I've got an 86 Z28 with stock 305 LG4 4bbl Rochester. The idle has started running upto 1800 RPM in park and when put into gear drops to around 1000. After driving for 15 min it will idle down to 800 in gear, but when put back in park idle will climb back up to around 1500.
It also will backfire when turned off once in a while.
Where should I start?

 

Start with the choke. It either isn't coming off like it should, which could be due to inadequate voltage at the thermostat; or it's gummed up so the mechanism doesn't release the fast idle cam properly.

Make sure the choke wire is properly attached to the thermostat for the first one. Check voltage with the ignition on if it is attached.

Spraying things down with carb cleaner will probably do if the thermostat is working properly and sticky mechanism is the issue.

 

Thanks... I'll start with the carb cleaner....How much voltage should be there? 12v?

 

Yep.

 

Alright heres what i found.
I Cleaned all the linkage. I can move the choke plate freely. While idling I can slow the idle by closing the plate with my hand and it slows down.
I'm getting 12.2v from the wire going to the electric choke however the choke does not appear to be working. Bad Electric Choke? Can I replace just the choke? It appears to be riveted on?!?
Thanks

 

Try holding the butterfly open all the way by hand and blipping the throttle. That should bring the idle down where it belongs when the fast idle cam, a little green weight on the passenger side of the carb, drops down (but it will come back up again when you let go of the choke and open the throttle again). What you did was bring the idle down by over-richening the mixture.

The choke thermostat can be replaced by itself. You'll have to drill out the pop rivets and install the replacement. If the replacement doesn't come with self-tapping screws to replace the rivets, you can use pop rivets again, or get some screws from a hardware store.

 

Thanks for the guideance. I just picked up the car a couple weeks ago. My dad bought it when I was a junior in HS. My 20 year old brother had it for the last 2 years and did all the things a 20 yr old does to a good car. It has taken me a month just undoing all the stupid things he did. I'm trying to restore it to good running order. I have a company car I drive for work so I keep it in the garage for now. If it were yours would you put in a new Carb/Manifold (Edelbrock or comparable) or try to rebuild the original.
I'm not going to be racing it or anything. Just a nice toy for the weekends.

 

Also, there's a solenoid on the driver's side of the carb, that may be holding the throttle open. It's supposed to do that when the A/C is on, and possibly when the ECM thinks the car should be idling and the RPMs are above 1500 or so (sort of a "dashpot" to limit the rate of throttle closing - an emissions thing). Try unpluggging that solenoid wire.

The electric part of the choke should open the choke, not close it. The choke thermostat is a bi-metal spring and will try to close the choke by itself when it's cool enough. The electric part heats up that thermostat and makes it open. So, even if the choke isn't "working", doesn't necessarily mean the electric part is bad.

You won't gain a whole lot by swapping the carb and manifold, over what the stock stuff will do when properly working. You'd have to change the distributor too. I would not change all that out, myself; and in fact, I am running the stock stuff on my own car. I'd strongly recommend trying to get the original to work right.

 

I'll give it a try this weekend. We're headed out of town tomorrow so I'll let you know how it works.

 

I would be glad to know what you find as well. I have a 1985 Camaro with LG4 and the same problem. The problem started getting worse after I put on an open element air cleaner.

 

I believe I have a Loose connector going to the choke. I'm getting 12 v on the wire but the connector itself is broken and when I try to squeeze the connector it will not hold the clip any more. Could this cause these symptoms?

 

Yes. If the thermostat isn't getting the juice, it won't open the choke properly.

 

Where would be the best place to find an electrical connector like that...I'm sure Advance Auto does not have something like that. A junkyard? I've tried a spade connector but thats not quite wide enough.

 

I've never tried to replace one. Just get them to hang on one way or another. But, if you try the junkyards, most any later GM carb'd cars should have that type of connector.

Depending upon how cobbled you're willing to go, you could cut off the existing connector, splice in a mating-type solderless terminal pair a couple of inches from the end of the wire, and solder the wire end to the thermostat terminal.

 

Yeah I was thinking about doing that until I can find one that will connect properly.
I won't ruin anything by doing that?
I hope this solves my problem.
It is idling so high that when i put in gear to back out of my garage it squeeks the tires. Not good on the tranny I'm sure.

 

Seems to have worked!
After about 30 seconds it began to drop about 200rpms every 10 seconds and was idling (very roughly) at 800 rpm after about 60 seconds. Is that normal? If I tap the throttle it seems to want to stall...then goes back up to 900-1000rpm.
There is a 3 prong electrical connector on the driver side. When i unplugged it it idled to 1500. Is this the TPS?

 

I don't know what I did but after trying to adjust the choke it will not shift out of first gear?!? Did I mess up something by messing with the choke?