R6T7

I can't seem to find the right balance with idle speed on my 67 PG car. Either it stalls when sitting in traffic while it's in drive, or it's idling at 1000 rpm while it's in park. Has anyone found a good way to get it right?

I always adjust idle mixture for highest rpm (or manifold vacuum) while the engine is hot, and in drive, and then when both mixture screws are set exactly the same number of turns out from bottoming, set the idle speed at about 600 to 750 rpm while in park. Problem is, when idle speed is that low, it will often stall out when it's in drive at a stoplight. When the A/C is on, it just gets worse.

I have the same problem with both PG cars - one is a 300HP, the other is a 390HP.

I know some have installed a idle speed compensator that adjusts idle when the A/C compressor is running, but I've got to believe that there's a way to get these cars to idle without stalling with the A/C off.

Would appreciate any advice -

Bob

stingrayl76

Check your initial timing and make sure it's at spec. Make sure you're using the correct vacuum advance can and that it's connected to manifold vacuum. Sorry, I don't know which vacuum can is correct for your applications.

JohnZ

I just went through this last week helping a guy set up his '57 2x4 with Powerglide for his P.V. test this week; on Powerglide C1's, idle mixture and idle speed are both set with it idling in Drive. With it set to the factory spec (600 rpm in Drive), shifting to neutral or park raises it to around 800 rpm.

Forget about "making sure both mixture screws are set the same number of turns out from bottoming" - the correct setting for each screw is based on the vacuum gauge reading, and they won't necessarily be the same.

w1ctc

If the mechanical advance is starting at too low an RPM, it would have that effect. That problem also happens when a hot cam is used with a stock converter.

MikeM

vark_wso

Bob

I have had this difficulty with my '66. Sometimes the idle speed achieved in neutral is high enough to bring in a couple degrees of mechanical advance, but when you shift into 'D', you loose that bit of advance. Similarly, the manifold vacuum in 'N' is 2-3" greater than in a 'D' position.

I'm using a B20 vacuum can now with good results. It is fully deployed at ~12" Hg, so even under the worst circumstances in idle/D I have all 16 degrees of the can in solution. (A B25 -- 14" -- should work also, but mine ran better with the B20.)

My distributor has stock weights, 24 degrees of mechanical advance available. I worked with Lars to get the right springs. You have to be careful not to get to light on the springs because of the rpm difference between 'N' and 'D'. I think he used Accel springs, but I'm not sure. I have one gold, medium-thin spring with about 10 coils & one natural finish, thicker, about 7 coils. This seems to hold off any mechanical even as high as 800-900.

Anyhow, before you settle on a 'compromise' idle speed that works 'OK' under all conditions, suggest you examine both vacuum and mechanical advance. Goal is to have the vacuum can fully deployed AND the mechanical advance fully retarded during all of your 'N', 'D', a/c-on, a/c-off situations.

jdk971

do these autos use a dash pot at the end of the linkage? i thought most
autos had a dash pot that keep the rpms up when the ac was kicked on.
jim

lars

This is exactly right. What frequently happens on automatic cars is that the timing changes dramatically (retards) from "P" to "D". This is due to 2 factors:

1. Centrifugal advance is coming in too quick. Make sure you have your timing curve set up so there is no mechanical timing difference between the rpm in "P" and the rpm in "D."

2. A vacuum advance control unit that does not pull in its full "range" in "D" is being used. If you are running the vacuum advance off of manifold vacuum (which is preferable from a performance standpoint to running off of ported vacuum) and the unit is too "stiff" to pull its full, "pegged out" range in "D", the vacuum advance can change (retard) the timing around 10 degrees-or-so when you drop it in "D." Check your vacuum in "D" and make sure your vacuum advance specs out for maximum deployment at or below that number. You can check for this problem by simply plugging the vacuum hose to the vacuum advance, re-setting the idle rpm for the new timing condition, and seeing if you still have a problem with excessive rpm drop - if the problem is solved, get a softer vacuum advance control unit.

Lars

JohnZ

Nope - A/C "idle kicker" solenoids weren't used until later in the C3 years.

A "dashpot" is a cushioning device that slows the throttle's return to idle; none were used on midyears. Some C3's with Q-Jets had them.

R6T7

Thanks for the great feedback - I had not considered the effect that timing and advance would have.

According to the owners manual, a 300HP powerglide car should be set to 4 degrees ATDC at 600 RPM if it has a distributor P/N 1111117, or 6 degrees BTDC if it has a distributor P/N 1111194. This sounds like they may have used a different vacuum advance (or maybe a different mechanical advance) in these two distributors. According to the NCRS judging manual, the 1111117 was used in PG cars, while the 1111194 was used in manual trans cars.

I'm not sure which one is in my car, since the amuminum band is missing, and I don't think these are any other numbers on it that would tell me what I have. However the engine is the original engine, and I suppose I could assume the distributor is also. Is there any way to tell which is which?

