Intermittent high idle issue

[69427]

Every since I swappped out the Holley 780VS for the 850DP last year I will frequently have a "stuck" throttle, in that when I let off the throttle the idle will stick at 1200-1500. A quick light blip of the throttle and it's back to 800 RPM. I've got two return springs (in addition to the built-in spring) on it, the heat isolator gasket has been hogged out so it shouldn't be hitting the throttle blades, and the secondaries are adjusted as closed as I can get them. (This shouldn't really be relevant as this high idle happens when I'm just driving at 5-10 mph.)
So, what am I missing here?

Thanks,
Mike

 

[Belgian1979vette]

Fuel leak in the carb ? Vacuum leak ?

 

[69427]

Fuel leak in the carb ? Vacuum leak ?

I don't believe so. I rebuilt the carb last year before I put it on (it had been sitting on the shelf for decades), so I don't believe, or have any evidence of, an internal fuel leak. Also, the engine will idle just fine once I kick it back down from its high idle, so I don't believe it's a vacuum leak. It just seems like the primaries don't want to fully close when the throttle is closed slowly (despite the return springs).

 

[DC3]

I had the same problem with the 650 DP on my '73. I had it in the shop for a different reason and asked them to take a look. They said it was the fast idle cam for the electric choke. They cleaned all the choke linkages and re-adjusted the fast idle cam and my problem went away. The mechanic said the clue that gave it away was that my throttle was sticking at exactly the normal rpm setting for the choke's fast idle (around 1500).

DC

 

[mrvette]

I had the same problem with the 650 DP on my '73. I had it in the shop for a different reason and asked them to take a look. They said it was the fast idle cam for the electric choke. They cleaned all the choke linkages and re-adjusted the fast idle cam and my problem went away. The mechanic said the clue that gave it away was that my throttle was sticking at exactly the normal rpm setting for the choke's fast idle (around 1500).

DC

SO similar to the chokes on most carbs, the pull off is set too rich, so relax the setting a few degrees...when you goose it, the inrushing air opens the choke and pulls off the higher idle cam associated with choke closing....Yeh, sticky linkages/shafts will stick it to you....

at least it's that way on a QJ and a AFB.....

I not a Holley man for reasons of previous comments on other threads...

:drink:

 

[69427]

I had the same problem with the 650 DP on my '73. I had it in the shop for a different reason and asked them to take a look. They said it was the fast idle cam for the electric choke. They cleaned all the choke linkages and re-adjusted the fast idle cam and my problem went away. The mechanic said the clue that gave it away was that my throttle was sticking at exactly the normal rpm setting for the choke's fast idle (around 1500).

DC

This thing doesn't have an electric choke, and the choke linkage is "locked" off, but I'll doublecheck the linkage stuff on the right side anyway.

Thanks.

Edit: Went out and looked at the choke linkage. I said THWI and pulled everything off. The car's in the trailer to take to a T&T evening so I'll see if the problem is still there later this week. If nothing else I've eliminated the choke stuff as a possible cause.

 

[DC3]

Edit: Went out and looked at the choke linkage. I said THWI and pulled everything off. The car's in the trailer to take to a T&T evening so I'll see if the problem is still there later this week. If nothing else I've eliminated the choke stuff as a possible cause.

Maybe you'll get lucky and removing the choke linkage will take care of the problem. I plan to remove mine as well. . . when I toss the carb and go with EFI.

DC

 

[69427]

Ran a test and tune evening at Gingerman on Wednesday. With the exception of the carb, the car ran great. The ambient temp was about 100*F most of the time, and I got a couple moments of (what I believe was) fuel percolation, and of course, the idle kept hanging up between 1200-2200 depending on the carb's mood.
I've eliminated all the choke stuff as a possible culprit, so I'll take another close look at the leftside carb linkage.

 

[big2bird]

Mike,
Is there the possibility the old throttle cable is just old and frayed, maybe the jacket. It is certainly pretty worn by now.

 

[69427]

Mike,
Is there the possibility the old throttle cable is just old and frayed, maybe the jacket. It is certainly pretty worn by now.

Well, it seemed that the problem showed up after I swapped out the 780 and put the 850 on. But given the age of the car it wouldn't be a bad idea if I lubed the cable (every 43 years ).

This should either cure it, or eliminate another factor.

Thanks!

 

[69427]

Well, the sticky throttle issue is still there. I've increased the return spring tension, but that hasn't cured it yet. I'm reluctant to add further spring tension as both the spring and the throttle cable are pulling the throttle shaft in the same direction (rearward), which just seems to increase the wear/bind on the shaft bore. I might see if I can somehow add a return spring in the forward direction (under the dropped filter base) to offset/neutralize the tension of the throttle cable on the throttle shaft.

I'd sure like to get this issue figured out. The Toyota floor mats are so comfy on my feet that I'd like to get the car on the road and put some serious miles on it again.

 

[SuperBuickGuy]

if it's not a new carb, pull the carb off and make sure there isn't any play in the throttle blade shafts.... heck, even if it's a new carb it could have that problem (although that would be something I've never seen or experienced)

 

[SmokinBBC]

if it's not a new carb, pull the carb off and make sure there isn't any play in the throttle blade shafts.... heck, even if it's a new carb it could have that problem (although that would be something I've never seen or experienced)

I bought a rebuilt qjet with similar problems...when warm it wouldn't return to normal idle. Turns out the the base was rebushed incorrectly. The primary shaft was hanging up.

 

[69427]

Well, got the problem resolved. I could push forward on the carb linkage and bring the idle speed back down which made me think that the shaft (and leftside primary blade) might be sticking. This made sense due to the rearward force of the return springs (plural). I always ran a single return spring over the years and never had a problem, but at a trackday last year the organizers required dual return springs. I added the second spring and ran the event. I got busy with other projects after that and just left the redundant spring on. I'm guessing that the rearward pressure (of the throttle cable and two springs) caused a touch of wear over the months on the butterfly shaft bore. So, I took both springs off and made a new bracket that would mount a single spring in front of (rather than behind) the throttle shaft. This will do the same return function, but also cancel out the rearward force on the throttle shaft. I started the engine up, and thank goodness the stuck idle problem is gone.

 

[SuperBuickGuy]

that's a good temp fix, but eventually you'll get a vacuum leak there - when you do, it's not terribly complicated to get the shaft re-bushed.... it is complicated pulling the entire carb apart, but the bushing is easy as pie :nuts: