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73-87 Chevy _ GMC Trucks => 73-87 Chevy & GMC Trucks => Topic started by: Smitty31 on October 04, 2017, 08:59:45 am
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Ok 1986 4.3 v6, A.T. W/a 2wd. Should the choke plate be straight up and down when the engine is fully warmed up? Mine is not I would say it's 95% but not straight up and down.also the idle screw is not touching backed all the way off and the truck seems like it idles a little to high. The high idle adult mentioned foe cold starts seems to work. When the truck is cold it idles high after a few minutes I tap the gas and it idles down.
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The choke plate should be fully open at operating temperature. Make sure the electric choke thermostat is receiving 14 volts with the engine running.
With the engine running, spray for vacuum leaks using aerosol carburetor cleaner, concentrating your efforts around the base of the carburetor and along the intake manifold flanges. Check the radial play in the throttle shaft with the engine not running by grabbing the throttle lever, rotating the throttle open 10° - 15° and wiggling the shaft side-to-side while you watch the throttle shaft where it enters the carburetor baseplate. There should be ≤1/32" of side-to-side play in the shaft. If radial play is greater 1/32", the throttle shaft bore is worn, allowing the throttle plates to shift sideways in the their bores. Mis-registration of the throttle plates in their bores prevent the throttle plates from closing completely, allowing air to bypass the throttle, keeping the base idle high (implied by the base idle screw being backed completely out). The solution for excessive radial play is to bush the throttle bore and recenter the plates or replace the carburetor.
Edit: corrected auto-correction ::)
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I will check the volts today. Had the carb rebuilt all new gaskets and repaired all vacuum leaks. The worn throttle shaft could be I'll check. Could the high idle cam be out of adjustment. Causing it to idle high.
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Of course. If the choke doesn't open sufficiently for the cold idle screw to clear the lowest step of the fast idle cam, then a high idle can result. Force and hold the choke plate all the way open and cycle the throttle to see if idle RPM drops. If the cold idle cam is preventing throttle closing and the base idle speed screw is backed out as you've described, idle RPM should drop exceedingly low, possibly to the point of stalling the engine. That is, unless the throttle plates are shifting excessively in their bores as a result of throttle shaft bore wear in the baseplate.
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Thanks for the info I will check tonight. Your not located in tennessee are you lol.
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I'm about 35 hours west of Tennessee. :)
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I backed the high idle for cold starts off @nd it idled down way down to the point I had to adjust the other idle screw back up.
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That's great news! Correct the choke operation and you'll be good to go.
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Here is a pic of the of the carb. You can see the choke plate is not straight up and down. The weird thing I can take my finger and move it all the way open and it will stay. I have replaced the choke thermostat and had the carb rebuilt.
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First, check the voltage at the choke stat as previously suggested. Is the choke stat riveted in place or retained by three screws?
(http://forum.73-87chevytrucks.com/smforum/index.php?action=dlattach;topic=35705.0;attach=37664;image)
The bimetal spring of the t-stat has a rectangular (box shaped) socket at its free end (RED ARROW) that engages a pin inside of the choke stat housing. The pin is integral with the linkage that operates the choke plate. The choke mechanism is designed for positive engagement and control of the choke plate. That is, the t-stat not only forces the choke plate closed when cold, but also forces the choke plate open as the engine warms. If the t-stat was inadvertently installed such that the linkage pin missed the spring socket, it might force the choke closed when cold, yet not pull it open as it heats, effectively causing gravity to open the choke instead of the thermostat.
OE electric choke stats generally are not adjustable, but instead designed to be indexed in a fixed position by a notch located on the periphery or outer circumference of the t-stat (BLUE ARROWS) and a corresponding tab on the mating inner circumference of the t-stat housing. There is more than one configuration that accomplishes the same task. If the choke stat was installed to miss the index, the t-stat simply might be rotated out of adjustment.
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It was fixed but the mechanic who built the carb fixed it with screws where it can be adjusted. I will check the volts and get back to you.
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Sorry for the long delay I've been busy at work and in my spare time working on my truck. I checked the volts going to the carb it was indeed at least 14 volts. Choke is still only opening up about 95 percent its oh so close just not straight up and down. I will try to take a clearer pic tomorrow and post it.
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Did you resolve this?
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Nope it still only opens up about 95 percent unless I take my finger and pull it straight up and down. It will stay then in less I push it back. Runs great warms up great also.
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Connect a DC voltmeter between the battery negative post and the choke t-stat housing then start the engine and measure the voltage. What is it?
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i did excatly that it was 14.3 volts
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Connect a DC voltmeter between the battery negative post and the choke t-stat housing then start the engine and measure the voltage. What is it?
^^^^ Read this again. The measurement validates the ground path, before proceeding to the next step.
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Ok let me start by saying I know very little about these meters. So I'm including a pic showing how mine is set up and it was this way when I took the readings. Black lead on negative post of battery and red lead on metal t housing on carb. 0.00 or on the metal spade 0.00 with engine off. Reading with engine on black to ground and red to elec. spade on t housing 14.49
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Good! Now, repeat with...
...Black lead on negative post of battery and red lead on metal t housing on carb ... with engine RUNNING....
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Ok it is 0.04 black lead on negative post and red lead on t housing.
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Good. Now, mark the choke t-stat adjustment with a pencil line across the edge of the t-stat and housing where they meet, then slowly remove the t-stat paying close attention to whether the link inside the housing is caged within the socket at the end of the bimetal spring (imaged earlier). Make sure there is NO GASKET between the t-stat and the housing, then reinstall the t-stat so the link is caged by the bimetal socket and the tang on the housing engages the notch at the edge of the t-stat (imaged earlier).
Was the link caged by the bimetal socket when you removed the t-stat and does the line you drew across the edge of the t-stat still line up?
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I will do it a little later and let you know.
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Ok did it everything aligned back up.
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1) Was the choke link caged by the bimetal socket when you removed the t-stat?
2) With the choke t-stat indexed to the tab and notch, the penciled mark lines up?
3) Did the choke plate opening change any?
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The socket and tab was aligned the pencil did match the tab on the housing and no it did not help.
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Take it back to the tech who overhauled and adjusted it to have him/her recheck the choke linkage adjustments (see the carburetor choke section of the 1986 Service Manual (http://www.73-87chevytrucks.com/techinfo/7387CKMans//Service/X8632_1986_GMC_Light_Duty_Truck_CK_G_P_10_to_30_Service_Manual.pdf#page=754), beginning on page 6C1-34). The problem with simply adjusting the choke t-stat is that the tab & notch won't align and the tab will prevent the stat from seating, possibly cracking the plastic cover when the screws are tightened.
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When he drilled the pop rivets out and put screws in he removed the tab on the choke thermostat so it could be adjusted manually. Thanks for all the info I'll have him take another look
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I downloaded manual in chapter 6 is about the choke. It said if the choke thermostat was receiving 12-15 volts replace the choke thermostat. I wonder why that is I thought 14 was good enough.
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Never mind I understand now.