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A slight hesitation stumble at speed

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Martinld123

Martinld123

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My 1956 100 BN2 with M upgrades has a miss at speed the same feeling you get when running out of petrol but just for a second and then smooths out. This happens while cruising in or out of overdrive while at a steady speed. The engine has a smooth idle with no sign of problems but miss seems to happen at lower RPM while cruising. Number 3 plug has more carbon then the other three plugs. I wonder if a valve seal could be leaking. The other three plugs have a clean coffee color. Maybe that is my problem. Any ideas. Thanks Marty
 
Hi Marty
Had similar problem with my BJ8 converted to 3 HD8 carburettors. I calibrated the mixture for maximum power at full acceleration, then cruising at 80 km/h I got similar symptom, simply too lean mixture. Suggest you try 1/4 turn richer mixture on the jet screw, quite easy and reversible test. Brg Lars
 
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In my experience, intermittent missing is usually ignition caused. Do the 'dark test:' run the engine in a dark garage--well ventilated--and look for arcing around the distributor cap and secondary wiring.

Just read an article in 'Auto Restorer' about a guy with similar issues with a '55 Chevy. He did all the fuel system checks--including a new carb--only to find it was an ignition issue.
 
Brg Lars thank you for responding. I richened both carbs (SU H6 for 100"M") two flats richer and it seemed to get worse. Long story short, when tuning carbs it calls for 12 flats out as starting point but mine would never run well at that starting point so I end up at about 16 flats out to get engine to run the way I believe it should. All has been well for sometime until just recently when this miss developed. Last year I sent carbs out to a shop to be rebuilt but later I found out knowledge of owner was not very good. I wonder it if could be too rich?? 60 year old distributor could be factor so I have cleaned and oiled it but electric problems could be cause. Will run car tonight in dark to look for any arcing. Thanks Marty
 
Bob_Spidell said:
In my experience, intermittent missing is usually ignition caused. Do the 'dark test:' run the engine in a dark garage--well ventilated--and look for arcing around the distributor cap and secondary wiring.

Just read an article in 'Auto Restorer' about a guy with similar issues with a '55 Chevy. He did all the fuel system checks--including a new carb--only to find it was an ignition issue.
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Bob, I just finished your"Dark Test" with interesting results. I did all of this at idle. There was a ring of light around each plug (at point where porcelain meets metal base) as each plug fires and when I touch the plug wires they glow but only at the point where I touch them. I was unable to get any kind of shock when running my fingers along plugs or wires. They are period correct, "Bumble Bee, black and yellow" plug wires sold in bulk from Moss and are unshielded wires. Maybe I should go to shielded wires but if I am not getting shock it can't be leaking too much? No arcing at distributor cap. What do you think? Thanks Marty
 
Martinld123 said:
Bob, I just finished your"Dark Test" with interesting results. I did all of this at idle. There was a ring of light around each plug (at point where porcelain meets metal base) as each plug fires and when I touch the plug wires they glow but only at the point where I touch them. I was unable to get any kind of shock when running my fingers along plugs or wires. They are period correct, "Bumble Bee, black and yellow" plug wires sold in bulk from Moss and are unshielded wires. Maybe I should go to shielded wires but if I am not getting shock it can't be leaking too much? No arcing at distributor cap. What do you think? Thanks Marty
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I have no idea what the rings of light around the plugs are but, theoretically, the only things that could cause them is electrical arcing or flame escaping, neither of which is good.

IMO, stranded core copper wires work best with original caps (with the pierce style connectors). I tried shielded, carbon core wires and got symptoms similar to what you are experiencing. I hear there is a TR6 distributor that will work in a Healey with the plug-in style connectors, but can't vouch for it.
 
Bob_Spidell said:
I have no idea what the rings of light around the plugs are but, theoretically, the only things that could cause them is electrical arcing or flame escaping, neither of which is good.

IMO, stranded core copper wires work best with original caps (with the pierce style connectors). I tried shielded, carbon core wires and got symptoms similar to what you are experiencing. I hear there is a TR6 distributor that will work in a Healey with the plug-in style connectors, but can't vouch for it.
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I have a "top loader" cap/wires from Advanced. They are working well in my Healey.
 
If you think it's the ignition system, I would suggest testing the strength of each spark which helps identify a ignition weak spot. I have a spark tester bought off eBay, use it many times on customers cars.

I set it up to jump a small gap, then continue to increase the gap until the spark fails to make the jump. Generally a good spark will jump a gap of about 1/2" before if starts to stumble. Another tester I have, I set up in series with each plug and I run four testers at the same time. This lights up a glow type plug. If you have a week one it shows up dull and occasionally it miss fires under load.

Just for reference, my 100 will jump a gap and remain steady at 19 to 20mm with a Lucas sports coil.
 
Martinld123 said:
Dcarlg, I bought a Sports Coil a few months ago from Moss with say 3000 miles on it now. I am using Pertronix electronic ignition. Thanks Marty
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Marty,
It's really frustrating to deal with defective parts after you correctly diagnose the problem. Good luck.
Douglas
 
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