Fast Idle - erratic tachometer

[Torxx]

'79 spider, Just installed new 34 adf, 1800 intake, no emission equip. Having a fast idle issue. Won't go much below 1500 rpm on the tach.

So, we were checking the vacuum connections, and spraying flammable brake cleaner around to see if would suck it in (raises rpm's when it does) and found that the carb connection at the base plate was sucking air.

Here's where it got interesting. ...The tachometer has always worked fine, and worked normally recently (including todxay)as we have been working on getting the car on the road. Out of nowhere, right after we tightened down the carb, the tach starts acting erratically. Jumping up and down and reading higher rpm than actual. sometimes it would settle down and seem to read correct, but would soon revert. we made several adjustments to the carb and timing, etc. seting the idle and the tach is still erratic. We may have the idle close to spec, but can't rely on the tach at all.

Also noticed a few times that the car would be idling, and then something would cause the engine to drop rpm's suddeny, and run like it was on two cylinders, kicking the throttle a couple times, or restarting would set it back to normal operation.

What's up???

[So Cal Mark]

try disconnecting the tach at the coil and see if the misfire goes away. You may have a tach with an internal short circuit. If the misfire is still there, remove the dizzy cap and look at the wiring to the pickup for breaks

[bradartigue]

open the distributor cap and inspect the wires to the pick up. if they are frayed they will do exactly what you describe. what may have happened is that at 1500 RPM the symptoms were masked.

[Torxx]

Great call on the wires in the distributor cap. Mark also deserves credit for the same suggestion in an archive post.

Prior to the tach problem, the dist. cap and wire was just replaced with new parts, so I fgured the wire should be good. Turns out that a spinning part inside the cap was rubbing on one of the wires and wore it bare. I taped it up and set it in the best I could to avoid rubbing.

All is well with tach. Idle is in range (from prior adjustments and vacuum diag.), good news , tach shows 1100 when cold / closed choke. I did not run it long enough to open the choke. I'm getting close to back on the street.

Thanks

[So Cal Mark]

great news

[backyardbill]

I know that this was an old post, but thanks a lot for it. I had weird idle issues too and when I pulled the distributor to replace the oil seal, there it was: chaffed wires. It runs like a top now!

[NJFIAT1981]

My tach has been acting up. It too has been giving erratic readings. I took it out the other day for a spin down the highway. The car began to bogg down and began to buck. Never stalled. When I pulled over the Spider would idle fine. Turned it off and waited a few minutes. Started it up and dove down the highway. Within 5 miles same thing happened. I will be checking the dist and pickup tomorrow am. Hope that fixes the problem. It's my 1981 Spider. Any suggestions

[NJFIAT1981]

UPDATE:
Wires in the dist. were frayed. Also cleaned up the heat sink, installed a new control module and was given a Master Blaster 2 ignition coil to try out. Runs great now. Tach. issues disappeared. Thanks to Ralph for all the help.