The 1947 - Present Chevrolet & GMC Truck Message Board Network







Register or Log In To remove these advertisements.

Go Back   The 1947 - Present Chevrolet & GMC Truck Message Board Network > General Truck Forums > Engine & Drivetrain

Web 67-72chevytrucks.com


Reply
 
Thread Tools Display Modes
Old 10-05-2015, 09:55 AM   #1
RyansToy
Registered User
 
RyansToy's Avatar
 
Join Date: Aug 2015
Location: Nova Scotia Canada
Posts: 145
Tuning carb with a Vacuum Gauge

So I bought a vacuum gauge at Princess Auto (canadian harbour freight equivalent). to try and tune my carb.

I am not very familiar with the vacuum gauge uses.

My vacuum seems to tune in around 18-19 in Hg. It does vibrate a bit.

Is this normal, or should I be looking at "spark plug gaps, distributor, coil"... as seen on some online things. Click the image to see the video.



http://vid14.photobucket.com/albums/...psik3aezrk.mp4

Last edited by RyansToy; 10-05-2015 at 10:22 AM.
RyansToy is offline   Reply With Quote
Old 10-05-2015, 10:05 AM   #2
RyansToy
Registered User
 
RyansToy's Avatar
 
Join Date: Aug 2015
Location: Nova Scotia Canada
Posts: 145
Re: Tuning carb with a Vacuum Gauge

So I just saw some video from Holley saying it may be caused by my ignition stuff... which is all new.

I haven't opened up my plug gaps to account for the HEI distributor yet. I guess that is on my list of to do...

Any other comments are welcome...
RyansToy is offline   Reply With Quote
Old 10-05-2015, 01:41 PM   #3
Ironangel
Senior Member
 
Ironangel's Avatar
 
Join Date: Feb 2010
Location: Falls City, Nebraska "100 Miles From Nowhere"
Posts: 2,219
Re: Tuning carb with a Vacuum Gauge

Howdy Ryan, it would help the guys here if you give some details like what motor, with what carb, 4bbl, 2bbl, any mods, and the initial timing as well...It's normal for vacuum gauges to flutter a bit, 18 hg aint bad! Someone with more knowledge than me will chime in soon.
__________________
Michael of the clan Hill,
"Two Seventy Two's"
71 1-ton Dually 350 4-Speed
71 C/50 Grain Truck, 350 Split-Axle 4-Speed
02 3/4 ton Express
14 Indian Chief Vintage
1952 Ford 8N, "Only Ford Allowed On The Property"
"Be American, Buy American"
Ironangel is offline   Reply With Quote
Old 10-05-2015, 02:34 PM   #4
RyansToy
Registered User
 
RyansToy's Avatar
 
Join Date: Aug 2015
Location: Nova Scotia Canada
Posts: 145
Re: Tuning carb with a Vacuum Gauge

My bad..

1968 LWB truck with 3 speed.
250 ci L6 engine (casting says 1975-6).
Has HEI of newer model L6.

Brand new plugs (@0.035"), wires, distributor cap, rotor, Fuel pump, fuel filters
RyansToy is offline   Reply With Quote
Old 10-05-2015, 04:08 PM   #5
chevy_mike
Never Ending Projects
 
chevy_mike's Avatar
 
Join Date: Jul 2006
Location: San Diego, CA
Posts: 3,836
Re: Tuning carb with a Vacuum Gauge

I thought I recall the plug gap for an HEI was .040 or .045. I recall it being bigger than stock when I converted. Also, did you upgrade your power wire to the coil? The stock one is a resistor wire and won't give you a full 12+ volts at the coil.
__________________
.
1965 C10 Panel, Tiki Express http://www.67-72chevytrucks.com/vboa...d.php?t=506580 SOLD
1968 Chevy C10, Long, Fleetside, Hot Rod Hauler http://67-72chevytrucks.com/vboard/s...d.php?t=313233 SOLD
1965 Chevy C10, Long, Fleetside, Hot Rod C10 http://67-72chevytrucks.com/vboard/s...d.php?t=415702 SOLD


We were given two ears and one mouth for a reason... listen twice as much and speak half as often...
chevy_mike is offline   Reply With Quote
Old 10-05-2015, 04:22 PM   #6
cableguy0
Senior Member
 
