Please help 1986 4x4 Suburban

Discussion in 'General Chevy & GM Tech Questions' started by gapzero, Mar 9, 2009.

  1. gapzero

    gapzero New Member

    I have a 1986 Gmc Suburban. It is 4x4 5.7. It starts right up with no problems. It idles fine when cold. Then when it warms up it wont idle on its own. I have to keep my foot on the gas to keep it running. Any help would be great cause this is my daily driver.
  2. KirkW

    KirkW Rockstar

    I have a 1986 Factory Service Manual and used to own an '86 Suburban, so I think I can help. In order to best answer your post I'll need to know the 8th digit of your VIN (it's either "L" or "M"), and also whether this was built with California or Federal emissions. This will tell me exactly which carb is found on your engine.

    My first thought is that the idle-speed screw is misadjusted. When the choke is closed, a stepped-cam rides up on the idle-speed screw (NOT the sealed mixture screws). This stepped cam forces the throttle open when the engine is cold. As the engine warms up, the choke coil begins to unwind. When you tap on the gas pedal you release the tension on the stepped cam and it rotates out of the way, forced by the choke coil. This allows the throttle plates to close more as the idle-speed screw lands on a lower step.

    If too low, then the engine will run too slowly and may stall.

    To gather more symptoms, try this. Remove the air cleaner on a cold engine. Press and release the gas pedal once. You should see both the primary and secondary barrels closed off by choke plates. Actually, the secondaries are closed off by an 'air valve', not a choke plate, so they'll always looked closed off.

    Have a helper start the engine. The primary choke plate should open up slightly as soon as the engine starts. That indicates the choke pull-off is operating correctly.

    As the engine warms up the choke plate over the primaries should gradually begin to open more and more (it takes several minutes). The idle speed will pick up as the engine warms, too. The opening choke indicates the choke coil is operating correctly.

    With the choke partly open, have your helper tap the gas pedal. The idle speed should drop somewhat lower. When the choke is fully open, tapping the gas pedal again should allow the idle speed to drop to its lowest value.

    That's what happens in a normal setup. If your carb is not behaving this way, please describe what it's doing differently and I'll try to help.
  3. gapzero

    gapzero New Member

    I am sorry it toome so long to get back. It is a L vin with a/c. I took it to the shop and they said the carb was shot. SO i guess I am going to get a rebuild kit and give that a try.
  4. KirkW

    KirkW Rockstar

    When rebuilding the Rochester QuadraJet, may I recommend the following books:

    HP Books: Rochester Carburetors,00.html?Rochester_Carb_Hp014_Karen_Roe#

    Haynes Techbook: Rochester Carburetor Manual

    Either book is a good resource for taking apart, cleaning and reassembling the carb, but the Haynes book is a little more 'step-by-step'. Your carb is going to be somewhat unusual because it has a "Dual Capacity Pump" - an accelerator-pump with an electric solenoid that varies the pump-shot depending upon engine temperature. Not a problem, just one more piece in the puzzle.

    When you go to buy the rebuild kit, the parts-counter guy will need to know the "carb number" (year/make/model won't cut it). The number is stamped in the metal of carb-body on the left side (throttle-linkage side), above the secondary-throttle shaft. It's an 8-digit number that begins "17085...". Post that number here so I can give you the exact adjustment settings from the factory manual (there are 39 different carbs offered for that year, depending upon exact engine and options).

    I also recommend getting a Carb Cleaner Dip Basket:

    This is MUCH better than using spray cans of carb-cleaner or gasoline. Very effective. The attached picture is from when I rebuilt the 2-bbl Rochester 2GV in my '68 Buick Skylark. It shows a venturi cluster before and after dipping in carb-cleaner. As you can see, it gets very clean, both inside and out. Trying to get the same results with spray cans is difficult and messy, and the overspray and spatter can get in your eyes, ruin nearby plastic pieces, spot paint.... The dip-tank is the way to go.

    (If you're curious, you can read my entire pictorial essay of rebuilding a 2GV here, though you have to register to see the pictures:

    Attached Files:

  5. Chris Miller

    Chris Miller Rockstar 100 Posts

    That carb dip is great stuff.
    Just don't knock it over in your garage like I did. :no: Took a couple of weeks to get rid of the smell.
    The '86, if memory serves, was the year they introduced the so-called "Frankenstein" carburetors, with a throttle position sensor and idle air control valve. Either of those will run about $50.
    If your choke is operating properly, check the idle air control valve next. On that carb, it sticks straight down through the right front top side. If it's full of junk, change that first. The throttle position sensor won't usually make it idle very badly, but it can make the idle jump up and down a bit.
  6. gapzero

    gapzero New Member

    you guys are really knowledgeable and helpful. Thank you so much. My carb is not the frankenstein yet. it has electric choke and a m/c solenoid which is mixture control. that is all. the number from my carb is 17085206. once again thanks
  7. KirkW

    KirkW Rockstar

    I don't think your carb has the 'dancing needle' electronic mixture control. What appears to be a mixture solenoid on the top of the carb is actually the Dual-Capacity Pump solenoid that controls the accelerator pump. If I remember correctly, the wires from the solenoid connect to a water-temp sensor on the engine, and then to power and ground. Basically, when the engine warms up the solenoid turns on and cuts the accelerator pump volume in half.

