Triumphrat

shoegaze

An otherwise perfectly tuned and groomed 2001 Bonneville develops a slight blurbling loss of power under highway speed at WOT one day. It is dismissed by the rider as being attributed to the 100% humidity and exposed K&Ns, and does not otherwise manifest itself negatively.
A month goes by, and the hesitation under WOT worsens, but under all other conditions, the bike is fine. Even low speed WOT seems unaffected, and the idle remains strong with acceleration off-idle being unaffected.
Another month goes by, and by now, the bike is displaying dangerous power loss at all RPMs...but only at speeds over 47MPH. (gearing is 18/43). On the interstate, there is no gear where the bike does not exhibit a loss of power. A wah--blurble--wah, wah--blurble goes on for miles and miles (as if the machine is out of petrol) until the owner returns to sub-47MPH speeds, at which point, the machine returns to perfect tune. What ails this particular Bonnie where ONLY VELOCITY dictates state of tune?

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"I'll take a chance, if you'll take the blame."
Ian Stephen McCulloch

Forchetto

Carb diaphragms perforated or damaged?.

alfacliff

possibly a plugged fuel line, or filter.

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2006 Bonnie Black
1980 Yamaha xs650 sold 4-30-2011

saphena

I'd change the ignition coil and/or HT leads

__________________
Bob - Ringer, Iron Butt, not dead yet
Don't worry about running out fuel, carry a spare can

"Just because you're offended doesn't make you right" Ricky Gervais

shoegaze

I had considered the same. I replaced the diapragms 2 years ago, preemptively, though I can certainly check them. What I find baffling (and partly why I did not already pull the carb tops to examine them) is that this is velocity based, not RPM based.

I cannot see how the diapragms would give trouble at speed, without deference to RPM as well.

Regards, and thanks.

__________________
"I'll take a chance, if you'll take the blame."
Ian Stephen McCulloch

propforward

Starting here makes sense. From the picture, it looks like you have aftermarket roundslide carbs?

I only mention that as I don't think those have diaphragms - although that was a good thought.

EDIT: posted at the same time as you - so they do have diaphragms. Fair enough.

I have experienced something similar where I had crap in the float bowls. Possibly you have water in there?

Maybe drain the float bowls - even remove and clean them - maybe blow the jets out with compressed air. Check all fuel lines for kinks and make sure they are clear. Check the fuel tap.

I'd start with fuel before going to the electrical. The eletcrical is a possibility, but work through it methodically.

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"A long distance motorcycle trip, outside of your comfort zone, will probably be a defining experience in your life. It's miles cheaper, easier and safer than the celebrities and tour operators want you to think it is. Do a big trip, come back and encourage someone else to do theirs. Remember the kindness shown to you by strangers, then pass kindness on in your world thereafter. Simple"

Austin Vince

shoegaze

I use no filtration, and the fuel flows freely. The carbs are clean, and there is no residue of any kind in the float bowls, nor is there any debris in the jets. Also, since fuel demand is based upon RPM, and not speed, I'm truly baffled.

__________________
"I'll take a chance, if you'll take the blame."
Ian Stephen McCulloch

shoegaze

The coil and leads are dry and operating well. Further, I believe the coil would give issue at low speed as well, if it were the culprit.

Thanks for your input, though.
Cheers-

__________________
"I'll take a chance, if you'll take the blame."
Ian Stephen McCulloch

saphena

not when my HT leads failed - high speed problem only

__________________
Bob - Ringer, Iron Butt, not dead yet
Don't worry about running out fuel, carry a spare can

"Just because you're offended doesn't make you right" Ricky Gervais

shoegaze

Thanks for your reply.

I've gone back to the stock CVK carbs, as I could not tolerate the bogging of the CRs until 3200RPM. Otherwise, the CRs were flawless carbs, save for the cold start issues, and the lack of decent high idle operation, but I digress...

The main reason that I've not pulled the tap on the tank is that I don't experience this issue--even in low speed, high RPM conditions-- where fuel demand is at its highest, and I do experience it at low RPMs where fuel demand is rather low.

I also truly suspect an electrical issue, but have very little knowledge of the operational variables that make up the electrical system as a whole.

If I had to guess the issue, I'd say that there is a speed sensor that sends a signal to the ignitor or coil, or reads from the crank position sensor that is malfunctioning. I believe that after I obtain a certain speed, a faulty signal is being sent to the ignitor telling it to retard timing, or some such thing.

I am a former professional mechanic and handy with guages, multimeters, and dynamic testing. Short of lashing the bike down, putting it on the centre stand and having someone run it through the gears whilst I test dynamic impulses from the coil, ignitor, etc., I am severely at a loss here.

__________________
"I'll take a chance, if you'll take the blame."
Ian Stephen McCulloch