Idling RPM Issue 67 327-300 w AC & PG

[Al R.]

Having an issue getting the RPMs to stay steady at 600. They will vary from 600 to 630 to 570. Have sprayed starting fluid on intake & carb base-no difference in engine speed was detected. Removed the vacuum hoses 1 at a time from the PB booster, AC, PG modular valve and plugged --no changes detected here either. Removed spark plug wires 1 at a time--engine stumbled as expected. All new wires plugs distr. cap, professionally rebuild dist. and carb, dist. has the copper tipped rotor. Adjusted air bleeder screws on carb-no help here either. I'm out of ideas. NEED HELP on this one. Thanks!! Al

[Gene M.]

Re: Idling RPM Issue 67 327-300 w AC & PG

Al,
Put a timing light on it and see if you have timing move a little. If it is you found the reason. But generally this is caused by a small vacuum leak. Carb throttle shafts ok too?

[Ken A.]

Re: Idling RPM Issue 67 327-300 w AC & PG

If you're basing this observation on the factory tach, then you're wasting your time. 30 rpm variation is acceptable specs per GM.

[Al R.]

Re: Idling RPM Issue 67 327-300 w AC & PG

Gene, the timing mark stays dead on the mark at 6* BFTDC. I'm using the RPM part of the timing light to check the RPM. I have not sprayed starter fluid around the throttle shafts, But I'll give it a try on Sun. Was going to tonight, but severe storms have knocked out power to a lot of NC, me included. I will post the results Sun. PM

[Timothy B.]

Re: Idling RPM Issue 67 327-300 w AC & PG

Have you warmed the car good and set the emulsion screws to max vacuum and rpm? Many times this adjustment is set when the plenum is not up to temperature and a small surge at idle.

How much vacuum does the engine make at idle?

[Al R.]

Re: Idling RPM Issue 67 327-300 w AC & PG

Tim, I have not checked the vacuum at idle. The engine was warmed up fully each time we have set the screws. Is it possible the carb to base gasket could be a problem? The one I installed had been LAYING AROUND THE SHOP A # OF YEARS. Sorry about the cap letters, no power here since 6PM last night due to the storms and computer is about exhausted the battery-no time to re type

[Timothy B.]

Re: Idling RPM Issue 67 327-300 w AC & PG

Al,
Anything is possible but I doubt the gasket is at fault if it's new. What rpm does the service manual spec the idle with A/C etc. like your car has. From your description it sounds to me like surging and that could be fuel distribution. Can you tune it out by adjusting the emulsion screws and does it also go away if you raise the idle to 700.

At 600rpm the throttle blades at almost completely shut on a 300hp engine and the 18" of vacuum is pulling hard on the curb idle ports on the primary side of the carburetor.

[John H.]

Re: Idling RPM Issue 67 327-300 w AC & PG

Tim, I have not checked the vacuum at idle. The engine was warmed up fully each time we have set the screws. Is it possible the carb to base gasket could be a problem? The one I installed had been LAYING AROUND THE SHOP A # OF YEARS.

Al -

Yes, it can, if it's not the correct gasket - the carb-to-intake gasket must always match the hole pattern on the intake; in your case, that's a 4-hole gasket (the gasket with one huge hole is only for use with single-plane "hot-rod" intake manifolds). BTW, you really need a vacuum gauge in order to properly adjust the idle mixture screws.

[Al R.]

Re: Idling RPM Issue 67 327-300 w AC & PG

OK back home with power and just checked the vacuum. It fluctuates between 11-12 inches at 600, 12 at 650, 14 at 850, 18.5 at 1350, 20 at 1850, and 20.5 at 2000RPM. John, I'm using the correct gasket between the intake and the thin SS plate, but an open 1 huge hole gasket between the carb and the thin SS plate. Should there even be a gasket between the carb and the thin SS plate? I was told by someone this weekend that he did not remember it required one. What should the vacuum be at 600 RPM for the 300HP with PG AC & PS? Thanks Al

[Timothy B.]

Re: Idling RPM Issue 67 327-300 w AC & PG

My 67 carries 18+" vacuum at curb idle speed (600)rpm, the vacuum readings you post sound like a L79 camshaft.

[Edward J.]

Re: Idling RPM Issue 67 327-300 w AC & PG

Al, It sounds like the cam is not stock, As Tim says,If so the Dist. vacuum canister should be tested to see if its all in with the 11/12 inchs of vacuum.Duke had always posted alot about the proper vacuum canister and driveabilty problems with out the correct one installed. my 63 327/340 car has the same as yours and has the B28 canister from standard ignition.

Another thing RPM'S that ranges of only about 50 RPMS is not really all that bad.Sometimes I will use my old school dwell meter which has RPMS and test that against the timing light with W/RPMS.

[Al R.]

Re: Idling RPM Issue 67 327-300 w AC & PG

You guys have got me to wondering if the engine builder installed the correct cam. I will double check with him. Will also check the can for the 11-12 inches of vacuum pull. The man who restored the distributor told me it had the wrong one on it when it was received by him and he would install the correct one when he restored it. But, if it has the wrong cam installed, then the can may need to be changed also. At this point, I don't think I would go to the trouble to change the cam, but rather try to find and correct what issues I have and at a later date install the correct cam, if, indeed that is the problem, which I hope is not the case. Other than asking the engine builder and hope he gives me an honest answer, if he even knows, is there an easy way to tell what cam is installed?

[Edward J.]

Re: Idling RPM Issue 67 327-300 w AC & PG

Al sometimes the rebuilders will ask you to take a reading of the engine vacuum at idle so the Dist. is set up for the vacuum.But If you relayed to the dist. rebuilder the stock 300hp cam was installed you likely have the wrong vacuum can.

[Al R.]

Re: Idling RPM Issue 67 327-300 w AC & PG

Edward or Tim, I didn't see in the AIM where a gasket is shown between the carb and the SS plate. Is this correct or should it have 1 like the gasket that goes down onto the top of the intake? If no gasket, should the SS plate be replaced each time the carb is removed or does it seal multiple times over? Thanks!

[Timothy B.]

Re: Idling RPM Issue 67 327-300 w AC & PG

Al,

There is no gasket between the carburetor and shim. You should not need to replace the shim when removing the carburetor and IMO, the gasket under the shim should be OK if it's not disturbed.

FWIW, the important thing to remember is the heat in the intake manifold slot can burn through the shim over time and cook the carburetor but I suspect that does not happen much any more as these cars are weekend cruisers. Look in the archives for some pictures of how most of us have plugged the slot to keep the heat out of this area. This year with the 10% ethonal gasoline I have found the need to wire the heat riser valve open to keep even more heat out of the plenum. After 30 minutes of driving the cast manifold gets hot and the gasoline in the Holley accelerator pump will vaporize causing difficult driving for slow stop and go.