72 TCS Idle Speed

[Dennis B.]

The TCS seems to be working correctly on both the temp below 85 and in 4th gear with about a 25 second delay, but I only get an increase of about 200 rpm instead of the 300-700 in both cases. Curb idle is between 800 and 900. If working but not in the 300-700 range, will there be a deduction on the Operations in Flight Judging? Any ideas or suggestions of things to check?

[Terry M.]

Re: 72 TCS Idle Speed

There should be a deduction for the condition you describe, but not all judges will catch that small difference.
Check ALL your distributor timing settings -- initial timing, centrifugal (especially when it begins), and perhaps most importantly the operation of the vacuum advance unit. Also check the vacuum to the distributor -- you might have a weak vacuum signal. I would bet you find one or more of these not up to specification.

[Jack H.]

Re: 72 TCS Idle Speed

I question the 'omnibus' spec of expecting a 300-700 RPM increase from the proper opertion of a TCS or CEC system. First, let's understand that cold engine idle RPM is a product of just how cold it is and that changes as the engine warms.

The Chassis Service Manual, tells you to set the cold idle RPM on carbs ONCE THE ENGINE IS FULLY WARMED UP. And those specs greatly exceed the contribution of an actuated TCS/CEC system.

So, judges and owners need to understand the cold idle RPM numbers published flow from GM tuneup specs and you will NOT see those figures on a stone cold engine whose choke is properly set immediately upon first fire. A truly cold engine can exhibit idle RPM's that are roughly 50% below what you dial in based on the tune up manual using a fully warmed engine and FORCE the high idle onto the specified cam step...

Now, I don't have a '70 or '72 CSM book here (TCS system), but I do have a '71 CSM (CEC system). If you look under Engine Fuel (Section 6M), it tells you NOT to adjust the carb's curb idle by using the emission solenoid adjuster. Then, it goes on to refer you to an adjustment table that's nowhere to be found in that section if you have to re-set the CEC/TCS solenoid.

If you turn ALL THE WAY to back of the book, there's a Specifications section and one of the tables pertaining to carb's is called 'Other Adjustments'. There, it lists Initial Idle Speed, Final Idle Speed, and CEC Value Engine Speed by specific car and configuration.

Consider a Holley based LT-1. The Final Idle Speed listed is 700 RPM and the CEC Valve Engine Speed is 900 RPM. That's a difference of 200 RPM which is what the CEC should contribute to a WARM engine when the solenoid fires AND the distributor is given vacuum advance.

There's NOTHING in this table that goes beyond a delta of 300 RPM with most engines falling in the 200 RPM range which dove tails with your finding(s). Watch out for judges who don't know how to interpret the original Chevy specs!!!!

[Terry M.]

Re: 72 TCS Idle Speed

Jack,
I can only recall two meets where we judged COLD engine conditions -- although I am sure there must have been more -- but I doubt many more. I judged at a Regional in Minnesota years ago where it snowed on the day of judging -- only light flurries though. I understand the Denver Regional last year exhibited some cold conditions. Beyond those and probably a very few other events, we judge Corvettes in more temperate conditions. Some, as at the National this past Summer, might be regarded as intemperate at the other extreme.

My point is that we rarely see truely cold start conditions, and often the opposite is the case -- we sometimes have to force choke operation. Of course then there are those who follow other suggestions on here and use 15W40 CJ4, but it may be best not to go there.

[Jack H.]

Re: 72 TCS Idle Speed

Yep, it was COLD at the Denver Regional and one car was actually PV'd on opening day going out into a driving blizzard! I think the owner got LOTS of slack...especially since his heater was working A-OK.

My comments on cold engine idle come from a personal experience with my '71 at an RMC/New Mexico joint chapter meet. It was COLD that morning and this was when there was no Operations section in the JG book...

The two ladies who conducted my Ops Check had me set the carb and fire the engine. They were using the PV manual because there was nothing to guide them in the JG book.

The engine fired and the tach read 750 RPM, or so. They, immediately took a deduction. I said, "Whoa, gal, you're reading the book wrong" and gave the little lecture I just did on where our published specs come from.

She didn't believe me.

So, I said, "Humor me and let's do other things while the car engine warms up. You'll see, once she's warm that tach WILL be reading 1350 RPM and I won't touch the accelerator pedal!"

WELL, as we worked other items the tach SLOWLY climbed... She still wanted to take a cold start deduct because the needle wasn't reading 1350! After a good 15 to 20 minutes, shore nuf, just as I predicted, the tach was PEGGED at 1350 RPM...

She was a bit ruffled, but begrudgingly erased the cold start deduction she'd taken. That's where I was coming from with my "Beware of judges" comment.

Not everyone who conducts an Operations Check is a qualified PV judge with the experience to understand where our published numbers come from + how a truly cold engine behaves differently from one that's fully warmed up.

