Tachometer fluctuation

[AUSMHLY]

Why does my Tachometer fluctuate?

I just tried a new british mechanic yesterday and when he finished tuning the carbs (which were just rebuilt by Joe Curto) the car idled at his shop at 7 1/2. But later that day, after driving around, the tach fluctuated.

When the cars warmed up, stopping at the lights, it will go from 5-12. Sometimes hitting the gas peddle will settle it from 12 to 10. Another time it may settle it from 12-7. Other times I may stop at the light and it reads 5, 6, 7, etc.

I have new wires, new plugs, new sports coil, Pertronics, cap and rotor look ok.

Negative ground car. Tach was rebuilt and has a chip board for negative conversion.

Bad tune up guy or is there some other area I need to look into.

Your thoughts?

Thank you.
Roger

 

[Roger]

Do you mean 100s RPM? I.e. 7 1/2 means 750rpm?
I'd suggest a bit of binding in your throttle linkage, but it's not unusual to have a bit of fluctuation. No ECUs in cars that old!

 

[Deleted member 8987]

Usually if it "hunts" it a vacuum-related issue.
If blipping of the throttle causes varying idle speeds (and you can actually HEAR the speed difference), how are the throttle shafts, bushings, and plate?

Seen loose plates do that, too.

 

[Bob_Spidell]

That's a lot of fluctuation. Offhand, sounds like an air (vacuum) leak.

Does the engine speed actually sound like the tach reading; i.e. the difference between 600rpm and 1,200rpm is considerable and you should be able to tell if it's the engine or just the tach. If not, check it against an external tach and if your tach's off I'd suspect the 'chip' (probably a DC-DC converter; could vary with temperature). Come to think of it, an electronic tach uses induction; the chip might be messing with the pulses from the ignition.

FWIW, my BJ8's idle will vary slightly depending on:

- air temp (slightly faster on cool days, slightly slower on hot days)
- density (faster at SL, slower at altitude)
- whether the clutch pedal is pushed in (slower) or not

Generally, the tach readout is rock solid for given atmospheric conditions (sort of surprising).

 

[Keoke]

Hi Roger, I think your peoblem is in the chip board put in the tach it may be sensitive to ignition noise on the main power lead. . Just a guess.--Fwiw--Keoke

 

[AUSMHLY]

Do you mean 100s RPM? I.e. 7 1/2 means 750rpm?
I'd suggest a bit of binding in your throttle linkage, but it's not unusual to have a bit of fluctuation. No ECUs in cars that old!

Yes, in 100's.
Today I took cleaned and greased all the linkages from the petrol pedal to the carbs.

Took it out for a spin. Does not appear to fluctuate like before.
But not sure of that, for I just drove for a little while to test the lubrication of the linkages.

Different situation now.
Car's warmed up. At a full stop, waiting for the light to change. In neutral, clutch is out, idle is around 9000. I'll watch the needle drop slowly. 9,8,7,6,5 and if I sit there long enough, the car will shut off.

I don't think this would be related to the chip in the tach. That would just be an off reading right?

A vacuum leak?
If this is a vacuum leak, where are all the places to look and how do I find the leak?

Thank you guys,
Roger

 

[TomFromStLouis]

I have read on this site that tachs with pertronix electronic ignitions installed are unreliable. I know mine is. Mine runs high and occasionally does the bouncing needle trick.

 

[Deleted member 8987]

Do you mean 100s RPM? I.e. 7 1/2 means 750rpm?
I'd suggest a bit of binding in your throttle linkage, but it's not unusual to have a bit of fluctuation. No ECUs in cars that old!

Yes, in 100's.
Today I took cleaned and greased all the linkages from the petrol pedal to the carbs.

Took it out for a spin. Does not appear to fluctuate like before.
But not sure of that, for I just drove for a little while to test the lubrication of the linkages.

Different situation now.
Car's warmed up. At a full stop, waiting for the light to change. In neutral, clutch is out, idle is around 9000. I'll watch the needle drop slowly. 9,8,7,6,5 and if I sit there long enough, the car will shut off.

I don't think this would be related to the chip in the tach. That would just be an off reading right?

A vacuum leak?
If this is a vacuum leak, where are all the places to look and how do I find the leak?

Thank you guys,
Roger

9000 rpm's unloaded at a stop light?
And it held together?
Amazing.

The droping off and then dying is not a tach chip-related iyem.
I'd be looking at vacuum leaks.
Did you disconnect something while cleaning?

 

[tahoe healey]

Yea, that's like double red line. No wounder he get 9 MPG.

 

[Deleted member 8987]

I wanna know who built that motor.

 

[AUSMHLY]

Do you mean 100s RPM? I.e. 7 1/2 means 750rpm?
I'd suggest a bit of binding in your throttle linkage, but it's not unusual to have a bit of fluctuation. No ECUs in cars that old!

Yes, in 100's.
Today I took cleaned and greased all the linkages from the petrol pedal to the carbs.

Took it out for a spin. Does not appear to fluctuate like before.
But not sure of that, for I just drove for a little while to test the lubrication of the linkages.

Different situation now.
Car's warmed up. At a full stop, waiting for the light to change. In neutral, clutch is out, idle is around 9000. I'll watch the needle drop slowly. 9,8,7,6,5 and if I sit there long enough, the car will shut off.

I don't think this would be related to the chip in the tach. That would just be an off reading right?

A vacuum leak?
If this is a vacuum leak, where are all the places to look and how do I find the leak?

