Fuel problems got me good this time.

[Joshua Kincaid]

Bad luck with the squarebody today (nothing new there). Changed out the battery and ground cable, no problems started right up no muss no fuss. Fast Forward 20 mins and two laps around the block and lo and behold the thing takes off on me, full rev dump change of Huggies required. Seems like my choke is fu**in off on me, popped the hood and the choke plate is about halfway shut. Its an 82 Sierra, 350 sbc, Qjet, and loads of miles. Carb is tight, new fuel filter, decently clean for a 30 year old truck. Anyone have any ideas right off the hop as to where to go from here? Card rebuild? New carb? Any input very much appreciated.

Also me being the goof that I am i went and busted a fitting towards the front of the engine, it wasn't hooked up to anything but it does have looks like vacuum line nipples. If someone who knows these things could help ID i would appreciate it ( please excuse the absolute mess of vacuum lines, currently replacing and recconecting)

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[Craig 85]

That is a ported vacuum switch that you probably don't need since none of your emissions are hooked up. By the way, if you're going to get rid of the vacuum pod on the left (driver's) front of the carb and it holds vacuum, let me know, I need a spare.

 

[Derrick]

ask @RustyPile about the carb he is a wiz from what i have heard

 

[Joshua Kincaid]

That is a ported vacuum switch that you probably don't need since none of your emissions are hooked up. By the way, if you're going to get rid of the vacuum pod on the left (driver's) front of the carb and it holds vacuum, let me know, I need a spare.

Awesome, I always like when I don't break important things. Also excuse my ignorance, im full of it, which part are you needing there boss?

 

[Craig 85]

Awesome, I always like when I don't break important things. Also excuse my ignorance, im full of it, which part are you needing there boss?

This one IFO the throttle lever.

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[Joshua Kincaid]

This one IFO the throttle lever.

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I'll take a saunt at it tomorrow see if it chooches and shoot ya a DM

 

[chengny]

Fast Forward 20 mins and two laps around the block and lo and behold the thing takes off on me, full rev dump change of Huggies required. Seems like my choke is fu**in off on me, popped the hood and the choke plate is about halfway shut. Its an 82 Sierra, 350 sbc, Qjet, and loads of miles. Carb is tight, new fuel filter, decently clean for a 30 year old truck. Anyone have any ideas right off the hop as to where to go from here? Card rebuild? New carb? Any input very much appreciated.

A 1/2 open (or closed) choke plate will not cause an engine to speed up to "full rev". No amount of choke would/could cause that. The throttle plates need to be open in order to supply the necessary fuel and air for such a dramatic increase in RPM's to occur

And it's extremely unlikely that the engine revved that high on its own. You opened the throttle plates as you made your test run and at some point they didn't close. So the engine was probably getting the normal amount of fuel/air for the speed you were driving. But when you stopped (or slowed down) the RPM's failed to come down because the throttle valves stayed open - at the point where you brought them to using the gas pedal/accelerator cable/throttle linkage. That degree of throttle opening was necessary when the truck was being driven. But with no load on the engine and the throttle still open, the RPM's ramped right up.

I would inspect and test the accelerator cable and throttle operating linkage for freedom of movement. Its best to release the end of the cable from the throttle lever and then check both for binding or sluggish operation while they are disconnected.

 

[RustyPile]

After 20 minutes of driving and the choke plate is still half closed tells me the thermostatic choke spring isn't being heated.. The spring has to get hot and unwind.. This pulls the rod down and opens the choke plate.. The reason for the high rpm is because the fast idle cam is still "engaged" because the choke hasn't fully opened. If memory serve me correctly, the choke on your truck is hot air actuated.. Meaning air passes through a tube in the crossover exhaust passage in the center of the intake manifold.. Passage could be blocked with carbon.. Tube could have a hole in it or it's disconnected.. Carburetor rebuild will not correct any of this..

[EDIT] Having re-read chengny's post, I need to clarify my "high rpm" statement.. a properly adjust fast idle - the rpm during warm up, should not exceed 1000 - 1200 rpm.. Could it be that in the heat of the moment, you just thought "full rev" condition??

 

[Joshua Kincaid]

A 1/2 open (or closed) choke plate will not cause an engine to speed up to "full rev". No amount of choke would/could cause that. The throttle plates need to be open in order to supply the necessary fuel and air for such a dramatic increase in RPM's to occur

And it's extremely unlikely that the engine revved that high on its own. You opened the throttle plates as you made your test run and at some point they didn't close. So the engine was probably getting the normal amount of fuel/air for the speed you were driving. But when you stopped (or slowed down) the RPM's failed to come down because the throttle valves stayed open - at the point where you brought them to using the gas pedal/accelerator cable/throttle linkage. That degree of throttle opening was necessary when the truck was being driven. But with no load on the engine and the throttle still open, the RPM's ramped right up.

I would inspect and test the accelerator cable and throttle operating linkage for freedom of movement. Its best to release the end of the cable from the throttle lever and then check both for binding or sluggish operation while they are disconnected.

Good to know on the choke thank you. Linkages are free and no binding on the cable either. The rpm dump I got wasn't anywhere near the throttle I was giving it (maybe a hair over idle through the turn) when it took off i was just coming off the clutch in 1st around a stop sign and bam goosed.

