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71 Bonneville.
It starts instantly with new plugs and runs great. When the plugs foul, it runs poorly and usually will not start. The plug in the picture was new and ran for about 10 min. Bad carbon fouling. Both plugs look the same. I have been thru the carbs many times and have replaced the floats, seats, jets. I have tried moving the clip at all 3 positions on the needleto lean it out and nothing changes. I tuned the carbs by shutting down a cylinder and tuning the opposite side. And it still fouls the plugs.
If the timing or the valves needed adjustments, it would not run as well as it does - correct?
Suggestions would be appreciated.
 

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I’m not saying this is you, but : Sometimes when you are trying to figure out something like this it helps to figure out what isn’t wrong to narrow down the possibility of what could be wrong. Like stating what has been done to the carbs already. The other thing that really helps is to do a complete tune up and check the compression. What could be wrong ? Anything that you’re not sure is right. If I ask a question on this forum it will help everyone if I do all of the normal adjustments first. It will save days of trading questions and answers back and forth. This is because when someone tells me their bike isn’t running right, I’m going to ask when was the last tune up, because that usually gets rid of half the problems.
 

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B8's are a little on the cool side. Try some Champion N3C's. Any evidence of oil fouling?
 

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The needle position has little to no effect on the mixture at idle. How are you setting the air screws?

Rod
 

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+1 to redmoggy above.
Had similar in my 77 T140 recently, plugs a sooty black as in the picture. Air screws set a 1.5 turns, so have just taken them out to 2 turns. after a couple of miles plugs were brown. Need the opportunity to put a few more miles at highway speeds to check all is still good.
Good luck.
Andy
 

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Three things to check:
Float levels:
Using aluminium, viton-tipped needles, set the float level to .080" below the cusp by pressing lightly on the TANG only. 'Stay-up' floats are easy to adjust.

Pilot jet
Get a .016" guitar string and insert through the jet (about 1 1/2 inches in) and file the end to remove any burrs. You may need a flashlight to see the jet hole. Spray carb cleaner through the jets afterwards.

Air filters.
Clean them in a bath of paraffin (kerosene), and leave to dry.
 

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I tried every method I read for setting my mixture at idle, I simply never managed to get it right. I eventually gave in and with some scepticism decided to try a Mates Gunson Colortune. Brilliant bit of kit.

Rod
 

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I also have a 71 Bonnie and I have previously had the same issue after i had rebuilt the engine. I ended up replacing the carbs and now the plugs don't foul to the point of not working anymore but they aren't great. I'm still working through the issue though because mine starts missing after I run it for about 10 min. I'm actually about to make a post about it. But I'm kind of in the same boat I'll let you know if i figure out anything
 

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NGK B8ES 2411
these are the correct plugs for your motor. in my experience, NGK is superior to any champion plug. there are lots of other opinions.

you've tried all sorts of carburation corrections.

how is your spark?

put a timing light on the plug wire and see whether the plug is firing a lot, a bit, or mostly not at all.
 

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Discussion Starter #14
Anyone considered the ignition coils (or other electrial component) might be dying ? ...J.D.
I thought that when a coil goes bad, it's like turning off a switch. Am I wrong?

The colour tune sounds interesting.could not seem to find a price for it.

I have tried the suggestions and am about ready for some professional help. I just want to ride the thing!

Thank you all for the responses.
 

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My experience with coil failures have always been missing, poor running, and stalling. I've had at least 4 bikes that had coil issues. I will admit yours is a little different in that you have problems on both cylinders at the same time. Could be an issue in the primary (low voltage) side of the ignition. But I would not rule out coil problems. How old are they ? Or the other electrics involved. Heat and age take thier toll. ...J.D.
 

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Coils don’t always just stop working. Sometimes they just act erratically. When it seems like both are behaving strangely, check your wiring. It’s not real likely they’ll both go bad at once.
 

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Coils don’t always just stop working. Sometimes they just act erratically. When it seems like both are behaving strangely, check your wiring. It’s not real likely they’ll both go bad at once.
I agree, but I would not rule out a coil problem. Points, condenser, wiring going back to the coils. I'd just take a long, hard look at the whole system.
When an engine runs rich, every one wants to rip in to the carburetors. If you do not have a clean spark, you are going to have too much fuel. ...J.D.
 

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i agree also. spark issues are common, and will cover the plug with dry carbon.

dan, bad coils, bad condensors, bad plug wires, and bad points can all cause the problems you describe.

all of them will cause the spark to be erratic, as displayed by an electronic timing light. forget the timing, just watch the flashing light.

if the spark is regular, as shown by the timing light, then your problem is elsewhere. otherwise, start looking at coils, condensers, wires, and points.
 

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Sometimes you just gotta replace that stuff. It only lasts so long. Points and condensers would be changed periodically “ just because”. Now everyone is trying to make them last forever. I replace my points , then clean up the old ones on the bench were I can see what I’m doing.
 

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I would recommend checking your jet sizes the main jet may be your problem based your elevation. Remember the pilot jet is only for starting and after a certain RPM the main jet takes over. Good luck.
 
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