I'll try to be as succinct as possible here.

1975 model with the engine from a 1980 Z-28. Lars saw pictures of the carb and said 75 or 76-ish so it's probably not be the carb that was originally on this engine.

Idle at 1100 in Drive, 1300 in park.

Indexed balancer and set timing according to procedure in Lars' papers. Total timing is at 36 at approx 3000 RPM. 16 Base.

I'm off the fast idle cam and have replaced and adjusted the choke according to Lars' papers and it seems to be working properly. Car starts cold just fine and runs pretty well other than high idle.

I thought I had a vacuum leak at the intake manifold ( for those that saw my other thread about carb gaskets) but it turned out to be the EGR gasket which has been fixed. I have gone through two cans of carb cleaner checking for vacuum leaks and have found nothing.

Idle mixture screws do nothing. All the way in doesn't affect the engine in the least. This leads me to believe I'm not running on the idle circuits at all. Likewise, I'm getting vacuum on the Ported Vacuum feed to the distributor at idle. If I pull the advance off the distributor, idle in park drops to about 1000 rpm. Curb idle screw is all the way out so it's not even touching anything.

All of this leads me to believe the throttle isn't closing all the way for some reason. I've looked down the carb bores but I admittedly don't know what it's supposed to look like down there. If I pull the carb off, is it likely I might find a culprit that a novice could fix or should I just live with it until I can get the carb to Lars after the first of the year which I'm planning to do whether I fix this or not? I was kinda hoping to get it sorted before then though.

PS. does anyone know what the allen wrench size is for the secondary air door set screw? Mine do not spring closed and the set screw is completely loose. I have a set of allen wrenches but none of them seem to fit the dang thing.

One final note. There is some vertical play in the primary throttle shaft and if I pull straight up on the linkage, Idle goes up a small amount and then back down when I release it. Is this indicative of warn bushings and could this be the cause of my high idle?

Thanks guys. I really appreciate any and all comments as I'm trying to learn how these systems operate so I that I can help myself and hopefully others in the future.

 

behing the choke housing and down at the throttle shaft is the high idle cam that sets the full choke high idle setting.
either that is mis-adjusted or the cam is stuck and the choke is not moving it so the regular idle screw can set the proper rpm when full warm.

Check that area out and make sure it is free and connected.

 

Thanks! I've gone through all of the choke settings and checked the fast idle cam to be sure that's all completely free and functioning and it seems to be. It is kinda hard to see down in there but I'm as certain as I can be (as a novice to this) that the throttle is completely off of the cam. I should have said in my original post that idle speed cold when on the choke is at 1600 or so and then once warm it'll drop down into the 1200 range. So as near as I can tell, it's going onto the fast idle cam when the choke is on and coming off once it warms up.

When I went through Lar's choke adjustment and setting the high idle, I did as the instructions say and set it to where the follower was just touching the high idle cam and then screwed it in 2.5 more turns. this yielded a cold idle of 3000rpm so clearly I had to back that off a bit.

 

Ok, good.
yes the bushings could be and sound like they are worn.
Jiggling the throttle shaft will allow it to close more if it is binding.

It does sound like the throttle blades are open too much.

 

That's what I thought when I started jiggling the throttle shaft, It made the idle go up a little more but never drop below where it's at so if they're truly stuck, jiggling the throttle shaft doesn't seem to be unsticking them.

 

that sucks.
guess the throttle plates and worn bushings
are binding.

I hate to advise any further because it requires
removing carb and looking from bottom up to see what is binding, also loosening the butterfly plate screws to allow them to center in the bore scares me.

I would hate for a screw to come out during engine running because it wasn't staked like from factory.

Lars is the one to deal with.
good luck

 

I've been un-Bubba'ing a 75 QJet and ran into similar symptoms, some of which masked the actual problem of a weak electric choke and missing parts.
Once I worked through vacuum leaks, missing linkages & weak electric choke I was able to properly setup the QJet.
All that said It appears that your fast idle cam is still contacting the throttle plates and controlling the idle RPM.
As you said it's hard to see so what you should do is once the engine is fully warmed up and you've verified that the choke is wide open (a necessity), I recommend to adjust the High Idle set screw out to make sure that it isn't still controlling your idle RPM when it shouldn't be. If you can turn it out and the idle drops you've narrowed the problem to the choke crkt (which may be as simple as being set too rich, or the choke itself could be weak or bad).
1st thing 1st is too determine beyond a shadow of doubt that the high speed cam isn't controlling the idle when the choke is wide open.
If the idle doesn't change at least you've verified the fast idle crkt isn't the problem.
Good luck

 

If you can wiggle the primary throttle shaft and change the RPM then the shaft holes need to be bushed. You will not be able to properly tune the carb without this being accomplished.

You can send it to Lars or learn to do it yourself from Cliff Ruggles' book.

https://www.ebay.com/itm/36341542940...AaAqyxEALw_wcB

He also sells the kit to do the busing install. either 5/16" or 3/8" shaft. Not sure which one you need. If you're in doubt call them and ask.

https://cliffshighperformance.com/pr...ft-bushing-kit

 

my thoughts as well. Thanks for all of the advice!

 

my assumption as well from the start was that the fast idle/choke circuit was to blame. So I backed out the fast idle (with the engine warm) screw until it physically came off the linkage with no change in idle speed. I did have a bad electric choke but I’ve replaced that and it bore seems to be working correctly.

Thanks so much for the input and advice.

 

I’ve been reading Cliff’s book for the last couple of weeks trying to decide if I’m up to the task of going through the carb myself but, not knowing anything about the history of this carb I feel like I’d rather have it built back to a known good baseline first by someone who knows what they’re doing so that in the future I can work on it knowing that what I’m starting with is what is supposed to be there.

I really appreciate the input.

 

Is the throttle lever tang on the idle screw? If not.....the choke cam is still on.....even if you adjust the fast idle it can still stick on. Even a Q-jet with a sloppy primary shaft will settle into an idle....it will just not be consistent....and it has to be very sloppy.....
If the throttle lever is not on the idle screw.....then move to the choke cam/lever....remove the carb and verify the plates shut with your eyes.....

Jebby

 

Try a fresh 3/32" on the secondary air door set screw.
Make sure the allen socket is clear.
It's possible that its rounded out some from previous adjustments.

 

When I tore into my Q-jet I knew nothing about Q-jets other than what I read in Cliff's book. I read it 5 maybe 6 times to get all the info in my head. It depends on your mechanical abilities to some degree as to whether you get into it yourself or not. But as long as you don't break anything or make permanent alterations it can always be put back together correctly.

I always figure "it's not working right now so I may as well try to fix it" kind of philosophy. It's a great way to learn. Tear into stuff and figure it out. Time permitting and ability/health and tools permitting.

 

I have a 1979 stock with what I think to be a stock Q Jet. After replacing a leaking fuel pump I also had sudden fast cold and curb idle and pretty much the same symptoms wouldn't seat on the idle cam as if there was a restriction but also ran extremely extremely rich!!! . Pretty certain the new pump was not the culprit. But decided to rebuild the carb. As I disassembled the carb I discovered the power piston retaining bushing was virtually destroyed. After rebuilding and reinstalling the carb. I addressed the running rich. Turns out the new pump was to high of pressure. According to my gauge 11psi, I insatalled a fuel regulator and gauge. set to 4psi where Q Jets seem to be happiest. Voila !!! Runs Great!!!

Please keep us fellow Vett Heads updated. One can never have enough info on these old Vetts
Cruise Safe!

 

Thanks everybody. I'm going to pull the carb this weekend and see what I can see. Hopefully something obvious will jump out at me.