Bouncing factory tachometer.

[620_Brian]

I finally got to drive my new 620. 

Sooo...what on earth would cause the factory tach to bounce around wildly?

it sort of goes up and down with RPM’s but it’s not “steady”. 

Any ideas?

 

it looks like the PO had just put a new Webber carb on..I definitely need to tinker with it and tune it. It backfires a little and just doesn’t have the OOMMPF I remember a webberized L20B should have. 

It does run and idle fine though. 

Ill do a search - I’m sure there’s threads on this. 

 

But that TACH!

 

Put the topper in storage and gave her a bath too. 

 

img upload

 

 

 

[distributorguy]

You could very weell have a distrributor problem, which would cause the tach to bounce wildly.  You can use a test light between the negative coil terminal and ground to see if the flashing (firing) is really corrupt, or if you have a tach problem.  The light will either flash consistently or erratically.  You can check the same thing by hooking up a timing light to each plug wire and see if it flashes consistently or erratically.  

[datsunwizard]

 I am assuming  a match box distributor? Do you have a ground from the matchbox module to engine block ground? No ground between the two will cause the bouncing tach, the factory wiring has a ground lug on the module, where the mounting screws are for the said module.

[datzenmike]

If it runs without stumble it could be assumed the matchbox is working properly. The '78 used a blue wire from the negative side of the coil to the tach, unlike earlier set ups where the tach is inline between coil and distributor. This wire could be loose on the coil or at the tach end and engine vibration making and breaking the contact. The engine would run unaffected.

[distributorguy]

Actually, most Datsun owners haven't experienced what it feels like to have a really well built distributor versus one with problems.  Look at all the folks on this forum who simply have a worn distributor that's maybe been cleaned and lubed, but not really rebuilt with new bushings and advance springs so that the timing curve is accurate and precise.  I'd bet almost everyone on this forum (who runs a distributor) has their timing off by more than 5 degrees from ideal setting at some point in their advance curve.  That's typical of this poster's statement:

"It backfires a little and just doesn’t have the OOMMPF I remember a webberized L20B should have. "

 

Dig through the ignition system and get it up to par before you mess with carb jetting, and adjust your float to spec!!!

[Draker]

I have the same thing on a matchbox dizzy.. well actually two of them. The first dizzy was really bad, it would bounce all over.. I swapped it out with another one and now I only have an occasional bounce between 2-3k. Car drives fine. I thought there was noise in the signal, so I put a filter on it. Didn't fix anything.. 

 

So basically, I think I've got two worn dizzys.. one worse than the other. 

[620_Brian]

Cool thanks for all the input. I’ll checkout the wiring.

But I guess I’m not sure what “matchbox” is?

[620_Brian]

Also...I took the air pump off today and plugged the one “injector” hose that was associated with it. It runs perfect now - no stumble or backfire.

I still think it has more power to offer though so I’ll be checking the timing and stuff. 

[distributorguy]

The "Matchbox" distributor is so called because of a black ignition module "box" that mounts to its side, about the size of a match box.  Check for shaft play side to side, as there should be NONE.   Sometimes these wear so bad that the plate inside the distributor is the only thing holding the shaft in place.  That's usually when the tach goes nuts.

[Ron1200]

Quoting myself :)

 

On 1/7/2017 at 11:10 PM, Ron1200 said:

Fixed my jumping tach needle... Turned out to be dirty or loose contacts.  Cleaned the slip-on connectors at the coil and tightened (pinched with pliers) the spade connectors at the matchbox on the distributor.  I don't know which actually corrected the issue as I did both at the same time. B)