too jumpy in first ~3% of throttle openi

my bike is gets very unsettled with the super small throttle opening, say when i am trying to open the throttle through a corner.

things i think i could adjust:

ignition timing (fully programmable)
jetting (pilot jet? air screw ? )

i am wondering is it getting too much fuel on the pilot jet? could too rich of a pilot jet cause this? what about too lean?

it’s making it difficult for me to be smooth through and out of corners...

i know two strokes are inherently surgey on the pipe, but i’m talking about even off the pipe it’s very unsettling


here’s an unrelated sort of related video where you might be able to hear it but mainly you hear me save a crash

 

here is a full lap that kinda shows it? more seat time would definitely help...

 

My WR'300'ZD is similar, that initial throttle turn just lifting the slide off the stop is surgy-sputtery hell. Maint throttle on one sweeper in particular at a local track is nerve wracking at times. I haven't figured out how to clean it up yet, I'm thinking with a YZ instead of WR gearbox I could be higher in the revs there which would help?

 

Yes to all the above, The question I would ask is what have you changed since the last time you rode it? I would start there, also do you keep notes of air temp etc, if you last rode it when it was hot out, and now it is much cooler your running lean.

 

tmx38 carburetor

i went from a 17.5 pilot to a 20 because my 17.5 was corroded almost completely clogged and i didn’t have a spare nor time to soak it clean

i went to a leaner needle but that hasn’t changed since i rode it last... the big change since i last rode it was i last rode end of summer... and it was 95 degrees out and now it was just 70.


i just started keeping a log book and i have obtained a relative air density meter and i take it with me to the dyno and the track to log everything, but i do not know how to take most advantage of it just yet. i know if the air is less dense i need less jetting.. that’s really it


going by what you’re saying...
it sounds like it’s too lean on the bottom just from the temperature change.

 

Install a dual egt guage.People were lean siezing in the cool weather at Roebling,due to the cool dry morning temps,and last years summer jetting.mine went lean, same symptoms as yours....egts saved me.

 

It could be to lean across the board, I would also run an EGT on each of your cylinders, also you can do a plug chop as well. But if you had it jetted right for 95 degrees and now it is 70 I would say you are probably on the right track, the 20 pilot could be causing it to be fat on the bottom and lean up top which would make it pretty jerky.


Depending on when you ride next, I would put everything back how you had it, and go up a size or two on the main and see what happens. This is probably the only time you will ever hear this but it is better to be to fat starting out than too lean.

 

I thought I knew a little bit about setting up carbs, then I read this:<https://www.google.com/url?sa=t&rct...omplete.doc&usg=AOvVaw3lUBTQxK_-OKcN3mkCowVS>

I plan on doing this to my bike to hopefully smooth the transition from closed throttle to WOT as I open the throttle to get the drive out of the corner. It *does* sound a bit time consuming, but if it helps get me out of the corner better, and gives much better throttle response, it *HAS* to be worth the time.

Another observation from the video: It seems that you were not ringing the bike out before shifting. May get better laptimes if you were. If I am off base on this, most humble apologies.

The condition you describe does make it sound, to me at least, like it is lean on the bottom end.

Good luck, hope you get it sorted.

 

so this is on my 125, not on the 250. the 250 just got a fresh top end, and is in the process of being put back together with other parts that were needing replacing.

the 125 has a full datalogger on it. i have exhaust gas temperature, cylinder head temperature, coolant temperature , gps, wheelspeed, angle, the whole nine yards except for wideband o2 ( maybe i'll add that this year )

i don't think i am getting on a straightaway long enough to see what the EGT peak will be. 4th gear in this transmission pulls look like this the day before I was at the track this past sunday



if you're familiar with dataloggers you'll know what everything is, but if not, basically the EGT peak at the end of a dyno pull is 386 C (converts to 727 F) at the end of a 4th gear pull. EGT and CHT are trending the same way, so I dont think i have a detonation or preignition issue

here is what the logs look like of the highest EGT i see over a fast lap (fast for me) at bushnell last year ( when the tip in was much better )



peak EGT 376 C ( 709 F )

fastforward to 7:20 for that lap from last year :::





My concern for EGT's is not to go over 600 C / 1100 F , but i am getting nowhere near that with the bike right now.

However if you listen to how the tip in is, listen to the new video again in the first post, and you dont hear the bike fourstroke hardly like at all when i come out of corners down low, and that alone says it is much much leaner than before because it aint four stroking at all like it used to.

