OEM carb erratic idle

[dimitrisprophet]

'83 king cab Z24, stock carb with every vacuum port plugged off besides the brake booster and distributor vacuum advance. 

 

To start I usually pump the gas 1-3 times before starting. It will always start right up, and will occasionally idle around 1.1k, while other times sitting at 2-2.5k. The idle never drops no matter how long I let it idle or drive, and it always sticks to whatever it initially got to on startup. 

 

Tonight I went out to get gas, it idled perfectly at 1k and drove fine. However, when I left the gas station and gave it 3/4 trottle (could be more but I didn't feel the pedal touch the floor), the idle started spiking up past 4k. I had to constantly turn it off at lights and quickly engage the clutch after starting again just to get home. 

 

I am planning on getting a Weber in a few weeks, but how can I fix/prevent this in the meantime?

[datzenmike]

Throttle must be sticking. Could be the cable. Disconnect and push/pull it from the carburetor end. It should slide back and forth effortlessly. If it does then must be the carburetor. WD-40 the linkages on the outside and work the throttle back and forth. Look down the primary with a flashlight to see if it is closed and at idle.

 

Try it but keep a flashlight handy. If it jams again pull over and look down the carburetor to see if either of the throttle plates is stuck part open and how much.

 

When warmed up is the choke plate fully open? like this???

 

[dimitrisprophet]

I'm starting to think the choke isn't working at all. The trottle will open it a little, but it doesn't move at any point after. The trottle doesn't seem to be getting stuck, and with the trottle plates closed + choke closed, it still acts like I'm flooring it as soon as it starts. 

 

Here is a video I took showing the carb and how it acts on startup. It doesn't usually sputter like that but it probably had too much gas in the carb. Also I manually turned it off because I thought it would go all the way past 6k if I let it. Also there is an exhaust leak at the headers so it sounds a little ratty

 

 

[datzenmike]

Choke is not connected. On a cold engine the choke plate should be partly closed. As soon as you step on the gas the choke plate which is spring loaded, should snap fully closed. When the choke closes there is a linkage that pulls a plastic cam in place that prevents the throttle from closing and produces a fast idle. The choke has an electric heater that warms it and turns it off at a set rate, about 10 min. As the choke comes off over that 10 min the fast idle reduces and eventually normal idle is reached.

 

This is the choke heater...

 

 

Yours may not have the three Philips screws that allow you to loosen and adjust the choke to richer or leaner. I think the Z24 chokes were set and riveted at the factory. You can easily enough drill out the soft aluminum and put back on with self tapping screws.

 

 

Inside is a by-metallic coil that contracts when cold (pulling the choke ON) and unwinds when warmed (releasing the choke to OFF) Make sure that J shape on the end of the coil is wrapped around the choke shaft tang.

[Thomas Perkins]

This is how I fixed mine long time ago...Never did me wrong again...Believe it or not,I took my 720 to the Nissan Dealer here in 95. I spent a fortune on trying to get mine right,paid three different places 200.00 each to rebuild,replaced every valve,that goes to it,that was when Nissan still had parts.No computers to seek help back then.The people at Nissan told me that the carbs are junk and they could not fix it and advised me to get a Weber.So I did.I only had 108,000 miles on it.It would always idle high.If you ever put a Weber on it.Nissan people will not touch it.I now have 375,000 miles on it...

Edited December 22, 2021 by Thomas Perkins

[dimitrisprophet]

Would the choke not connected or adjusted wrong cause it to WOT on startup? Can I test this by unplugging the choke?

[datzenmike]

The choke looks disconnected and floppy so maybe starting with the fast idle randomly engaged. You could wire it open and then this won't happen but it's better to have the choke working properly.

[dimitrisprophet]

So I did some messing around and put wd-40 on basically every exterior moving part on the carb.  

 

Here is a video of startup, and I didn't touch the gas to set the choke before starting. 

https://imgur.com/a/ISW4PTW

 

You can see the choke gets pushed open, so much to the point where it was revving way too high. I turned it off as soon as it hit about 3k because it didn't seem like it was going to stop going up.  

 

So I plugged off the vacuum line like this to prevent the choke from touching the trottle. 

 

This caused the engine to run like dog water, but it wasn't revving away and actually idles right at 2k. Adjusting the idle screw had no effect. Because of this, should I still drill the rivets off the choke and clean/adjust it? Or is there another way of coming at this besides plugging the vacuum line?

[datzenmike]

 

HOLD the throttle 1/2 open and open and close the choke flap. This plastic part is the fast idle cam. It should turn counter clockwise as the choke is closed and return clockwise as the choke is opened.

 

 

When choke is on, the throttle closes and rests on one of those steps and cannot close to it's normal idle position, causing a fast idle. As the choke comes off it goes down the steps lowering the idle down till normal. Around 600-700.

 

Make sure that plastic cam is free to move.

[dimitrisprophet]

I believe you found the culprit. The fast idle cam is sticky and when I manually turned it fully counter clockwise, the throttle did not effect/reset the cam. I'm not sure if the spring on the back of the plastic came undone or what, but I pulled the carb off this morning and plan on tearing it apart, cleaning it, and inspecting for missing or broken parts. 

 

If all parts are not broken and accounted for, do you believe that cleaning it and putting it back together would solve this sticking issue? I don't really want to pull the trigger on a Weber quite yet if I can help it 

[datzenmike]

If all the parts are there.

 

 

 

In this picture above the two hoses with screws in them should be connected. Probably the top long one will reach the bottom one with the round canister on it. This is the vacuum break assembly and when the engine starts it pulls the choke slightly open to let in air. It should never have been disconnected but you know previous owners, right?

[dimitrisprophet]

Like I mentioned in the previous reply, I removed the line and blocked off the ports you're referring to. This was the only way that I could get it to start and not rev away.

 

I have the carb half torn apart and it looks like the fast idle cam was indeed sticky, and the float was sticky too. At this point I'm honestly not interesting in putting any more time or effort into fixing the stock carb just so it can idle fine and still run shitty so I'm going for a Weber now

[datzenmike]

A lot of very poor knock offs. Pierce Manifolds are still good. Expect around $300. For this you get everything needed for a proper trouble free swap. Lock Tite the adapter plate stubs and bolts they are known for shaking loose causing a vacuum leak.