Semi-automatic transmission and idle speed

[DonaldSmith]

Somewhere in my memory, from having this 47 DeSoto for 20 years, an idle speed of 450 rpm seems to pop up, in regard to proper transmission shifting. 

 

I just solved a failure to downshift, due to a poor connection at the fuse at the Transmission Relay.  Now I'm wondering what my idle RPM are.   Have I set it too low to keep running? 

 

My venerable Cen-Tech multi-meter, even with fresh batteries, gives me a low reading, like in the tens.  I know we are to multiply the reading by ten, but still way too low.  Besides, why different settings for the number of cylinders?  For a 4-cycle engine, each plug fires once every two revolutions.  The number of cylinders shouldn't matter.  Clip the test lead onto the coil wire?  That makes sense, but against the directions.  And still too low. So maybe the meter is toast.  

 

Then I bought a small, cheap  inductive meter, suitable for any engine with a spark plug    Wild readings.  Probably too much induction from other plug wires.  So I bought a Digital Photo Sensor Tachometer.  Sounds easy.  Stick the reflective tape on the shaft, point and read. But the hand instrument must be within 2 to 8 inches away, and perpendicular to the shaft.  So, I can put a chunk of reflective tape on the edge of the damper. But the fan blades are menacingly within the 2 to 8 inch range.  maybe I can get the meter lower, below the lower radiator hose.  Not ready to try it. 

 

So I'm going to stew on this for a while.   Comments and insights welcome.   

[Bryan G]

I wouldn't mind knowing my RPM; in the other current thread there was a mention of a kit to make a 12v neg ground tach work on our cars, and I may dig into it. I have a 70s vintage Sears engine analyzer that I thought would work but it's 12v only. I've been keeping my eyes open for an older model. 

 

The timing mark on my crank pulley was butchered long ago; almost impossible to see down there, anyway. I just set the timing with a vacuum gauge, then dialed it back slightly so it didn't knock. (I'd been running it for quite a while with the timing way too retarded, which explains the hesitation I kept getting when accelerating.) As for RPM, I've adjusted it by ear and sought that sweet spot where it won't stall. I need to up it slightly right now as it tends to conk out on occasion when feathering the go pedal while warm. My ear tells me I have it set at 450 or so but I really don't know. What I do know is that these engines can truly impress with their low idle speed (and smoothness.)

[Art Bailey]

12 minutes ago, Bryan G said:

I wouldn't mind knowing my RPM; in the other current thread there was a mention of a kit to make a 12v neg ground tach work on our cars, and I may dig into it. I have a 70s vintage Sears engine analyzer that I thought would work but it's 12v only. I've been keeping my eyes open for an older model. 

I rigged a 12v wallwart to provide power to my Sears 12v engine analyzer, did the trick. 

[Doug&Deb]

Any engine analyzer with a tachometer will work. I use the cheapest 12 volt riding mower battery I can find to power my timing light. My analyzer works fine with the 6 volt battery. I set my idle at 475

[Sniper]

A four cycle engine does indeed only fire, each cylinder, once every two rotations.

 

A single cylinder 4 cycle engine has one ignition even every two revolutions

 

A two cylinder 4 cycle engine has two ignition events every two revolutions.

 

A three cylinder 4 cycle engine has three ignition events every two revolutions.

 

A four cylinder 4 cycle engine has two ignition events every four revolutions.

 

Etc, the tach needs to know how many cylinders you have so it can read the proper rpm by counting the ignition events and dividing by the number of cylinders.

[DonaldSmith]

I gave up on the brand-new Digital Photo Sensor Tachometer.  No way to get it close enough to the damper, with the fan blade and belts too close.  I can use the meter to check the RPM of my drill chuck, if I ever need to know that.  

 

I clipped the venerable Gen-Tech Automotive Meter to the coil wire, and was getting readings of 81, 82.  Multiply by 10; idling too fast.  Back out the idle screw, more like 690, 720.  Still some lowering to do. 

 

I clipped the pocket-size Pulst Engine Tachometer to the plug boot, and was getting readings of 400, 450, 600 plus; pick a number.  Not too bad. 

 

I twiddled the idle speed and mixture screws till the engine sounded about right.  But the proof of the pudding is in the eating. 

 

A short neighborhood cruise had good upshift, downshift at stops, and kickdown working.  What more could a fella want?  Starting off could get touchy.  Too much pedal?  I got used to the feel. 

 

So the idle $PM is not set in stone; it's whatever works

 

First goal is reliable travel 40 miles each way to the Orphan Car Show in Ypsilanti on September 18th.  It's looking good for that. 

 

The second goal is to do full maintenance and tuning over the winter, for a reliable trip across the border to Chatham, Ontario, next Summer, for the National DeSoto Club Convention, 70 miles, or 114 km.   

 

 

[keithb7]

My mid-70’s Snap-on Dwell/Tach

meter box works well. However I have to hook it up before I start the engine. Hook it up while running? The tach is erratic and way out. I couldn’t figure out why. A friend gave me this tip.  Now it works perfectly. 
 

 

Edited August 27, 2022 by keithb7

[DonaldSmith]

Keith, do you hook your meter to the coil wire, or any spark plug wire? 

[Sniper]

18 minutes ago, DonaldSmith said:

Keith, do you hook your meter to the coil wire, or any spark plug wire?

 

If there are three wires, black, red and something else.  Black goes to battery negative terminal, red goes to battery positive terminal and the other wire hooks up to one of the two small terminals on the coil, specifically the one connected to the points. 

 

Two wire ones, black hooks to the negative post of the battery, or a ground, and red goes to the points terminal on the coil, for - ground engine, positive ground systems you put the red to the ground,, the black to the points terminal on the coil.

 

Nothing for the dwell meter gets hooked to a coil or plug wire.