using vacuum gauge to adjust carb

[trailblazr81]

OK, so Im not new to working on engines, or cars/trucks... but how does leaning the air fuel mixture raise the vacuum?

Also Ive checked the Krangen's and Auto Zones in my area and I cant find a simple vacuum gauge with the hose to connect to carb. All I can find are those vacuum gauge/ brake bleeders and those are kinda pricey. Ive used 'em a couple times for brakes but they keep braking on me and I dont want to spend that much just for a simple vacuum test gauge.

BTW I'm trying to tune my Barry Grant Speed demon with 4 corner idle adjustment on my 383 with Comp Cams XE274 (230/236 duration @ .050). It seems to be running a little rich.

 

[diesel4me]

mmmmmmmm...ok...

A good vacuum gauge is hard to find in some stores..A NAPA or a "real" parts store usually stocks them--they are a combo vacuum/fuel pump pressure tester ..I bought a gauge from an industrial supply place that sells all kinds of gauges for 15 bucks,its 6" in diameter,and much more sensitive that the "combo" one I have..it only measures vacuum though..

I usually run it at idle,and adjust each mixture screw until the highest,steadiest reading is reached..(why tuning the carb raises the vacuum is a mystery to me--lets just say when the engine is running the best it ever will,that is where the vacuum is highest and steadiest!)..I then turn the scews OUT a bit,until the vacuum drops 1 inch..if you leave it at its high steady reading setting,it wont run good cold (a BIT too lean)--so "fattening" up the mixture helps offset that some..if the timing is right,it should read between 18-22 inches,unless you have a hot cam,or a vacuum leak!..

I never messed with a Barry Grant carb--only 1 Holley that had 4 idle mixture screws..sounds like a nightmare!..

 

[trailblazr81]

Ya Ive heard and now experienced that the Barry Grant carbs can be a real pain to tune. Mines a stree/race carb. But once they are tuned (and theres alot of stuff you can adjust) its supposed to ROCK!! Tho now that this motor and carb is in my 73 K20 (was originally going to go into 72 Nova) I will someday just get a truck avenger or something similar. I see I can order a vacuum gauge out of summit but was hoping to do the work over the weekend. I'll check the 2 NAPA's in my area...

 

[mrk5]

I don't think the Demons are hard to tune.

I've had real good luck using a handheld tach. I just have someone sit in the car with it in drive and the brake applied. Keep air cleaner and everything hooked up to the carb. Start with all 4 screws 2 turns out. Then adjust them 1/8 of turn at a time to get the highest idle. I do all for screws at the same time.

Check RPM. Turn all 4 screws in 1/8 turn and check again to see what if RPM changed.

I've never liked using vacuum gages.

Their tech line is very helpful, too.

I have a Holley Truck Avenger, and I wish I would have bought Demon's new Trucker carb.

 

[trailblazr81]

I just looked at that truck carb from Barry Grant... too bad its $570 out of Jegs catalog. Thats the trucker speed demon 650... a little too much for me.