Carb Adjustment... why?
Guest
Posts: n/a
Carb Adjustment... why?
What would cause a carb to require adjustment? I've noticed that while crusing I'm getting a slight hesitation.. She starts right up and has pickup with no problem.. She just not running smooth.. I have to reread Lar's "How to Tune a Q-Jet".. again.. I've never tried this before but just wondering if this is a regular thing with Q-Jet's or Carb engines.. Why would you need new rods? Do they wear out? I'm guessing it's my carb.. Any other ideas?
Le Mans Master
Joined: Mar 1999
Posts: 6,744
Likes: 1
From: delmont pa
Cruise-In I Veteran
Cruise-In II Veteran
Re: Carb Adjustment... why? (Koz)
check float level first then try a heavier spring in the power piston,drill the idle tube restrictions, or back out the idle air bleed screws. :chevy
Race Director
Joined: Jun 2000
Posts: 10,039
Likes: 669
From: FL
Re: Carb Adjustment... why? (Koz)
The spring on your secondary air door may have lost some tension. This is an easy adjustment.
Carbs just tend to get gunked up with age. I rebuilt mine then Lars showed me how to tune it. Using his info, I went from a 72/46 primary jet/rod combo stock to 74/47 with a cam and exhaust change to a 75/48 combo with the rebuilt motor (bigger cam and heads).
Any mods that affect airflow (intake, cam, exhaust) will probably require some carb tweaking to get the fuel flow to match.
Carbs just tend to get gunked up with age. I rebuilt mine then Lars showed me how to tune it. Using his info, I went from a 72/46 primary jet/rod combo stock to 74/47 with a cam and exhaust change to a 75/48 combo with the rebuilt motor (bigger cam and heads).
Any mods that affect airflow (intake, cam, exhaust) will probably require some carb tweaking to get the fuel flow to match.
Race Director
Joined: Mar 2001
Posts: 11,322
Likes: 1
From: Living in the Hartland
Re: Carb Adjustment... why? (clem zahrobsky)
Clem
That cured my hesitation but I have an awful smell at idle. How stong should the exhuast smell be at idle? Also, I have read the drilling small holes in the main butterflys helps increase air/fuel ratio. I actually did that on my 750 DP but had so many other problems not sure it did much.
Thanks
That cured my hesitation but I have an awful smell at idle. How stong should the exhuast smell be at idle? Also, I have read the drilling small holes in the main butterflys helps increase air/fuel ratio. I actually did that on my 750 DP but had so many other problems not sure it did much.
Thanks
Guest
Posts: n/a
Re: Carb Adjustment... why? (SteveG75)
OK.. I did replace the Cat so that could explain something. Are these adjusting rods easy to come by? Autozone type place or special order? Expensive? I haven't gotten through all of Lar's guide but when I read about taking off the top of the carb I get a little weak kneeded.. Anything to go boinnng when doing that? How hard is this to do?
Le Mans Master
Joined: Mar 1999
Posts: 6,744
Likes: 1
From: delmont pa
Cruise-In I Veteran
Cruise-In II Veteran
Re: Carb Adjustment... why? (Koz)
Q jet rods and jets are available from edelbrock(your local speed shop) or try http://www.weber-carburetors.com/quadrajet.asp :chevy
[Modified by clem zahrobsky, 3:25 PM 5/7/2002]
[Modified by clem zahrobsky, 3:27 PM 5/7/2002]
[Modified by clem zahrobsky, 3:25 PM 5/7/2002]
[Modified by clem zahrobsky, 3:27 PM 5/7/2002]
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
2027 Corvette vs The World: Every C8 vs Its Closest Competitor
Joe Kucinski
Drifting
Joined: Feb 2001
Posts: 1,603
Likes: 0
From: San Francisco, CA
Re: Carb Adjustment... why? (Koz)
Koz- I just rebuilt mine with no prior experience and If you are going to open it and remove the jets anyway, you would be foolish not to do the same. Get the Rochester Carb. book by ??? Roe regardless of what you do.
Build a little stand as shown in the book. Label all vaccuum hoses before you remove them. Be really organized and work slowly. I had seperate trays for parts from the airhorn, fuel bowl and throttle body so I couldn't get the stuff mixed up. Install a rebuild kit, filter and a new float (set- it up correctly). Clean everything up good with carb. cleaner and clean all passages and parts with compressed air if available. Then readjust the mixtures, choke, etc. and see where that leaves you. Coincidently, I put a new cat on prior to doing this and did not have to change any jets, metering rods etc.
I spent way too much time on the rebuild, but I am sure I did a more meticulous job than any shop would have done, (except Lars of course). My car has never started or run better. :D
Build a little stand as shown in the book. Label all vaccuum hoses before you remove them. Be really organized and work slowly. I had seperate trays for parts from the airhorn, fuel bowl and throttle body so I couldn't get the stuff mixed up. Install a rebuild kit, filter and a new float (set- it up correctly). Clean everything up good with carb. cleaner and clean all passages and parts with compressed air if available. Then readjust the mixtures, choke, etc. and see where that leaves you. Coincidently, I put a new cat on prior to doing this and did not have to change any jets, metering rods etc.
I spent way too much time on the rebuild, but I am sure I did a more meticulous job than any shop would have done, (except Lars of course). My car has never started or run better. :D
Safety Car
Joined: Jan 2000
Posts: 4,143
Likes: 0
Re: Carb Adjustment... why? (Koz)
Carb Adjustment... why ? - 'Koz' ( 'cuz or 'cause )....