Join Date: Apr 2000
Location: Delta,Pa
Posts: 14,950
Re: Tuning carb with a Vacuum Gauge

Hei plus gap needs to be .045. Vacuum at18hg aint bad at all. Where is your timing set at? Are you using a vacuum advance? As mentioned you CANNOT use the factory coil wire to power and hei. You need to run a new 12 gaage wire from and ignition source on the fuse block.
__________________
Owner of North Point Car Care in Dundalk Md. We specialize in custom exhaust on both modern and classic vehicles. We are a full service auto shop from classics to modern vehicles. Feel free to contact me with questions. I will give a 10% discount to any board member.
cableguy0 is offline   Reply With Quote
Old 10-05-2015, 06:17 PM   #7
RyansToy
Registered User
 
RyansToy's Avatar
 
Join Date: Aug 2015
Location: Nova Scotia Canada
Posts: 145
Re: Tuning carb with a Vacuum Gauge

I didn't bother with the 12 gauge wire on the fuse block. I ran the original wire to a relay and used pretty thick gauge wire to run the source right from the battery (all of 1.5 feet).

I actually didn't notice any difference after doing this, but because i work for an electronics company - at least it cost me nothing.

its a factory HEI (probably from a 1984ish 6 cylinder). I am using the vacuum advance. I checked the can is working ok (used a vacuum pump). Not sure about the weights on the mechanical advance.

I almost went for the gap tool today, but am not certain that would be a cause of my hesitation... I don't want to change anything till I sort the acceleration thing out.

Do you guys think just the gap alone would cause such a significant stumble?
__________________
1968 GMC truck
910 Trim (Canadian)
127" WB
3 on the Three Trans (Saginaw SM330)
250 ci L6 Engine
HEI ignition (early 80s L6)
RyansToy is offline   Reply With Quote
Old 10-05-2015, 06:24 PM   #8
cableguy0
Senior Member
 
Join Date: Apr 2000
Location: Delta,Pa
Posts: 14,950
Re: Tuning carb with a Vacuum Gauge

The gap needs to be .045 to get a complete fuel burn. Without gapping the plugs properly doing the hei isn't improving much. The point of the hei is hotter spark. Without proper plug gap you aren't getting all of the benefits of the switch over. You need to get timing correct also. Make sure the mechanical advance is even working. What is timing set at idle?
__________________
Owner of North Point Car Care in Dundalk Md. We specialize in custom exhaust on both modern and classic vehicles. We are a full service auto shop from classics to modern vehicles. Feel free to contact me with questions. I will give a 10% discount to any board member.
cableguy0 is offline   Reply With Quote
Old 10-05-2015, 06:35 PM   #9
RyansToy
Registered User
 
RyansToy's Avatar
 
Join Date: Aug 2015
Location: Nova Scotia Canada
Posts: 145
Re: Tuning carb with a Vacuum Gauge

sorry, forgot about the timing.

I started with the 4 degrees from the factory manual. It ran like a bag of $hit. After a long time reading I finally set it up at around 10 or 12 degrees advance. I can't remember where I left it.

It runs good at 60 mph, and at 70. It just stumbles when I accelerate. Lately it is when you go WOT. Half throttle is ok. Also it stalls at idle here and there...
__________________
1968 GMC truck
910 Trim (Canadian)
127" WB
3 on the Three Trans (Saginaw SM330)
250 ci L6 Engine
HEI ignition (early 80s L6)
RyansToy is offline   Reply With Quote
Old 10-05-2015, 07:01 PM   #10
cableguy0
Senior Member
 
Join Date: Apr 2000
Location: Delta,Pa
Posts: 14,950
Re: Tuning carb with a Vacuum Gauge

You need to find out if your mechanical advance is working. Remove the distributor cap and grab the rotor. You should be able to move it and it should spring right back. The effort required should be minimal. Timing at 10-12 is about right. What kind of carb do you have and what condition is it in? Stumbling on acceleration could be lack of mechanical advance working. It could also be an issue with the carb. Timing and ignition system needs to be right before you even touch the carb though. What kind of spark plugs are you using?
__________________
Owner of North Point Car Care in Dundalk Md. We specialize in custom exhaust on both modern and classic vehicles. We are a full service auto shop from classics to modern vehicles. Feel free to contact me with questions. I will give a 10% discount to any board member.
cableguy0 is offline   Reply With Quote
Old 10-05-2015, 08:11 PM   #11
RyansToy
Registered User
 
RyansToy's Avatar
 
Join Date: Aug 2015
Location: Nova Scotia Canada
Posts: 145
Re: Tuning carb with a Vacuum Gauge

Just got in from gapping the plugs. Wholey crap, what a difference that made. I can't believe the difference in power now.

