BKbroiler

Yesterday I was at Englishtown for some bracket racing. I had run 12.45 last week but yesterday the air was not as good so I expected to go slower. The first time shot it ran 12.72 with a miss for a second while in high gear, at only 104. The second time shot, it ran clean all the way and turned 12.60 at 106.5 but I noticed the tach was jumping up to 7k and down to 4k while I was going thru the lights. Its normally right at 5,500 at that point reading steady. I wasn't happy but, other than check for loose connections there wasn't much I could do. BTW - my tach is the original electric unit, no problem before this. So for Round 1, I dialed 12.59, but ended up running 12.80, at 104, and lost of course. My eighth mile speed was down 3 mph as was the final speed. I could hear a very slight miss during the run. Tach was bouncing all over. During my 50 mile drive home the tach continued to jump around, even at idle. The engine seems to run fine at part throttle, but the performance and tach problems must be related. Any ideas?

myk7

I've never heard of a tach causing engine problems, but it's possible I guess. I'd check out the coil first.

BKbroiler

I don't think the tach is causing the problem either, but it must be related to it. I have spare parts to swap so I guess I will start with another cap and coil and see what happens.

ajrothm

Is there an MSD box on the car or just the stock HEI?

If it is the HEI, check the tach wire where it plugs into the HEI cap, trace that wire back to the tach and make sure that wire is not melted/grounding out on anything. That will wreak havoc on the module in the HEI can cause some screwy stuff...

Obviously, it is an ignition related issue so you need to check everything. Something is grounding out, arcing causing the misfire and the tach is picking it up....

BKbroiler

Thanks for the suggestions guys.
Allan - It is a stock type HEI, with a Pertronix distributor. Your theory about the tach wire sounds good. I'll check that before I change any parts.

gkull

The tach counts ignition pulses. If the ignition is faulty the tach will read screwy. If you have an MSD take it out of the system and retest. Stock hei units are very reliable, so it would be the last item on my list.

69427

You lost me there. What exactly do you have?

BKbroiler

The distributor itself is an aftermarket unit. Pertronix is the brand name. It is an HEI type, just like the original, with the coil on the cap, no extra boxes or anything like MSD.

BKbroiler

I found loose wiring on the tach filter which is the original for the car. Wiring is frayed at each end of the can, That probably explains the tach problems but could that affect the performance of the engine?