Tachometer Dances Like It Wants to Audition for DWTS...

[ACME A12]

Not RR-related, but here's a tachometer issue for someone more electronically savy than I:

Put a new Summit (Mallory) Electronic Distributor in my '74 Duster today. The one that was in the car was one that I had run in one of my other Dusters back in the '80's; It served me well an couple of different engines but had gotten a little worn... I did not change the ECU or make any other changes to the ignition system. I did install new plugs, and I disabled the vacuum advance so I am relying solely on the mechanical curve in the distributor now. Initial timing was dialed in at 18 degrees with 34 coming in at 2,000 RPM. I then hooked up the vacuum gauge and piddled with the carb (Holley 750 0-80508S) until I was pulling about 15"Hg at idle - decent for a 340 with a Comp XE274H in it I thought. The huge off the line bog that I had been experiencing lately is now gone and it is running great again. It's now ready for the trip over to Kissimmee to hang out with Johnny McMoPar next weekend.

So here's the rub - suddenly my tach (Autometer Street Tach) jumps all over the place at idle (1,000 RPM in neutral or park)... :huh: It was always steady as a rock before, but at anything less than 1,500 RPM it's jumping like a mexican jumping bean. One irregularity that I noticed when I set up the distributor was that the air gap between the pick-up and the reluctor was not uniform. I set it at .008" (Ma MoPar recommended) on the two tightest points and have about .011" on the two widest points. Any idea if perhaps this might cause what I'm seeing, and if so, why does it clean up when the engine speed is increased? If anyone has any thoughts please let me know. Just kind of aggravating...

:jester:
ACME

 

[69hemibeep]

I would go back to working on the phord, you were making progress on it. Hope I was of some help :jester:

 

[ACME A12]

I would go back to working on the phord, you were making progress on it. Hope I was of some help :jester:

You were an immense help, Mr. HemiBeep. Now I will be able to sleep tonight...

:jester:

 

[A31PKG]

The making and breaking of a magnetic field is all the control unit is looking for so it can trigger the coil...As long as there is a votage pulse generated, the clearance really shouldn't be an issue as long as it's close - when we're only talking about .003 of an inch difference, I think your're in...I'm not familiar with the schematic or operating criteria of your system, but in general terms, a tach is nothing more than a fancy voltmeter. It should be easily checked with a DVOM...Do you have another (known good) car you could temporarily hook the tach up to?

 

[69hemibeep]

Ray do you have an old tune up test set with tach? Set that up and see what happens. I have one with dwell, amps, volts and tach high and low

 

[ACME A12]

Do you have another (known good) car you could temporarily hook the tach up to?

No; it's hard wired into the car. Are you assuming that something harmful happened to the tach today that just happens to coincide with the dist change? I see this as highly unlikely...as it still functions normally above idle...

 

[ACME A12]

Ray do you have an old tune up test set with tach? Set that up and see what happens. I have one with dwell, amps, volts and tach high and low

Not currently; I had and old Penske/Craftsman unit that the old man bought new at Sears back in the early '70's...works like a champ...but he borrowed it back so it's a few states away at the moment...

 

[A31PKG]

As a troubleshooting strategy, I was suggesting to substitute a "known-good" source to see if it was the tach or not. Since your tach is not easily removed, that would be difficult. A better idea, would be to hookup a test unit (Dwell/Tach/Volt meter) which has been suggested. FWIW, If it was an input or ground issue (excessive resistance, loose connection, etc) then I believe that the tach would DWTS at any RPM. If the tach checks out, then another possibility is that there may in fact be an air-gap clearance issue (either at the pick-up, or between the rotor and dist cap) caused by worn dist shaft bushings...This problem could tend to minimize as RPM's increase. Picture a tire that vibrates at 40 mph, and smooths out at 60. I recall the time when we used to observe the secondary occilliscope patterns back in the 70's. When there was an excessive variation in the dwell (points closed) pattern (caused by worn point cam or dist shaft bushings) that portion of the pattern would not line up at the 30 degree mark on the screen (spec for V8's). One cylinder would be at say, 20 degrees, and the values could vary all the way to 80...But, when the RPM's inceased, you would see the patterns tend to line up. This "slop" in dwell reading did not always cause a terribly rough idle, but could effect how the tach worked....Just offering a thought or two...