The carb is a fresh rebuild. I did it.... I had no issues, except that the metering rod measurement was twice what it should be according to the specs on the adjustment table. It did however match the measurement for a 230ci engine. I figured if it ran ok before as it was, I would leave it for now.

I am using a champion copper plug. RN14YC original gap was 0.036" I have now changed it to the 0.045"

When I had the plugs out they looked really good to me.
__________________
1968 GMC truck
910 Trim (Canadian)
127" WB
3 on the Three Trans (Saginaw SM330)
250 ci L6 Engine
HEI ignition (early 80s L6)
RyansToy is offline   Reply With Quote
Old 10-05-2015, 08:35 PM   #12
cableguy0
Senior Member
 
Join Date: Apr 2000
Location: Delta,Pa
Posts: 14,950
Re: Tuning carb with a Vacuum Gauge

Champions are for dodges and lawnmowers. Grab a set of ac delco plugs. Told ya the gap would help.
__________________
Owner of North Point Car Care in Dundalk Md. We specialize in custom exhaust on both modern and classic vehicles. We are a full service auto shop from classics to modern vehicles. Feel free to contact me with questions. I will give a 10% discount to any board member.
cableguy0 is offline   Reply With Quote
Old 10-05-2015, 10:00 PM   #13
RyansToy
Registered User
 
RyansToy's Avatar
 
Join Date: Aug 2015
Location: Nova Scotia Canada
Posts: 145
Re: Tuning carb with a Vacuum Gauge

someone needs to write a book about owning a C10... and how to take care of it...

I decided i would start a truck diary to note down all the alterations, and research findings... so two weeks from now I can look back and realize how much falls out of my head when i am not looking!
__________________
1968 GMC truck
910 Trim (Canadian)
127" WB
3 on the Three Trans (Saginaw SM330)
250 ci L6 Engine
HEI ignition (early 80s L6)
RyansToy is offline   Reply With Quote
Old 10-05-2015, 11:06 PM   #14
cableguy0
Senior Member
 
Join Date: Apr 2000
Location: Delta,Pa
Posts: 14,950
Re: Tuning carb with a Vacuum Gauge

Quote:
Originally Posted by RyansToy View Post
someone needs to write a book about owning a C10... and how to take care of it...

I decided i would start a truck diary to note down all the alterations, and research findings... so two weeks from now I can look back and realize how much falls out of my head when i am not looking!
This site is a book. Well actually its a massive set of encyclopedias. The info here couldn't be housed in a single book.
__________________
Owner of North Point Car Care in Dundalk Md. We specialize in custom exhaust on both modern and classic vehicles. We are a full service auto shop from classics to modern vehicles. Feel free to contact me with questions. I will give a 10% discount to any board member.
cableguy0 is offline   Reply With Quote
Old 10-05-2015, 11:27 PM   #15
Ironangel
Senior Member
 
Ironangel's Avatar
 
Join Date: Feb 2010
Location: Falls City, Nebraska "100 Miles From Nowhere"
Posts: 2,219
Re: Tuning carb with a Vacuum Gauge

Quote:
Originally Posted by RyansToy View Post
Just got in from gapping the plugs. Wholey crap, what a difference that made. I can't believe the difference in power now.

