It sounds more and more like you have a tiny tiny tiny rip in 1 diaphragm.
Jetting is related to throttle position.
Electrical unless its on/off type, is related to temperature. As in - doesn't work at all, or works only when cold, or works when hot only.
9500 rpm is the same flow in the exhaust regardless of any other parameter.

Cool.
Srinath.

Well, next up is to re-check the carbs but I'm still pretty close to convinced that it's ignition.

BTW this type of ignition failure is not an on/off problem. If the coil had a broken primary then it'd be on/off but there are other failure modes for a coil, for example a breakdown of insulation I the wiring wherein vibration at a certain frequency would cause intermittent shorting between windings, if on the secondary then in that case it would only reduce spark intensity when this happened. However, my theory had to do more with the time constant of charging and discharging a coil. If you put resistance in series with either the primary or secondary by introducing corroded wiring or connectors then it increases the time required to both charge and discharge which would manifest as a failure to spark at high rpm (high frequency, insufficient time to charge/discharge).

But since my bike runs on the side stand in the garage up to 12k rpm then this eliminates the time constant problem as a potential problem. The fact that it only fails under load makes me think maybe it's fouling the spark plug and can't clear quickly enough (way too rich?) or more likely, since the problem increase in rpm after I upped the jet size from 125 to 127.5 makes me think it's actually lean at high rpm. I turned the choke on while riding and it smashed my low-mid rpm performance but the high rpm didn't really change any, and the 9.5k rpm thing didn't change at all, so I am still not convinced this is fuel delivery related. Who knows, maybe it's air flow. I have never changed the air filter except after I first got the bike but it's only had 4k put on it since then so it shouldn't be clogged...

I was reminded while working on it over the weekend that the stock airbox truly sucks very bad. Mostly the problem is that the inlets are angled and the carbs are not angled so getting it to actually fit onto the carbs is incredibly difficult. That's why I bought some pod filters to try but I don't have big enough jets to try them, but if I pull it all apart again and double-check everything in the carbs, then I think I'm putting pods in. Not the cheap K&N knock-offs I have, but Uni pods. This is just to reduce the pain of maintenance on this thing. I'm guessing my 135 jets may actually work with Uni pods. But at the very least it'll reboot the whole problem and give me something else to hunt down.

Talked myself into the idea that this is still too lean at this RPM/throttle, so I ordered 132.5 jets.

Story continues in my build thread:
http://gstwins.com/gsboard/index.php?topic=71055.0

But I'm now basically convinced this may not be ignition.

I'll be interested in what you find out. I had this problem but it was sorted out with going up in jet size. I bought my bike with the Yoshi slip on and stock jets and just used the wiki recommendations for the rejet. Of course its somewhat apples and oranges because I have the late model 3 jet carb. But, I had almost the exact same problems you are describing, where the bike would could cut out around 9k rpms. I just short shifted back then all the time but I can rev freely to 11k if I want now with no issues. Good luck!

If it is throttle position related - it is carbs.
If it is RPM related - its flow rate - and likely to be exhaust but anything to do with air flow (weirdly on the intake side it is throttle position - as in if you had a choked off air filter - it will present related to throttle position when its air flow related)
If its electric it will be related to heat and sometimes it can work when cold not when hot, or when hot not cold - semi conductors have lower resistance when they heat up, wires and conductors do the opposite.
It its road speed related - yea its friction, you got brakes that are binding - you'd get to 30mph fine and then never move past that - don't even laugh, I had a Vulcan that did this. I killed myself trying to find the problem ... duh, binding front brake when the thing got warm cos the fluid was all water.

Cool.
Srinath.

I just think now that the big/cut-down Yoshimura muffler along with the header wrap are having a much larger effect on airflow/jetting than I expected they would. The wiki advice for "slip on" muffler probably doesn't apply to this particular exhaust setup.

With any luck the jets and new air filter will arrive tomorrow and I can swap it all Saturday. Rain is supposed to go away and Sunday is supposed to be a gorgeous day so maybe I can take it for a test ride.

No in mufflers - less is more. No way you're running out of flow in anything unless it was stock.
BTW 12k idle is same flow on the muffler side as 12K under full load. 9.5K is 20%+ nope.
But you can be lean just cos you're dealing with a different flow pattern from the cleaned up ports and head - I think it may make for a higher velocity, but I have been wrong before on this.
Simple way to do - without doing a plug pull @ full load and hence likely to get hit on the highway - Jet up 1 size at a time, and its the only change you do, till it gets worse, then back 1 step.
Cool.
Srinath.

FYI 132.5 jets along with maybe contributions from a) new air filter, b) reset float height, c) replaced vacuum hose all fixed this issue. Bike runs like it should now.

so... first drive this year, have same symptoms as mr72 here, wont rev above 9k on load, and stutters on acclerations, i have 136 mains, should i go even higher? kn lunchbox and somewhat sporty sounding slipon.

