First... sorry for the jetting question.  I know there are a ton of carb / jet threads (plus the wiki) and I feel like I've read them all, but I am still in need of some advice.

You would think that insofar as I rescued this bike from oblivion (fairings, paint, graphics, engine covers, bars, pads, brake lines, tires, sprockets, chain, etc...) it would be a little more appreciative of my efforts.... but such has not been the case.  It ALWAYS has a little something wrong with it... sometimes easy things (neutral sensor, regulator/rectifier, both brake light switches, front caliper, fork seals, etc...) and sometimes not so easy things.

So here we go. I've re-jetted a few of my GSs in the past, but the carbs have me stymied in this case. The bike has never run "right" since I bought it. It had been re-jetted by the PO and it took FOREVER to warm up (like literally 10 minutes) when I got it. I tolerated that for a (very short) while and eventually put the stock jets back in, sonic cleaned the carbs, checked for vacuum leaks, etc... etc... thinking I would just start from scratch and move on from there. It ended up going from bad to worse and developed a terrible hanging idle.  I fiddled with it for a few weeks off and on and finally decided to take it to my local shop (you know... to get it done right. lol).  They eventually (don't get me started on how long it took) determined that the valves were w-a-a-ay out of spec and after what seemed like another eternity, adjusted the valves and re-jetted the carbs (20 pilot, 125 main, stock mid and stock needle set-up). They told me it was AGAP based on the age and mileage of the bike. What... with less than 6000 miles on it??? C'mon, it's not a piece of fruit... it shouldn't just "go bad."  lol.

At that point it was back to the 10 minute warm-up cycle but rode "ok" once it was warmed up... not a lot of get-up-and-go compared to my other GSs, but rideable.   

I decided to take another stab at jetting it and bought the DynoJet Stage 1 Kit. I've used their kits before and had good success.  Based on an extensive conversation with a tech from DynoJet, I installed an 18 pilot, 128 main, kept the stock mid, and installed the DynoJet needle (circlip on the 4th groove from the top which positions the needle fairly high). The air screws are three full turns out. 

Now it starts right up (like a champ!) and rides fantastically up until about 6500 RPM at which point it hesitates under hard acceleration (maybe 3/4 to WOT). Not unbearably so, but enough to affect the "fun factor" for sure, and enough to make it difficult to change lanes with any sense of urgency (or under more precarious circumstances... to get me out of harm's way). 

Here are few more salient points:

- I'm running a Delkevic exhaust and mid-pipe with the stock airbox
- It runs better with the exhaust baffle taken out than left in (i.e. it seems to like more air).  With the baffle out, the trouble doesn't start until about 7000 RPM
- When I tape off half the air intake the trouble starts at about 5500 RPM (so less air seems to make it worse)
- If I add a little choke it becomes virtually unrideable above 4000 RPM and hesitates under almost any throttle
- If slowly approach the RPM at which the trouble starts (say from 5000 RPM) it runs fine. It will cruise at 7000 - 8000 as long as I take my time getting there. But....
- If I'm riding aggressively and open the throttle hard out of a corner or from a dead stop it races right past 6500 without a problem. And...
- If I wind it out from the bottom up it does much better and usually won't hesitate until it gets to about 8000. So...
- The trouble seems to be if I "goose it," as my Dad would have said, from the midrange to the high end. 
- It burbles and pops just a "little" bit when down shifting, but not much, and certainly not a full backfire.  My other GSs do this, so it doesn't really concern me too much, but in the interest of providing as much info as possible I thought I'd mention it.

Based on the baffle and air intake (and choke) behavior it seems like it's running rich. But with 128 mains, that doesn't seem right, at least not based on what everybody else is running in the other threads I've read.

So... any thoughts on the next steps? And more specifically, if it needs to be leaner what's the preferred method? Air screws in??? Smaller main jets??? Move the needles down??? 
 
And finally, if you've made it to the end of my way-too-long post, you must be interested in helping me out so thanks in advance.  lol

B. 

Hey Bruce. I have been through precisely this problem, oddly enough with DJ needles. Solving it required returning to regular needles.

You are correct, it's too rich. The DJ jet sizes are not really comparable to Mikuni jet sizes, and the needles are very different than stock needles. If you MUST stick with the DJ kit, then I'd recommend moving the needles down until the problem goes away. It's such a chore to get in and change this, if it were me I'd probably knock them all the way and then see if it doesn't fix it, and then move the other direction as a remedy. But in any case, I think you'll wind up using the highest circlip position right before you decide to chunk the DJ kit and go back to stock.

With the mods you list, I think you'd be best off with 20 pilot jets and +2.5 vs. stock whatever it is main jet and then shim the stock needles with a washer or two and tune from there with needles. Your hanging idle problem before was probably due to the pilot mixture being wrong (probably way too rich) so you had to crank up the idle speed so the main venturi was flowing some air to lean out the idle, which left it with enough airflow to pick up some fuel from the main jet and keep it from returning to idle. Could be clogged idle air passage in the carb throat causing very rich idle or more likely, leaky pilot needle o-rings, causing it to leak fuel no matter the pilot setting. Fix that, and return to stock(ish) jetting and needles and I bet Bob's your uncle.

Careful not to confuse pilot mixture with running condition. What you are talking about, 3/4 throttle and midrange revs, is transition to main jet. If it hesitates or just runs bad for a few rpms then clears and continues revving right to 11K then it's a needle position problem. If it just kind of hits an rpm wall and won't rev past it unless you reduce the throttle, it's too rich on the main jet and you need to jet down.

