Need help on a topic beat to death TTR125 jetting

Alright, I'll start by saying sorry.

I've searched and read, here, thumper, planetmini, everywhere.

So I'm needing more help.
Here goes.
01 ttr125 w/ 05 motor (doesn't matter)
Motor had slapping piston. Bored and replaced piston with wiseco 50.5mm piston. I did all the work per yamaha. I can't remember if it ran well after rebuild bc it has been sitting years.

It has a new fmf powercore 4 w/ spark arrestor, stock air filter with only spark arrestor removed. Bike bogged before exhaust.
Bought genuine mikuni vm24 from niche. 1.5 slide. Needle unknown size.
Came with 30,35,40 pilot and 140,145,150 mains. I bought 15-37.5 pilots.

The problem is this. At idle, no matter slow jet size, plug gets black as shit. Fat. But quickly blipping throttle it bogs (lean), removing air filter makes it worse, so lean.

It revs and runs awesome with rpm's up.
Smaller pilot cleans the plug up some, and really makes the bog no worse.
Current settings. 17.5 pilot 2 turns out, needle on second from top clip. Main 150
I've changed pilots from 30 all the way down to 15. Moved clip from top to bottom.
Main @ 150 has stayed the same size.
New plug
Carb immaculately clean.
Fresh shell 93 octane
This big was why the bike was not put in service years ago.

Niche tells me this should run fine with the pilots and mains they provided. It won't. Any tips?
Can a too big main make it flood at idle? I know supposedly no, but can it?

Things I need to double check: compression and leak down.

 

Yes, the main has some influence on idle mix, just like the pilot is contributing a small amount to the WFO mix. I'd ignore the plug read at idle, dial in the idle response from blips to get close on the pilot/idle mix, then get the main set at WFO. From there it's needle time.

That said, nearly every TTR I've seen with an aftermarket / larger carb has bogged from a dead idle, save for a couple camm'd 150 kitted examples. Even the stock carb can be a bit of a PITA right off idle. These are VERY mild motors out of the crate, going straight to WFO kills the velocity through the carb. You may need to aim for a bad idle to get good snap throttle response.

 

Thanks. My next attack is to buy 110, 120 & 130 mains. Jet it lean on slow jet and 110 on main. See if that helps. Thanks for the confirmation on the main. I've read so many posts on this. Some are as lean as 17.5 & 105 up to 40 & 160. Some have a large gap. Like 22.5 & 150. I've never had a carb foul the plug but bog lean. My hair is falling out. If I get so big on idle I'll just ride it like a smoker when I'm idling and clear it out when idling.

 

how is the intake manifold? Any cracks?

 

Not that I found. I sprayed starter fluid while running. No sucking in or idling up. I cut the nubs off and put good clamps. But I'll put that on the recheck list.

 

TTRs engines are not complex. Sounds like the pilot jet. What size needle jet are you running?

Also, are all your o-rings and gaskets present and in good shape?

I have the same carb on my 2007 TTR. I run a 17.5 pilot and 112.5 main jets with stock exhaust. I found those to be optimal after trying many sizes for both jets. Not sure how many turns out I did for the idle screw through. A 120 will kill my throttle at 3/4 up.

 

Not sure what orings you're referring to, but everything is sealed up when I checked it with starter fluid last time.

As far as jetting. It got better as far as starting and idling with a 17.5, but bogged. The needle, I don't know. I'll have to look tomorrow. It's whatever niche shipped it with. That's good to hear about your jetting. I bought a 110, 120 & 130 main yesterday to try and get in the ballpark. Gonna start back with 17.5 and 110, then go from there. Hopefully tomorrow, but gotta see how work goes. The difference in jet sizes on these motors with basic same components are crazy. Some people say theirs run good with a 35 pilot, some as low as 15. But, one has to wonder if those 35 pilot folks know how to tune a carb, bc it'll start and run good with a 35, but it fouls the plug after 5-10 minutes of idling.

 

A couple of jets -- let's just say 'copper pieces' -- have small o-rings. And a washer and spring for at least one fitting. Niche cycle is reputable and I've had no issues with their parts nor their service, so good choice there.

Your bike mods SHOULD mean that it breathes better and you would be fighting a 'too lean' condition instead of a 'too rich' and 'fouling plugs' condition. Since this is an "at idle" issue, have you checked your choke's operation and health? It's a funky apparatus with its own small plug to enrichen the fuel mix at idle. And a WFO throttle will mask the effect of a choke that is stuck on -- more fuel BUT also more air. Try either severely loosening the choke cable or yank it out of the carb orifice and plug it. Then run your bike. Sometimes, eliminating one variable at a time can help find the problem area. You'll probably have to adjust the idle screw.

Having not changed the main jet just proves the main is not the issue, obviously. But stranger things have happened (leaking in the main jet orifice, worn o-ring, too narrow of a needle).

I bet if you put your TTR back to stock, we could quickly resolve this as we'd be comparing apples to apples. But you're close to resolving your carb issue.

Last question: At WFO, does the engine run steady or do the revs sound uneven or pulsate?

 

I appreciate all the input. The only O'rings I see that aren't there from the exploded view are on the air screw and idle screw. This is a new carb from niche and that is how it was shipped to me. I noticed on the VM that it does not have an oring on the main jet like the stock unit does. But the exploded parts breakdown doesn't show one either, so it's right. Let me preface with, being a mechanic I know mechanical stuff does funky shit even when you think it's good, the choke functions (opens/closes) properly when off the bike during inspections. You can see it slide open and closed, but I guess the circuit itself could be screwy?
That being said, Ill put removing choke in the pile of things to check.
At WFO I can't say bc it's been years since I messed with it (but the carb is still brand new with meticulous cleaning and new gas with inline filter), but next time I get to run it I'll run it out the drive and see how that acts.
I do know that if you move the needle clip down(fatter), open throttle to 1/4 ish, then rev it, it puffs black as it picks up on the needle. So the needle clip all the way fat does look too rich.

