[JJKJ]

The new engine is running great but I have a slight issue which may be a common problem or not but I haven't run into it before. After I drive the car and it warms up, if driven docile, the car comes back down to idle as normal. However, if I get on the car during the drive (going WOT or close), the car sometimes does not return to normal idle. Instead it idles about 200-300rpm higher than normal. It is not on the high idle cam and the primary throttle linkage seems to be down tight on the idle set screw. If I remove the vacuum advance, there is no change, but I'm wondering if my mechanical advance may be sticking? Tapping the throttle has no effect bringing the idle back to normal. Here's a few other details:

The distributer is a new HEI unit

I rebuilt the carb and installed bushings on the primary throttle shaft pre Cliff's book so the primary throttle shaft is operating smoothly and seems to be going closed completely

I have my TH350 governor set for pretty conservative part throttle shifts as I was breaking in the engine so part throttle easy shifts rare gets the engine above 2500. If I keep the car driven in this manner, I don't recall having the high idle problem.

Thanks,

[67drake]

Did you check to see if your secondaries are being held open slightly?
I had this happen to me after I did my quad rebuild,and found the secondary blades not seating all the way,only after I had opened up the secondaries though. It drove me nuts until I started looking into the secondary side of the carb.

[guccieng]

this happens to me with any 'air gap' style intake, including my chevy. the heat-soaked manifold is vaporizing the fuel better than when it's cool, and letting it sit at idle for a few minutes cools mine down and the idle drops down to normal accordingly. i don't think it's a problem; just how it goes...

[JJKJ]

Originally Posted by 67drake

Did you check to see if your secondaries are being held open slightly?
I had this happen to me after I did my quad rebuild,and found the secondary blades not seating all the way,only after I had opened up the secondaries though. It drove me nuts until I started looking into the secondary side of the carb.

Hmm, haven't looked at the secondaries. I'll look at them next time around. What was causing your secondaries to stay open? Bent shaft or other?

[JJKJ]

Originally Posted by guccieng

this happens to me with any 'air gap' style intake, including my chevy. the heat-soaked manifold is vaporizing the fuel better than when it's cool, and letting it sit at idle for a few minutes cools mine down and the idle drops down to normal accordingly. i don't think it's a problem; just how it goes...

I may have to install an idle stop selonoid then. When I set my idle to a nice idle speed at normal idle, then when it idles up and I try to shut it down, I get run-on. If I set my idle speed based on the higher idle, then unless it gets warmed back up, idle is too low. I guess an idle stop selonoid would fix the run-on. Otherwise it's not really a problem as it isn't enough of a change to cause problems with idle in gear.

[guccieng]

the run-on is more frequently an ignition problem, not carb. putting a diode in the alternator harness will keep from firing the plugs when you shut it off. the way to test is to pull the coil wire when running. if it's dieseling, it will continue to try to run, if it dies immediately, it's not your carb, it's the ignition. report back and we can help you with the diode. i did it to my chevy and can't remember everything, but i can look it up pretty quick.

[lust4speed]

I would say that the run-on would be a good indicator that the throttle blades are being held open further, and not the result of heat or ignition. It should be pretty easy to find -- just get under the hood while the idle is abnormally high and first push the secondaries closed harder. If this doesn't work then apply pressure to the primary throttle shaft.

Most carbs will pick up a few RPM when obtaining operating temperature (probably less than 100 RPM) and this type of higher idle won't have an effect on the throttle blades.

[JJKJ]

Well tonight I'm going to check operation of both sets of throttle blades, reset idle speed and mixture as well as check/adjust timing. Hopefully I can narrow it down from there.

[67drake]

Originally Posted by JJKJ

Hmm, haven't looked at the secondaries. I'll look at them next time around. What was causing your secondaries to stay open? Bent shaft or other?

Other.
I actually had a small piece of tape that I had used to block off a vacuum port on the back of the carb get sucked in and stuck just above the secondary throttle blades. When the secondaries would open a corner of the tape would not let the butterflys close all the way. Don't tell anybody though,it was embarrising!
It drove me nuts for awhile though,I kept thinking the choke was set wrong,and it was going on the high idle cam. I would think that anything binding in the secondaries would do the same thing.

[GTOLou]

small vacuum leak can do it also..

[Cliff R]

Re-center and index the secondary throttle plates. Also make sure there is clearance between the linkage that actuates them and the lever on the secondary throttle shaft. I see this problem on occassion, and it's easily corrected with a few minor adjustments.....Cliff

[JJKJ]

Thanks for the additional suggestions. Hopefully tonight I'll get a chance to look at things and figure out the issue.

[just501]

did you get your problem figured out? I have had the same issue, but tapping the accelerator brings the idle back down. the primary throttle blades seem to be sticking or catching just prior to idle. i have an e-carb though.

[mysticmissle]

also weak advance springs may not be pulling advance out at lower speed (rpm)

[Kenth]

All of above, and

What size engine?
Camshaft duration at .050" lift? LSA?
Q-jet number?
Idle tubes size?
Upper idle air bleeds size?
Idle channel restriction size?
Lower idle air bleeds size?
Idle needle ports size?
Idle air bypass ports size?

[JJKJ]

Originally Posted by mysticmissle

also weak advance springs may not be pulling advance out at lower speed (rpm)

I know this thread has been dead for a while, but I thought I'd give an update. The car ended up sitting in storage most of the last 7 months due to moving to a different home and all the fun that goes with that (as well as winter). I finally got some nice weather and time to play with the car this last weekend.

I put a dial-back timing light on the car and watched the advance. I had initial timing set at 10*. Idle set at 800 initially. Almost immediately off idle, the advance would start to come in. After an easy wind up to 2500rpm, I'd bring it back down to idle. Timing would hang up at 14-16* and idle would be up over 1000rpm. I inspected the weights and springs--the weights were free and had lube. I should note now this distributor is an eBay $50 HEI. The springs were my problem; they're very light. I did an experiment and used the heavy springs from my original points distributor. Yes I know they won't allow full advance but I wanted to see if they would solve my problem. I ran the engine up several times and each time it would idle back down to the correct timing and rpm. Bingo, identified the problem.

Now I just have to get the issue resolved. I have sent an email to Sun Tuned to get info on having my distributor recurved; haven't gotten a response yet. I may look at other options if I don't hear back--not sure if he's still doing recurves.

A side note: I was using an Actron dial-back light with rpm display. There is about a 300rpm difference between my tach and the rpm display on my timing light. Just curious if anyone else has experienced this?

[Greg Reid]

I'm glad I decided to read this thread. I may be in store for the same problem as I have a "$50 ebay distributor" too. I'll bet it's the same. Now I'll know what to look for.
You might also find a couple of posts by Jim Hand in this thread of interest...
http://forums.maxperformanceinc.com/...=622086&page=2