 

[Joshua Kincaid]

After 20 minutes of driving and the choke plate is still half closed tells me the thermostatic choke spring isn't being heated.. The spring has to get hot and unwind.. This pulls the rod down and opens the choke plate.. The reason for the high rpm is because the fast idle cam is still "engaged" because the choke hasn't fully opened. If memory serve me correctly, the choke on your truck is hot air actuated.. Meaning air passes through a tube in the crossover exhaust passage in the center of the intake manifold.. Passage could be blocked with carbon.. Tube could have a hole in it or it's disconnected.. Carburetor rebuild will not correct any of this..

[EDIT] Having re-read chengny's post, I need to clarify my "high rpm" statement.. a properly adjust fast idle - the rpm during warm up, should not exceed 1000 - 1200 rpm.. Could it be that in the heat of the moment, you just thought "full rev" condition??

I'll have to do some research on that crossover tube. And oh yeah this thing took off like a runaway diesel...actually it took off like my runaway diesel did lol

 

[RustyPile]

I'll have to do some research on that crossover tube. And oh yeah this thing took off like a runaway diesel...actually it took off like my runaway diesel did lol

It's not a crossover "tube".. It's a passage in the intake manifold that crosses from one side to the other directly under the carburetor.. I's below where the intake runners converge at the carburetor.. There's corresponding passages or ports in the both heads.. The tube I spoke of enters this passage, makes a U turn and exits right back out.. The moving air inside this tube is heated by the exhaust gasses passing over it..

If the engine did indeed take off, I suspect binding throttle linkage.. Here's something to think about.. You mentioned replacing battery cables.. I have seen situations where the negative cable failed in some manner, separated or came loose.. All the current being used by the vehicle HAS TO HAVE a return path to the battery.. Sometimes the throttle cable becomes the return path.. The plastic melts or softens from the heat generated from the current passing through the metal part of the cable.. The cable binds and the throttle sticks..

 

[Joshua Kincaid]

It's not a crossover "tube".. It's a passage in the intake manifold that crosses from one side to the other directly under the carburetor.. I's below where the intake runners converge at the carburetor.. There's corresponding passages or ports in the both heads.. The tube I spoke of enters this passage, makes a U turn and exits right back out.. The moving air inside this tube is heated by the exhaust gasses passing over it..

If the engine did indeed take off, I suspect binding throttle linkage.. Here's something to think about.. You mentioned replacing battery cables.. I have seen situations where the negative cable failed in some manner, separated or came loose.. All the current being used by the vehicle HAS TO HAVE a return path to the battery.. Sometimes the throttle cable becomes the return path.. The plastic melts or softens from the heat generated from the current passing through the metal part of the cable.. The cable binds and the throttle sticks..

Understood, beg the pard still getting up to speed with lingo. I'll have to take another stab at the linkage/cable side of things seems like. On the ground cable I just copied the way it was set up before which in hindsight might not be the best. Main cable is grounded from the battery to the block and there was/is a smaller diameter wire running from the neg terminal to the body where the headlight ground is which is run down to the frame.

 

[RustyPile]

Understood, beg the pard still getting up to speed with lingo. I'll have to take another stab at the linkage/cable side of things seems like. On the ground cable I just copied the way it was set up before which in hindsight might not be the best. Main cable is grounded from the battery to the block and there was/is a smaller diameter wire running from the neg terminal to the body where the headlight ground is which is run down to the frame.

No need to apologize.. I'm here to teach not scold or reprimand.. Did you attach that smaller ground to the original location?? What about the braided ground strap between the rear of the passenger side head and the body? I couldn't see it in your pics.. If you don't have one, get one. Disconnect the throttle cable from the carburetor.. The cable should slide very, very freely back and forth in it's housing.. I mean absolutely not even the slightest bind.. Move the throttle arm on the carburetor back and forth.. It too, should be free of any binding.. Check to be sure the return spring is attached at both ends in such a manner that it doesn't pass "over center" thus holding the throttle open..

 

[Joshua Kincaid]

No need to apologize.. I'm here to teach not scold or reprimand.. Did you attach that smaller ground to the original location?? What about the braided ground strap between the rear of the passenger side head and the body? I couldn't see it in your pics.. If you don't have one, get one. Disconnect the throttle cable from the carburetor.. The cable should slide very, very freely back and forth in it's housing.. I mean absolutely not even the slightest bind.. Move the throttle arm on the carburetor back and forth.. It too, should be free of any binding.. Check to be sure the return spring is attached at both ends in such a manner that it doesn't pass "over center" thus holding the throttle open..

The smaller ground is running from the battery to a bolt right next to the pass headlight where the headlight is grounded. Im not for sure on the ground strap on the head I'll have to check on that. Throttle linkage feels good, just the standard weight of plates moving no restriction, iirc cable may have a wee tad of stick but never felt like enough to hang up, will check.

 

[RustyPile]

The smaller ground is running from the battery to a bolt right next to the pass headlight where the headlight is grounded. Im not for sure on the ground strap on the head I'll have to check on that. Throttle linkage feels good, just the standard weight of plates moving no restriction, iirc cable may have a wee tad of stick but never felt like enough to hang up, will check.

Read my last post again.. "a wee tad" is not acceptable.. You can't see inside the housing to check the cable itself.. The cable is made up of several strands much like a rope.. If just one of those strands break it will jamb, it's just a matter of time.. What you describe as a "wee tad of stick" could be a broken strand.. Check again carefully..