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BJ8 misfiring at speed

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Robert53

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At 60-70mph (~2500- 3000rpm) under load my BJ8 sarts to run rough and then backfires through the exhaust. Runs fine when I back off on the pedal. Runs fine at idle and lower speeds. Accelerates smootly.
New Grose jets installed. New SU fuel pump. Plenty of volume of gas to carbs although I haven't measured delivery pressure. Just ran a test where I turned off fuel pump (ignition) when problem started at ~55mph going up slight incline and then checked to see if there was gas in bowls - there was. New ditributor cap, rotor, condensor. Vac advance works. Solid core ignition wires checked - all good with low (no) resistance measured. Coil checks out OK. Any ideas on what to check next? Does this sound like a fuel delivery problem or electrical problem? Thx
 
I had an engine miss at constant speeds over 2500 that I thought was fuel related but turned out to be an ignition timing issue.
 
My last missing trouble at that rev range turned out to be faulty plug leads.
 
Did your problem start after you did the tuneup? Did you change the points? What brand of distributor cap did you use? I've had non-Lucas caps arc across the surface, shorting spark to ground.
 
I had a similar problem twice. It turned out to be a condenser not tightend sufficiently to the dizzy base plate.
 
My high speed miss was traced to the plastic spark plug caps that attach the plug wires to the spark plugs. The caps had a carbon rod inside(for some reason unknown to me) that had burned through causing an open circuit. The problem was not visible from the outside. Replacing the plug wires and caps solved the issue.
 
Was there a new Lucas cap involved? I just got a new lucas cap last month and the brass contacts in the cap were molded 1/8 inch farther apart than orginal. Put old cap back on and NO breaking up after 2,000 rpms. Bought a old NOS Bosch cap off of ebay and it measured same as orginal and no break up after 2,000 rpms when installed. I sent new Lucas back. Wheeguy
 
Just because you bought new tune up parts...does not mean they are good! Lately I have been getting nothing but crap from the big venders! I had bad sport coils, bad point sets, bad condensers, and really cheap caps and rotors! If I where you I would change out the whole lot with original parts if you can find them! I had my new tune up go south after just 1000 miles... I had to change out to another whole tune up including replacing the new sport coil to get my Healey running smooth again.
 
drambuie said:
Just because you bought new tune up parts...does not mean they are good! Lately I have been getting nothing but crap from the big venders! I had bad sport coils, bad point sets, bad condensers, and really cheap caps and rotors! If I where you I would change out the whole lot with original parts if you can find them! I had my new tune up go south after just 1000 miles... I had to change out to another whole tune up including replacing the new sport coil to get my Healey running smooth again.
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Last year I had an in-person conversation with a quality-control director from one of the major suppliers.

He told me battery/points/coil ignition has not been standard on any car for more than 20 years and is obsolete. They are having problems getting quality capacitors and coils from their suppliers who don't really care that much due to the small numbers involved. It's become a niche market. The people who did it for a living are long gone. The manufacturing companies that are making Lucas products have bought the rights to the name and are making the stuff in China from a variety of suppliers. Some guys are keeping a stock of 5-10 capacitors because of the failure rate.

IMHO a Pertronix, 123 or Mallory setup might be the way to go for reliability. These people do it for a living.

Just saying...
 
Thanks for sharing your information Steve ... I have been contemplating switching to a Pertronix or a Mallory for years, I remember someone telling me when they installed a new Mallory duel point unit it really made a big difference in mid range and higher end performance! If I can no longer locate new old stock tune up parts I will defiantly make a change. When I think about it...I never had problems with my old original ignition parts as they lasted a average of 12,000 miles or more. I guess the only thing stopping me is all the negative reports of Pertronix failures from other BCF members. Most likely due to faulty negative ground conversions before they bought the unit.
 
drambuie said:
Thanks for sharing your information Steve ... I have been contemplating switching to a Pertronix or a Mallory for years, I remember someone telling me when they installed a new Mallory duel point unit it really made a big difference in mid range and higher end performance! If I can no longer locate new old stock tune up parts I will defiantly make a change. When I think about it...I never had problems with my old original ignition parts as they lasted a average of 12,000 miles or more. I guess the only thing stopping me is all the negative reports of Pertronix failures from other BCF members. Most likely due to faulty negative ground conversions before they bought the unit.
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I've fried a couple of them myself - shorting spark plugs troubleshooting, wiring backwards, leaving the ignition turned on for more than 30 secs or so. Except for user-error they are very reliable. Pertronix has a pdf of troubleshooting dos and donts.

I carry a spare distributor all setup with points & condenser just in case - I've loaned it out to a driver who lost a condenser out on the road.
 
You know Steve, you would think by now after all the complaints Pertronix gets they would have some sort of safety circuit built in to avoid there units being fried so easy.
 
drambuie said:
You know Steve, you would think by now after all the complaints Pertronix gets they would have some sort of safety circuit built in to avoid there units being fried so easy.
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I believe the Pertronix II has such a protection circuit (along with variable timing and other features). Unfortunately, I think it's too large to fit under a Healey distributor cap.
 
Side note on Pertonix: the 'insert' type (Pertronix I) for the 4-bangers don't need/use the magnet sleeve that goes on the distributor drive. Apparently, the shape of the cam is enough to trigger the unit.
 
Thanks to all for the responses! I've been away from the Forum for a bit whilst Down Under. I'll need time to digest them. Then I'll probably have a few questions about what to pursue next.
 
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