    The electronic carbs were used on California-emissions trucks.

    Bit of trivia - your carb is known as a Rochester M4MED - the "M" for Modified (open-loop, non-electronic) fuel metering, the "4M" for 4-bbl., the "E" for electric choke, and the "D" for dual-capacity pump. A 'frankenstein' carb would've been known as an E4MED, the first "E" for electronic.

    But back to yours. A carb number of 17085206 uses these adjustment values:

    Float Level: 10mm (13/32")
    Pump Rod Setting: 7mm (9/32")
    Pump Rod Location: Inner
    Air Valve Spring: 7/8 turn
    Choke Coil Lever: .120
    Fast Idle Cam (Choke Rod): 46°
    Vacuum Break (Front): n/a (no front-vacuum break)
    Vacuum Break (Rear): 26°
    Air Valve Rod: 0.6mm (0.025")
    Unloader: 39°
    Propane Enrichment Speed: 20rpm

    What they mean - most of these settings are fixed or adjusted by bending metal tabs. Which also means that unless someone has monkeyed around with the carb, they should all be correct. Simply cleaning and reassembling the carb with all new gaskets should solve your problems. But check those settings as near as you can. Most rebuild kits come with a cardboard ruler and a description of the above settings and how they are measured.

    The Propane Enrichment Speed is GM's way of tuning the carb for best emissions. At idle and in Drive, adding propane to the air cleaner should cause the idle to drop by the specified RPM. This shows that the mixture screws are adjusted to an ideal, lean state.

    In practice I haven't had much luck with that method. When I rebuilt my Rochester I used a Gunson's ColorTune spark plug:

    Basically, it's a see-through spark plug that allows you to see the color of the flame inside the engine while it's running. Turn the mixture needle until the color is a nice blue. If it's yellow it's too rich, blue/white and/or flickering and it's too lean. It got my old '86 Suburban to pass the emissions test with flying colors, and my '68 Buick has tuned up and run quite nicely.

    Anyway, have fun rebuilding the carb. Feel free to post with any questions.
  8. gapzero

    gapzero New Member

    You guys are awesome. I do have a question. When I took it apart and started cleaning it , two little hollow brass tubes fell out of the main body. I now have no idea where there go. PLEASE HELP. Cause it is clean and ready to go back together. Also, I build custom and restoration quality wiring harness' for any year vehicle. So if you ever have and wiring or electronics questions I can help.
  9. KirkW

    KirkW Rockstar

    Without pictures it'll be hard to tell. But most QuadraJets have four brass tubes on the underside of the air horn (the top main piece). These are driven in place from the factory and should not be loose. If you have the HPBooks "Rochester Carburetors", look in page 136 - it will describe the brass air bleed tubes and how to make a special tool to drive them home. If you simply tap them in place, you'll distort the precise hole at the tip and create driveability problems when the secondaries kick in.

    I'll try to make a scan of the pages at work today and post them tonight.
  10. KirkW

    KirkW Rockstar

    Ok, I have a PDF of that page from the "Rochester Carburetors" book. Unfortunately it's too big to attach so I'll have to e-mail it to you (check for a private message with contact info).

    If the brass tube you mentioned looks like the one in the attachment, you'll need to drive it back into place in the airhorn. I would ignore the text about drilling holes and orifice changes - you're not trying to tune a race-winning Corvette, just get your Suburban to run correctly. The pictures show the brass tubes and where they go. You'll need a setting tool to drive the brass tubes back into the airhorn. The text describes very precise measurements to make a tool that will drive the air-bleed tubes to an exact depth.

    However, you could probably get away with a simple block of aluminum, drilled with a 1/16" drill bit to about a 1/4" in depth. This block will then bottom on the 'shoulder' of the air-bleed tubes and not the tip. This will allow you to drive the tube back into the airhorn by very gently tapping with a hammer on the aluminum block. Work slowly, measure frequently, stop when it's standing at the same height as the other tubes.

    If the larger fuel-feed tubes have come loose, then a slightly countersink on hole drilled in the aluminum block will probably provide enough surface to bear on the rounded edges of the fuel-feed tube and not directly on the tip. Tap it in place in the same manner to about 1-1/4" in height. In a stock carb, all tubes sit at the same height.

    The other worry is why the brass tube fell out in the first place. I would apply some horizontal scratches to the end of the tube that sticks into the airhorn before tapping it in place. The idea is to scratch the surface enough to raise a ridges of metal that it will make a tight fit and keep it in place.

    Good luck!

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