Can low vacuum effect idle circuit on Q Jet - Chevelle Tech
Engine General Engine Discussion.

 9Likes
Reply
 
LinkBack Thread Tools Search this Thread Display Modes
post #1 of 22 (permalink) Old May 30th, 20, 10:19 AM Thread Starter
Tech Team
Stan
 
Join Date: Jan 2004
Location: Alberta, Canada
Posts: 539
Can low vacuum effect idle circuit on Q Jet

New cam in my small block and ended up with 8-9 in of vacuum. Everything seems OK but cannot get sustainable idle below 1100 rpm. Does a Q Jet require a minimum amount of vacuum for the idle circuit to work properly? If so, can a Q Jet be modified to work with low vacuum?
stanski is offline  
Sponsored Links
Advertisement
 
post #2 of 22 (permalink) Old May 30th, 20, 1:58 PM
Senior Tech Team
 
Join Date: Apr 2012
Location: Yerington, Nevada
Posts: 2,244
Re: Can low vacuum effect idle circuit on Q Jet

Complete ignition degrees needed for all parameters, and cam degree specs (NOT lift), especially intake closing point, and overall compression ratio, PLEASE.

Very end of the process, not usually needed, but, if the cam is seriously duration, valve timing big, it might need bleed holes in the primary throttle plates, VERY, VERY RARE.
Dave Ray is offline  
post #3 of 22 (permalink) Old May 30th, 20, 3:37 PM Thread Starter
Tech Team
Stan
 
Join Date: Jan 2004
Location: Alberta, Canada
Posts: 539
Re: Can low vacuum effect idle circuit on Q Jet

Lift .310/.325 Duration 225/235

Timing at 12 degrees right now

Compression 10:1
stanski is offline  
 
post #4 of 22 (permalink) Old May 30th, 20, 4:08 PM
Lifetime Premium Member
Jim
 
Join Date: Nov 1999
Location: International Falls MN
Posts: 6,582
Re: Can low vacuum effect idle circuit on Q Jet

You're gonna have to open up the Idle restrictions & down channel restriction & maybe the Idle air bypass.
You probably have the mixture screws out so they are just about to fall out?
A little more timing too.

Jim

77 Monte
421 Profilers ported by Eric Weingartner, Straub HR 3200 conv. from Jake 200-4r w/ 3.73's
Machine work by Torvinen's machine
11.93 @ 112.7 & tuning
67 Imp.SS for the wife 275 hp 327-TH350
77 cruiser is offline  
post #5 of 22 (permalink) Old May 30th, 20, 8:51 PM
Senior Tech Team
 
Join Date: Apr 2012
Location: Yerington, Nevada
Posts: 2,244
Re: Can low vacuum effect idle circuit on Q Jet

Lift .310/.325 Duration 225/235

Timing at 12 degrees right now

Compression 10:1

This tells me virtually NOTHING of how the timing is set up, nor the overall duration of the cam, let alone the closing degree of the intake valve. Hard to get the right info out, when there is nothing to go by.

Now:
INITIAL timing: 12 deg/BTDC
Vacuum advance degrees added by full manifold vacuum, unknown
Full IDLE timing, unknown
Mechanical advance curve, unknown
TOTAL initial and mechanical timing, unknown

Cam overall degrees, unknown
Cam degrees @ .050, 225/235
Lift: not important
Intake valve closing degrees, unknown
Lobe centers, unknown

Engine idle speed, way too high

Info would help.
Dave Ray is offline  
post #6 of 22 (permalink) Old May 30th, 20, 9:07 PM
Boldly procrastrinating
Tom Terrific II
 
Join Date: Oct 1998
Location: Glendale, Az
Posts: 28,798
Garage
Re: Can low vacuum effect idle circuit on Q Jet

What Dave said. and 12 degrees is not going to be enough for a cam like that. You need to be looking at about 20 or so. try that first, it will speed up the idle and allow the the throttle blade angle to be returned to more normal position. try hooking the vacuum advance to manifold vac. it may help.

The idle feed restrictions can be drilled but it takes special extra long .038 or .039 drill. not many have these.
hydro462 likes this.

Tom Terrific or Terrible Tom, depending on the phase of the moon, passing cosmic rays or other factors not fully understood except by my wife.
Tom Mobley is offline  
post #7 of 22 (permalink) Old May 30th, 20, 9:27 PM
Lifetime Premium Member
Jim
 
Join Date: Nov 1999
Location: International Falls MN
Posts: 6,582
Re: Can low vacuum effect idle circuit on Q Jet

Should ask too what is the carb from, number.

Jim

77 Monte
421 Profilers ported by Eric Weingartner, Straub HR 3200 conv. from Jake 200-4r w/ 3.73's
Machine work by Torvinen's machine
11.93 @ 112.7 & tuning
67 Imp.SS for the wife 275 hp 327-TH350
77 cruiser is offline  
post #8 of 22 (permalink) Old May 31st, 20, 7:09 AM
Senior Tech Team
Jeff
 
Join Date: Jun 2009
Location: Pittsburgh, Pa.
Posts: 2,770
Re: Can low vacuum effect idle circuit on Q Jet

Your going to have to raise the initial timing. I had to run straight timing on a few cars to get them to idle okay. If you have a vacuum booster you will have to get a pump, hydro boost or convert to manual.. Also you can change the brake pedal feel if you move the rod to the upper hole (6:1) manual ratio..
hydro462 is offline  
post #9 of 22 (permalink) Old May 31st, 20, 7:33 AM
Lifetime Premium Member
Jeff
 
Join Date: Jan 2002
Location: western ma.
Posts: 686
Re: Can low vacuum effect idle circuit on Q Jet

225* @ .050 isn't all that big of a cam. You should have more vacuum than that. You sure you don't have a vacuum leak?
427L88 likes this.
nitrofox is online now  
post #10 of 22 (permalink) Old May 31st, 20, 8:22 AM
Gold Member
Gene
 
Join Date: Sep 2000
Location: Buffalo NY
Posts: 15,571
Re: Can low vacuum effect idle circuit on Q Jet

Keep in mind y'all, @ 8-9" Hg, the power piston is likely raised a bit by the stock spring, and increasing the surface area of the primary jet. QUESTION: if so, would Stan be seeing dripping fuel from the primary boosters @ idle? Or only if the pri throttle plates are open too far?
Alan F and 71350SS like this.

Gene
ACES 3112/Team Chevelle Gold #62
Be big, be a 'builder'!
427L88 is offline  
post #11 of 22 (permalink) Old May 31st, 20, 9:18 AM
Lifetime Premium Member
Jim
 
Join Date: Nov 1999
Location: International Falls MN
Posts: 6,582
Re: Can low vacuum effect idle circuit on Q Jet

Power valve wouldn't make the boosters drip, but opening the throttle too far will or too high fuel level.
427L88 likes this.

Jim

77 Monte
421 Profilers ported by Eric Weingartner, Straub HR 3200 conv. from Jake 200-4r w/ 3.73's
Machine work by Torvinen's machine
11.93 @ 112.7 & tuning
67 Imp.SS for the wife 275 hp 327-TH350
77 cruiser is offline  
post #12 of 22 (permalink) Old May 31st, 20, 12:26 PM
Senior Tech Team
 
Join Date: Apr 2012
Location: Yerington, Nevada
Posts: 2,244
Re: Can low vacuum effect idle circuit on Q Jet

The @.050 numbers are not a good indicator of how radical, or not a cam is. It all depends on the ramps on the lobes past the .050. Fast opening, more degrees under curve, less engine vacuum, higher overall degrees number. Slower ramps, less overall duration, better vacuum, more sedate, easier to control with timing and carb.

So, once again, what are the numbers we asked for, so we can help you accurately?
Dave Ray is offline  
post #13 of 22 (permalink) Old May 31st, 20, 12:46 PM
Senior Tech Team
Gary
 
Join Date: Feb 2007
Location: Kansas City,Mo.
Posts: 4,532
Re: Can low vacuum effect idle circuit on Q Jet

First thing I would do is get the timing up to 18 220 degrees or so then drop the idle speed. Keep it high enough where it will idle. Once it's warmed up take your hand and gradually choke the primary side of the carburetor a little at a time and see what this does to your idle speed. If the RPM Rises its lean. If it raises much more than 50 you need to work on the idle circuit. Have you tried to adjust the mixture screws?
gnicholson is offline  
post #14 of 22 (permalink) Old May 31st, 20, 1:17 PM
Senior Tech Team
 
Join Date: Feb 2019
Posts: 1,116
Re: Can low vacuum effect idle circuit on Q Jet

Here's some things I did years go when I ran Q-jets. First put your cam on a 114 lca and that will help with vacuum. Second use a spring from an ink pen (might need several for the right one) but out flat out works with big cams. Third, if all else fails but a new Q-jet form Jet performance slap it on and enjoy the ride.


To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.
68Chevele is offline  
post #15 of 22 (permalink) Old May 31st, 20, 1:48 PM Thread Starter
Tech Team
Stan
 
Join Date: Jan 2004
Location: Alberta, Canada
Posts: 539
Re: Can low vacuum effect idle circuit on Q Jet

Carb # is 1708224
stanski is offline  
Sponsored Links
Advertisement
 
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Chevelle Tech forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address. Note, you will be sent a confirmation request to this address.

Email Address:
OR

Log-in










Thread Tools Search this Thread
Show Printable Version Show Printable Version
Email this Page Email this Page
Search this Thread:

Advanced Search
Display Modes
Linear Mode Linear Mode



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome