Tuning carb with a Vacuum Gauge - The 1947 - Present Chevrolet & GMC Truck Message Board Network

RyansToy · 10-05-2015, 09:55 AM

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

RyansToy · 10-05-2015, 10:05 AM

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...

**Ironangel** · 10-05-2015, 01:41 PM

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.

RyansToy · 10-05-2015, 02:34 PM

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

chevy_mike · 10-05-2015, 04:08 PM

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.

cableguy0 · 10-05-2015, 04:22 PM

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.

RyansToy · 10-05-2015, 06:17 PM

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?

cableguy0 · 10-05-2015, 06:24 PM

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?

RyansToy · 10-05-2015, 06:35 PM

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...

cableguy0 · 10-05-2015, 07:01 PM

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?

RyansToy · 10-05-2015, 08:11 PM

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.

cableguy0 · 10-05-2015, 08:35 PM

Champions are for dodges and lawnmowers. Grab a set of ac delco plugs. Told ya the gap would help.

RyansToy · 10-05-2015, 10:00 PM

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!

cableguy0 · 10-05-2015, 11:06 PM

Quote:

Originally Posted by RyansToy

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.

**Ironangel** · 10-05-2015, 11:27 PM

Quote:

Originally Posted by RyansToy

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

Champions are for dodges and lawnmowers. Grab a set of ac delco plugs. Told ya the gap would help.

Now that was funny!

ranger danger · 10-06-2015, 06:59 AM

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?

cableguy0 · 10-06-2015, 06:41 PM

Quote:

Originally Posted by ranger danger

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

GASoline71 · 10-06-2015, 07:20 PM

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

ranger danger · 10-06-2015, 07:37 PM

Quote:

Originally Posted by cableguy0

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.

cableguy0 · 10-06-2015, 07:40 PM

Quote:

Originally Posted by ranger danger

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.

RyansToy · 10-07-2015, 06:40 PM

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...

67 Burb · 10-08-2015, 12:56 AM

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.

Twisted78SS · 10-08-2015, 11:10 AM

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

prostreetC-10 · 10-08-2015, 04:50 PM

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.

mike16 · 10-08-2015, 06:28 PM

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

10-05-2015, 09:55 AM	#1
RyansToy Registered User 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.

10-05-2015, 10:05 AM	#2
RyansToy Registered User 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...

10-05-2015, 01:41 PM	#3
Ironangel Senior Member 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"

10-05-2015, 02:34 PM	#4
RyansToy Registered User 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

10-05-2015, 04:08 PM	#5
chevy_mike Never Ending Projects 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...

10-05-2015, 04:22 PM	#6
cableguy0 Senior Member Join Date: Apr 2000 Location: Delta,Pa Posts: 14,948	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.

10-05-2015, 06:17 PM	#7
RyansToy Registered User 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)

10-05-2015, 06:24 PM	#8
cableguy0 Senior Member Join Date: Apr 2000 Location: Delta,Pa Posts: 14,948	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.

10-05-2015, 06:35 PM	#9
RyansToy Registered User 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)

10-05-2015, 07:01 PM	#10
cableguy0 Senior Member Join Date: Apr 2000 Location: Delta,Pa Posts: 14,948	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.

10-05-2015, 08:11 PM	#11
RyansToy Registered User 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)

10-05-2015, 08:35 PM	#12
cableguy0 Senior Member Join Date: Apr 2000 Location: Delta,Pa Posts: 14,948	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.

10-05-2015, 10:00 PM	#13
RyansToy Registered User 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)

10-06-2015, 06:59 AM	#16
ranger danger Registered User 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.

10-07-2015, 06:40 PM	#21
RyansToy Registered User 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)



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10-08-2015, 12:56 AM	#22
67 Burb Registered User Join Date: Dec 2011 Location: Brandon, Manitoba, Canada Posts: 548	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

10-08-2015, 11:10 AM	#23
Twisted78SS Wide n'Low 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

10-08-2015, 04:50 PM	#24
prostreetC-10 My Carbon Footprint 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. __________________ A quick "pedalfest" *Still getting loose (carb test)* 496 Initial fire up video!

10-08-2015, 06:28 PM	#25
mike16 Registered User Join Date: Dec 2009 Location: bisbee, arizona Posts: 1,530	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.

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