But if I set initial timing to 4 degrees ATDC, it seems like I'm starting with a retarded advance which would make matters even worse. Or should I set it to 6 degrees BTDC and go from there?

Bob

Powershift

Bob:

To keep from guessing and to do things correctly, you need to determine what the vacuum advance and mechanical advance are for your distributor. Suggest getting a dial back timing light, or use a standard light with tape/chalk and degree your harmonic damper. Then see what your timing is every 500 rpm from idle speed (500 RPM) to 4000 or so RPM. The vacuum advance should be disconnected during this test. This should tell you what distributor you have and how well the mechanical advance is performing. It will also tell you if the distributor has been modified sometime during its life. Also get a Mighty-Vac, or a vacuum gage, and manually apply vacuum to the distributor vacuum advance unit with the engine running to see at what point its starts to advance and at what point it is maxed out, along with the degrees of advance.

You mentioned two distributor numbers: 1111117 and 1111194. The #17 (for automatic transmission) mechanical curve starts at 900 RPM and is 19 degrees at 1200 RPM, 25 degrees at 1500 RPM, with a max of 40 degrees at 5100 RPM. This is the one that requires 4 degrees ATDC initial setting.

The #94 (for manual transmission) mechanical curve also starts at 900 RPM. It is 9 degrees at 1200 RPM, 15 degrees at 1500 RPM, with a max of 30 degrees at 5100 RPM. This one requires 6 degrees BTDC initial timing.

The vacuum advance is the same for both #17 and #94 distributors and is 0 degrees at 6 inches of mercury vacuum and is 15 degrees at 12 inches of mercury vacuum.

All of the values above are crank degrees at engine RPM and are from the 1967 Chevrolet Chassis Service Manual Specifications.

If you were to start at 6 degrees BTDC initial advance and had the correct #17 distributor for auto cars, your total mechanical advance could be as high as 46 degrees, which can cause engine damage. So you really need to determine what you have before we can advise further.

It also appears that your car has the California Air Injector Reactor emissions package. Please verify if this is correct and whether or not it is still operational. Also tell us what your engine manifold vacuum is in PARK and in DRIVE, along with the corresponding RPMs. Also if the engine vacuum is steady.

Larry

R6T7

Larry -

That information helps a lot - you are correct - this is a California car, and it has all of it's A.I.R. components installed and operational. I have removed the air pump belt as the car tends to backfire a bit on deceleration. (The backfire could have been caused by a very rich running carburetor which I have corrected by rebuilding it, and I have not tested it with the belt installed)

How did you guess it was a California car - other than my address?

The engine has just been rebuilt, and it pulls about 21" to 22" of vacuum at 700 rpm idle - I have not measured it in Drive, and won't be able to do that for a day or two. Vacuum is steady as you would expect for a fresh engine.

I've got a dial back timing light and vacuum guage, so I will be able to run the tests as you described. I'll try to tackle that tomorrow after the car comes back from a local car show -

Bob

Powershift

Bob:

The #1111117 distributor is listed in the 1967 Chevrolet Service Manual only for California cars with K-19 Air Injector Reactor (air pump). Plus you live in California.

You also need to determine if your distributor vacuum advance vacuum source is manifold or ported vacuum. Both originate from the Holley carb, but if manifold vacuum it is active with the carb throttle plates closed. If ported vacuum, it will not be active until the throttle plates are opened up. This can make a big difference in how your engine responds. So you need to check the amount of vacuum coming off the carb rubber hose to the distributor at engine idle. It will either be your 21-22 inches of vacuum (manifold) or zero (ported) . Let us know.

Also, was your recent engine rebuild done to stock specs, or did you make changes?

Larry

JohnZ

Forget the 4* ATDC setting - that was an emissions crutch to increase EGT (by retarding the timing) to make the A.I.R. system more efficient in "afterburning" the exhaust stream in the manifolds. The "ported" vacuum setup on the K-19 further retarded idle timing by providing zero advance at idle.

It will run MUCH better with 6*-8* BTDC of initial timing and full manifold vacuum on the vacuum advance unit. You'll need to take out some centrifugal advance, as the stock curve in your distributor adds 40* at 5100 rpm, and you don't want to exceed 36* total (initial plus centrifugal).

The "backfiring" you noted on overrun (which is actually "afterfiring") is a sure sign that the rubber diaphragm in the mixture control valve has failed - they all do. When it fails, the valve defaults to open, and dumps air into the manifolds full-time.

R6T7

John -

Thanks for that perspective - I think I now have the information I need to get this car set up for best operation. Unfortunately, I won't be able to work on it for a few weeks as we'll be out of town.

Thanks to everyone for the input on this issue -

Bob

JohnZ

Bob -

In the meantime, stick a BB in the vacuum signal hose that goes to the mixture control valve - when the internal diaphragm fails, it also creates a vacuum leak through the valve; the BB will stop it.