On Start Engine Revs to Infinity and does not idle

[trentdoubledown11]

1984 C10 305
New 4 barrel quadrajet (although it sat for 5 years)
New Fuel Pump
New dual 16g tanks
New Sending Units and fuel lines (between tanks and tank switcher)

About 5 years ago the truck died while driving. I replaced the fuel pump and fuel filters (on carb and 2 back at the tanks). Then I replaced the Tank selector valve. Then the tanks (from 16g to 20g) and fuel lines (all but the lines from the selector valve to the fuel pump. After that did not work, I replaced the carb but was not able to get it running at the time.

Prior to the dying, I also replaced the plugs, plug wires, distributor and cap.

The truck sat for the remaining 5 years until recently. Since then, I replaced the tanks (back to 16g), both sending units, the fuel pump (old one seized), fuel filters, blew out the fuel lines with air, and bought another air cleaner w/filter. I had the original air cleaner with vacuum ports, but most of the emission control stuff was removed by the previous owner so I got one without all the ports. I also put in a new battery, flushed the coolant, and changed the oil for good measure.

I bought a new gas can and added 5 gallons to the passenger side tank. The tank selector switch and valve work as I can see the fuel gauge move when I switch over.

Yesterday, after 5 years, I was able to get fuel to the engine and it started. Woohoo!

The issue:

The engine turns over about twice then starts. But, the engine revs up like I have the accelerator pressed to the floor. Originally, I thought I did, but tried it several times after that with my foot on the brake just to be sure.

I want to be clear that I am not talking about a high idle. The engine never gets to the point of idling. It immediately revs and I have to turn it off because it sounds like it will explode. I'm also seeing a faint amount of white smoke.

What could cause this?

I've capped off all vacuum ports that are not used although

The Carb gasket looks fine.

The throttle and choke have some rust, but we are getting movement and the plates are closing as best that we can tell.

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[trentdoubledown11]

Additional Photos

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[Bextreme04]

Its a brand new carb... have you set it up yet? Make sure you turn the idle mixture screws all the way in until they lightly bottom out and then turn each one out 2 turns. That will get you in the ball park for starting idle mixture.

Are you pressing the accelerator pedal to the floor and gently releasing before you start it? That should set the choke on and also puts the carb up on the high idle cam. It sounds like the high idle set screw is set WAY too high. I'd back that off quite a bit and try again. It will be a small screw right behind the electric choke. Go counterclockwise a few turns and then start it up, you should see about 900rpm at fire-up and it should work its way up to around 1300rpm after a minute or so. Then go drive it around to get it nice and warm and set the timing, idle mixture, and hot idle.

 

[DoubleDingo]

When you converted the carb to electric choke did you plug the vacuum port between the can and the carb? Or did you buy the carb already converted over? Have you adjusted the fast idle to be at a lower rpm? The screw is under the choke.

 

[trentdoubledown11]

I bought the truck from my grandfather who made some mods that I don't know about. I'm not sure if the original carb was converted to electric choke or not but it was there when I got it. I purchased the carb to match and plopped it on there. Which vacuum port am I looking for? Front or back of the carb to the can? (you'll have to forgive me as I am pretty terrible at know what everything is called).

I don't think I adjusted anything when I put on the carb. I was expecting to have to adjust but I thought it would at least idle first.

DoubleDingo and Bextreme04. Thank you both! I will try a couple things over lunch and see if it works out.

 

[trentdoubledown11]

So I tightened the screw under the electric choke all the way down then loosened it by 2 full turns.

The screw on the opposite side (I think is the idle mixing screw) was snug so I loosened it a couple turns too.

I tried to start it again with the same results.

I've noticed now that when I manually pull the choke down using the piece labeled "T" (middle image of additional photos) it will stay open until I pull the throttle and then it snaps closed.

I think there was a moment when it was going to start normally but then it seems like it is getting stuck open and revs really high again. Once it does that, it continues to rev high when started until I "fiddle" with the choke and throttle.

Did I do the adjustments right?

Would there be a misplaced spring that could cause this?

Would it be too much rust on the moving parts? Would cleaning help that?

 

[Bextreme04]

No.... the idle mixture screws are in the baseplate on the front of the carb. You have a left and a right. They might be capped off. The screw behind the electric choke needs to be unscrewed to lower the idle. Rebuild manual says unscrew all the way out. Then do the following.

• Make sure high idle cam is on the top step
• Screw in stop screw until it makes contact and then screw in 3 full turns.

Once it starts up you can then adjust from there until you get 1600 RPM in neutral according to my sheet.

Before you do that, make sure your regular idle screw on the driver side is fully unscrewed so that it isn't holding the throttle open at all either.

Also tighten the idle MIXTURE screws all the way in until they LIGHTLY bottom out and then unscrew two full turns. See the pics below for cam position, idle set screw, and idle screw positions. The high idle screw is on the passenger side behind the electric choke module and would look like the picture if you didn't have the electric choke module mounted over the top.

 

[Bextreme04]

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[Bextreme04]

I would back off the Idle speed screw all the way and then screw it back in until it just lightly touches the throttle stop also. The idle mixture screws will be on the very bottom plate in front. One of them is under the fuel inlet in your pics and the other will be under where the PCV hose is going in. The high idle cam is the "T" item you talked about in a previous post.

