My son and I just rebuilt the q-jet on a L69 engined '84 Z28. When we started her up she idled at about 1500 which I thought wasn't too high for cold start and only adjusted on the bench. What seems strange though was that after a couple of minutes the idle seemed to be climbing to around 2500 and still going up when I shut her off. If I immediately restarted it would start at 1700 again and then start climbing after about 20 seconds.

When we rebuilt the carb. the parts store gave us the wrong kit but most of the parts looked right so we used it. I think the kit they gave us was for a non-cc carb. I just put a new throttle position sensor in as well.

Before the rebuild, it idled high but didn't increase from the starting idle speed.

Any ideas?

 

me too just started to do it and I cant figure it out

 

2500 isn't terribly high (but still up there!). Have you tried tapping the gas when it starts revving the idle?

 

Did you epoxy the float bowl plugs on the bottom?

 

I have nothing bring the idel below 1800 rpm's. it was fine b4 I replaced the intake manifold gasket now its screwed up thnks for the response.

 

Say what?

 

Did you epoxy the float bowl plugs on the bottom?

No, I did not epoxy the plugs. What would be the symptoms if they were leaking?

I've been doing some searches at lunch today and I think I may have found at least part of the problem. I believe we threaded the lean stop rod in too far (controls the MCS). I want to back it out about 5 turns and try that.

Thanks

 

Srry 99hawk must have posted the same time u did I didnt rebuild mine it was fine now just wont kick down

 

did u solve your problem? my choke connection was bad. I replaced it and all is well for now

 

No not yet. Haven't pulled carb. apart yet.

 

ok good luck

 

Symptoms of the float bowl plugs leaking, in my case, was an inability to get the engine to idle below 1800 in neutral. If you're pulling the carb back off, you might as well epoxy them.

 

Well we pulled the carb. and set the lean mixture limit. I don't know exactly how many turns out I set it to since I made a gage out of a finish nail 1.304" long and used that per the book. I'm guessing it was around 2-3 turns out.
We also epoxied the plugs as suggested. Just waiting for the JB weld to harden up so we can put her back on and try it.
Thanks for all the advice!

 

She still idles at about 1700 rpm then gradually drops down to 1400 after a couple of minutes but then after about 5 minutes starts increasing to 2000+. At that point we shut her down each time.
Is there some thermal switch or device that would send a signal after running about 5 to 7 minutes?

Theres an odd fitting at the end of one of the vacuum hoses that I not sure about. I'll post a pic in a while.

Also remembered when I was rebuilding the carb I installed a float needle seat (cylindrical piece the float needle seals against) that had diametrical holes in it whereas the original did not. I'm not sure this has anything to do with these symptoms though.

Thoughts?

 

What speed does it idle at if you manually close the choke?

 

My GUESS is a mis-matched throttle plate to bowl gasket which
would cause a vacuum leak on the front of the carb above the idle mixture screws.

 

Try to forget about the engine speed change as it warms. If you let the car warm up without kicking the choke down chances are you'll hear it increase in speed. Could be that the computer system is going into closed loop. Hate to say it but cjances are good that It's gonna be an assy problem. As far as the adjustments go inside. They won't have any effect on speed as per say, just for mixture. That would be to the effect of just turning the mixture screws on an old style carb.

Make sure the fast idle cams not binding and the throttle closes all the way. If so then get the right kit and do it again.

BTW the basic setting for the internal mixture control lean screw is 2 and 3/4 turns out from bottom.

The Rich mixture screw is the one in the lid. Not the TPS. I like to take it to the very top and turn it down about 1/3 to a 1/2 turn.. Gets the best H.P. that way.

Hope it's been some help.. I've built a ton of these in both 2 and 4 barrel models as well as about every other kinda carb there is.

 

I bought another carb. kit and will have another go at it. It may not happen until the weekend though. I'll let you guys know what happens.
Thanks

 

She still starts with a fast idle and increases after a few minutes.
I've replaced all the vacuum hoses.
What I'm noticing is that the choke plate is not opening after a few minutes. There is 12 volts to the choke thermostat so I replaced the choke thermostat.
Started her up and the choke still does not open. The thermostat does not feel like its getting warm. Should it be warm to the touch after running a few minutes?
If I open the choke plate with my fingers the idle decreases so I'm pretty sure the choke is the problem.

Where is the choke heater relay? Is there a way to test it?

I think I've narrowed it down to the relay or the new choke thermostat is bad.

Thanks.

 

Well, we bench tested the old choke thermostat by putting 12V on the tab and negative on the back. It never moved. The new choke wasn't moving the linkage on the car so we tested it too. We found that we had put a gasket between the thermostat and the choke housing so the thermostat was not grounded. Picked up that gasket from the first incorrect carb. rebuild kit. We also had to clean up the contacts on the wire attaching to the choke.
Now it opens but it still idles high and the choke light is not going off on the dash.
I suspect the choke heater relay may be the culprit. I also have heard intermittent buzzing from the dash. I believe that might be the relay.
Can someone verify how to get to it?

 

The choke light functions as an alternator warning light. In fact, very rarely will any sort of choke failure trigger the light...

I'd suggest getting the alternator checked.

 

Replacing the choke heater relay took care of the high idle. Now she'll idle around 1000 rpm. Took her for a test drive and she threw a code 44 but at least we're getting closer. Many thanks to this board.