM

well on a regular engine you need vacuum for power brakes....
18 in. is what you would want.....
the more cam the less vacuum......

SOUNDS like your still advanced to far if it's got a knock.....
not sure about your cam question....but i would think not....

sure you did not have a 2 barr. carb and went to a 4barr.

ckenwright · 01-31-2004, 12:30 AM

The carb it had was described as a 1 barrel 3 venturi carb. And yes vacuum goes to 0 under full throttle. What do you guys have the timing set at with the quadrajets. I know there will be differences with engines but I would like to get a ballpark. To see if I am even close. This is a 350, truck originally had a 307. 69 c-10

*rage'nrat638* · 01-31-2004, 12:33 AM

how about a little info

engine size
trans
year

ckenwright · 01-31-2004, 12:41 AM

It is 69 C-10 originally had the 307 V-8, three on the tree. Everything is still the same except the engine was changed out with a 350 when the 307 needed rebuild. Then, one day I was changing gaskets on the carb and broke a bolt off in the manifold good. So I decided since I had to change out the manifold to switch to a Quadrajet.

bigjimzlll · 01-31-2004, 03:16 AM

OK..are you sure you are on full manifold vacuum? not ported? Set the timing to 8º BTDC with the vacuum advance disconnected and the line plugged. Hook up the vacuum gauge to full manifold vacuum...it will have constant steady vacuum at idle...turn in the idle enrichment screw a 1/4 turn...does the vacuum go up or down? If it goes up..turn another 1/4 turn..if it goes down..turn the screw out 1/2 turn...continue this until you get max vacuum...test drive with out the vacuum advance hooked up..if no knocking..hook up the vacuum advance to ported vacuum(the higher of the two vacuum ports)

ckenwright · 01-31-2004, 03:24 PM

Thanks for the help. I will let you know how it goes.

lukecp · 01-31-2004, 03:28 PM

Bigjim is right, simply hooking the vaccum advance cannister up to a full manifold vaccum port will dramatically increase the idle vaccum. I did this when working on a friend's truck, it had about 9" at idle, and simply switching the advance canister hose to a full manifold port brought it up to 20". With no changes in idle mixture or timing.

ckenwright · 02-01-2004, 12:29 AM

If you have any pictures of where you hooked the vacuum up to that would be real helpful. I am not sure I hooked everything up correctly when I installed this carb.

*dtlilly* · 02-01-2004, 02:14 AM

I never use a timing light, vac gadge, etc. I have a friend step on the gas and the brake at the same time, while I turn the distributor until it pings and or runs crappy, then I turn it back just a hair and your as good as you can get. A tired motor with a stretched timing chain will never be set to factory specs, so go by the ear method, and your set. I once had a motor so bad, that I had to take the distributor out and turn it one tooth, just to get it to run right.

01-30-2004, 11:50 PM	#1
ckenwright Registered User Join Date: May 2003 Location: Phoenix, AZ Posts: 104	only 12 in hg Today, when I got home from work, I tried to tune the truck. I used a vacuum guage, timing light, and external tack. No matter how hard I tried, I could not get the vacuum below 12 in hg unless I went way advance. I finally settled on 6 or 8 BTC and test drove it to find that it knocked under full throttle. Could all this be happening as a result of having put a 4 barrell carb on a motor that had a one barrell on it with out changing the cam also? Any input would be appreciated.

01-31-2004, 12:09 AM	#2
stelth2002 Young Gun wth Fast Inline Join Date: Jan 2002 Location: Idaho Posts: 1,475	The more vacum at idle the better. I assume that this must be an inline if it had a monojet on it before. Back the timeing off if its knocking under full load. Your vacumm goes to zero when your at full throttle. That is if you have enough carburation __________________ 1967 Chevrolet LWB with built 292! 415 ft. lbs of torque, 4 speed :O with 27% overdrive too, 3.73 Posi. Frame and body done... getting painted!!!