When you push the throttle all the way down and then let it up, it should be moving the flat piece onto the high spot on the T as shown in the pictures I posted above. When the truck warms up the choke will pull that cam off and let it sit on the regular "idle speed screw" while letting the choke butterfly's open up.

 

[DoubleDingo]

I mentioned the electric choke conversion because of the plugged vacuum port on the top/back of the carb. When I converted my hot air choke to electric, I had to plug that port, and the port I mention inside the choke can is literally inside it. If that carb was originally a hot air choke carb, there is a vacuum port that goes into the side of the carb, it is to draw the hot air through the tubes that go into the intake and down to the head. The electric choke conversion kit has a plug that goes in that port, then you install the choke can to hold the plug in place. The choke can you have is an electric choke can, but that doesn't mean the carb didn't come originally as a hot air choke carb. The only way to know if that port exists and is plugged if it does exist, is to remove the choke can. But make sure you note where your choke setting is, especially if it is operating properly now.

 

[DoubleDingo]

So I tightened the screw under the electric choke all the way down then loosened it by 2 full turns.

The screw on the opposite side (I think is the idle mixing screw) was snug so I loosened it a couple turns too.

I tried to start it again with the same results.

I've noticed now that when I manually pull the choke down using the piece labeled "T" (middle image of additional photos) it will stay open until I pull the throttle and then it snaps closed.

I think there was a moment when it was going to start normally but then it seems like it is getting stuck open and revs really high again. Once it does that, it continues to rev high when started until I "fiddle" with the choke and throttle.

Did I do the adjustments right?

Would there be a misplaced spring that could cause this?

Would it be too much rust on the moving parts? Would cleaning help that?

@Bextreme04 explained it well, perform those checks and adjustments and go from there.

 

[trentdoubledown11]

I went back to it again.

I found all of the screws and adjusted them. It is still revving up like it was before. I'll make sure the screw under the choke has the light touch that was mentioned.

A few things I noticed this go round:

It seemed to be building up to backfire. I heard about a half pop before I killed it.

Another thing was the back plate on top of the carb was slightly wet with what seemed like fuel.

When I opened up that back plate some sort of smoke or vapor was floating around (light/white in color). I do not remember it doing that previously.

This may be unrelated, but I took another look at my vacuum hoses and noticed that the manifold vacuum fitting had one port broken off and another one that was routed down beside the distributor to the oil pan and was open. This can be seen in the 4th picture of Additional Photos. You'll see the black hose connected to the top of the fitting that switches to a metal hose going down. Is there any relation to this fitting and starting/idling?

I apologize if I am moving in the wrong direction here. I haven't learned the skill for troubleshooting carb related stuff yet.

 

[DoubleDingo]

If there are any vacuum leaks, whether it is from an unplugged port, carb not tight to the intake, broken fittings, cracked hose, etc, the engine will not settle down. That line going off the top of the one fitting and going under the truck most likely going to your transmission modulator valve. Both ends of that line, are also a potential vacuum leak. Until you know there are zero vacuum leaks, you cannot fully tune or dial-in the carb or the timing

 

[Bextreme04]

I went back to it again.

I found all of the screws and adjusted them. It is still revving up like it was before. I'll make sure the screw under the choke has the light touch that was mentioned.

A few things I noticed this go round:

It seemed to be building up to backfire. I heard about a half pop before I killed it.

Another thing was the back plate on top of the carb was slightly wet with what seemed like fuel.

When I opened up that back plate some sort of smoke or vapor was floating around (light/white in color). I do not remember it doing that previously.

This may be unrelated, but I took another look at my vacuum hoses and noticed that the manifold vacuum fitting had one port broken off and another one that was routed down beside the distributor to the oil pan and was open. This can be seen in the 4th picture of Additional Photos. You'll see the black hose connected to the top of the fitting that switches to a metal hose going down. Is there any relation to this fitting and starting/idling?

I apologize if I am moving in the wrong direction here. I haven't learned the skill for troubleshooting carb related stuff yet.

Lets start at the most basic of basics.... Push open the choke flap and look down inside. Is the primary throttle plate closed? Now push open the big secondary flap on the back(big "A" stamped on it) and look down in there to make sure those throttle plates are closed. If those are not closed with it sitting there and not running, you will never be able to properly adjust the idle.

 

[Bextreme04]

If there are any vacuum leaks, whether it is from an unplugged port, carb not tight to the intake, broken fittings, cracked hose, etc, the engine will not settle down. That line going off the top of the one fitting and going under the truck most likely going to your transmission modulator valve. Bothell ends of that line, are also a potential vacuum leak. Until you know there are zero vacuum leaks, you cannot fully tune or dial-in the carb or the timing

This is also true and for the same reason. If you are not controlling the flow of air into the engine via the carburetor, no adjustments on the carburetor will be effective. Generally speaking, if you have the carb set to the "close enough to run" settings I gave you, and have a massive vacuum leak(unmetered air), it will often refuse to run or idle at all. This is because it will be running super lean, due to lots of air getting sucked in with almost no fuel being added. I would recommend you unplug and cap all of those fittings until you get the carb dialed in. You need to leave the PCV connected and also the hard line to the brake booster. Other than that, I would unplug and cap everything until you get the engine running right and dialed in. Then I would slowly start connecting lines to identify any broken/cracked hoses that are vacuum leaks.