Here's what I'm gonna do bc I was getting frustrated with it.
I'm going to get a baseline again.
Check compression and leak down on cylinder
set float again
start with pilot at 17.5/2 turns out
main on 110
needle in top clip

I'd rather fix a lean plug condition than a fouled plug condition. Just how my brain works.
I also have a new factory yamaha air filter for it.

I will let you all know how I fix it. IT WILL GET FIXED! lol

 

But the niche cycles exploded view does NOT have the orings I spoke of in above post. So the niche carb is correct as far as orings and washers per their parts exploded view.

http://www.nichecycle.com/ncs/parts.../vm24-512-parts-list-vm24-512-parts-list.html

 

I didn't see mention of an airbox mod? I recall, reading on the internet, that you want to open that up with a few drilled 1" holes or something. You can google it. In the minimoto section on here, someone posted a pretty extensive minimoto setup. Not recalling if it was for an XR100 or TTR125. But it was a good read.

 

You'll figure it out though. I bet that when you find the problem, it will be something that you looked at before or opted to skip over thinking it was too small to make a difference.

 

Old post but just picked up one of these myself. First thing is get a decent filter, remove the snorkel from the airbox. For a quick test just remove the side over off it. Those things are starved for air. I ended up making my own version of what bbr sells for these things as it flows a bit more and uses an off the shelf filter. Easy to do when 3D printing something and not having to have them manufactured like bbr. If I didn’t have access to the resources I do I’d have bought the bbr setup.

 

OP, did you ever figure this out? Sounds like you and I have/had similar issues (buying the Niche VM24 and having an extremely rich condition at startup). I *assumed* it was from too large of a main/pilot as they ship with a 150/40 and like you, everything I see around the webs says a 110/20 is more the ballpark we need to be in.

Anyhow, I'm going to swap out the mains (I don't know if I have any other pilots, but mine bogs like shit upon startup) and see what happens.

http://forums.13x.com/index.php?threads/ttr-125-problems.364658/

 

I did not. This got put on hold. I bought pilots all the way down to 15's. Went from 15 (won't start), 17.5 (starts good but lean pop/backfire), went all the way up to 32.5 using 2.5 increments and it only seemed to get richer idling, but the pop turned to a rich bog. It seemed the best at 22.5. I took the stock filter out, no change. I was in the process of rechecking timing after I checked compression when the timing chain adjuster bolt stripped. Gotta helicoil that but I'm currently out of the country. I have the bike completely apart with motor out. Gonna check my build work, head, valves, valve springs for any discrepancies. Then reassemble and try again.
It idles so rich that there is fuel left behind the carb in the intake tube when I remove my to change jets.
I think the niche come with the recommended 1.5 (going off memory not sure) needle. We're not the only ones with this problem and I contacted niche years ago right after I bought it from them about the issue. They said, blah blah blah, it should run right. Bull shit, it doesn't. I've check for vacuum leaks using starter fluid, I've covered the filter, removed the filter, many new plugs, checked compression.

One thing I wasn't sure I had accurate was float height, but that will be done before I start again, also I never blocked of the choke assembly to see if it's malfunctioning.
Fyi, my motor is stock head with .5 over wiseco piston. Stock filter with fmf full exhaust. If I lean the main, it won't run on top. It runs out well on top with the 150. I think I bought a 110, 120, 130 main and they would not run on top. I think a small pilot with the 150 main might get you closer. I tried almost every combination of pilot, main, needle position I had. Pulled almost all my hair out.

 

If you find a combo, please post so I can try it.

Thanks

 

Ugh - reading your post was discouraging because you've got more mechanical ability in your pinkie than I've got in my entire being.

Anyhow, decided to give it a go this morning - the main culprit that it *could* be is how I installed the throttle cable, I thinks (or at least that's probably the issue with the stumbling). Going to change out the main to something smaller and maybe set the float to be intentionally "lean" - just looking at the size difference between this carb and the stocker, this one is much larger. I wonder if setting it intentionally lean may help even things out

 

I'm not understanding your throttle cable pictures but that's okay that I don't. Look at an exploded parts view of the carb and route the cable and place the gold plate in the positions they should be.

Just bc it's bigger doesn't mean you lean it. Think this way, size of carb doesn't matter (if it's in the ballpark). This engine will only suck so much air. That amount of air determines how much fuel gets pulled from your tank through your carb in to engine. You have to match the jet sizes of the carb to match what YOUR engine flows in air. Too big of jets, you saturate your air/fuel mixture. Too small jets and the af ratio is too lean. Your motor flows the same amount of air no matter the jet size. Jet size determines how much fuel the engine air flow (piston pulling in air) allows in to the mix. The carb is just a metering block. Your motor determines how much air and fuel it need. If it's fat/rich, it just means that your jet is too big allowing too much fuel in to the mix. I'm repeating myself.

Jist is. Motor pulls air, a set amount based on the build of it, i.e. bore size, valve size, head flow, etc. It pulls that air whether the carb is there or not. Carb is just a metering system of fuel. It's hard to explain in typing. I hope that makes sense.

 

I officially gave up.

Will drop it off at a local carb specialist this week and have him cleanup my mess.

 

Dang. Probably need to take him the bike so he can make the engine run with it.