01-31-2004, 03:16 AM	#7
bigjimzlll Senior Member Join Date: Dec 2001 Location: Redding,CA...USA Posts: 4,736	OK..are you sure you are on full manifold vacuum? not ported? Set the timing to 8º BTDC with the vacuum advance disconnected and the line plugged. Hook up the vacuum gauge to full manifold vacuum...it will have constant steady vacuum at idle...turn in the idle enrichment screw a 1/4 turn...does the vacuum go up or down? If it goes up..turn another 1/4 turn..if it goes down..turn the screw out 1/2 turn...continue this until you get max vacuum...test drive with out the vacuum advance hooked up..if no knocking..hook up the vacuum advance to ported vacuum(the higher of the two vacuum ports) __________________ It's called "drag racing" if they called it "tic..tic..WHAM!..BANG! F*&K!!!", they'd have to keep the magazines under the counter with the other men's publications click the clicky to join the site.... http://67-72chevytrucks.com/vboard/payments.php 67 lwb..first hotrod in 25 years..540 best ET is 9.45 @ 141.44 Anderson,CA

01-31-2004, 03:28 PM	#9
lukecp Formerly yellow72custom Join Date: May 2001 Location: Austin, TX Posts: 7,531	Bigjim is right, simply hooking the vaccum advance cannister up to a full manifold vaccum port will dramatically increase the idle vaccum. I did this when working on a friend's truck, it had about 9" at idle, and simply switching the advance canister hose to a full manifold port brought it up to 20". With no changes in idle mixture or timing. __________________ '72 Chevy C10 Mild 350/TH350/3.07. Ochre/White. Old high school ride. '70 GMC C2500 '62 327 4bbl/SM465/4.56-geared Dana 60. White/White. Project or parts truck. '97 Saturn SL DD. 1.9/5-speed. 40+ highway mpg

01-31-2004, 12:11 AM	#3
rage'nrat638 Account Suspended Join Date: Mar 2003 Location: THE FALL GUY CHICAGO IL Posts: 5,883	well on a regular engine you need vacuum for power brakes.... 18 in. is what you would want..... the more cam the less vacuum...... SOUNDS like your still advanced to far if it's got a knock..... not sure about your cam question....but i would think not.... sure you did not have a 2 barr. carb and went to a 4barr.

01-31-2004, 12:30 AM	#4
ckenwright Registered User Join Date: May 2003 Location: Phoenix, AZ Posts: 104	The carb it had was described as a 1 barrel 3 venturi carb. And yes vacuum goes to 0 under full throttle. What do you guys have the timing set at with the quadrajets. I know there will be differences with engines but I would like to get a ballpark. To see if I am even close. This is a 350, truck originally had a 307. 69 c-10

01-31-2004, 12:33 AM	#5
rage'nrat638 Account Suspended Join Date: Mar 2003 Location: THE FALL GUY CHICAGO IL Posts: 5,883	how about a little info engine size trans year

01-31-2004, 12:41 AM	#6
ckenwright Registered User Join Date: May 2003 Location: Phoenix, AZ Posts: 104	It is 69 C-10 originally had the 307 V-8, three on the tree. Everything is still the same except the engine was changed out with a 350 when the 307 needed rebuild. Then, one day I was changing gaskets on the carb and broke a bolt off in the manifold good. So I decided since I had to change out the manifold to switch to a Quadrajet.

01-31-2004, 03:24 PM	#8
ckenwright Registered User Join Date: May 2003 Location: Phoenix, AZ Posts: 104	Thanks for the help. I will let you know how it goes.

02-01-2004, 12:29 AM	#10
ckenwright Registered User Join Date: May 2003 Location: Phoenix, AZ Posts: 104	If you have any pictures of where you hooked the vacuum up to that would be real helpful. I am not sure I hooked everything up correctly when I installed this carb.

02-01-2004, 02:14 AM	#11
dtlilly Account Suspended Join Date: Jan 2003 Location: Shadow of Lambeau Field Posts: 5,597	I never use a timing light, vac gadge, etc. I have a friend step on the gas and the brake at the same time, while I turn the distributor until it pings and or runs crappy, then I turn it back just a hair and your as good as you can get. A tired motor with a stretched timing chain will never be set to factory specs, so go by the ear method, and your set. I once had a motor so bad, that I had to take the distributor out and turn it one tooth, just to get it to run right.



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