HI,
I am new to Carburetors and recently picked up a 1984 Pontiac Trans Am 305. Automatic. Quadrojet. All stock and well maintained.

Car runs exceptionally well, almost like new but every now and then I notice that when I turn the car off it "diesels" or runs a few second afterwards. Now the funny part is this does not happen all the time. Most of the times, it idles perfectly in gear and when I put it in park, it is at about 800 RPM and turns off fine. Sometimes, when I put it in park, it is at about 1100 rpm and then it diesels. This is after it is hot in both situations (i.e. when it turns off fine and not) I am guessing this is because its idling too high at 1100 rpm but again that is rare and usually sits at 800 rpm in park.. The choke is wide open, I don't have any vacuum leaks (I hook a gauge up). Trying to figure out why it occurs sometimes. No codes or check engine light.

Any thoughts on what to look at or why this happens occasionally? Appreciate any help on where to look. Thanks.

 

Caused by engine deposits and or bad gas. Try using higher octane fuel.

 

Your idle is actually a bit too high. At operating temp, it should be at 600-700 in drive. If your idle is at 1100rpm when in Park then something isn't right. That high of an idle means the choke is engaged or the fast idle cam is engaged. I had this happen on my car (1985 LG4) and it was because the idle was too high. These engines run very lean to begin with, which is why the timing and idle settings are what they are. Yes, deposits can cause a run-on condition but I think it is not the cause.

 

An idle speed that is too fast, or ignition timing that is too far advanced can cause the "dieseling" you're refering to. I like 800 RPM In Neutral, although the factory setting is lower. Probably wouldn't hurt to check the timing...

 

Thank You for all the replies. My only concern is this happens occasionally ,not all the time. Is it possible that one of the electronic components of the carburetor is going bad? I believe the 2 i see one is MCS which controls the richness and the other I believe is an idle speed component? I would think if it was timing and/or a idle speed screw this would happen all the time, not occasionally?

 

If the "kicker" solenoid is still mounted on the throttle linkage side of the carb, and is still active, try screwing the head of it, that contacts the linkage, inward, by maybe ½ turn or so. Might take a giant set of pliers, if it's the variety that has what looks like splines on it instead of a hex, since it's a kind of locking thread situation. Of course, cover the jaws of the pliers with something so you don't leave a bunch of p***** tracks all over it.

 

when it is idling high, have a look and see if anything is holding the throttle up off of the curb idle adjustment screw.

a common cause is the throttle kicker, as Sofa mentioned. disconnecting the power lead to this solenoid should allow the throttle to drop back down to the curb idle adjustment setting if this is the case. a bad TPS can cause the ecm to keep the throttle kicker engaged at idle.

choke linkage on the other side of the carb could also bind and keep the idle up.

if it's idling high AND the throttle is resting comfortably against the curb idle set screw, a vacuum leak is possible, although I wouldn't normally expect one to be intermittent.

 

Thanks All. I found the issue, just not sure how to fix. See the picture with the red circle. I am guessing that is the fast idle cam. I started it this am cold. here is what happens.

1) Start it with one pump, is running at about 1800 rpms cold.
2) I blip the peddle after 20 seconds and it calms down to a nice 1000 rpm.
3) Put car in great and drives perfect. Drive it for 10 to 15 minutes put it in park and it is idling at 1200 rpm.

I then read some old posts and took air cleaner off when it was running and just touched it every so soft where that E flange is and it dropped down and the idles were perfect. near 600-700 in park.

I have sprayed that area with carb cleaner in the past and it is really clean (no dirt, grease, etc.) why would it be just need a very minor push to drop down? Is there an adjustment I need to do? If so any guidance would be greatly appreciated. Thanks

 

Yes that is the fast idle cam.

If the carb has been worked on recently, someone probably assembled the fast idle linkage incorrectly. Look down at the end of the throttle shaft, under the choke thermostat bowl. The problem is probably in that area.

This is of course, assuming that the choke itself, is working properly. Which if it's not, specifically if it's rotated too far towards "rich", it may never be pulling completely off.

 

Thanks. Yes choke is working perfectly. This happens sometimes, not all the time, so guessing something is making it stick at times...maybe I just need to spray a ton more cleaner on it......but I did spray close to an entire bottle all around it....

 

Inspect the linkage closely.

 

Will DO. Thanks for all the help!

 

I would add, the single most common assembly mistake I've seen in this area, involves the choke's secondary lockout.

Part callout 34 in this blowup. It's REEEEEEEL EEEEEEEZIE to put it on upside down.

There's a bunch of these that people have posted over the years. This is the 3rd version I've put up here myself. Some of them (the source, not the posting) are absolutely horrible in one area or another, or great where others suck. I've never seen one of these that was completely satisfactory. It always helps to look at as many as you can, and figure out for yourself which one shows what you need to know the best.

 

All that carb cleaner probably dried everything out, including the lubrication that should be on the linkages. Give them a small shot of WD-40 or equivalent.