Manifold Vacuum

Moderator: Moderators

Re: Manifold Vacuum

Postby monza355 » Tue Nov 09, 2010 1:30 pm

It is actually rich as hell. It is a Holley 750. I figured it is a little too big for the motor but the 650 Speed Demon is a piece of crap. The plugs are black. You can smell the fuel. I thought smaller jets were needed.
monza355
 
Posts: 133
Joined: Thu Nov 05, 2009 1:35 pm


Re: Manifold Vacuum

Postby 72 SS PNL » Tue Nov 09, 2010 3:46 pm

well choking it limits the air going into , which helps with a vacuum leak or lean condition

ive got a 750 double pumper on my 68 elco , and a pretty decent cam , and i had to richen up the carb , it always smells rich but the plugs are a nice tan brown color

have you checked the power valve ? also when you start to close the choke , does it raise the idle speed up ?
first and best car owned 72 GT(changed to SS) panel wagon

pictures of the car from 86 - http://www.myrideisme.com/Garage/06_IBM_GOAT/955
User avatar
72 SS PNL
 
Posts: 2716
Joined: Sat Mar 14, 2009 1:18 am
Location: phx az

1973 Chevrolet Vega Kammback GT

Re: Manifold Vacuum

Postby monza355 » Tue Nov 09, 2010 3:58 pm

The idle did go up when I started to close it.

My plugs are black and sooty (full of soot).
monza355
 
Posts: 133
Joined: Thu Nov 05, 2009 1:35 pm

Re: Manifold Vacuum

Postby 72 SS PNL » Tue Nov 09, 2010 7:12 pm

was the carb brand new , or used when you got it ?
first and best car owned 72 GT(changed to SS) panel wagon

pictures of the car from 86 - http://www.myrideisme.com/Garage/06_IBM_GOAT/955
User avatar
72 SS PNL
 
Posts: 2716
Joined: Sat Mar 14, 2009 1:18 am
Location: phx az

1973 Chevrolet Vega Kammback GT

Re: Manifold Vacuum

Postby monza355 » Tue Nov 09, 2010 10:34 pm

It was used. It was clean. Somewhat reliable source. Should I go through it with a kit? Maybe just spend the money for a new one.
monza355
 
Posts: 133
Joined: Thu Nov 05, 2009 1:35 pm

Re: Manifold Vacuum

Postby monza355 » Tue Nov 09, 2010 10:39 pm

I have a new 650 Speed Demon but it was worse. Could not get it to not backfire through the carb no matter what I tried. Put the Holley on it and can burn em good, with a little choke of course.
monza355
 
Posts: 133
Joined: Thu Nov 05, 2009 1:35 pm

Re: Manifold Vacuum

Postby starfire383 » Wed Nov 10, 2010 12:44 pm

I doubt any of this is related to your carburetor.

Two things to remember......a vacuum leak makes one cylinder lean and the others rich as hell.

Weak spark and vacuum leaks are to blame for 95% of "carburetor" problems. If the carburetor is doing it's job properly, weak spark or weak vacuum will make the carb seem all floody. I take a fair amount of carburetor work into my backyard shop, but rarely do I actually rebuild or replace the carburetor. It's usually igniton work, fixing vacuum leaks, or decipering that a couple cylinders are lazy and no amount of tuning will make it run right.

If you're 110% sure about the ignition system and intake gasket, consider doing a compression & leakdown test to look for troubles like bad valve seats, etc. There are a couple deep mechanical things that can screwup vacuum significantly. How certain are you about lifter adjustment???? Set an exhaust lifter a smidge tight and all bets are off when it comes to vacuum response, instead of building vaccum in the intake manifold that cylinder will also pull exhaust back in, which also makes your plugs black as hell.

One other thing.....the cfm of a carburetor will not make an engine flood. GM used 800cfm Qjets on 305s. If all mechanical components are healthy, the carb calibration sets the plug condition. Not carb size. Size is more related to low speed driveability and WOT power. You control CFM with your foot, the carb's not flowing 650/750 when idling or cruising. When it comes to tuning, carb size is the icing on the cake. These issues go deeper than icing.
User avatar
starfire383
 
Posts: 952
Joined: Tue Jul 04, 2006 4:06 pm

Re: Manifold Vacuum

Postby 72 SS PNL » Wed Nov 10, 2010 1:09 pm

being used , that can put the power valve at higher risk of being blown out , and the jetting being far off even if its only a couple sizes different from out of the box

whats your timing at idle ? and total timing ? i like to keep it 8-12 , and usually on the higher end with out going over 16
first and best car owned 72 GT(changed to SS) panel wagon

pictures of the car from 86 - http://www.myrideisme.com/Garage/06_IBM_GOAT/955
User avatar
72 SS PNL
 
Posts: 2716
Joined: Sat Mar 14, 2009 1:18 am
Location: phx az

1973 Chevrolet Vega Kammback GT

Re: Manifold Vacuum

Postby 3pedals » Wed Nov 10, 2010 1:51 pm

monza355 wrote:Comp Cams XE268H. The needle moves up and down slightly. No drastic movements.

