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Old 10-05-2015, 09:55 AM   #1
RyansToy
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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

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Old 10-05-2015, 10:05 AM   #2
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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...
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Old 10-05-2015, 01:41 PM   #3
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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.
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Old 10-05-2015, 02:34 PM   #4
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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
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Old 10-05-2015, 04:08 PM   #5
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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.
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Old 10-05-2015, 04:22 PM   #6
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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.
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Old 10-05-2015, 06:17 PM   #7
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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?
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Old 10-05-2015, 06:24 PM   #8
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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?
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Old 10-05-2015, 06:35 PM   #9
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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...
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Old 10-05-2015, 07:01 PM   #10
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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?
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Old 10-05-2015, 08:11 PM   #11
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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.
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Old 10-05-2015, 08:35 PM   #12
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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.
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Old 10-05-2015, 10:00 PM   #13
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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!
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Old 10-05-2015, 11:06 PM   #14
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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.
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Old 10-05-2015, 11:27 PM   #15
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Re: Tuning carb with a Vacuum Gauge

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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!
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Old 10-06-2015, 06:59 AM   #16
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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?
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Old 10-06-2015, 06:41 PM   #17
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Re: Tuning carb with a Vacuum Gauge

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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
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Old 10-06-2015, 07:20 PM   #18
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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!

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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.
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Old 10-06-2015, 07:37 PM   #19
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Re: Tuning carb with a Vacuum Gauge

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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.
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Old 10-06-2015, 07:40 PM   #20
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Re: Tuning carb with a Vacuum Gauge

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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.
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Old 10-07-2015, 06:40 PM   #21
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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...
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Old 10-08-2015, 12:56 AM   #22
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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.
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Old 10-08-2015, 11:10 AM   #23
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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
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Old 10-08-2015, 04:50 PM   #24
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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.
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Old 10-08-2015, 06:28 PM   #25
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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

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