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Gents,
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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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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Probably nothing at all to do with your problem but.

I ride a BSA unit single and at an event earlier this year I had experienced some "popping" in the exhaust during deceleration. It really didn't sound like what I've had happen with a leak at the exhaust.......more like it was "deeper" in the engine.....if that makes any sense?

Well for me it turned out to be a bad spark plug.........just a thought and it would be easy for you to check.

Gordon


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I have adjusted the valves within the last 1000 miles


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Originally Posted by Bryan Holmes
I have adjusted the valves within the last 1000 miles

I’m asking something more like: “Are they correct right now?.”


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ive had the adjuster nuts back off if i was sloppy about tightening them correctly.


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B6 plug is at least one step too hot. If a colder plug plug fouls something else is wrong.


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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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I am on the exhaust pipe /air leak side, more info on main jet no s, slide nos and exhaust , air filter systems would help
Use NGK B8s , as others have warned B6s are too hot.

If the pipe is leaking around the port it can be re sealed, strip, clean to shiny port and pipe metal, final wipe with vinegar after degreasing, let the vinegar evaporate off then smear with plumbers bath silicon, if its really loose add shims cut from beer cans to take up clearance, let it all set up for 24 hours."Mikalor" make superior silencer clamps, that actually clamp properly.


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Blubber fire! LOL
Stuart Kirk, you da man!

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Blubber fire! LOL
Stuart Kirk, you da man!

Digging beer cans out of the weekend trash too.


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