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| [GMCnet] Carb rebuilding [message #84689] |
Sat, 15 May 2010 09:39  |
emerystora
Messages: 4442
Registered: January 2004
Karma: 13
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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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| Re: [GMCnet] Carb rebuilding [message #84701 is a reply to message #84689] |
Sat, 15 May 2010 11:26  |
Chr$
Messages: 2690
Registered: January 2004
Location: Scottsdale, AZ
Karma: 1
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Senior Member |
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One aside, the current version of WD-40 is no longer flammable. Not sure If I'd use it. The Carb Cleaner cans come with a red straw too. Probably better to use that.
-Chr$: Perpetual SmartAss
Scottsdale, AZ
77 Ex-Kingsley 455 SOLD!
2010 Nomad 24 Ft TT 390W PV W/MPPT, EV4010 and custom cargo door.
Photosite: Chrisc GMC:"It has Begun" TT: "The Other Woman"
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| Re: [GMCnet] Carb rebuilding [message #85056 is a reply to message #84742] |
Tue, 18 May 2010 02:57 |
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mike miller
Messages: 3576
Registered: February 2004
Location: Hillsboro, Oregon
Karma: 0
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Senior Member |
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| fred v wrote on Sat, 15 May 2010 16:19 |
what's a "lean roll" idle test?
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I did not know ... did a search. This (LINKED AND BELOW) seems to sound right.
I have done this on single barrel carbs, just didn't know what it was called and never thought about it on a multi-barrel carb.
<http://teambuick.com/forums/showthread.php?p=31555>
| Quote: |
Also, to perform a "lean roll" adjustment on a multi throat carburetor do the following:
Open both mixture screws about 3-4 turns.
Then start the engine.
Turn one mixture screw completely closed. Which one does not matter.
Turn the other screw slowly in until the engine just starts to "lean roll" and then back it out 1/2 turn. The engine should run smoothly.
Now, open the other screw 3-4 turns.
This is important: Turn the previous screw in until it is completely closed AND COUNT THE NUMBER OF TURNS EXACTLY. Note the angle of the screw slot as you want to return this screw to this position after you lean roll the other screw.
Now, turn in the second screw until the engine lean rolls and then back it out 1/2 turn. You are finished with this screw.
Open the first screw the exact number of turns that you noted and make sure the screw slot is at the same angle as when it was set prior.
You are now finished setting the mixture to the leanest setting that will run the vehicle efficiently.
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Mike Miller -- Hillsboro, OR -- on the Black list
(#2)`78 23' Birchaven Rear Bath -- (#3)`77 23' Birchaven Side Bath
More Sidekicks than GMC's and a late model Malibu called 'Boo'
http://m000035.blogspot.com
[Updated on: Tue, 18 May 2010 02:58] Report message to a moderator |
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