Quote:
Originally Posted by cableguy0 View Post
Champions are for dodges and lawnmowers. Grab a set of ac delco plugs. Told ya the gap would help.
Now that was funny!
__________________
Michael of the clan Hill,
"Two Seventy Two's"
71 1-ton Dually 350 4-Speed
71 C/50 Grain Truck, 350 Split-Axle 4-Speed
02 3/4 ton Express
14 Indian Chief Vintage
1952 Ford 8N, "Only Ford Allowed On The Property"
"Be American, Buy American"
Ironangel is offline   Reply With Quote
Old 10-06-2015, 06:59 AM   #16
ranger danger
Registered User
 
ranger danger's Avatar
 
Join Date: Jan 2015
Location: Vacaville CA
Posts: 896
Re: Tuning carb with a Vacuum Gauge

I dont mean to hyjack this thread but I replaced my stock Ignition on my mid 70's 454 with a MSD pro billet. Plugs are gapped at 35. Should I re-gap the plugs to 45?
__________________
1969 C20 Suburban "Rez Dog"
Rebuild America one testicle at a time.
US Army 1977-1979 12B Combat Engineer
US Navy 1979-1983 Gunners Mate Guns (GMG)
NRA Life Member

Unrestrained Children in back seats cause Accidents.
Unrestrained Accidents in back seats cause Children.
ranger danger is offline   Reply With Quote
Old 10-06-2015, 06:41 PM   #17
cableguy0
Senior Member
 
Join Date: Apr 2000
Location: Delta,Pa
Posts: 14,950
Re: Tuning carb with a Vacuum Gauge

Quote:
Originally Posted by ranger danger View Post
I dont mean to hyjack this thread but I replaced my stock Ignition on my mid 70's 454 with a MSD pro billet. Plugs are gapped at 35. Should I re-gap the plugs to 45?
Absolutely. If your also running an msd box like the 6al you can go as far as .060
__________________
Owner of North Point Car Care in Dundalk Md. We specialize in custom exhaust on both modern and classic vehicles. We are a full service auto shop from classics to modern vehicles. Feel free to contact me with questions. I will give a 10% discount to any board member.
cableguy0 is offline   Reply With Quote
Old 10-06-2015, 07:20 PM   #18
GASoline71
"I ain't nobody, dork."
 
GASoline71's Avatar
 
Join Date: Sep 2007
Location: Whidbey Island, Washington
Posts: 8,921
Re: Tuning carb with a Vacuum Gauge

I am running an MSD Pro-Billet with a Digital 6AL box in my 1972 GMC with a pretty snotty 355. My plug gaps are set at .058

Nice big fat hot spark!

Gary
__________________
'cuz chicks dig scars...

My 1972 GMC 1500 Super Custom (Creeping Death) "long term" build thread.

The Rebuild of Creeping Death after the wreck

Quote:
Originally Posted by LONGHAIR View Post
I would never rebuild a 305.
Quote:
Originally Posted by prostreetC-10 View Post
I love using vacuum gauges as part of the carb tuning process. I hook the gauge to the inside of my garbage can and leave it there.
Quote:
Originally Posted by Marv D View Post
Remember Murphys 2nd law of mechanical relationships... "OPPOSING COMPONENTS ATTEMPTING TO OCCUPY THE SAME SPACE, AT THE SAME TIME, GENERALLY END UP OCCUPYING ADJOINING SPACE AT THE BOTTOM OF THE OIL PAN"
Quote:
Originally Posted by cableguy0 View Post
Its cheaper to listen to advice given when you ask for help than it is to ignore everyone and wait for carnage.
GASoline71 is offline   Reply With Quote
Old 10-06-2015, 07:37 PM   #19
ranger danger
Registered User
 
ranger danger's Avatar
 
Join Date: Jan 2015
Location: Vacaville CA
Posts: 896
Re: Tuning carb with a Vacuum Gauge

Quote:
Originally Posted by cableguy0 View Post
Absolutely. If your also running an msd box like the 6al you can go as far as .060
Quote:
I am running an MSD Pro-Billet with a Digital 6AL box in my 1972 GMC with a pretty snotty 355. My plug gaps are set at .058

Nice big fat hot spark!

Gary
Thanks guys! I'm not running the 6AL and the motor is basically stock. Ill try re-gaping to 45 and see how it works out.
__________________
1969 C20 Suburban "Rez Dog"
Rebuild America one testicle at a time.
US Army 1977-1979 12B Combat Engineer
US Navy 1979-1983 Gunners Mate Guns (GMG)
NRA Life Member

Unrestrained Children in back seats cause Accidents.
Unrestrained Accidents in back seats cause Children.
ranger danger is offline   Reply With Quote
Old 10-06-2015, 07:40 PM   #20
cableguy0
Senior Member
 
