C2 Hot Idle Speed Too High

[Tim E.]

Getting ready for a PV attempt in Waco, TX this weekend and am having trouble with one last thing - I can't get my 66 base motor (327/300) with Holley 4160 to idle slow enough. The engine runs smooth and strong but won't idle slower than 700 RPM. Timing is set at 6 degrees BTDC, idle mixture is set 1/4 turn lean of high vacuum, dwell is right at 30 degrees, and choke & fast idle work perfectly. In the past, I've been able to adjust it down to 400 RPM but I can't seem to anymore. I've studied the linkage and it appears the primary throttle plates are fully closed.

Any ideas what I can look for next? Thanks, Tim

[Dick W.]

Re: C2 Hot Idle Speed Too High

Tim, is something holding open the secondary plates? Do you have a vacuum leak? Just a couple of things to check.

[Albert P.]

Re: C2 Hot Idle Speed Too High

Have you checked to see if the accelerator ground strap is not creating a drag on the accel rod?

[Tim E.]

Re: C2 Hot Idle Speed Too High

I've got 23" going into the vacuum can and I definitely get a vacuum advance. With the vacuum disconnected and the hose plugged, the engine runs smooth at 400 RPM. I did not check the tightness of the carb nuts to ensure I don't have a leak at the base of the carb. I'll eyeball the vacuum hoses one more time. Thanks Dick, Tim

[Tim E.]

Re: C2 Hot Idle Speed Too High

Garth - The linkage appears free and I'm able to pull the throttle rod towards the firewall as far as the throttle plates will allow. One thing you've got me thinking about is to disconnect the linkage altogether just to make sure I don't have a bind someplace. Thanks, Tim

[Pat #25912]

Re: C2 Hot Idle Speed Too High

Just a thought, if you have a new accelerator pedal, it may be a little stiff and not comming all the way back.

[Gerard F.]

Re: C2 Hot Idle Speed Too High

Tim,

I think I would check whether the secondary throttle plate is too far open at idle. There is an stop screw on the underside of the throttle plate on a 3810 or 3367.

If it is too far open, you will have trouble setting your mixture on the primary side. As you turn the mixture screws in to lean, you should get a drop in RPM called a lean roll. Once you get the lean roll, then back out 1/4 to 1/2 a turn. If you can't get the mixture to adjust to a lean roll, probably the secondary throttle plate is too far open, or at worst, the primary block is warped.

23" is pretty high in vacuum, my 67 327/300 pulls about 17" at about 650 idle. It is tough to get them to idle down to 500 spec with the newer gas.

Have you got the correct distributor and vacuum can? Maybe Duke will chime in on this.

Jerry Fuccillo
#42179

[Tim E.]

Re: C2 Hot Idle Speed Too High

Thanks for the reply Jerry, nice to talk to you again.

Looking down the secondary bores, the throttle plates appear to be fully closed at idle (as do the primarys).

I get a very noticeable lean roll at about 1.25 turns out on each side of the metering block. 1/4 turn out from that point produces a very smooth idle. I actually fiddled with the mixture screws wondering if I could force down the idle speed by leaning the mixture but the engine would stall too easily.

The distributor housing itself is not original because it has a screw-in button to adjust end-play on the driven tach gear (don't know what advance weights are on it). I have a reproduction (Corvette Central) 355 can on the distributor now. With all that vacuum, the engine speeds up from 400 RPM to 700 RPM when I reconnect the vacuum line to the can.

What affect would wrong jet size have (if any) on idle speed? Do you think I could be drawing in unneeded fuel and therefore increasing idle speed?

[Gerard F.]

Re: C2 Hot Idle Speed Too High

Tim,

I think we are getting into Duke's expertise which is far greater than mine.

I would suspect the distributor type, and advance at idle with the amount of vacuum you have. Would make a difference if the car is an automatic or manual transmission, and whether the distributor is matched to the transmission. I'd see if I could check the history on the distributor. Maybe the can is not right for the distributor, or the initial is too advanced for it.