That's why cold idle is set at roughly twice the RPM of normal curb idle. Internal engine friction and sluggish oil flow WILL reduce first fire engine RPM! The choke's high idle is there to prevent a truly cold engine from stalling on initial cold start where it might be running at 'half speed' until it warms up...

[Patrick H.]

Re: 72 TCS Idle Speed

Now, I don't have a '70 or '72 CSM book here (TCS system), but I do have a '71 CSM (CEC system). If you look under Engine Fuel (Section 6M), it tells you NOT to adjust the carb's curb idle by using the emission solenoid adjuster. Then, it goes on to refer you to an adjustment table that's nowhere to be found in that section if you have to re-set the CEC/TCS solenoid.

Jack,

On a 72 the idle speed is set using the solenoid on the left front of the carb. It surprised me too (having owned my 71 much longer than my 72) but the idea is that when the car is shut off, the solenoid drops and the throttle blades close even further to stop any "dieseling" effect.

Of course, as a side note, you have to go thru the correct sequence to start the car. Be sure you depress the accelerator to set the choke after you've turned the key to the "on" position but before you attempt starting it. The solenoid doesn't have enough power to push the accelerator lever and attached springs, so you'll start the car at its 400 rpm "shutdown" point if you depress the accelerator before turnign the key to on and energizing it. One of those "I know because I do it" things. And yes, I've discovered that my car can run just fine at 400 rpm when warm.

Patrick

[Terry M.]

Re: 72 TCS Idle Speed

"Not everyone who conducts an Operations Check is a qualified PV judge with the experience to understand where our published numbers come from + how a truly cold engine behaves differently from one that's fully warmed up."

I absolutely agree Jack.

But I couldn't resist tweaking you a little about COLD meets.

Hey they tell me it is gonig to be 90* here in North Carolina tomorrow. Sure beats the snow we had at the Illinois/Wisconsin border Monday night.

[Jack H.]

Re: 72 TCS Idle Speed

We have that reputation out here in Colorado... You may remember a few stray flakes of snow falling on opening day at our Breckenridge Regional too, Terry!

No offense taken, I just wanted to clarify where I was coming from with my 'beware Judges' comment. As we DO typically judge during warm weather conditions and rarely see cars under truly cold situations, there are some judges who don't understand the in/out details of evaluating cold engine operation...

Also, Pat's comment about turning the ignition ON and THEN doing cold start carb priming is SPOT ON! These idle stop solenoids have a limit to how much plunger force they can deliver. When you press the pedal all the way to the floor to set the choke, that frees the solenoid plunger from trying to overcome the throttle cable's return spring force.

In actual operation, the solenoid kicks in when the tranny is in high gear with the car in motion. So, the solenoid plunger is free to travel without 'fighting' the throttle cable spring. I see few judges who advise owners on this method of cold starting the engine and/or testing the idle stop solenoid once the engine is running...

This can be a vital consideration with the one year only CEC system from 1971. Why? Well, the solenoid has a dual function (switch vac AND switch curb idle). When the solenoid vents vac, it takes in engine compartment air via the filter foam surrounding the plunger.

That air is typically dirty (laiden with grease/oil) and the plunger shaft area of these solenoids tend to 'load up' with time/use. So, it's not uncommon to see a difference between a used original and a low mileage NOS part. But, the performance question is really does the solenoid engage properly when the throttle is moved off the carb's curb idle setting?

[Jack H.]

Re: 72 TCS Idle Speed

Hum, that's INTERESTING, Pat! I'll have to check out a copy of the '72 CSM book because that doesn't make full sense to me...

The way the idle stop solenoid works to cut emissions is by controlling the DELTA RPM between normal curb idle (mechanically set) and the elevated idle RPM dictated by the solenoid plunger. You can't get there from here if you only spec ONE of the two RPM levels.

I CAN understand the solenoid is capable of delivering an anti-diesel feature that would be beneficial. But, that too doesn't work without there being a second tune up spec for the carb's mechanical curb idle speed. So, my 'go do' is to read more...

[Patrick H.]

Re: 72 TCS Idle Speed

Hum, that's INTERESTING, Pat! I'll have to check out a copy of the '72 CSM book because that doesn't make full sense to me...

The way the idle stop solenoid works to cut emissions is by controlling the DELTA RPM between normal curb idle (mechanically set) and the elevated idle RPM dictated by the solenoid plunger. You can't get there from here if you only spec ONE of the two RPM levels.

I CAN understand the solenoid is capable of delivering an anti-diesel feature that would be beneficial. But, that too doesn't work without there being a second tune up spec for the carb's mechanical curb idle speed. So, my 'go do' is to read more...

Jack,

There is a second spec for the mechanical curb idle speed.