 

[Hoosier Bird]

What is DWTS?

 

[69hemibeep]

What is DWTS?

You know what it is, you watch it along with survivor

 

[A31PKG]

Doesn't Work Tachometer Syndrome

 

[george68hemirr]

Doesn't Work Tachometer Syndrome

this guy is good

 

[69hemibeep]

Doesn't Work Tachometer Syndrome

this guy is good

Yeah, he danced right around it

 

[Big John]

Check all your connections. You could have loosened the one at the coil.

 

[ACME A12]

As a troubleshooting strategy, I was suggesting to substitute a "known-good" source to see if it was the tach or not. Since your tach is not easily removed, that would be difficult. A better idea, would be to hookup a test unit (Dwell/Tach/Volt meter) which has been suggested. FWIW, If it was an input or ground issue (excessive resistance, loose connection, etc) then I believe that the tach would DWTS at any RPM. If the tach checks out, then another possibility is that there may in fact be an air-gap clearance issue (either at the pick-up, or between the rotor and dist cap) caused by worn dist shaft bushings...

Distributor is brand new so the bushings should be fine. The variance in air-gap clearance was an immediate concern. Just typical shoddy craftsmanship as we have all begun to expect.

This problem could tend to minimize as RPM's increase. Picture a tire that vibrates at 40 mph, and smooths out at 60. I recall the time when we used to observe the secondary occilliscope patterns back in the 70's. When there was an excessive variation in the dwell (points closed) pattern (caused by worn point cam or dist shaft bushings) that portion of the pattern would not line up at the 30 degree mark on the screen (spec for V8's). One cylinder would be at say, 20 degrees, and the values could vary all the way to 80...But, when the RPM's inceased, you would see the patterns tend to line up. This "slop" in dwell reading did not always cause a terribly rough idle, but could effect how the tach worked...

Makes perfect sense to me. Now I need to figure out how to mimimize the variation.

Just offering a thought or two...

All input is appreciated. Thank-you! :cheers:

:jester:
ACME

 

[ACME A12]

Check all your connections. You could have loosened the one at the coil.

Check. Will do. Sometimes it is the simplest things...

:jester:

 

[george68hemirr]

[quote="Big John":5heit1gg]Check all your connections. You could have loosened the one at the coil.

Check. Will do. Sometimes it is the simplest things...

:jester:[/quote:5heit1gg]
you talk about simple things .....my buddy drops in a brand new motor and everything else new too and ran into a problem with the coil wire[brand new too] shorting out on the firewall......stupid things.....good luck ray finding it

 

[Hoosier Bird]

What is DWTS?

You know what it is, you watch it along with survivor

I haven't watched Survivor for over a year........but my wife watches Dancing With The Stars....so it sounds like Ray is a fan......

 

[ACME A12]

[quote="Hoosier Bird"8f65sk7]What is DWTS?

You know what it is, you watch it along with survivor

I haven't watched Survivor for over a year........but my wife watches Dancing With The Stars....so it sounds like Ray is a fan...... [/quote8f65sk7]

I only have peripheral knowledge from seeing the ads and promos - much the same as my limited knowledge of your beloved Survivor program... Just figured you and Stuie would get a kick out of a DWTS reference...just one more reason for you two to bust out the chiffon and the boas and parade around the living room... :lmao:

:jester:

 

[69hemibeep]

I only have peripheral knowledge from seeing the ads and promos - much the same as my limited knowledge of your beloved Survivor program... Just figured you and Stuie would get a kick out of a DWTS reference...just one more reason for you two to bust out the chiffon and the boas and parade around the living room... :lmao:

:jester:[/quote]
I have a mental picture of that. Get it out of my head :ack: