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Tach jumping, rough idle.

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Lionheart

Senior Member
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The tach on my spit tends to intermittently lurch and jump about 500-1000 rpm above the actual rpm. It's much more prevalent at higher rpms.

Here's what I got:
Flamethrower coil
Pertronix ignitor
Otherwise, stock

Idle is a bit rough as well...was that way with the Stromberg, and is still that way with the single Weber 40DCOE. You can see the steering wheel shaking, even at 1000 rpm. Car runs good on the road, lots of pep.

Could this be indicative of a bad ignitor module? Could it be a bad tach?

Len
 
I think you have an issue with the ignition.

Tell me, is it a 12 volt coil or a 6 volt coil, and year of your Spitty?

This is a just a s.w.a.g (Spitfire Wild Arse Guess) and I am probably way out in left field, but I often see 12vdc coils used where there is an external ballast resistor either in the harness or on the coil bracket - a 12 vdc coil needs 12 volts when the key is in "Run" to operate properly.

Am I wrong by saying that Triumphs mainly use 6 vdc ignitions that apply 12 vdc to the coil only during the starter cycle to enhance starting?? So what is happening is, you start up fine, but when you release the key, the ballast resistor is placed in the circuit and the coil is getting only 6 volts to operate the engine on. Low RPMs is when the engine needs the best spark to run smoothly.

Good test, use your old Lucas Coil and see how it runs.

If you decide to use a 12vdc coil, you have to loose the external or hidden ballast resistor so the coil gets 12vdc all the time.

Pertronix sales places automatically assume you are using an all 12VDC ignition system.

Second, older Smiths tachometers - say pre 1973 to be safe, do not like electronic points because it does not take the volts pulse to ground. The pulse pickup sensing coil in the tachometer does not see the amount of volt pulses to make the tachometer needle respond correctly. Those older tachometers need a second loop or wire run through the pickup coil on the tachometer circuit board.

Hope this helps ...
 
Thanks Glenn.

It's a 77 Spit 1500.
The coil is running on full-time 12vdc.

I really wish I still had an oscilloscope, so I could view the breaker signal.
 
Lionheart said:
Thanks Glenn.

It's a 77 Spit 1500.
The coil is running on full-time 12vdc.

I really wish I still had an oscilloscope, so I could view the breaker signal.
Click to expand...

No local clubs have a SunTune?? /bcforum/images/%%GRAEMLIN_URL%%/frown.gif

So how about the plugs? Maybe locate a spark tester and stick that inline with the plug wire. You could have a dead plug.

Compression? Maybe a tight valve?
 
I just put new plugs in it. They are ALL a nice cappuccino color.

I'll do a compression test tomorrow. I have no idea what the valves are like...it shows 85k on the odometer, and I have no evidence that there was ever any engine work done.

I just bought the car last August, so I'm still figuring things out. (This is my first 'proper' LBC...my first LBC was a 71 Plymouth Cricket...it's the same as a Talbot Avenger in the UK.)
 
Well smoothing out running on the road, that is usually a quality of combustion. Yeah, do a compression check. What is running like under load like a hill? Oh right, you are in New Jersey /bcforum/images/%%GRAEMLIN_URL%%/frown.gif

Maybe check for vacuum leaks too, and pop the oil cap while idling to see what happens.

Weber's are a bit tricky to get right, but if you have a nice light brown deposit on all the plugs, that is a good sign the fuel is right, and no oil burning.
 
I did a compression test today. Here's what I got:
143
140
140
146

They all rose about the same; taking about 3 comp strokes to build the majority of the pressure, and a few more to max out.

I don't have a leakdown tester, so didn't test that.

I realize that the tach jumping and the rough idle may or may not be connected. If they are, then it's ignition-related.
Is the Pertronix Ignitor prone to this type of failure?
 
Worn-out end on your tach cable? Try going back to points and condenser and see how she runs.
 
TR6BILL said:
Worn-out end on your tach cable? Try going back to points and condenser and see how she runs.
Click to expand...
This is an electric tach car (at least I think it is...you got me wonderin' now). I believe the original ignition was an optical breaker system. I'm not really sure about that, but in the box of extra parts that you always get with an old car, there is an optical pickup and rotating assy.

I would install an aftermarket distributor in a heartbeat if they didn't cost $300.
 
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