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Another BJ8 Misfire Question

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CLEAH

CLEAH

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BJ8 gurus, need your help. My car has developed a worsening misfire, a bit at idle, very bad at higher RPM's. I've been going through all the usual suspects--replaced the cap, rotor, condenser, points, disconnected the white and black wire from the boot switch, checked compression, looked at plugs. My numbers 3 and 6 plugs look fouled, and I suspect that is where the misfire are happening because all the other plugs look perfect. Compression at 180 in those cylinders. Newly rebuilt engine, and the carbs have been gone though.

But anyway, here is something I have not seen reported on the forum: When revving the engine and getting bad misfire, the tach dies. Rev to say 2500, and the tach needle drops and bounces up and down while the engine is misfiring.

I have not yet replaced the coil (my restorer is sending a new one). Thoughts?
 
On a BJ8 the tach is electronic and takes a feed from the ignition somewhere, so the fact that it falls suggests an intermittent connection in the area.
 
A while back I heard there was a slug of bad condensers going around; if you got both from the same source you could have gotten two bad ones. The quality of points is suspect, too; not to be flip, but if you install a Pertronix unit you'll never have to worry about either again (NFI). I may have just been lucky, but my unmodified tach works fine with the Pertronix. Oh, and there were some bad rotors going around, too, but--in my personal experience--they fail outright.
 
Agree with everything so far. Does anyone know where in the ignition system the tach picks up its signal?
 
I should wait and fit the new coil it solved the problem for me, I think the tachometer gets its signal from the coil, I have known some GM engines lose spark due to a faulty tach could this be a possibility.
 
CLEAH said:
Agree with everything so far. Does anyone know where in the ignition system the tach picks up its signal?
Click to expand...

It HAS to be in series with the primary circuit to the coil. The pickup on the tach is inductive, and the signal is pulses of 12V from the timing mechanism (points or other). It's the only timed signal that isn't 15,000 volts or so.
 
Thought I would update on this. After changing out essentially everything (points, rotor, condenser, cap, wires, coil, even the voltage regulator), it turns out it was the plugs.

I mentioned above that my numbers 3 and 6 plugs were sooty, so I replaced those. But the misfire reappeared (usually after the car warmed up) even though the new plugs from numbers 3 and 6 looked fine. We finally went from Champion RN9YC plugs to Autolite 64, and all seems solved.
 
Just curious why you were running Champion RN9YC plugs; 12s are recommended, which are hotter running (this would explain the sootiness with the 9s)?

Did you fix your tach problem?
 
Hi Bob,

Good question regarding the plugs. We really don’t know. Fresh out of restoration, the car was delivered to me with NGK BPR6ES plugs. It ran really well. The car needed to go back to the restoration shop for a bit of post-restoration fettling. The outside mechanic who does carb set-up and final adjustments was doing some further work on a problem with the carbs (ultimately determined to be a bad float), and as a part of the problem diagnosis, changed out the NGK’s. Why he put in Champion RN9YC’s I don’t know. When the car was returned to me, it was running fine on those plugs, until it didn’t. It never occurred to me that the plugs were not the “right” ones, even as I replaced a few of them because of sooting. Just did not register with me. My shop has since switched from their previously preferred NGK BPR6ES plugs to Autolite 64’s, which is what I now have in the car and it is running robustly.

I note that the NGK BPR6ES and the Autolite 64 plugs do not match (on the conversion charts) the recommended Champion RN12YC’s, but both result in excellent running.

What I learned is to give attention to plugs along with all the other usual-suspect components when tracking down an ignition problem.

As for the tach going wonky, it only did that when it misfired. It is otherwise fine.
 
Plugs - they are a whole new world when it comes to running smooth, I remember replacing mine many years ago and a guy in the Self Fit shop could not match the old ones that I had, so gave me what he said was the Bosch equivalent. Well in no time at all I found out that they were not. Then I delved into the black art of sparking plugs and had my eyes opened for me. It turned out that the man gave me cold plugs when I needed hot plugs, one by one they sooted up and kept failing, they were ok when new or freshly cleaned. I then changed them out for a new set of NGK BP5ES and have had no trouble since.

:cheers:

Bob
 
You cannot beat the NGK plugs for reliability.
 
Over the many years of forum reading, I have been impressed with number of posts of people with problems by using nonoriginal design ignition parts, distributors in particular. If the ignition system is properly installed with setting from the manual it will and does for everyone do just fine. Sometimes the reason for miss at high revs or starting from cold is there to be be found and rectified with a worn or misadjusted part. I would stay away from putting in new type whizzbang anythings in ignition systems.
Jay, '65 3000. Usual qualifier, I am referring to Healeys use for normal use as opposed to any form of racing.
 
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