C2 Holley throttle hang only with engine running?

[Timothy B.]

OK, I need some advice from the carb experts. I searched the achieves and found some reference to this but not a complete story. My 67 327 with Holley 3810 carb has a funny throttle plate problem. Alum intake. Manual trans car. With the engine warmed up but NOT running, the throttle plates close fully and easily with the normal return spring in place. Returns to throttle stop without any hesitation with the engine off. But, with the engine idling, it doesn't always want to close all the way without a slight extra push from my hand on the linkage. If I open the throttle manually and let it close quickly, it will close all the way to the throttle stop. If I slowly open the throttle and slowly let is close (like you might do during actual drives as you approach a traffic light for example), it will hang about 300-400 rpm high sometimes. An extra push by hand will bring it down to normal idle.

It behaves almost like the throttle plates aren't properly balanced to engine vacuum like they should be, causing a force at idle in the open direction. I can see the secondary is setup to purposely load the secondary plates against the closed stop, but the primary is supposed to be neutrally balanced I think.

I can't see that my carb base throttle holes are particularly worn. Does anyone have a suggestion as to what I should check or would cause this? I can put on a stronger spring but I don't think I should have to do that. Thanks.

[Albert P.]

Re: C2 Holley throttle hang only with engine runni

Just an outside shot, but if you have ground strap connected to accel linkage and it's pretty new and stiff, it may cause the problem you have.

[Gerard F.]

Re: C2 Holley throttle hang only with engine runni

Tim,

I would check this:

Is the spring connected to the correct position on the accellerator lever. Through the hole in the end of the rod.

Check the bushing between the the lever and the rod.

Try adjusting the accellerator rod for no load on the rod at idle.

If you can push it back to idle by hand, something seems to be hanging.

Try some WD40 on all the rotation points of the accellerator system from the gas pedal to the carb.

Jerry Fuccillo
#42179

[Timothy B.]

Re: C2 Holley throttle hang only with engine runni

Gerard and all, check out this earlier thread:

https://www.ncrs.org/forum/archive23.cgi?review=194778

Note the comments towards the end of the thread about throttle plate pivot hole wear, and how it could cause this? Mine don't seem that worn, but I might try investigating. My spring is connected correctly, bushing is in good shape. As I mentioned, with the engine off there is no restriction or hanging, only occurs with engine running.

[Mark B.]

Re: C2 Holley throttle hang only with engine runni

Tim, I have found that most of the springs used to close the throttles on carburetors are way too light and will actually be too weak to resist the vacuum pull on the butterflies at idle. You might try a heavier spring, or try two light springs. I suggest the latter as a safety measure. If you've ever had a throttle spring break, you will know what I mean. A very scary moment, indeed. I always run two springs on my carbs; ideally, one inside the other. Good luck.

[Timothy B.]

Re: C2 Holley throttle hang only with engine runni

Update: I took apart the base of my carb today and disassembled the butterfly valves on the primary side. There is a small white plastic bushing on the shaft only on the drivers side. No similar bushing on the passengers side but it looks like one would fit there. Since most of the loading from the linkage is on the drivers side maybe they only installed one? Bushing didn't seem that worn, and since I don't have a replacement I reinstalled it. I can feel some play in the shafts, but it doesn't seem to be any more than is present on the secondary side which hardly ever rotates. I lubed things with motor oil (which I realize won't stay put) and all operates smoothly. Can you buy repacement plastic bushings and if so how many are installed per carb? Supplier?

I did find something that might be the actual cause of my problem. I noticed by inspecting the bore closely as I rotated the butterfly valves full shut then open, that there seemed to be a spot of slight raised metal in the bore. The location was right at the idle holes/slot. I could see by the shiny metal at that location that the valve was contacting the spot and slightly hanging - only slightly. I used some super fine paper to smooth out the area and now the valves close without contacting the high spot first. I can tell the difference when moving the valve by hand.

All back together and running now and working great. I did have to back out the secondary stop screw slightly from the target setting of 1/2 turn past contact in order to lower the idle speed down to where I want it. At least right now the throttle closes cleanly all the way to the idle speed screw all the time. My theory is that with engine vacuum on the valves, the raised metal spot might have been hanging slightly but without engine vacuum it didn't hit the spot as hard. On the other hand if may just be the oil making things work now and once fresh fuel clears that out my problem might return. I will drive it a bit and see how it does.

The comment about having a second spring inside the other is a good idea. I think I might look into that as a backup.

[Clem Z.]

those teflon sleeves are included in holleys

master rebuild kits. some holleys have them from the factory and some do not but most all throttle shafts are grooved for their use and they can be used to tighten up a worn shaft hole

[Timothy B.]

Conclusion to problem

OK, after a few days of running the car and multiple drives, it appears my problem is fixed. As I described, the problems appears to have been the raised metal in the primary throttle bore right at the closed position. The raised metal was very slight, and would only cause a problem in certain conditions. If you experience a similar condition, I would suggest that you inspect the throttle bore carefully, with particular attention to the area near the idle slots. The edges of these slots can become deformed and can hang the plates in certain condition. Be very careful when trying to remove raised metal as you don't want to damage the bore. The throttle plates should close all the way to a hard stop as they contact the bore walls evenly.