I have a curious problem which has just occurred.I have a 85 Z28 with a LG4,
and the stock quadrajet carby.I think it is anyway.So onto the problem,i took the car for a run and everything was ok,but about 10 miles from home,i was coming down a hill and it started to rev harder with no throttle on.When i got back on the flat it was running about 2200 rpm and then dropping back to about a 1000 rpm.The rest of the way home i didnt have to use the throttle at all.Now when i start it cold,it goes straight to 2000 rpm,with a rough idle and blowing lots of black smoke.I've checked the fast idle setting,and even with the screw backed right off it makes no difference to the idle.The only changes i've made recently are i put a new exhaust on,headers,2 1/2 inch off the headers into a 3 inch y pipe,and single 3 inch for the rest.I also installed a msd 6a box,but i dont see that having anything to do with this problem.Does making the exhaust bigger ,mean you need to run leaner or richer?.Also does the choke pulloff adjustment screw have a certain number of turns out from fully seated or is it just set by feel.Sorry for the long post,but i've checked for all things i can think of,ie vacuum leaks,i have power at the choke wire,i dont know a great deal about these carbys and i'm
not sure where to go from here.Any help would be appreciated.

Thanks,
Alicat.

 

Sure sounds like a choke problem. The thermostats can go bad, meaning not heating up and/or not opening, even with 12 v to it.

Are you sure you're adjusting the high idle? It's under the choke thermostat housing. If the cam is disengaged, screwing that out isn't going to affect idle because the main idle speed adjustment is going to take over. Check throttle cables while you're at it, as a messed-up cable can hold the throttle open - try disconnecting them and see if that makes any difference. Sure sounds like a choke problem, though.

The pull-off is adjusted with the engine off, cold, and choke closed. Apply vacuum to the pull-off, and adjust the choke butterfly opening to spec (which I don't have in front of me). It's something like an eighth of an inch gap as measured from the top of the butterfly to the choke tower.

A good resource would be either Doug Roe's, or the Haynes shop manual, both called Rochester Carburetors. I got mine from Amazon.com, although I've seen the Haynes manual in Pep Boys.

 

Well i got it to run better,but only by luck i think.I played around with it for a while with no change then all of a sudden it came right?.I have noticed that the choke plate does'nt snap closed when i blip the throttle,so maybe the fast idle cam is out of adjustment,is there a certain way of adjusting that.The choke does'nt seem to open any further as the engine warms up,so would the thermostat be rs?.I'm thinking that mabye the original problem was caused by something stuck in the needle and seat because it was just like lots of fuel going in all the time.Thanks for any help.

Alicat.

 

It could be a bad thermostat, or it could be gummed up linkage.

 

Linkages seem fine.Acceleration seems smooth,either with the pedal or by hand under the hood.Just to be sure i sprayed carby cleaner inside the carby and out,so it could well be the thermostat in the choke as you say.Is the thermostat fixable or do you just replace it?.

 

You replace the element, the round plastic plate on the outside which has the bi-metal coil attached to it.