 

Assuming your tuning for track/race conditions and want healthy HP, not conservative rich jetting. I’d say you’re way too rich. I’d also highly recommend you get your bike on a dyno and tune it properly to find out where it wants to perform in a controlled environment with empirical test data.

Based upon my own experience tuning a Honda RS125 with Roland Cushway and on MCE racing dyno w/Kevin Murray I set my carb setting to result in the equivalent EGT = RPM/10 in Fahrenheit degrees

More or less like this.
RPM = EGT
5,000 = 500
6,000 = 600
7,000 = 700
8,000 = 800
9,000 = 900
10,000 = 1000
11,000 = 1100
12,000 = 1200 degrees

You would never want to go beyond 1250Degrees F, due to it being too lean to make good HP. This is also assuming your EGT gauge is 6” from the piston where it is recommended by MyChron. If you are using a SpeedTune system or some other variant, start with their recommendations and tune from there.

My 0.02 worth

 

I did about 30 pulls on a dynojet dyno last saturday, including swapping out the previuos intake manifold and reeds setup for a vforce reeds setup with a 3d printed nylon intake manifold that i printed up the night before.... ( it made 1.5 hp more )

However i didn't realize i had a cracked header until i got back home and unloaded the bike.

EGT probe is 150mm from the piston skirt, per aim / mychron instructions

spark plug is white, and my dad's identical bike is running a 420 main jet while i'm running a 340... i dont think i'm way too rich. maybe a little too rich but i don't think way too rich, at least not on the main jet. ..

peter: what gear are you dynoing in? what gear ratio? for me 6th gear is 1:1 so i am thinking about dynoing in that instead of 4th gear going forward, which is like 1.286

 

I honestly have no idea what gear the dyno runs were in. I’d guess 4 or 5th but I’m not really sure.

A crack in your expansion chamber can throw your jetting way off. Get it fixed before doing anything else.

Peter

 

yeah, i fixed it that night. i’m working out of state for the next two months so it will be back to square 1 with everything when i return home.

 

I’d also comment that due to the rediculous simplicity of the 2 stroke carb., 3% throttle opening isn’t worth adjusting your tuning for (much). More or less, it’s going to be your air/fuel screw and throttle slides you need to adjust.

Make sure your carb slides are very well synced
PL

 

well on this bike i only have one slide, so i am sure it is synchronized with itself quite sufficiently.

I have not been able to find different slides available for this carburetor. I have the part numbers, but no longer in production I suppose. (TMX 38-27-k)

So that may leave me with the pilot jet and air screw i can adjust for 0 to 1/8" throttle tuning.

 

That's a big carb for a 125cc engine I suspect.
I just read that this carb is designed for a 250cc engine, this maybe difficult to get working correctly on that smaller displacement engine.
Info was found here:
https://www.bikebandit.com/aftermar...mikuni-tmx-38-27-k-series-carburetors/p/29440
Pat Mooney.

 

Just out of curiosity, is a Lectron a common upgrade for these bikes? I know the TMX is a very frowned upon carb in the offroad world. I just replaced my TMX on my Husky with a Lectron and it ran much better and stays in tune better through temp and atmospheric changes.

 

125s often use big carbs if they're expected to have their neck wrung. (The Honda RS125s I've poked at have 38mm PWKs for example)

If swapping carbs, your upgrade options would be:

- Keihin PWK 'Air striker' / 'Quad vent'
- Lectron
- Tech Elevated Smartcarb (Evolution of the Lectron)

Leccys / TE Smartcarbs are longer than PWKs, possibly longer than TMXs, shorter than ye olde VMs. I tried fitting my 36mm Smartcarb on a friend's RS125 but the bowl fouls the cases before you can get the carb in the boot. A different intake/boot might make it work? I dunno how Leccys compare on that front.

 

Here is the intake setup that I 3d printed for the bike to match up with the vforce reeds. Perhaps some new printed parts would be needed but that wouldnt be difficult.













I'm pretty interested in the smart carb after reading peoples reviews. i wonder how it would do in a road race application... seize on decel ? LOL

 

I've got one on my 1992 WR250ZD motard, did fine at Loudon other than the cylinder casting cracking on the outside. (Had it looked over, suspicion is the crack had been forming for years, casting flaw.) That said, I have the same issues trying to dial in maint throttle / initial slide lift as the OP. I need to find a two stroke guy to work with dialing it in. When I've used the bike in the dirt the carb worked brilliantly. Never had to think about it, never loaded up or fouled a plug, never needed to be 'cleaned out' before it'd rip, just kick and GOOOOOO.