Thank you guys,
Roger

9000 rpm's unloaded at a stop light?
And it held together?
Amazing.

The droping off and then dying is not a tach chip-related iyem.
I'd be looking at vacuum leaks.
Did you disconnect something while cleaning?

OK, you guys having fun?
Thanks for letting me slide on the 9000, Mr. chip-related iyem....
I've checked everything I did, and I have not disconnected anything that should not have been disconnected. Only the metal links for cleaning, and lubing.

So, now for a little help with:

A vacuum leak?
If this is a vacuum leak, where are all the places to look and how do I find the leak?

 

[Deleted member 8987]

Tight intake valve.
Manifold to head gaskets.
Carb base gaskets.
This one have a brake booster?


BTW, whatever happened with your mileage test?

 

[Bob_Spidell]

I have read on this site that tachs with pertronix electronic ignitions installed are unreliable. I know mine is. Mine runs high and occasionally does the bouncing needle trick.

I have a Pertonix on my BJ8 and the tach is spot on and rock solid (wish I could say the same about my speedometer).

 

[AUSMHLY]

Tight intake valve.
Manifold to head gaskets.
Carb base gaskets.
This one have a brake booster?


BTW, whatever happened with your mileage test?

Thanks for pointing me in the right direction.
I'll be looking in those areas this week.

Yes, I have a brake booster.

How do I test for leaks at all the area you mentioned.

The mileage test is down the road. I just got the carbs on and now I'm trying to work though the new issues of just getting the car to run properly. Once that is done, then I'll be reporting back that I wave to the Prius's while their filling up at the pump!

 

[Deleted member 8987]

In the shops, I used to always use spray carb cleaner.
That stuff will remove engine paint, so sparingly (with the wand attached).
New carbs, probably not shaft bushings.
Did 100% or the old carb base gaskets get removed from the manifold?
Undo the main vacuum line to the booster, plug it and run it.
If the rev up and slowly dying goes away, I'd be looking for a diaphragm issue in the booster.
Valves....when again was the last time they were adjusted?

 

[glemon]

The question asked about whether the motor seems to be fluctuating in speed or just the tach, as noted you can here a fluctuation in 100 rpm or so pretty easily in a Healey motor, if it is really going from 600 to 1200 rpm that should be very apparent, expecially with the hood open, will sound like it is hunting for a spot, speeding up and slowing down. If it is not doing that something electrical in the tach, if it is as noted vacuum leak is a good guess. are the air filters on tight, if you put some hand pressure on the carbs up or down while the motor is running does the sound or speed change (if so carbs and or manifold is loose).

Good luck!

 

[Deleted member 8987]

This was what caused th direction to change:

"Different situation now.
Car's warmed up. At a full stop, waiting for the light to change. In neutral, clutch is out, idle is around 900(0). I'll watch the needle drop slowly. 9,8,7,6,5 and if I sit there long enough, the car will shut off."

 

[tahoe healey]

These beasts do seem to increase idle when hot as the oil gets thinner. Too hot and they slow down and die. Can you correlate the changes with temp changes? You might also try going back to points (temporarily) to see if this still happens. Carbs should be good as they were done by the best. Check the timing as well. I agree that the first thing to look at is vacuum leak.

 

[red57]

"Different situation now.
Car's warmed up. At a full stop, waiting for the light to change. In neutral, clutch is out, idle is around 900(0). I'll watch the needle drop slowly. 9,8,7,6,5 and if I sit there long enough, the car will shut off."

Does the idle become progresively rougher as it slows down? If so, does it smooth out when you give a little throttle?

I ask because it sounds to me like the carbs may be set a little rich. The slowing idle could be plugs loading up from being too rich (which will usually mean running rougher as the plugs misfire), and if it clears out and smoothes when you bring it up to 1000-1500 this could be the plugs cleaning up. After idling hot for a while, shut it off and pull a couple of plugs and see if they are sooty. You mentioned you had a 'new' mechanic tune it and maybe he just set them a bit too rich.

This would not explain the eratic different idles you mentioned initially but your later posts suggest that the eratic part is better after you lubed the linkages.

Just some random thoughts,
Dave

 

[Keoke]

Do you mean 100s RPM? I.e. 7 1/2 means 750rpm?
I'd suggest a bit of binding in your throttle linkage, but it's not unusual to have a bit of fluctuation. No ECUs in cars that old!

Yes, in 100's.
Today I took cleaned and greased all the linkages from the petrol pedal to the carbs.

Took it out for a spin. Does not appear to fluctuate like before.
But not sure of that, for I just drove for a little while to test the lubrication of the linkages.

Different situation now.
Car's warmed up. At a full stop, waiting for the light to change. In neutral, clutch is out, idle is around 9000. I'll watch the needle drop slowly. 9,8,7,6,5 and if I sit there long enough, the car will shut off.

I don't think this would be related to the chip in the tach. That would just be an off reading right?

Well that would depend on the design. For example if it failed toinclude amissing pulse detector it could run at its fre runinf speed trigger circuits that ae to be externally excited usually have very high free running speeds. But why achip was used in the first place is a mystery to me


A vacuum leak?
If this is a vacuum leak, where are all the places to look and how do I find the leak?

At the carb shafts and the mountings to the manifold as well as the manifold to the engine

Thank you guys, You are welcome---Keoke- :laugh:
Roger

9000 rpm's unloaded at a stop light?
And it held together?
Amazing.

The droping off and then dying is not a tach chip-related iyem.
I'd be looking at vacuum leaks.
Did you disconnect something while cleaning?