Yet another off-idle popping question

[ChrisMA]

I know there's a post just like it next to mine, and a dozen or more in the search results. I've read them, and know popping on decel likely indicates either a lean condition or an exhaust leak. But different bikes may have different problems, so I figure it can't hurt to ask.

80 GS1000G, all stock, airbox, factory jets etc, original exhaust with no obvious leaks. Valves adjusted, carbs clean and unblocked, new orings and rubber, and I've done everything I can to eliminate air leaks (full list of work below). Carbs are synced with Carbtune pro. Pilot screws are 3 1/2 turns out on #s 1&4, and about 3 turns on 2&3. With the BS carbs it's really tough to fine-tune by ear, but my plugs had hinted that the outside cylinders may have been leaner, so I turned them out "all the way" to 3.5 to see if it eliminated the popping (it didn't). I didn't go any farther than that based on what I've read here. Plugs are now a uniform lightish tan (pic below), though I haven't done a proper chop test yet.

The bike starts up with about half choke, no backfiring, and settles down within a minute or two, and I can ride off with little or no choke. Lots of power all the way through the rev range. Holds idle, and returns to idle quickly and consistently. Idle is set to a hair under 1000. Exhaust is a little light gray when warming up, which I assume is just moisture as it's gone after I've ridden it.

Along with fairly consistent popping when I let off throttle (not very loud, but obvious), there's a very minor repetitive popping (almost like a "burbling") when holding a cruising speed. I don't see fluctuation on the tach needle, but I can kind of feel it in the movement in the bike, almost like a subtle rocking movement forward and back, like there's a repeating hesitation or maybe a misfire. It's that rhythmic pattern that has me wondering. Does that sound like it's running lean? Or could it be a float / fuel level issue, something to do with fuel sloshing around? Could that point me at something that I've missed?

The exhaust is stock, no obvious holes, but a rattle on the right side tells me it has internal problems. I've already bought a set of exhaust seals, but don't plan to install them until the winter (because I'm sure to shear some bolts and don't want to lose any more riding time this season). I don't want to replace the exhaust if I can avoid it.

Here's everything I've done: adjusted the valves, full carb teardown and cleanup (twice) following the instructions on Basscliff's site, replaced all o-rings, and basically all of the fuel system rubber (boots, intake pipes, fuel and air lines). New petcock (2 years old). Adjusted floats with a micrometer: PO had measured them to the top ridge, I readjusted them correctly, which moved them "up" (so, on the bike, they're lower), though I haven't directly measured fuel levels yet. Float springs seemed fine, the valve held the floats in position. Resealed the airbox twice (had some leaking after the first time, redid the job with additional tape and some silicone). Spray test doesn't show any obvious air leaks, but I know that's not a perfect test. New Uni filter, oiled per the instructions. New plugs, gapped to spec, with recent plug terminals, new battery. Electrical system is stock, charging seems fine (but haven't done any electrical tests yet).

What have I missed?

Thanks,
Chris
IMG_20180813_101923 (1).jpg

[standswithabeer]

popping on decel and steady throttle "hunting" does sound like a lean condition, and it looks like you have "room" to make the carbs a little richer before the plugs are too far from tan. but am not familiar with your carbs. can you move the needle clips down (raising the needle to enrichen) one notch, or just a smidge using shims made for this purpose?

[ChrisMA]

Thank you, standswithabeer (and great handle!). Throttle "hunting" is a good description. I've been trying to search for other descriptions of what it feels like. It's not really racing, it's not really bogging, it's just... not quite steady. Definitely not like other bikes I've ridden. It's helpful to know that it's a sign of a lean condition.

No, as I understand it, my BS / CV carbs don't really allow much mixture adjustment at all. No needle clips, just a single screw per carb to open up the idle/slow circuit. Beyond that, I believe I'd be replacing jets, but I don't think that's the problem here. If I'm lean, I think it likely means I haven't found all my air leaks yet.

Air leaks suck.

[standswithabeer]

hey chris,
stumbled across a pic-heavy and informative link (below) on rebuilding your carbs, and about 3/4 of the way down it shows how to adjust needle position by replacing the original plastic washers with thinner shims. but before that, you could carefully for a torn diaphragm.

check out note 5 on the trouble-shooting page (last page), and good luck!

https://thexscafedotcom.files.wordpress.com/2014/06/mikuni_bs-cv_carburetor_rebuild_tutorial.pdf

[Big T]

What does "carbs unblocked" mean?

[ChrisMA]

What does "carbs unblocked" mean?

Yeah, unclear wording on my part. I just mean that they were dipped and cleaned, with all the small passages cleared out ("unblocked") following the two guides on Basscliff's site. #1 in particular had been gunked up by rusty sediment from the tank. Oh yeah, and I then EvapoRusted the tank, so that problem is gone (for now).

[ChrisMA]

hey chris,
stumbled across a pic-heavy and informative link (below) on rebuilding your carbs, and about 3/4 of the way down it shows how to adjust needle position by replacing the original plastic washers with thinner shims. but before that, you could carefully for a torn diaphragm.

check out note 5 on the trouble-shooting page (last page), and good luck!

https://thexscafedotcom.files.wordpr...d_tutorial.pdf

Thank you! Yes, I did refer to that document when I rebuilt my carbs, but I did not notice the part about adjusting the needle position. That does give me something to think about. And while the diaphragms did look to be in good shape, next time I have them apart, I'll go over them again.

One other thing that I didn't notice in that document is point 4 on the troubleshooting page:
"If the engine stumbles just off idle, try opening the pilot screws another ½ turn and try again.The maximum number of open turns is four, otherwise you should get larger pilot jets."

I've read many times on the forum not to go beyond 3 1/2 turns, but perhaps for these carbs I still have a little room for adjustment. Easy enough to try...

Thanks!
Chris

[Tom R]

Those little o-rings under the mixture screws are easy to bugger up. I once put a blob of thick grease on top of the screws to help seal them up when I knew mine were bad.... seemed to help. Anyway, it’s an easy check. I used a lab grease meant to seal desiccators, but I imagine anything could work and it cleans up with a rag.

[ChrisMA]

Reporting back, in case someone finds this when researching similar problems...

In my case, opening up my pilot screws out to 4 full turns did seem to alleviate the subtle hesitation and popping off idle. I realize that some of my carbs are probably now too rich, and will soon hook up the carbtune again to get them all dialed in better. But it does seem that for these BS/CV carbs, there's a bit more headroom after 3 1/2 turns.

Next time I have the carbs out, I'll take a closer look at the needle position and mixture screw o-rings. Thanks again, guys!