CrimsonDragon

Hope you guys have some ideas, we are stuck. 85 Z28, 305, Quadrajet. It was undriveable as the float went bad. Took it to a local retired carburetor gurus house where it was rebuilt. He installed it on a test engine where it ran great and idled perfect. After installing it back home on the 85, idle starts at 1200, choke kicks off then goes to 2000 and will not come down. The carb guru came over to the house, checked the install then took it apart twice and checked it again on his test engine where it runs perfect. At the same time, we installed a new fuel pump, fuel lines and fuel filter. It doesn't put off any codes and runs great but will not come down below 2000. We have eliminated all the mechanical reasons - choke, choke linkage, high idle cam, stuck throttle blades, vacuum leaks, MCS solenoid resistance is at 22, fresh tuneup, timing is perfect. Any ideas would be greatly appreciated. It is my sons car and he has been sick his entire life, is now 20, finally healthy and driving the Camaro to college. Unfortunately, he inherited the 3rd Gen sickness as we have had an 89 IROC his whole life. We stumbled into a great deal on this and he is totally in love with his car. Thanks again,

tealman92

Hows the gasket between the carb and intake manifold? Have you checked for any cracks in the intake manifold or leaking gasket between the head and intake? Have you tried to smother the carb to choke all air from entering it to see if the car still runs? Only thing that will cause it to idle high is a vacuum leak if you have ruled out the carb so next is the intake manifold.

CrimsonDragon

Thanks for the reply. Yes we have sprayed two cans of carb cleaner around there are no leaks at the manifold or anything between the carb and manifold or intake manifold leaks. No vacuum leaks at all. The carb guru , Joe, came by and he has an incredible ear and said for sure there are no vacuum leaks.Yes we can choke it out and it will just about die with the choke closed.

zed-028

i had an issue like this with mine, turned out to be the PCV valve was stuffed, it had worn/jammed on an angle and was locked open letting in a crazy amount of unmetered air into the carb sending it insane.

pull the pcv hose and plug it with your finger, see if it drops down, Vac leaks are really all that i can think of that could cause such a massive high idle.

tealman92

It has to be a vac leak. If you have put that carb as it is on another engine and it idled down properly and you bolt it to your engine and it is idling high it has to be a vac leak. Check all the ports that connect to it even the brake booster line. Hell try plugging all the vac lines to the carb even the timing advance line and see if it idles out. If it does then start to hook each one back up 1 by 1 to find the leaker

CrimsonDragon

Thanks all for the replies. Tried plugging the PCV valve this morning no change, will pull all the vaccum lines , plug each and reconnect 1 by 1 tonight. Will let you know. Thanks again.

paulo57509

How far advanced is the base ignition timing?

CrimsonDragon

Timing is set to the stock specs. Funny thing is it ran great, idled great prior to the rebuild, it just started flooding from the bad float.

paulo57509

What's different from the rebuilder's setup and yours? The devil is in the details....

Is an idle stop solenoid being used on both? Is it the same idle stop solenoid?

zed-028

what im going to say may sound a little condescending but its not intended that way (i just believe in tackling the simple stuff), is the carb fitted at the moment with the throttle cables attached? if it is, can you disconnect the throttle cable from the arm but leave the return spring on. if someone has bent the bracket or the cable has been inadvertently tightened it might be cracking the throttle enough to give you the crazy revs.

back the idle screw way off and make sure the blades arent opening mistakenly

norton750

I had a similar problem. turned out to be the distributor

CrimsonDragon

I tried retarding the timing today which brought the idle down after disconnecting the computer connection to the distributor - connected it back up to the computer and the idle went back up to 2000. What part of the distributor went bad on yours and what did you you replace to fix it? Thanks for the reply

CrimsonDragon

Also, here's the answer to the other questions. We've ruled out everything mechanical seemably, to the Carburetor when mounted to our manifold we've eliminated all the throttle cables vacuum lines the bracket tree on the Carburetor are sitting against her stops the choke is off . Worked with Moates today trying to data log it but 85, LG4 with a computer controlled Carburetor can't be data logged. I have a message into Bruce at Hawks Third Gen to pick his brain a bit. Thanks again for all the great replies, it's seeming like it has to be something electronic that's raising the idle up because nothing mechanical is doing it. It's not putting up any codes beside 12 and it runs great from 2000 on up to Redline but just doesn't want to idle below it. We have an 89 IROC that the computer went bad and it just put everything down to limp mode and it would drive but not drive well it didn't rev things up it did the opposite . Thanks again for all the replies, our son really appreciates it

naf

iTrader: (7)

the ecm does not control the idle speed except to engage the idle bump solenoid during certain conditions (decal, etc.). if the bump solenoid is not holding the throttle off of the curb idle set screw you can rule that out.

if the primary throttle stop is resting on the curb idle set screw and no further adjustment of the set screw (turning it out) will lower the idle, there is only one thing that can allow the idle speed issue: TOO MUCH AIR.

No distributor issue or ecm issue is going to allow the engine to idle at 2000 rpm in the absence of enough air to support that idle (the ccc-qjet carb will try to add the additional fuel needed).

the extra air is either passing through your throttle blades, primary OR secondary side; bypassing the throttle blades through the carburetor's internal passages or getting into the carburetor/manifold system through a leak.

since this carb 'apparently' worked well on another motor, I'd look at the things that have changed: carburetor mounting gasket; vacuum connections (including the accessory connection at the manifold, throttle cable assembly (is the throttle REALLY resting on the curb idle set screw?).

Did an Ape torque this down onto this motor and tweak the base so that one of the throttle blades is hanging slightly open? Things like that.

Go away from the distributor and the ecm. no amount of timing change is going to make a motor idle high in the absence of the air and fuel to support that idle.

norton750

I don't remember what was causing the dist to advance like that, it was a long time ago. What I did to fix it was change out the distributor and carb for non computer controled units. A little tweeking on those made a lot more power too.

naf

iTrader: (7)

Yes. you can retard the timing until the engine barely runs (or quits altogether) but don't do that. the timing is supposed to advance at idle to around 22 degrees or so of advance. it's a good thing. you want as much advance at idle as you can get without so much that it pings at cruise tip in or detonates at WOT.

set your base timing with the esc disconnected so it reads around 4 degrees. THEN reconnect the esc and let it advance the idle timing. YES with the esc disconnected and the timing retarded the idle will be lower BECAUSE the engine is running much less efficiently at the retarded timing.

zed-028

Did this ever get resolved?

CrimsonDragon

80 hour work week last week then got hit with the flu this week....next week is looking better to try a few more things. if we can't find it there's a local mechanic that has this exact engine in his car and guarantees that he can find it so I might be going that direction. If I find out what it is I'll definitely post and let you guys know. Thanks again for the reply