Tachometer Questions

[Matt in Japan]

Recently, I have noticed that my tach likes to jump around a bit. This usually happens during acceleration - while the engine sounds like it's accelerating smoothly, the tach will pause at (for example) 2000 RPM and then jump to about 4000, or whatever speed I'm running. Almost like it's having a delayed reaction.

Does anyone know what causes this and how to cure it?

Thanks...

Matt
1977 Porsche 911S

[Alan Cottrill]

do you still have points?

[Matt in Japan]

Yes. I have them gapped properly at .35 mm and I have my timing set at Z1, since mine is a ROW model.

[Alan Cottrill]

I had the same problem and when I changed to a petronix ignitor it went away.

I didn't change to the petronix to fix the tach problem but it seemed to fix it.

does it seem to be worse on rainy or very humid days?

[Jim Sims]

On my '76 California spec 2.7 a "bouncing" tach means the point dwell is too far off; I found that it's hard to get the dwell set with just a feeler gage. The gage was just a starting point and to finely set the dwell required a dwell meter. Jim

[Alan Cottrill]

i've always wondered to set the dwell do you have to pop the cap, make an adjustment, put the cap back on start the engine check the dwell stop the motor pop the cap make an adjustment and so on and so on?

[Jim Sims]

Yeah; it's too bad Bosch didn't use the nifty (GM?) distributor/point design that had the little window or shutter in the side that allowed one to stick a small hex wrench in and adjust dwell while the engine was turning/running. Jim

[Alan Cottrill]

I'd buy 10 ignitors not to go through that

[RoninLB]

Haven't done it yet on the stock CDI/points.. but on the old ignition systems I used to remove the cap, hook up dwell meter, and have someone turn the key while I set the points as I read the meter.........Ron

[CamB]

I do what Ron does (on my BMW), except on a front engined car you can set up the dwell meter so you can see it when you crank...

Also interesting to leave it connected and rev the engine. My BMW's dwell changes a lot, indicating the distributor is very very tired...

[Matt in Japan]

Looks like I'll be picking up an ignitor pretty soon. From the discussions I see here, it seems to be the best solution for many different reasons. I have heard differing opinions on the Pertronix and the Alison-Crane. Any suggestions?

BTW, thanks to everyone for helping me out...

Matt

[Alan Cottrill]

if your distributor shaft has any play side to side I'd recomend the crane.

the crane unit uses a rotating disc with 6 slots cut in it. The optical sensor senses the light through the slots to trigger the ignition. If you have any play it won't be affected.

with the petronix if you have play in the distributor shaft you may have a problem with the gap between the rotating unit and the pickup changing.

if you have vertical play in the shaft (shaft moves up and down/in and out) you may also have problems lining up the rotating part with the pickup.

I've had good luck with the petronix but I see the merits of the crane unit.