GS450 miss/stumble while cruising (another carb guru request...)

[kxpower?]

Hey guys,

This is on my 82 GS450 TXZ, with the BS34SS CV carbs. Pretty similar to our beloved Ninjette carbs. Running stock pilots, 135 mains, and needles shimmed up a washer thickness. Running K&N pods and a nice quiet stock exhaust.

Here's my problem: When cruising in top gear at 60+ mph and steady throttle (it takes 1/2 to 3/4 throttle at 4500 rpm, not the ballsiest little bike), I get this miss that I can't really explain. It's like I completely lose a cylinder for just a few revolutions, then it comes right back. But it is ONLY at steady throttle, if you roll on or off it smoothes right out. Riding around town or playing in the twisties you'd never know a thing is wrong, it idles and runs like a top. But it's really annoying anytime you're out just getting down the highway. It's enough that you slow down pretty noticeably, then you just roll the throttle on a tiny bit and it wakes right back up.

I don't think it is anything electrical, and fuel supply to the bowls doesn't seem to be an issue since it has no problem under sustained hard acceleration and climbing (like 1000 feet at a time on the Cherohola--no sweat at all). And it seems to be intermittent enough that the plugs don't get a chance to change at all, they consistently look just like the photo.

Everything I know points at the needles needing adjustment, but how would I find out whether to go up or down? Or am I overlooking something else?

[Ghostt]

Paging @ducatiman

[VaFish]

I think they look awfully white, which I have always thought means running too lean.

Also this Jetting diagram shows that Main Jets control from 3/4 throttle to WFO. Although 1/2-3/4 could also be needle taper or clip position.

I'd raise it up in the clip or add a washer first. Then go to a larger jet.

[ducatiman]

Jesse-

these are very early Mikuni CV carbs..many more parts than your later model Keihin CV.

80's vintage Mikuni carbed GS twins were notoriously jetted lean from the factory, requiring long warm up times before being able to accept throttle.

stock pilots 17.5
stock mains 115 (you are up 8 sizes at 135!)
stock needle 4C2

What is your fuel consumption (MPG) like?

Have you accessed and set your pilot screws for highest idle and best throttle response?

Does the bike exhibit lean running at low throttle openings or require long warmup?

[kxpower?]

Originally Posted by ducatiman

Jesse-

these are very early Mikuni CV carbs..many more parts than your later model Keihin CV.

80's vintage Mikuni carbed GS twins were notoriously jetted lean from the factory, requiring long warm up times before being able to accept throttle.

stock pilots 17.5
stock mains 115 (you are up 8 sizes at 135!)
stock needle 4C2

What is your fuel consumption (MPG) like?

Have you accessed and set your pilot screws for highest idle and best throttle response?

Does the bike exhibit lean running at low throttle openings or require long warmup?

Mileage has consistently been 45-55. I put the 135's in and raised the needle to get it running well with straight pipes, when I got a stock exhaust and installed it, it still ran fine so I didn't mess with it.

I've barely messed with the idle screws, just enough to get it idling smoothly. It requires full choke to start, I usually let it warm up on choke while gearing up then turn it off when I take off. It acts a little cold-natured for the first mile or so then smoothes right out.

Oh, and I forgot to mention that this stumble pre-dates the new exhaust, it's done this since I brought it home. Only now I've whittled down all the other issues to the point that it's one of the few remaining problems.

[ducatiman]

true to standard procedure, larger mains accompany pod installation, but 8 sizes over with stock pipes? quite a jump

some experimentation called for....so add a needle shim, look for + or negative results

long term..i'd up the pilot 1 size to a #20...followed by setting the pilot screws
this may require tweaking the needle yet again (there is a crossover point)

Note that upping the pilot was standard procedure (using the stock airbox) back in the day to correct the cold blooded nature.

Lacking a dyno and exhaust gas analyzer...you're forced to jet the old fashioned way....some trial and error and seat of the pants.

Post your results, PM me if you choose, too.

[jkv45]

Seems like those Mains might be too big.

That, or a diaphragm might be damaged and letting a slide drop slightly during constant throttle position. Closing the throttle would cut the air and let it run better, opening the throttle would give it a surge to help lift it.

Just a thought.

[ducatiman]

an undeniable benefit to pods is to enable lightning quick carb pulls and access to carb innards. Diaphragm examination and internal changes with test rides between should be fairly painless.

Regardless, I'd encourage Jesse, even though he may be doing multiple changes...to perform one at a time...to access and easily reverse if need be.

Step by step...should be an interesting, fun challenge for him. And rewarding when he nails it. Meanwhile accumulating knowledge along the way.

Also note...any carb evaluation and changes are always predicated on carbs with truly clear circuits and correct setup.

[ducatiman]

an additional thought...taking JKV's thinking a step further...make sure the slide to carb working surfaces are free and clear of any corrosion, burrs...no impediments to motion.

Hmmm, jet kits instructions commonly encourage cutting the return springs short....is this the case with your carbs?

[kxpower?]

Hey I'm sorry to let this sit, on the road a ton for work and this has gotten pushed to the back burner. I just went through the carbs, confirmed 17.5 pilots and 135 mains, idle screws are both 2 turns out. Diaphragms are perfect, springs are both 4-1/16" free length, and slides rise and fall smoothly and at exactly the same speed. Nothing weird going on with the needles either, I was a little suspicious that maybe one of the circlips wasnt fully seated causing a needle height mismatch but everything seems good there. I'm ordering some#20 pilots and 120 mains, will try each change separately and post with results.

[kxpower?]

Paging @ducatiman

OK so I'm on jets r us looking for the pilot jet, and all they have listed under the version without crossdrilled holes (N151.067) is 30-60. If you go to the pilot with the same dimensions with 8 crossdrilled holes ( VM22/210),then they have from 10-140. The pilot in there now is the OE mikuni 17.5, and there aren't any crossdrilled holes.



Is the crossdrilled pilot interchangeable? Or how do I get the next upsize pilot?

[ducatiman]

kx-

this being an oldie will require some homework

JET, PILOT (17.5)
09492-17005 OEM Suzuki part # this from ronayers.com online fiche

Googling the above, I came upon an obscure GS owners forum post where a member encountered the same issue you describe, but no final answer was ever posted

OPTION A:
Pull your jet and get to a Suzuki dealer, see if they've got an old microfiche.
The microfiche may have Suzuki part numbers for alternate sized jets, if I remember correctly, they did. You get the right parts guy, he'll figure it out. With a correct part # for 20...order from Suzuki.

OPTION B:
call jets r us...they may be aware, having run into this issue before

OPTION C:
I'll PM you shortly

[kxpower?]

I sent an email to Jetsrus, we'll see what they have to say.

Originally Posted by ducatiman

kx-

this being an oldie will require some homework

JET, PILOT (17.5)
09492-17005 OEM Suzuki part # this from ronayers.com online fiche

Googling the above, I came upon an obscure GS owners forum post where a member encountered the same issue you describe, but no final answer was ever posted

OPTION A:
Pull your jet and get to a Suzuki dealer, see if they've got an old microfiche.
The microfiche may have Suzuki part numbers for alternate sized jets, if I remember correctly, they did. You get the right parts guy, he'll figure it out. With a correct part # for 20...order from Suzuki.

OPTION B:
call jets r us...they may be aware, having run into this issue before

OPTION C:
I'll PM you shortly