Jerry Fuccillo
#42179

[Joe C.]

Re: C2 Hot Idle Speed Too High

Tim,

23 in-hg is too high for your engine. Jerry's 17 in-hg sounds more like it. Jet size will have no effect on idle speed. The only thing that will affect idle speed (other than timing) would be position of throttle plates and/or vacuum leaks. If, indeed, your idle vacuum is 23 in-hg, then with the proper vac. can, you'll get far too much vac advance at idle, which will compound the problem.
Check your PCV valve. Is it the right one? Is it passing too much air? Is it stuck?
Do you have the distributor specs for your engine? If not, then I can send them to you.
What have you done to the engine to cause this condition, and when was the last time that it idled properly?
Is the primary butterfly binding when the engine heats up? This is a common problem with Holleys, and the slightest sloppiness or misalignment of the primary shaft will cause this condition. If you back out the idle stop screw, and are able to manually close the throttle fully, then that is your problem.

Sounds like a vac leak, and the PCV valve is the prime suspect.

Joe

[Tim E.]

Re: C2 Hot Idle Speed Too High

PCV valve could certainly be suspect....it's a cleaned up original so age-wise it's very old. I'll pull it tonight and see if it's stuck or leaks.

This condition is pretty recent, it hasn't been more than a few months ago when I could adjust the idle down as far as I wanted.

I'll look at the butterflies one more time. Both primaries and secondaries seem nicely aligned and smoothly operating. Both also seem fully closed with the idle stop screw fully out and the engine idling at 700 RPM.

Thanks Joe, Tim

[Duke W.]

Re: C2 Hot Idle Speed Too High

I'm highly suspect of "reproduction" VACs and PCV valves - whether they actually conform to the OE specs.

I don't know Holleys in great detail, but certainly the advice given should be followed.

You should check the VAC with a vacuum gage to be sure it meets specs. Also use a timing light to at least verify that the centrifugal curve doesn't start until at least 700 and advances from there near to spec. A dial back is best so you can check the entire curve.

There's no easy why to check whether PCV valves actually meet the design flow spec, but buying a correct OE replacement from NAPA is a good way to cross check what's installed. If the idle speed changes, the two don't have the same flow rate.

I suspect your vacuum gage may be inaccurate as a 300 HP engine should pull in the range of 17-19" at about 500 in neutral on a standard sea level day.

And I know of no reason why a properly set up 300 HP engine should not idle as new with today's gasolines. They aren't THAT much different.

The devil is always in the details, so systematically checking all these fine details will usually find the problem.

Duke

[Tim E.]

Re: C2 Hot Idle Speed Too High

I want to thank everyone for their diagnostic help. Your suggestions were spot on and as a result, I was able to determine the primary cause of the high idle speed situation.....excessively worn primary throttle shafts. As Joe suggested, I started by cleaning and checking the PCV valve which seemed to perform fine according to the test procedure I followed.

I could see play in both the primary and secondary throttle shafts as I moved them fore and aft in their bores. Not much, but enough to see. At one point, a little fuel even flowed out of the gap since I was operating the accelerator pump a little each time.

I was fortunate to have a parts carburetor that had a tight throttle plate so I cleaned it up and installed it. The vacuum reduced to 21" (on my unverified vacuum gauge), I was able to reset the fast idle and then the hot idle down to 400 RPM with a little room to go! The engine didn't idle bad to begin with (a sputter here and there, but generally smooth) but it's noticeably smoother now.

A new or rebuilt throttle plate may produce even better results. I'm inspired now to check other fine details as Duke suggests: distributer advance curves, timing, PCV valve, vacuum advance can, and vacuum.

Thanks again Guys...I think I've crammed for this final (this weekend's PV) enough! Hopefully the PV Gods will be on my side this weekend!

[Tim E.]

Re: C2 Hot Idle Speed Too High

Joe - You were right on the money! A worn primary throttle shaft was the culprit....see my more detailed note elsewhere in this thread.

Thanks a bunch, Tim

[Gerard F.]

Good Luck on your PV *NM*