You made a funny - ha ha :lol: :lol: :lol:
Don't know about the Quadra-junk, but Holleys require routine adjustments & especially after setting for a few weeks.... :jester
You made a funny - ha ha :lol: :lol: :lol:
Don't know about the Quadra-junk, but Holleys require routine adjustments & especially after setting for a few weeks.... :jester
Guest
Posts: n/a
Re: Carb Adjustment... why? (R. Bruno)
Is this a fairly easy job to do.. I've never removed a carb.. What special tools will I require? In Lar's write-up he talks about measuring the float level between .375 and .400 .. I know there's a trip to sears for a tool for that? what? My car starts easy.. idle's pretty good too and has pickup. It's at cruzing speed.. 50-60mph I almost feel a hesitation where she's not getting a steady feed of fuel. Last night I loader her up with carb cleaner but still felt the rough cruize. She's not bucking.. just not smooth.. From what I'm reading I can adjust the idle with a screw turn but all other adjustments are done with new rods? It can't be that hard to do if that is the only adjustments availalbe. I'm not interested in drilling wholes in the butterfly. It may work but not what I'd like to experiment with.
Burning Brakes
Joined: Mar 2002
Posts: 803
Likes: 0
From: South San Francisco CA
Re: Carb Adjustment... why? (Koz)
I suffered through that same problem for awhile. What seemed to have worked was to change the plugs and plug wires and open the gap on the plugs to .045" (.010 over). The car has been running fine with those settings for the last six or seven months. No loading up, no hesitation, no bog at all. You might experiment and give this a try.
Drifting
Joined: Feb 2001
Posts: 1,603
Likes: 0
From: San Francisco, CA
Re: Carb Adjustment... why? (Koz)
FONTKING makes a good point. Have you replaced wires etc? As to the carb., you can adjust air/fuel mixture with mixture screws. I really don't think you want to get into changing metering rods and/or jets. I am not an expert, but if your car is not modified and was running ok at some point in the past I don't think rods and jets are the problem. You don't need any special tools for the carb. work. The rebuild kit includes a little ruler for measuring float level. If the float level is not right or if it has become saturated with gas and sunk a little, then you have a problem. (It works on a similar principal to a toilet tank.) The carb may seem like black magic, but once you read-up on it and take one apart, you will see that it is not all that complicated.
Safety Car
Joined: Feb 1999
Posts: 3,631
Likes: 14
From: Fountain Hills AZ
Re: Carb Adjustment... why? (Koz)
There are lots of other things besides the carb that can cause hesitation at cruise, mostly ignition problems. A real common thing is having too much advance. What is added to initial is also added to total and you can wind up with too much, especially under light load cruise when you have max vacuum advance included. Another possibility is a weak spark. Lean mixutres can be harder to ignite and carbs are usually tuned their leanest at the conditions you described.
Instructor
Joined: Jun 2000
Posts: 226
Likes: 0
From: Southlake Texas
Re: Carb Adjustment... why? (Vetterodder)
This is how I felt when I started - STOP what you are doing - READ Lars tuning guide - ALL of it - do not drill holes in anything unless you have written Lars a mail and he says so - the QJET can feed an engine up to a 454 or better without many mods if any. My Vette ran me out of the garage it was so rich when I started - if you can't stand to be near it - it is too rich.
Race Director
Joined: Mar 2001
Posts: 11,322
Likes: 1
From: Living in the Hartland
Re: Carb Adjustment... why? (Koz)
Koz
Think I got people off your problem a bit plus I have a Holley and you have a q-jet. I adjusted my idle mixture a bit richer which got rid of my hesitation but leaves me with a rich mixture at idle. I read in a Holley tuning book that if your drill small holes in the butterflies it can lean up idle mixture and get rid of some of the smell at idle. I am not a q-jet expert but I would NOT do it a q-jet, not much there to drill for the primaries. I think I might have too small a power valve, power valve feeds a little extra gas when under load.
Granted carbs aren't black magic but diagnosing a problem takes some experience since more than one problem may give very similar symptoms.
Sorry about causing the confussion
Think I got people off your problem a bit plus I have a Holley and you have a q-jet. I adjusted my idle mixture a bit richer which got rid of my hesitation but leaves me with a rich mixture at idle. I read in a Holley tuning book that if your drill small holes in the butterflies it can lean up idle mixture and get rid of some of the smell at idle. I am not a q-jet expert but I would NOT do it a q-jet, not much there to drill for the primaries. I think I might have too small a power valve, power valve feeds a little extra gas when under load.
Granted carbs aren't black magic but diagnosing a problem takes some experience since more than one problem may give very similar symptoms.
Sorry about causing the confussion
Safety Car
Joined: Jun 2001
Posts: 4,004
Likes: 2
From: Severna Park Md
Re: Carb Adjustment... why? (Fevre)
Weather, air density, altitude, many things not related to the carb itself can warrant an adjustment. This is where E.F.I. is superior. The computor adjusts for these differences for you automatically.
Guest
Posts: n/a
Re: Carb Adjustment... why? (fontking1a)
Thanks everyone.. That is what I was looking for.. Basically since I haven't change anything (other than the Cat) I would suspect that the carb is either dirty or my plugs/wires need attention.. Since I haven't change my points then my timing shoudn't have changed either... right? Rods should not be causing this problem.. I'll try pulling the plugs first to see what I find. :cheers: