Okay, this is a bit odd...

Just got back from driving from my girl's grandma's house. The drive isn't a particularly long or hard one, but it involves twisty country backroads that all RX-7's know and love. We make a stop really quickly, and we get back on the road. I end up pulling out in front of a car (you have to back into the road, and you can't see far behind you) so to prevent him from slowing down I floor it.

I shift from first to second at about 6k rpms, no problem. I let it get a little higher for the next shift, but something odd happens. I'm looking down at my tach and it's going up past 6k like normal, but once it gets around 7500, the tach bounces a little between 6500 and 7500. I'm thinking..wtf? The engine itself is running normally, it doesn't seem to be dipping down in revs, but the tach goes nuts for a bit, and when it's at the height of bounching, it buzzes slightly, like the redline buzzer. Thing is, it's not redlining.

I shift to third, and take it to the top of third, and it does the same thing. Very odd. I slow it down a bit and drive normally the rest of the way home. When I get to my driveway, I leave it in neutral and rev it high, it stumbled a bit up top, but not as badly as it did under load.

Here's my car:

'91 coupe, 30k miles on new engine
K&N cone filter, Bonez superflow cat, Racing Beat Catback. No other mods.

The only things I've done to my car lately are changing the spark plugs, which I did a couple of days ago, and I put the Bonez cat on a couple of weeks ago. I don't push the car near redline that often, but I have done it a couple of times since I put in the cat so I don't think that has anything to do with it.

Anyone have any thoughts?

 

May be just the trailing ignition system breaking up on you a little bit at the high revs, which shows up on the tach. Check the hardware that's holding down the trailing coil pack, if it's all old & crusty clean it up a little for a better bond to ground.

 

prolly a loose connection

 

also, gravity can pick up that needle pretty easily, its not weighted to be accurate if you're pulling F1-Car g's. Also if yuou don't have the cover to the gauges, the wind will pick up the speedo and tach and bounce them around on a windy day

 

Don't tell me you removed your gauge cluster lens cover for weight reduction, Kenteth, lol...

 

im having the same problem but my tach bounces when i turn on the signal switch or hazard lights, but the engine runs fine

 

That sounds like you have one of your lighting wires running right beside the tach signal wire somewhere in the bundle, and every time your power the lighting circuit, a small EMF pulse causes your tach to twitch. I thought there was a small capacitor in the gauge cluster for the tach circuit to prevent this...

 

..welll....

naww, it was cracked, so I just removed it

 

Heh heh heh

Well checked out my spark plug wires...thinking maybe one of them isn't tight enough after I plugged them back in last week. The contacts themselves look fine. Not brand new, but not corroded either. However, one of the wires doesn't do the little snap that they usually do when they go on, so that may be it. Maybe I'll try running a new wire to see if that helps any..

I took my car out to get some pizza, and it did not have any hesitation this time, but I was in the middle of town so I didn't spend a lot of time in the 7000 rpm neighborhood. I guess time will tell...

 

The tach lead has nothign to do with the plugs, but the ignititors themselves. You could leave off two plugs and so long the car could actually run, you'ld get a fine tach reading. Granted it will be a bit rough, but thats because the engine is running like **** from the two missing plugs, not because there is somethign wrong with the tach lead.