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Allan G, Bustednukel, The Bonneville Shop
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Original Post (Thread Starter)
by Bryan Holmes
Bryan Holmes
I’ve got a 1969 BSA Lightning. When going full throttle, then turning throttle off about 1/4 turn, there is a missing or sputtering sound. Like a popping out the exhaust, possibly right side. I can hold the throttle there and it will keep doing it, until I further reduce or increase throttle. I’d say it’s 3/4 throttle. It doesn’t do it on the way up to full throttle. Since I’ve had it, I replaced the secondary wires because they were hard and misfired when wet. I replaced the points with a mark4 electronic ignition. The carbs never seemed to idle correctly,and would almost stall off idle. I swapped the set of premiers off my ‘66 bonneville and the sputtering continued. I installed a brand new set of wassell evolution carbs on, just for the better idle issues being resolved. It had 12v coils wired in series, due to the electronic ignition. I swapped on brand new Lucas 6v coils and still have the issue. I’ve replaced the plugs in the past. NGK B6ES. Also, the paperwork from the previous owner shows the harness was replaced. Any suggestions? Thanks in advance.
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by Stuart Kirk
Stuart Kirk
Originally Posted by Bryan Holmes
.....When going full throttle, then turning throttle off about 1/4 turn, there is a missing or sputtering sound. Like a popping out the exhaust, possibly right side.........
Your description sounds just like a "too rich" main jet check. IE: at WOT, quickly reduce to 7/8 or 3/4 throttle and notice what happens. If the engine picks up a little power, the main jetting is too lean, but if the jetting is too rich, power will fall off and in an extreme case, the engine can eight stroke or blubber (fire every other cycle). This can sound just like a misfire.

So, how to check this possibility..... How about try it in the morning when the air is cool. If the symptoms are less bad, that suggests an over rich condition. Do it again in the heat of the day and compare your seat of the pants afternoon results with how it ran in the morning. If morning was better and afternoon was worse, you are jetted too rich.

So then, I would confirm the main jet size is correct and if it isn't, put the correct jets in it. And second, I would replace the needle jets because they do wear out rather quickly and richen the mid range enough to reduce power and if they are super worn out, that could be your whole trouble.

Oh, DavidP is right about that B6 plug. It's too hot and you are risking engine damage. Put B8ES's in it at least. If it needs 6's so the plugs don't foul, that's another sign of being too rich.
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by triton thrasher
triton thrasher
Could an exhaust pipe be loose or leaking in the cylinder head? That could cause banging in the exhaust without a real engine miss.

If not, then for a suspected mixture problem at 3/4 throttle, I would raise the needle by putting the clip in a lower notch, to see if that enrichment makes the problem worse or better.

If worse, try lowering the needle.

Can we assume you attend to service items such as valve clearances?
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