Home » Public Forums » GMCnet » [GMCnet] Quadrajet carbs for GMC motorhome
| [GMCnet] Quadrajet carbs for GMC motorhome [message #340911] |
Mon, 11 February 2019 14:25 |
gmc-email-list
Messages: 124
Registered: February 2019
Karma:
|
Senior Member |
|
|
Bruce—
this might be of interest to you.
This was posted16 years ago:
At 2:01 PM -0700 7/7/03, Brent Covey wrote:
> From: Brent Covey [ ] Vancouver, BC [000636]
>
> Hi guys!
>
> I have been doing a bunch of carb work this past few weeks on some big inch
> GM engines and have something you might want to try out-
>
> GM p/n 07034838 Metering Rods
>
> These are primary rods that work well with stock GMC jets. They appear to be
> at the very cusp of blowing the smog test for Smog II and the BC test but
> should squeak thru OK and are a very good match to most of the big engines.
>
> They're very easily obtained at any GM dealer for about $5. I would be
> interested in some feedback from anyone else who tries them out. I think
> you'll be quite surprised at the tremendous power increase, my experiments
> suggest the fuel economy remains good.
> Brent
I had also posted this some time back for the jets:
At 1:22 PM -0700 11/2/02, Emery L. Stora wrote:
> Part numbers: 7031971 and 7031972
> Just change the last two numbers for the size you want.
> The parts places should be able to order them and I am really surprised that they don't have some in stock. Try an Olds, GMC, Chevy or Caddy dealer parts department if the auto parts stores don't have them.
Or, going even farther back Brent posted this:
At 6:31 PM -0700 3/3/01, Brent Covey wrote:
> Theres a few things to pay extra close attention to-
>
> All carb kits and parts within them are ordered by looking them up relative
> to the carb number stamped into the carb body. This number will start out
> with "70xxxxx" or "170xxxxx" and you'll need that to order the goodies for
> the rebuild.
>
> Most GMC carbs will have very little wear as this is something that occurs
> more in high city traffic mileages, not just zooming down the highway.
>
> Every time you encounter a black coloured foam float, replace it. Some GMC's
> use a brass one which you can shake and see if its pinholed and allowed
> liquid inside. Most are fine.
>
> ALWAYS buy a new vacuum break diaphragm, this is the little bellows thing
> that pulls the choke open, located at the passengers side front of the carb.
> These are the #1 failure in carbs, and cause flooding and hard starts when
> they fail. A used one is OK to use, but make sure you have a spare anyhow,
> and carefully check that it holds a vacuum and pulls the choke open when
> vacuum in applied.
>
> Never remove the throttle plates or shafts or choke plate/secondary air
> valve unless you are replacing them, as the screws often break off and are
> in a position if they fall out the engine will inhale them, which is bad
> news.
>
> The vacuum break adjustment is altitude sensitive. The kit instructions do
> not make it clear how its adjusted, but the essence is, it should open about
> 3/16" when the vacuum break diaphragm has vacuum applied to it. This is
> approximately the distance the little groove embossed into the choke flap is
> wide. Once its on, you may need to fiddle a little with this setting, it
> needs to open less at low altitudes than higher ones.
>
> Main jets need a PERFECT fit screwdriver to remove them. They can strip and
> tear up with the wrong one, so get a really tight fitting screwdriver for
> them. A sharp blow to the handle of the screwdriver with a light tool will
> usually aid in breaking them free, they like to get stuck in there.
>
> Retain the original GMC main metering rods, they're perfect for the GMC.
> Jets can be changed to increase mixture strength a little, all GMC's should
> use at least a '70' jet, and some could benefit with as much as a 74.
> Basically as a rule of thumb go 3-4 jet sizes richer than whats stock and
> you should be OK. At altitudes over 3000', #71 is fine for most people. At
> 5000'+ 70 will do. Jets have a part number ending in a two digit suffix that
> designates their size, a 0.070" opening in a jet is a "XXXXX70" jet as
> example. The prefixes are determined by carb family, and can be looked up in
> the book, jets are about $3 each from GM.
>
> The secondary air valves are opening too soon on most carbs, you should have
> a close look at yours and make sure you understand where the adjustment is
> located. Theres a set screw beneath the carb top, upside down that locks the
> screw visible inset in the passengers side rearward edge that adjusts spring
> tension. Double check lockout operation, not solely for locking out, but
> also for releasing the air valve again as well.
