The 1947 - Present Chevrolet & GMC Truck Message Board Network







Register or Log In To remove these advertisements.

Go Back   The 1947 - Present Chevrolet & GMC Truck Message Board Network > 47 - Current classic GM Trucks > The 1947 - 1959 Chevrolet & GMC Pickups Message Board

Web 67-72chevytrucks.com


Reply
 
Thread Tools Display Modes
Old 05-22-2018, 12:21 AM   #1
evilokc
Registered User
 
Join Date: Mar 2013
Location: albuquerque New Mexico
Posts: 522
carb tuning question

my truck is running pretty well but I have some questions about tuning the mixture on my holley. to start I know my carb needs larger main jets. I used a vacuum gauge to set the mixture. i plugged into the constant manifold and set the screws (all4) out 1&1/2 turns then adjusted then while watching the vacuum gauge. once it was at the highest reading i took it out and it went lean on me.

1) the highest reading is the correct mixture?

2) if the main jets are to small will i be able to get the mixture right?

3) when you change the jets do you need to reset the mixture?
evilokc is offline   Reply With Quote
Old 05-22-2018, 12:32 AM   #2
geezer#99
Registered User
 
geezer#99's Avatar
 
Join Date: Aug 2009
Location: Bowser
Posts: 13,732
Re: carb tuning question

What 4 corner idle carb do you have?
Mixscrews are for idle setting.
At elevation you need to lean your jets, not richen them.

Can you screw any of the mix screws in all the way without the motor quitting?
geezer#99 is offline   Reply With Quote
Old 05-22-2018, 11:08 PM   #3
evilokc
Registered User
 
Join Date: Mar 2013
Location: albuquerque New Mexico
Posts: 522
Re: carb tuning question

Quote:
Originally Posted by geezer#99 View Post
What 4 corner idle carb do you have?
Mixscrews are for idle setting.
At elevation you need to lean your jets, not richen them.

Can you screw any of the mix screws in all the way without the motor quitting?
I'm not at elevation anymore. I'm at sea level. when i was adjusting the idle mixture screws all it was trying to die on me when i started screwing them in
evilokc is offline   Reply With Quote
Old 05-22-2018, 04:17 AM   #4
mr48chev
Registered User
 
mr48chev's Avatar
 
Join Date: Jul 2007
Location: Toppenish, WA
Posts: 15,710
Re: carb tuning question

I'd say we first need to know the model number of the carb you are dealing with.

Is this an original out of the box carb or is it an older carb that possibly someone mixed and matched parts on?

Yes the highest vacuum reading you can get is normally what you want that and a steady vacuum reading. Normally secondaries don't have an idle mixture system in them. In the past I have mixed and matched Holley parts to build a carb that didn't exist But I don't know if a primary metering block can be put on the secondary side.
__________________
Founding member of the too many projects, too little time and money club.

My ongoing truck projects:
48 Chev 3100 that will run a 292 Six.
71 GMC 2500 that is getting a Cad 500 transplant.
77 C 30 dualie, 454, 4 speed with a 10 foot flatbed and hoist. It does the heavy work and hauls the projects around.
mr48chev is offline   Reply With Quote
Old 05-22-2018, 09:02 PM   #5
Black_Sheep
Registered User
 
Black_Sheep's Avatar
 
Join Date: Jun 2017
Location: upper midwest
Posts: 1,129
Re: carb tuning question

Some Holley carbs have reverse idle mixture screws, the “list” number off the choke air horn will identify what carb you have. Unless you are running a big cam, the carb jetting should be close right out of the box.

I like to set for peak vacuum then lean it out until it just begins to lose rpms.Reset the curb idle and tweak it again.
Black_Sheep is offline   Reply With Quote
Old 05-22-2018, 09:05 PM   #6
Black_Sheep
Registered User
 
Black_Sheep's Avatar
 
Join Date: Jun 2017
Location: upper midwest
Posts: 1,129
Re: carb tuning question

What makes you think it goes lean?
Black_Sheep is offline   Reply With Quote
Old 05-22-2018, 11:06 PM   #7
evilokc
Registered User
 
Join Date: Mar 2013
Location: albuquerque New Mexico
Posts: 522
Re: carb tuning question

its a new model quickfuel hotrod series carb. I believe the part number is hr-580-vs. they have 4 mixture screws. one on each corner. it also has a traditional singe idle speed screw. the truck surges and struggles at constant throttle which is a sign that the jets are off. i did the vacuum test to see what size power valve i needed and then pulled the metering block to see what was in it. the factory jets are 66 but it has 64s in it now. holley tech said the carb is good size for the motor but since its on the small side needs bigger jets. they suggested i try 72s. the power valve IS the size they suggested. i guess what I'm asking and I'm sorry if this was answered and went over my head. will the known problem of small jets be more apparent once the mixture is leaned out?
Attached Images
 
evilokc is offline   Reply With Quote
Old 05-22-2018, 11:50 PM   #8
geezer#99
Registered User
 
geezer#99's Avatar
 
Join Date: Aug 2009
Location: Bowser
Posts: 13,732
Re: carb tuning question

So what’s the rest of your motor’s specs?
The struggling and surging isn’t always the carb.
Most of the time it’s timing.
What’s your timing curve like?
geezer#99 is offline   Reply With Quote
Old 05-22-2018, 11:59 PM   #9
Black_Sheep
Registered User
 
Black_Sheep's Avatar
 
Join Date: Jun 2017
Location: upper midwest
Posts: 1,129
Re: carb tuning question

The idle circuit supplies fuel as the carb transitions to the main jets, it can effect the air/fuel ratio as engine rpms increase
Black_Sheep is offline   Reply With Quote
Old 05-23-2018, 12:27 PM   #10
mr48chev
Registered User
 
mr48chev's Avatar
 
Join Date: Jul 2007
Location: Toppenish, WA
Posts: 15,710
Re: carb tuning question

I pulled up the instruction sheet and it has a pretty good section on setting the idle mixture screws. http://documents.holley.com/hr_slaye...ons_rev._1.pdf

There are a lot of tuning points on that carb that may affect the off idle transition including the choice of air bleeds. It seems to be infinitely adjustable in the right hands.

Still you are back to basic carburetor theory and how each circuit of the carb works at what time under what conditions. Meaning is the going lean condition at an off idle point of mild acceleration, is it under hard acceleration or is it at higher road speeds?
__________________
Founding member of the too many projects, too little time and money club.

My ongoing truck projects:
48 Chev 3100 that will run a 292 Six.
71 GMC 2500 that is getting a Cad 500 transplant.
77 C 30 dualie, 454, 4 speed with a 10 foot flatbed and hoist. It does the heavy work and hauls the projects around.
mr48chev is offline   Reply With Quote
Old 05-26-2018, 02:33 PM   #11
evilokc
Registered User
 
Join Date: Mar 2013
Location: albuquerque New Mexico
Posts: 522
Re: carb tuning question

I believe I have the initial issue solved. I swapped jets and its running great. put about 60 miles on it in the last 2 days and its pretty happy now. next question. I found some trash in the main jets when I changed them. its possible that had always been there but I want to add a post pump smaller filter. every website seems to say 10 or 20 micron. the 10 micron guys says that 20 is too large and the 20 micron guys say the 10 wont flo well enough. what say you guys? 10 or 20 micron between pump and carb?
evilokc is offline   Reply With Quote
Reply

Bookmarks


Posting Rules
You may not post new threads
You may not 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

Forum Jump


All times are GMT -4. The time now is 10:19 PM.


Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2024, vBulletin Solutions Inc.
Copyright 1997-2022 67-72chevytrucks.com