Few days ago i cleaned carbs and set the floatheight as manual tells, the startup was a longshot, emptied my battery and when it finally started after recharge it runned on just one cylinder, after <1min both started rocking. plugs are semi-new.

Quote from: Meukowi on April 23, 2020, 07:26:25 AM
so... first drive this year, have same symptoms as mr72 here, wont rev above 9k on load, and stutters on acclerations, i have 136 mains, should i go even higher? kn lunchbox and somewhat sporty sounding slipon.

Few days ago i cleaned carbs and set the floatheight as manual tells, the startup was a longshot, emptied my battery and when it finally started after recharge it runned on just one cylinder, after <1min both started rocking. plugs are semi-new.

What jet is a 136 ? DJ ? Those things have different needles. Remember the needle is a tapered obstruction in the emulsion tube. The main jet is at the inlet to the emulsion tube, the needle is at the exit. Smaller obstruction at the exit functions the same as a larger entry etc etc.
Anyway larger than 136 ? since we cant call them and ask for a 138 and a 140 - I suggest you bench mark it to the mikuni numbers. Make a few runs making notes per throttle position etc etc with the jets you have now.
Try a 137.5 - making no other changes. Make the same observations.
That is now your new bench mark. From here on, work in the mikuni jet range, and take it from there.

With the stock needle, the K&N and slip on would need a 147.5 mains with 20 pilots on a 01+ bike. I doubt you're going to need much bigger than a 140 maybe - but again experiment and know.

Cool.
Buddha.

Quote from: The Buddha on April 23, 2020, 07:33:14 AM

Quote from: Meukowi on April 23, 2020, 07:26:25 AM
so... first drive this year, have same symptoms as mr72 here, wont rev above 9k on load, and stutters on acclerations, i have 136 mains, should i go even higher? kn lunchbox and somewhat sporty sounding slipon.

Few days ago i cleaned carbs and set the floatheight as manual tells, the startup was a longshot, emptied my battery and when it finally started after recharge it runned on just one cylinder, after <1min both started rocking. plugs are semi-new.

What jet is a 136 ? DJ ? Those things have different needles. Remember the needle is a tapered obstruction in the emulsion tube. The main jet is at the inlet to the emulsion tube, the needle is at the exit. Smaller obstruction at the exit functions the same as a larger entry etc etc.
Anyway larger than 136 ? since we cant call them and ask for a 138 and a 140 - I suggest you bench mark it to the mikuni numbers. Make a few runs making notes per throttle position etc etc with the jets you have now.
Try a 137.5 - making no other changes. Make the same observations.
That is now your new bench mark. From here on, work in the mikuni jet range, and take it from there.

With the stock needle, the K&N and slip on would need a 147.5 mains with 20 pilots on a 01+ bike. I doubt you're going to need much bigger than a 140 maybe - but again experiment and know.

Cool.
Buddha.

i remembered the jet size wrong, its 134. oh yeah ive got those dyno jet weird needles, so i put an order on 138s, sizes available here are 133, 135 and 138. guess one size up on 134 wont make too much difference? the plug colour aint too bad in my opinion.. cant tell if its too rich or lean judging by it

plugs look fine. DJ needles taper is weird and I could not get rid of a stumble off idle on my bike with what I suspect are DJ needles in it.

The thing is, the plugs will look fine if it's running fine at the last condition where you were riding. like part throttle 5k rpm, which has a lot more to do with needle position than jet.

If you take a wrench with you and go ride, accelerate WOT until it sputters and then immediately kill the engine, coast to a stop, and then pull the plugs, you can read the plugs under the WOT/high-revs condition. But you have to kill the motor while it's dong its sputtering. Not really practical for most folks. Now would be the time to do it, abandoned highways and all that.

Quote from: mr72 on April 23, 2020, 08:22:37 AM
plugs look fine. DJ needles taper is weird and I could not get rid of a stumble off idle on my bike with what I suspect are DJ needles in it.

The thing is, the plugs will look fine if it's running fine at the last condition where you were riding. like part throttle 5k rpm, which has a lot more to do with needle position than jet.

If you take a wrench with you and go ride, accelerate WOT until it sputters and then immediately kill the engine, coast to a stop, and then pull the plugs, you can read the plugs under the WOT/high-revs condition. But you have to kill the motor while it's dong its sputtering. Not really practical for most folks. Now would be the time to do it, abandoned highways and all that.

yeah i could do that, probably order then 140+ jets too for stock needles, if it turns out that dynojet is the issue here

so.. to the jets, does Mikuni N102/221

sound familiar? no clue what the proper size is
edit. googled it up, so for everyone else N100.604 series is the factory size, but also N102.221 will fit, just a little smaller head on it

Plug is an idle reading. It can nearly always be compensated for everything down at idle.
There is a reason why they say get the mains right before fiddling with down the throttle range.
WFO - you have 1 control - Mains.
1/2 - 3/4 - You have 2 Main and needle.
1/4 - 1/2 - You have 3, main, needle, and float level - oddly float level cant go over the top of bowl and less than 1mm down from it is acceptable.
1/8 - 1/4 - add pilot to the above. So 4.
idle - 1/8th - add air screw so 5.

Anyway all those numbers 133, 138 etc are all non mikuni numbers. No idea what they relate to in the real world. You have 134 - so start with 135 mikuni and benchmark your observation.
The key here is - 1 change and only 1 change. Don't even touch anything else. Make detailed observations related to throttle position.

That high rpm plug pull is a feasible idea - especially if you live in an low traffic WFO highways area, probably helped along by this quarantine and shutdown.
But really you don't have to, you can tell by feel and symptoms and rpms it pulls etc.
Either way - don't kill yourself trying to get the jetting just right, there are other ways.
Its basically put in larger and larger mains till your max rpm drops.
Then back up 1 step.
Then needle and all the rest can be dialed in from there.

Cool.
Buddha.

Quote from: Meukowi on April 23, 2020, 08:44:41 AM
so.. to the jets, does Mikuni N102/221

sound familiar? no clue what the proper size is
edit. googled it up, so for everyone else N100.604 series is the factory size, but also N102.221 will fit, just a little smaller head on it

Wait - what year is your bike ?
Cool.
Buddha.

Quote from: The Buddha on April 23, 2020, 09:44:35 AM
Plug is an idle reading. It can nearly always be compensated for everything down at idle.
There is a reason why they say get the mains right before fiddling with down the throttle range.
WFO - you have 1 control - Mains.
1/2 - 3/4 - You have 2 Main and needle.
1/4 - 1/2 - You have 3, main, needle, and float level - oddly float level cant go over the top of bowl and less than 1mm down from it is acceptable.
1/8 - 1/4 - add pilot to the above. So 4.
idle - 1/8th - add air screw so 5.

Anyway all those numbers 133, 138 etc are all non mikuni numbers. No idea what they relate to in the real world. You have 134 - so start with 135 mikuni and benchmark your observation.
The key here is - 1 change and only 1 change. Don't even touch anything else. Make detailed observations related to throttle position.

That high rpm plug pull is a feasible idea - especially if you live in an low traffic WFO highways area, probably helped along by this quarantine and shutdown.
But really you don't have to, you can tell by feel and symptoms and rpms it pulls etc.
Either way - don't kill yourself trying to get the jetting just right, there are other ways.
Its basically put in larger and larger mains till your max rpm drops.
Then back up 1 step.
Then needle and all the rest can be dialed in from there.

Cool.
Buddha.

ok cool, i have no idea what brand they were, but cheap 3$ each, found those mikunis 137.5s, was double the price. But yeah, the accleration is very bad currently, doesnt do really anything except bog if i wot or even half wot it + backfires on startup, might felt a little difference on the good side when i choked during acc.

Quote from: The Buddha on April 23, 2020, 09:50:01 AM

Quote from: Meukowi on April 23, 2020, 08:44:41 AM
so.. to the jets, does Mikuni N102/221

sound familiar? no clue what the proper size is
edit. googled it up, so for everyone else N100.604 series is the factory size, but also N102.221 will fit, just a little smaller head on it

Wait - what year is your bike ?
Cool.
Buddha.

1992 slingshot

OK you need 40/150/1 washer and 3 turns - but these are large round slotted mikuni's, don't just put in small round slotted - just don't.

OK you're 134, DJ needle - yea bench mark it with the right type of 135 mikuni main jet.
Your symptoms seem rich - not lean BTW.
Opening throttle problems like poor response and wet sneeze type misfires are rich.
Steady state problems like lean surging, and misfiring on steady throttle but this will be a hard - donk - type misfire are lean.

How about you don't touch nothing - check the float level and post back.
The 89-00 carbs have floats that drift up and can cause problems in a bike that sat over winter.

Cool.
Buddha.

Quote from: The Buddha on April 23, 2020, 01:32:51 PM
OK you need 40/150/1 washer and 3 turns - but these are large round slotted mikuni's, don't just put in small round slotted - just don't.

OK you're 134, DJ needle - yea bench mark it with the right type of 135 mikuni main jet.
Your symptoms seem rich - not lean BTW.
Opening throttle problems like poor response and wet sneeze type misfires are rich.
Steady state problems like lean surging, and misfiring on steady throttle but this will be a hard - donk - type misfire are lean.

How about you don't touch nothing - check the float level and post back.
The 89-00 carbs have floats that drift up and can cause problems in a bike that sat over winter.

Cool.
Buddha.

yeah i'll chevk that up tday after work, the pipe  method, since i already measured the floats to right level with a caliper, it was running good last summer tho, or somewhat good. now it sneezes itself forwards. When i think about it now it sounds very much of rich running, even tho the dj kit tells to use 134 mains with slipon and hiflo filters, have to check if i have a washer below the needle, i bet i shouldnt have since the shape of the needle is slimmer than oem