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| [GMCnet] Carb rebuilding [message #84689] |
Sat, 15 May 2010 09:39  |
emerystora
Messages: 4442
Registered: January 2004
Karma:
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Senior Member |
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Brent Covey has posted some good information way back in 2001:
> 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 enuf 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 judicuious
> 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 yeild to Oven Cleaner, but this
> will eat the castings fast if not supervised, washed in water thouroughly,
> and de-activated with acid, such as white vinegar. You MUST pay close
> attenbtion 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. Dont 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 neccesary, 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
Billy Massey also have some good info on carbs posted on his site that was provided by Bob Drewes.
http://www.bdub.net/oddsnends.html
It about 64 items down on the list.
Emery Stora
77 Kingsley
Santa Fe, NM
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