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Troubled by results adjusting air/fuel mixture using vacuum gauge.


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cnewton
Novice

May 8, 2022, 1:19 AM

Post #1 of 5 (262 views)
Troubled by results adjusting air/fuel mixture using vacuum gauge. Sign In

The engine is a GM 1984 5.7L V-8 with a Rochester quadra-jet carburetor. The vehicle is actually a 21' cabin cruiser but I believe the information I'm looking for would be the same regardless of the vehicle type. I replaced the original salt water damaged carburetor with a rebuild. They were both Rochester 4-barrels, same bolt pattern, same linkage... so that so far was a quick task. The only difference was the rebuild had a small vent like tube in the front where a hose attached. I believe for a PVC valve hook up. The old carburetor didn't have this. So I put a tube on it and plugged it. I backed both air/fuel mixture screws out 2 1/2 turns and it started right up, sounded great let it idle for 5 minutes or so and shut it down. To fine tune air/fuel mixture, I was told to get a vacuum gauge and watch the you tube videos. So I used the Pcv tube in the front of the carburetor to get manifold vacuum instead of ported since it is constant.
I started the motor and the gauge instantly went to 30, buried it. I started closing the air/fuel mixture screws one at a time until both were all the way in. There was no change in the way the engine ran or the vacuum reading. When I unplugged the gauge from the valve, the engine accelerated and that's where I shut it down. I'm obviously new to this. The first vacuum gauge I've ever had and still watching You tube videos but what I really don't get is how adjusting the air/fuel mixture screws had no effect at all. I had the idle adjustment set a little high but it didn't move. I'm at a loss as to what to look at next and I would sure appreciate any suggestions or ideas you could throw this way.Thanks again


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

May 8, 2022, 4:28 AM

Post #2 of 5 (258 views)
Re: Troubled by results adjusting air/fuel mixture using vacuum gauge. Sign In

I'll tell you. I've been a mechanic for 60 years and rebuilt my share of carbs but I have NEVER heard of adjusting mixture screws with a vacuum gauge.
Before I start I set both screws to 1.5 turns from inner stop. Start the engine and let it get to operating temp (important), then screw them in one at a time until the RPM drops and back it off half a turn.

I think the vent thing was a difference between a marine carb and an automotive carb. One of them would use a sealed vent system to prevent explosion from fumes building up in the bilge. If you bought an automotive carb, then you need to be real careful starting the engine. Vent the bilge first.



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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



cnewton
Novice

May 8, 2022, 6:51 PM

Post #3 of 5 (235 views)
Re: Troubled by results adjusting air/fuel mixture using vacuum gauge. Sign In

Appreciate it Hammer. Short, to the point easy to follow instructions. It's running great. Rather, it's idling great. Didn't make it out today. Turns out the vacuum gauge I bought was defective and wouldn't zero out, it's starting point was 12" Hg. Spent too much time worrying aboutthe possible problems coming all based on a bunch of illegitimate vacuum readings.
Thanks again


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

May 8, 2022, 6:54 PM

Post #4 of 5 (232 views)
Re: Troubled by results adjusting air/fuel mixture using vacuum gauge. Sign In

You can't adjust idle mixture with a vacuum gauge.



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

May 8, 2022, 8:10 PM

Post #5 of 5 (227 views)
Re: Troubled by results adjusting air/fuel mixture using vacuum gauge. Sign In

Quick read, sorry. Owned and a specialty then and still have all specialty tools.
Yes to 2+1/2 out from bottom not 1.5 could pretty much leave them there other adjustments are final when in use vs on bench. Vacuum gauge Hgs are incorrect IMO beware of that out there What OP post is WRONG. That might be 1/2 throttle when in use and watching almost impossible and useless now. Yikes but vacuum would be for sea level marine in widespread use can and is over 500 ft. If more altered specs above 1,000 marginally would help - BTW and in person - done that - endless algorithm this for water would be fresh water (non salt) few would so real sea level is the spec is good enough



If/when adjusting these things (IMO best carb of all time) hand vacuum gauge is handy to check pull off's measure angles of top plates or just as easy use drill bits.
Vacuum applied to pull offs some with metered leaks intended wild assortment of these to set the known specs or now move in order the listed items to spec - all on a bench no engine needed.
DANG LOWER SCREWS ARE ONLY FOR IDLE MIXTURE AND LOW/INTERMEDIATE mixture are NOT setting air ratio of fuel and air when these work hard dammit the whole world couldn't figure them out to save a life.
Mixture is level of fuel in float bowls with knew weight of float a plastic or brass can be too heavy good luck now finding weight specs.
Takes three hands or tools hold and watch with fuel when top is off see where it stops via needle valve or altered to a check ball.


100s of assorted ones look the same are different - trucks/marine would lack electrical parts - a hazard to do in some so bad made a fire didn't do it.
Specs when unsure and where to adjust and how you could find GM trucks for era ~1969>1972 counter rotating didn't matter for this all GM large and small blocks + others as carb was amazing.


Final adjustment are for when on the operating engine BTW.
I'll let folks search for application they are used for close specs and if rods to bend, tabs to bend is how to alter in order or just quit and start all over.
Fuel has a weight with now alcohol in it varies so would change how buoyant floats are get rid of the alcohol for proper fuel or adjust if stuck with it.


BTW - instructions are fine print pages long last of the techs that could read and do these (me) were rare then almost extinct now.


End this about adjusting mixture with vacuum gauge no point and not useful would use just to hold choke pull-off to open with cold spring pushing measure gap up top/front plate can do same without vacuum pump/weights/angle gauges.
These are a specialty all by them selves would or could be a thick book to understand need to use your head/brain and know what item is for what or it just isn't for you it wasn't when new and in use so they ended because of it,


Tom
> Can't help wild long post, sorry for typos understanding these is a full course of study alone months long not one page!



(This post was edited by Tom Greenleaf on May 8, 2022, 8:29 PM)






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