>
> Float settings aren't especially critical, go with the book on those. Be
> sure the needle is hanging off the rearward edge of the float arm, not
> hooked thru the holes in the arm which many people do. Be sure to stretch
> the float hinge axle (C shaped heavy wire thing) open a bit to ensure when
> the carb top goes back on it compresses the axle enough to ensure the float
> has a good fulcrum to seat the valve with.
>
> Make sure you install the choke seal and plastic hollow pin between the
> housing and carb body. The choke must be adjusted so that it is strong
> enough to just close the choke well at room temperature, you need to
> slightly open the throttle for this. Check that the fast idle cam behind the
> choke is pulled to the top step when the choke is closed and throttle is
> released.. Adjustments of the choke spring are best carried out with the
> plastic choke housing cover loose enough to turn with your thumbs, and the
> screws out. The apparent screwdriver slot in the plastic cover will break if
> you attempt to use a screwdriver usually.
>
> The plugs in the bottom of the carbs can work loose, and this usually
> manifests itself as high fuel consumption, and very slow starts after you
> have let it sit a few hours with a hot engine. You can inspect them for
> signs of leakage, usually they're OK, and a dollop of some non-fuel-soluble
> sealant will keep them dry usually. You can fix a leaking plug by judicious
> restaking of it, but if you drive the plug too far in, you'll possibly block
> the passage its for. The plugs are self evident from their grey colour.
>
> Be sure to double check screw tensions just before you install the carb as
> well.
>
> Its imperative of course that you take your time and have very clean parts
> to work with. Invest in some carb cleaner and let it soak overnight.
> Extremely stubborn deposits will usually yield to Oven Cleaner, but this
> will eat the castings fast if not supervised, washed in water thoroughly,
> and de-activated with acid, such as white vinegar. You MUST pay close
> attention if you use a caustic cleaner, and be sure you've soaked it in
> vinegar etc very well after, and rinsed in water.
>
> WD-40 makes a reasonably good source of 'compressed air' for cleaning
> passages. The engine will also start and run OK on WD40 if you want to prime
> it a little thru the bowl vent. Whatever you do, make sure its clean as you
> can get it, before you reassemble it.
>
> Most carbs will look pretty good and only need a kit and slight tweaking of
> the adjustments.
>
> The big fuel filter nut is 1" and must be tightened carefully so as not to
> strip the threads. When you reconnect the smaller fuel line to it, use a 1"
> wrench (many crescent wrenches will reach 1" satisfactorily) and bend the
> tube slightly in the direction of 'loosening it' while you tighten the flare
> nut on the end the last bit, which will put some tension in the direction of
> 'self tightening' in the tube and help keep it tight. You will need a flare
> nut wrench to loosen the fuel line, buy a TOP QUALITY one, the cheap ones
> are as good as useless. If you just cant get it apart, taking it off in the
> first place, Vise Grips will usually do it OK, or you can cut the line and
> use a six point socket to get the flare end out and replace the steel fuel
> line later. Don't re-use a steel line with a damaged flare, rounded off or
> manhandled nut, or a kink in it, its a fire risk.
>
> Set Idle mixtures once the new carb is on by 'lean roll'. Test secondary
> airvalve operation, and adjust as necessary, and next day on a cold engine,
> check the cold starting and fast idle, and tweak if need be.
>
> Thats basically most of it. I am sure I've forgotten something I'd have
> thought of if I had one right here, and of course, if you see anything awry,
> find out what you need to do.
>
> As always, any carb or fuel system component needs doubly careful assembly
> as any leak is a terrible fire risk. Be extra careful when you do this sort
> of work and make CERTAIN that if you smell fuel you stop AT ONCE and
> investigate it. This is doubly important now that the Reformulated fuels
> with heavy Oxygenate content are causing deterioration of soft carb
> components at a high rate, and much faster than we were used to in years
> past.
>
> Good luck with your QuadraJet, you should be able to do a very good job at
> home if you take your time.
>
> Brent Covey
> Vancouver BC
Emery Stora
77 Kingsley
Frederick, CO
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
|
|
|
|
Current Time: Sat Nov 16 06:29:44 CST 2024
Total time taken to generate the page: 0.01035 seconds
|
GMCforum
Search
Help
Register
Login
Home