Oh, well you might also need to dial in the float level. Man, I hate carbs.

Taking 10 minutes to warm up, BTW, is pretty normal. If yours is not taking like 5-10 minutes of actually running it on the road to warm up, then you have the entire thing set up way too rich. If you set it up to start and run easy with little or no choke or turn off choke before you are underway, then it will run horribly once hot. Hear me now and believe me later!

Quote from: mr72 on June 11, 2023, 05:46:26 AMTaking 10 minutes to warm up, BTW, is pretty normal. If yours is not taking like 5-10 minutes of actually running it on the road to warm up, then you have the entire thing set up way too rich. If you set it up to start and run easy with little or no choke or turn off choke before you are underway, then it will run horribly once hot. Hear me now and believe me later!

Yes, I should have been more specific about the warm-up issues and I did thoroughly review your excellent information on adjusting carbs. When I say it needed to warm up, I mean I couldn't even twist the throttle without it dying, to say nothing of actually making it out of the driveway. Lol. At least at this point I know the exact state of the carbs and the contents and can work from ground zero rather than compensating for what I speculate might have been done. I was really hoping it was going to be a "reach under there and give a half turn on the air adjustment screw and you'll be fine" fix. Lol. I may still try that, but I was also leaning toward the needles or the jets.  Thanks for your input and being such a great resource.

B. 

Quote from: mr72 on June 11, 2023, 05:46:26 AMCareful not to confuse pilot mixture with running condition. What you are talking about, 3/4 throttle and midrange revs, is transition to main jet. If it hesitates or just runs bad for a few rpms then clears and continues revving right to 11K then it's a needle position problem. If it just kind of hits an rpm wall and won't rev past it unless you reduce the throttle, it's too rich on the main jet and you need to jet down.

Testing, Testing...

Ok, because I value economy of effort (which I feel obligated to point out is completely different from being lazy. lol) I decided to run one more quick "test" before tearing everything apart again. I reached under and turned the air / fuel screws 1/2 turn in on each carb.  My understanding is that will change the air / fuel mix across the range, not just in one spot (please correct me if I'm wrong on that). 

Et voila! My 6500 + RPM problem is more or less solved.  There is still some barely perceptible sluggishness from 8000 and up, but it's so slight that if you weren't looking for it, you'd probably not notice it.

However, the popping upon deceleration has increased markedly (not a full-blown backfire, but more noticeable) which indicates to me that it's now running lean on the low end.

Additionally, my smooth "BAH-DAH-BAH-DAH-BAH-DAH-BAH-DAH" of an idle, has now changed to more of a "BAH-DAH-BAH-DAH-BAH-DAH-bah-doo-BAH-DAH-BAH-DAH-bah-doo-bah-doo-BAH-DAH" almost like it misfires occasionally... starved for fuel now?

My conclusion is that by changing the overall air / fuel mixture I've compensated for the rich high end and now the low end is lean... but this was just a test. 

So, my plan now (I think) is to lower the needle rather than change the main jet (and of course return the air/ fuel mix screws to 3 turns out where they started.  The thought being that since the bike can handle full throttle at lower RPMs the issue is really that once the RPMs get going and the slide raises, the needle is the culprit that's responsible for letting in too much fuel on the high end.  I'd welcome any input on this hypothesis.

Unfortunately, whatever the eventual solution ends up being, it will have to wait until at least mid-week as I am on "renovate the bathroom" duty this weekend. Our daughter's boyfriend from college is visiting for the first time, and we are (due to some logic with which I am unacquainted) obligated to renovate our guest bathroom prior to his arrival.  lol.  I informed the ladies in my house that since he has never actually seen the bathroom before (and it's actually quite a nice bathroom that I've already renovated once), that we could simply tell him that it was just renovated, and that might serve the same end.  However, I was not unexpectedly, outvoted by the ladies.

Back to my plumbing...

Thanks again for any input.

B.   

p.s. For the moment I'm going to stick with the DJ needles only because the circlip and groove set-up makes them easily adjustable, and the needles that came out of the carbs looked a bit rough.  I'll revisit that decision however, if things don't improve in the next round of adjustments.

Well, to keep from confusing yourself, best not to call those "air screws". They actually adjust fuel, not air. Three turns out is too much. If you need three turns out, you need a bigger pilot jet.

This affects mixture primarily at idle and low rpms with small throttle.

Usually you turn main jet at wot and high revs for max power, then idle with pilot jet and mixture. Once those are set correctly, midrange issues are sorted with needle taper or position.

So, go back and set the pilot mixture properly, see my blog post linked below for more details. Then fix the midrange issues by adjusting the needle. If leaning the pilot mixture reduced the issue, then it was too rich on the needle so lowering the needle is the next logical step. You'll have to re-tune the idle mixture after tinkering with the needle position.

Thanks again for the info. It seems just when I thought I was almost "there," there has now moved. Lol. And you are correct. I know what those screws do, but for some reason I am stuck calling them "air screws" in my head.  I'll make the changes you suggest and report back with some results.

B.

Fixed! So I finally got a chance to jump back into the carbs... went down two sizes on the main jet and bumped the needle down one groove (or moved the circlip up depending on your perspective). It now runs better than all of my other GSs.  I did stick with all the Dynojet components as opposed to mixing and matching.

Whoo hoo and thanks!