Join Date: Apr 2000
Location: Delta,Pa
Posts: 14,950
Re: Tuning carb with a Vacuum Gauge

Quote:
Originally Posted by ranger danger View Post
Thanks guys! I'm not running the 6AL and the motor is basically stock. Ill try re-gaping to 45 and see how it works out.
You should instantly notice more power. Less fuel smell in the exhaust and better gas mileage that is unless the new found power makes your right foot a little heavier.
__________________
Owner of North Point Car Care in Dundalk Md. We specialize in custom exhaust on both modern and classic vehicles. We are a full service auto shop from classics to modern vehicles. Feel free to contact me with questions. I will give a 10% discount to any board member.
cableguy0 is offline   Reply With Quote
Old 10-07-2015, 06:40 PM   #21
RyansToy
Registered User
 
RyansToy's Avatar
 
Join Date: Aug 2015
Location: Nova Scotia Canada
Posts: 145
Re: Tuning carb with a Vacuum Gauge

So on Monday I talked to a chevy mechanic dude... older guy use to the carbs. We set up tonight to have a look at the truck.

I was reasonably happy with the truck prior to going, the gapped plugs really made it much nicer. Brought him a case of beer and offered some money...

15 minutes later he adjusted the distributor without the timing light, asked me to restart the engine. Started easily. Ran well. Went for a drive. It was perfect....

So when I got home, I looked at the timing with the light. The advance (vac blocked) is way off the markings. I estimate the timing to be around 16-20 degrees advance.

Can't complain, but i really wish I understood the process.

He didn't take any money...
__________________
1968 GMC truck
910 Trim (Canadian)
127" WB
3 on the Three Trans (Saginaw SM330)
250 ci L6 Engine
HEI ignition (early 80s L6)
RyansToy is offline   Reply With Quote
Old 10-08-2015, 12:56 AM   #22
67 Burb
Registered User
 
67 Burb's Avatar
 
Join Date: Dec 2011
Location: Brandon, Manitoba, Canada
Posts: 545
Re: Tuning carb with a Vacuum Gauge

Ryan, does the vacuum gauge still bounce back and forth like it did before?

Here's an old vacuum gauge chart I found. It might help you figure out what's going on.
Attached Images
 
__________________
Bob

1967 2wd Burb, 350/TH350, 3.73 posi
67 Burb is offline   Reply With Quote
Old 10-08-2015, 11:10 AM   #23
Twisted78SS
Wide n'Low
 
Twisted78SS's Avatar
 
Join Date: Jan 2007
Location: La Center, WA
Posts: 1,511
Re: Tuning carb with a Vacuum Gauge

Yup very similar to the one I was getting ready to post.. I'll toss it up here anyway

http://www.classictruckshop.com/club...ts/vac/uum.htm
__________________
Trippin Hazard.. 67 swb,
Gold Member.. 68 Suburban
Air Force Retired
Twisted78SS is offline   Reply With Quote
Old 10-08-2015, 04:50 PM   #24
prostreetC-10
My Carbon Footprint
 
prostreetC-10's Avatar
 
Join Date: Oct 2008
Location: Orygun
Posts: 5,527
Re: Tuning carb with a Vacuum Gauge

I love using vacuum gauges as part of the carb tuning process. I hook the gauge to the inside of my garbage can and leave it there.
prostreetC-10 is offline   Reply With Quote
Old 10-08-2015, 06:28 PM   #25
mike16
Registered User
 
Join Date: Dec 2009
Location: bisbee, arizona
Posts: 1,529
Re: Tuning carb with a Vacuum Gauge

some of the older chiltons and motor manual have pretty large sections on rading a vacuum guage. Also some of the mid fifties troubleshooting books used in vocational automotive courses up through the early 70's have lots of info on using vacuum guages.

Not for thothin but

you want a guage with as large in diameter face as you can find and avoid ones that have fluids in side the guage to dampen needle movement.

those slight little twitches of the needle can tell you alot.

its a lost art for the most part.

I bought one with a 8" face from McMasters Carr back in the early 90's. Still use it. and know how.no computer does that

Last edited by mike16; 10-08-2015 at 06:35 PM.
mike16 is offline   Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -4. The time now is 05:58 AM.


Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2024, vBulletin Solutions Inc.
Copyright 1997-2022 67-72chevytrucks.com