So, my idle is high.
1975 automatic, stock everything except for full exhaust and headers... idles at 1300 hot or cold. Idle is smooth, and turns on easy.

What I tried:
waited for choke open completely, so engine is hot.

- turned the accelerator screw all the way back, lever is not touching it, still too high.



-disconnected hose to the vacuum system ( wiper, headlight) right by the engine and plugged it. No change.

-break hose was very tight and couldn't take it off on either side ( i don't suspect a leak there)

- forced the choke closed, died instantly. Is that a sign of no leak?

Should I move on to checking the timing? If so, does any mechanic know how to do it or should I tell him how much advance I want?

also, on the picture below, this circle is for the choke, not for the mixture righ? ( it does say lean / rich on it)

 

first, make sure the fast idle screw isn't touching the fast idle cam. if it is make sure the cam isn't hung-up or binding. then disconnect the throttle linkage and see if that releases any tension and the hot idle screw makes contact with the flat. there is a little play in the postion of the throttle cable. don't mess with the choke setting yet. also, put a washer under the carb hold down bolt-and DON'T over tighten it-just snug.

 

Your timing is probably too far advanced, does it have run on when you shut the engine off ?

 

My money is on a vac leak... if you spray carb cleaner/propane/etc around, do you get a surge in RPM?

Could be throttle shaft leak, bad gasket somewhere etc.

 

+1. My guess too

 

if the stop screw isn't contacting the flat, something mechanical is holding the carb butterfly open.

 

Not necessarily true. These carbs have some room before the stop screw actually contacts the throttle linkage to open the throttle blades.

It *looks* OK to me.

 

You are getting some good advice. A couple of troubleshooting questions,

* When did this problem start?
* Was it like this when you bought the car?
* Was anything changed prior to the problem starting? upgrades, repairs or adjustments?

If the source of the issue is unknown, first, isolate the carb. Disconnect the throttle cable and plug all the vacuum connections on the carb and the manifold with vacuum caps. See if that changes anything.

 

thanks for all the help,

Jnd5101, I checked the fast idle screw and... I don't have one, just the hole where it goes. As for the fast idle cam, ( I am assuming its the lever that pushes against the fast idle screw), it's moving ok, doesn't seem to be stuck; moves freely. I will be disconnecting the throttle linkage tomorrow. I'll let you know. As for "put a washer under the carb hold down bolt-and DON'T over tighten it-just snug." I am a bit unclear on what you mean, do i put a washer between the bolt and the carb for every screw that holds the carb to the engine?

Motorhead, I will be cheking the timing tomorrow. I have no engine runoff. My brother has a gun. I will be reading more about it tonight so I am well informed (hopefully) before I attempt anything. I did it on my bother's old beetle a while back.

Also, as some of you mentioned, I did trow some carb cleaner around, no peak in rpm. I am still not fully convinced, since it seems to be quite a common problem to leak around the gasket.


Lastly, I noticed that the accelerator doesnt go back all the way. It only goes back if I manualy pull on the lever, from (1300 goes to 1250). I cleaned, but still not going all the way by itself. Will be cleaning some more tomorrow, but flaps look very clean.

SO tommorrow:
remove throttle cable, see if it goes back.
Cleam carb some more.
Use gun to check timing.
If no avail, I'll be buying a gasket.

I'll let you guys know tomorrow.

 

johnt365,
Problem started after I fixed the rpm gauge. haha
When I bought the car, 2 weeks ago, the rpm gauge wasnt working. Idle seemed fine by ear, but it clearly wasn't. I wasn't used to corvettes at all, and now that I hear it, it is as high as the rpm gauge says it is. I installed the rpm and did the gauge set, works perfect (pain in the a** to change it)

The car is stock, exept for the exhoust.
As for isolating the carb, I was attepting that today, but I'll buy plugs. (i was isolating one at a time). But can I plug the pcv too? How about the vaccum advance hose ( the one that goes to the distributor? that one too?

Also, the line that feeds the car vaccuum comes of from the top of the engine, not from the carb, so do I need to plug those too?