My issue is the engine idles fine and revs fine but when you put it in gear and try to drive, it sputters and misses. I'm going to play with the timing a little and see if I can clear it up some. It smells very rich. I tried adjusting the carb with a vacuum guage to get it a little leaner but it didn't seem to change.

I agree with everyone so far. That said, as unlikely as it may be... what do you know about your valve adjustment? Low vacuum, idles fine & revs fine, sputters, misses when put in gear, smell of gas... :?: Valves that are to tight will lower your vacuum and the needle on the guage will flutter slightly. With the combination of low vacuum (valves to tight :?: ) which causes an over all lack of torque at low speed, putting an auto trans in gear can make it difficult to drive off idle. Just a thought....
--- Dan ---
'72 GT Hatch: 406, 4spd/M21, 12 bolt/3.73/posi.
User avatar
3pedals
 
Posts: 234
Joined: Wed Jun 16, 2010 1:02 pm
Location: Turlock, California

1972 Chevrolet Vega GT

Re: Manifold Vacuum

Postby monza355 » Wed Nov 10, 2010 3:28 pm

I guess I need to go back through the valves. I thought I had them right, but I'm finding more and more that I don't know chit. So does anyone have good procedures for adjusting valves?
monza355
 
Posts: 133
Joined: Thu Nov 05, 2009 1:35 pm

Re: Manifold Vacuum

Postby 3pedals » Wed Nov 10, 2010 3:36 pm

This is one way to do it:
*
DSCN0397.JPG
You do not have the required permissions to view the files attached to this post.
--- Dan ---
'72 GT Hatch: 406, 4spd/M21, 12 bolt/3.73/posi.
User avatar
3pedals
 
Posts: 234
Joined: Wed Jun 16, 2010 1:02 pm
Location: Turlock, California

1972 Chevrolet Vega GT

Re: Manifold Vacuum

Postby starfire383 » Wed Nov 10, 2010 6:41 pm

Get the idle as low as you can and let the car warm up slightly. Like 100 degrees. Remove one valve cover.

Start engine, work front to back (or back to front) one valve at a time. Back off til it clatters, tighten til it stops, add a half turn. Move to the next valve. If you're new to valve adjustment it's the closest to foolproof there is. Unless it's a full on show car you should be able to clean the mess without too much trouble.

I hear a lot of talk about blown out power valves on holleys. My stuff sneezes a lot, mostly due to having to be moved around in cold weather without a choke. Have probably rebuilt a hundred holleys at this point and had maybe 2 blown power valves in the whole lot. And those honestly didn't look that blown, more like dry rot and age cracking. It's the first thing everyone thinks of, but on my troubleshooter list it's bottom of the page. Maybe even page 2.

good luck
Last edited by starfire383 on Wed Nov 10, 2010 7:20 pm, edited 1 time in total.
User avatar
starfire383
 
Posts: 952
Joined: Tue Jul 04, 2006 4:06 pm

Re: Manifold Vacuum

Postby monza355 » Wed Nov 10, 2010 6:55 pm

I was just going to ask if there is a good way to do it with the engine running. I definitely do not have to worry about it being a show car. Thanks for all of the great information. I will let you all know what I come up with this weekend.

Quick question - Does it need full exhaust or is open headers ok? I have Sanderson shorty headers.
monza355
 
Posts: 133
Joined: Thu Nov 05, 2009 1:35 pm

Re: Manifold Vacuum

Postby starfire383 » Wed Nov 10, 2010 7:21 pm

Open hedders will make it hard to decipher the click of a single valve. If you can slip some mufflers or pipe on the thing it'll help you be confident in your work.

good luck
User avatar
starfire383
 
Posts: 952
Joined: Tue Jul 04, 2006 4:06 pm

Re: Manifold Vacuum

Postby BillPappy » Wed Nov 10, 2010 10:45 pm

Just wondering about something.. What kind of intake do you have?? Is it a chevy stock 4 barrel intake?
I had this problem. After doing everything I could think of. One day I pulled the intake for the 8th time. I notice a plug in the side.
I never noticed it before.
Someone had drilled thu the side.. Making a big hole in the middle of the Duel plane stock intake. Then plugged the outside..
So I flipped it over and pulled the pan off the bottom. The pan is to keep hot oil from hitting the underside of the intake.
Thats when I seen a big crack And where they had drilled Made a very big intern vacuum leak.
Best time for adjusting valves is with the intake off.. YA can see what your doing. You can watch the lifter As you set the lash.
But these guys are dead on... adjusting Valves with Just headers is dang nere imposable.
Me,, I hate adjusting Valves that way now days.
Oil Smoke,,All the splashing oil.
It's hard for an old street racer to stay on the safe side of stupid

Pappy
User avatar
BillPappy
 
Posts: 675
Joined: Fri Feb 01, 2008 6:35 am

PreviousNext

Return to Carb/Intake/EFI Induction Tech

Who is online

Users browsing this forum: No registered users and 1 guest

cron