Troubleshooting from stem to stern until Hell won't have it.

[eddy]

'97 Standard, bought with ostensibly 24K on the clock.  Chewed-up rubber on pegs & shift lever, but upholstery is flawless and the inside of the timing enclosure is shiny and pristine. Stock belts - although aged past expiration date - show no wear.

*Cold* compression nearly identical on all six cylinders - about 175-180 PSI.

Early on, a faint stumbling sound was observed, slightly perceptible amid the normal drone when cruising at any speed.  Maybe a little worse at mid-rpm range.  Disappeared under accel / decel.

Desmogged, tightly capped off all six air injection ports and unused vacuum nipples.

Replaced visibly leaking exhaust gaskets.  Two or three rotten studs, replaced all twelve with hex cap screws of correct length, carefully retorqued by feel, trying not to overtorque (half of them are unreachable with a torque wrench).  Is there some good reason why the original fastening was with chrome-plated cap nuts, or is this just a shiny vanity thing?  Is it OK to leak-check with a spray bottle of soapy water if the engine isn't hot?

Previously had done the "Ragnar"-style baffle drilling (that supposedly precludes the need for rejetting) on the exhaust stubs for a bit more of that Porsche-like exhaust note.  The relentless backfire and hard-starting seemed to occur later on. Blocked off two (one on each side) of the opened exhaust outlets to slightly increase back pressure, no apparent effect.

By the way, how many fake exhaust outlets do these bikes have? Web search yields conflicting info; some say 1 per side, others say 2. Can't remember from checking with a tape measure months ago.

Did away with the petcock vacuum shutoff, diaphragm, spring, etc., confirmed good fuel flow with middle plate replaced by a tightly sealed copper gasket & hi-temp RTV that should be solvent-proof.  Cleaned tank filter, which was almost spotless to begin with.  Confirmed no leakage.

Went through the whole intake system, but didn't do full carb rebuild. Cleaned sediment out of float bowls just for good measure. Fresh intake o-rings, boots (they were a little stiff) and sealed up the airbox/velocity stack junctions with RTV. The problem seems global, as all plugs are exactly the same dark color w/o soot or oil fouling, so the search is mainly for some single item that would affect everything equally.

Replaced all of those short (cutoff?) hoses, which were uniformly petrified with visible cracking at the ends.  Now she starts a lot easier, but still has the horrible, almost continuous backfire, especially a few RPMs off-idle. Also replaced all other hoses that are still in use, which isn't very many after desmogging.

All ignition component resistances test perfectly to factory spec - transformer ("coil") primary & secondary windings, HV leads, (resistors are in the spark plugs rather than what the Gooble says) sensor coils, etc. using a very accurate Fluke true RMS multimeter. Being a retired electronic bench tech, this part is a piece of cake unless something has gone thermally intermittent - such as the ignition control module behind the coolant overflow tank? That seems unlikely, thankfully.  There are three output transistors in it, of a sort that often fail in audio equipment, but this would have had catastrophic effects.  The large connector on the bottom had some loose contacts, probably not an issue, but were easily cleaned and slightly tightened anyway.

Confirmed new (as of 1 year ago) timing belts hadn't jumped a tooth, and tension looks OK.

Sprayed a little carb cleaner here and there with engine idling, looking for vacuum leaks per tradition.  Found none.

Stumped.  All feedback greatly appreciated.   

[Thunderbolt]

Read again through what you have done so far.  Sounds like you have covered most all the bases.  When we had Valkyries, we don't any longer, I rebuilt carbs several times.  Always removed all the carbs and cleaned everything well and reassembled.  If memory serves me correctly the usual culprit on popping especially on deceleration was intake O-rings or vacuum lines.  Sounds like from your description that you have covered those possibilities.  Spraying carb cleaner is something I never did but others have used it to find vacuum leaks.  Just answering your post with some encouragement and a pat on the back so to speak as I don't like to see one with the amount of input or effort that you put into it and no one responds.

[t-man403]

Man, I hate to sound like the simpleton here but to me it sounds like you have a fuel problem. Have you tried riding with the choke on at various settings? You did the fuel filter so I'm assuming you put a new gas. If you did, I'd dump in a full can of Seafoam. If you didn't fill the tank, I'd probably dump what fuel is in it and fill with premium to help clean things out along with the Seafoam.
Also, I'd observe the headers. Any color change, maybe a timing belt is out a tooth.

[eddy]

The new o-rings might be a tad thin, they're generic. But they visibly compress a bit, and I gooped them up pretty good with sealant.

May try fuel additive just for S&G.  I always use only zero-ethanol 91+ gas.

[Valker]

I had what seemed like fueling issues on an old GoldWing years ago. It turns out that switching from 90+ octane back to 86/87 cured the issues. Just my experience, not a mechanic's opinion.

[t-man403]

The new o-rings might be a tad thin, they're generic. But they visibly compress a bit, and I gooped them up pretty good with sealant.

May try fuel additive just for S&G.  I always use only zero-ethanol 91+ gas.

If your running hi-test, all should be clean unless it sat a lengthy period of time hence the Seafoam suggestion. Best snake oil ever!