 

One at a time is fine and probably easier to keep track of. Removing the headlights from the equation was a good move. There are plenty of opportunities for a leak in that system.

I realize there are many variables in this but went out and took a couple pictures for you as a reference. My car is a small block with a medium cam and uses manifold vacuum advance. The car idles just over 600 rpms in gear.

You can see several threads on the screw.


I flicked the choke on and took this shot to show the gap that the fast idle cam leaves when it is engaged.

I don't know if that helps or not...

 

Loosen the 4 carb mounting bolts; then wiggle the carb around to find the 'center' of movement (best location). It could be that the primary throttle plates are interferring with the lower gasket or intake manifold. Once reset, tighten the 4 screws and fire it up again.

Also, disconnect the throttle cable from the carb linkage and exercise that cable and accelerator mechanism to see if there is any binding in it. You may just need a stronger throttle return spring, if you don't have complete freedom of motion.

 

I removed the throttle cable, it is holding it back a little. I guess I need a stronger spring, I inserted some oil in the cable.

Anyways, what I have done is:
-drove the car around to reach good temp,
-removed the throttle cable, lever went as low as possible ( 1200 rpm)
-then I used the gun to check the timing, It gave me 29.
-Removed the vacuum advance and plugged it, Idle lowered a bit (1100 rpm)
-measured with the gun and it gave me exactly 10 degrees.

So, before I open the distributor and put a rubber band on the centrifugal advance to check if its going in too early, I would like to know if 10 degrees is ok, i see people use it at 12.




**I checked for vacuum leak, it has a little tiny bit of a throttle shaft leak. I sprayed with carb cleaner, and right on the lever, it raises a little. When I toggle the shaft, it has an up down play.

 

i don't think timing is the problem, but at idle with the vacuum advance connected, you should be getting about 50*. in any case, set the tiimg for 36* advance at 3000 rpm (vacuum off) and let the initial timing fall where it may. it's difficult to get full advance too early unless someone has 'doctored' the dist. you should be getting full centrifugal advance by 3000.
are you saying the idle screw is touching the flat and it's still at 1200? look down into the primary butterfly and see if it's totally closed. if it's totally closed, you may have one of those 'commercially rebuilt' carbs that lars has talked about that has been 'reengineered' and has had the idle and primary circuits 'modified'.

 

10 deg. advance with vacuum advance plugged is fine. It won't be optimized for performance, but it's closer to that than the factory setting.

With the throttle cable removed from the carb, can't you dial down the idle speed further? If not, you have a significant vacuum leak somewhere. That additional intake air is allowing the idle to increase. You need to locate that leak.

 

I really hope I don't have a re-engineered carb.
I will leave the timing as it is. Not interested in performance, but would love to have that low idle sound.

Shaft goes as back as it can and seems to close the primary butterflys pretty well.

I will proceed to look for leaks once again.

As for the throttle shaft, is it normal to have a little bit of leak like I have? Maybe that's my problem. (up down play, rests on down)

 

the throttle shafts cause the shaft bores to enlarge and bushings are installed when a carb is rebuilt. but lars posted recently that he doubts that there is enough air leakage to affect performance.

 

50* of advance at idle? That's ridiculous. That would give him around 70* once the mechanical advance kicked in. Maybe you mistyped it? Anyway, it's bad info.

bem, don't worry about the timing at this point, it sounds like it should be fine where it is, for now. Once you get your idle problem sorted out, we'll get your timing optimized.

Scott

 

I had the same problem with my carburetor for quite while and I went through all the above suggestions.
Finally found that the secondary throttle plates were not seating properly in the bores, After re-aligning, the idle dropped down to where it should be.
Some may agrue that this isn't possible, but it sure worked for me.

 

I'm going to agree with the guys here who say you have a vacuum leak somewhere. Your primary throttle plates are about as closed as they can get. If you didn't have a vacuum leak, your car would die out with the throttle screw that far out. Start looking for that vacuum leak. If your brake booster vac hose is as stuck as you said, I'd start by replacing that one first. It sounds like it's rock hard from age and it could have a crack or a hole where you can't see it. After that one's done, you can continue checking the others.
Good luck!
Duane