Two Stroke Motocross

Two Stroke Motocross Forum => Technical => Topic started by: Sapper on June 03, 2012, 03:24:17 PM

Title: carb sputter...at a loss here.
Post by: Sapper on June 03, 2012, 03:24:17 PM: Since we bought 03 KX125 2 years ago. I've been fighting a bad sputter on the bottom. If it's on the pipe it runs hard and doesn't miss a beat. It has a hanging idle sometimes and it's is not an air leak. I've narrowed this down to the cable itself going into the top of the carb. If I move it, the idle drops back down. This bike has been rebuilt at least 4 times with new seals everytime si I'm sure it's not an air leak. I even did the carb cleaner spray down multiple times with no change.

Last year I had it fixed by moving the clip to the top position. Now, We just put it back together with a new topend and it's worse than ever. I changed all jets every which way...up, down & every in between.

The kicker is, when I pull the choke back up, it doesn't kill the motor or even bog it. The idle will get higher. But it runs just the same as it is off. I've been through this carb twice, cleaning blowing it out, etc and gonna do it again tonight. The choke doesn't stick up anymore. It's wornout but does seat completly.

Any ideas from anyone??
Title: carb sputter...at a loss here.
Post by: eprovenzano on June 03, 2012, 05:25:53 PM: My 1st thought is jetting. What needle and clip position are you running?
Title: carb sputter...at a loss here.
Post by: Sapper on June 03, 2012, 05:55:28 PM: I need to look at needle but it's stock. Top position. If I go lower, it gets worse.

Right now for jetting, I have 35 pilot, 400 main.
Title: carb sputter...at a loss here.
Post by: GlennC on June 03, 2012, 06:35:33 PM: Here are the FMF specs. May be a good place to stert depending on your altitude....

Jetting Results
Main Jet
410
Pilot Jet
37.5
Needle
STOCK
Needle Position
2ND
Power Jet
STOCK
Air Screw
1.5
Fuel Screw
N/A
Comments
Title: carb sputter...at a loss here.
Post by: eprovenzano on June 03, 2012, 08:18:07 PM: Have you tried to adjust the air screw? I know with my katoom, as the weather changes, I need to adjust the air screw to get it to idle properly, and remove any bogging.
Title: carb sputter...at a loss here.
Post by: Sapper on June 03, 2012, 09:59:12 PM: Air screw changes very little and runs best at about 1-7/8 out.

Glenn, I started with that jetting when we first put on the FMF pipe. I had to lean it out a little. It was running a little rich and spooging a little. Once I got it cleaned up and fought the sputter and moved the clip up, it ran great and zero sputter. Like I said above, it's worse than ever and moving the clip and and down makes a little difference and it's all the way up. If I do get another needle, Which one, there's 8 optional needles from OEM. I need to check the manual for which one is stock then research which way is leaner.

Why can't they use a Holley?!!! It'd be a whole easier.
Title: carb sputter...at a loss here.
Post by: GlennC on June 03, 2012, 10:33:54 PM: I hear ya, but a tired top end requires leaner jetting, when you put in a new piston and rings you probably need to richen it back up.
Title: carb sputter...at a loss here.
Post by: Sapper on June 04, 2012, 12:39:22 PM: If I drop the clip or raise the Pilot size, it gets worse. The main is fine at 400. It's gotta be in that choke circuit or something. I wish I had another carb to swap out and try.
I'd like to do a Keihen swap but that's more $$$ than it's worth for a stock setup.
Title: carb sputter...at a loss here.
Post by: VintageBlueSmoke on June 04, 2012, 02:50:12 PM: I hate to say it but it seems like you are chasing ghosts. Sometimes all your changes add up to the same sum but a different answer.

I would personally start over with the FMF setting - and forget about what you know about your area and bike. Then make small changes until you get it dialed in.
Title: carb sputter...at a loss here.
Post by: SachsGS on June 04, 2012, 03:22:42 PM: Check your float level and needle and seat assembly for wear.Makes sure your choke is bottoming properly and that the rubber seal at the base of the choke plunger is not worn or damaged.
Title: carb sputter...at a loss here.
Post by: Sapper on June 04, 2012, 03:37:20 PM: Some things I double checked last night with it still being on the bike. (Was working on another home project too). Choke bottoms out all the way because as I thread the plunger back down in, the knob/shaft comes up. Did another spray test, No air leaks. Choke still will not shut down engine.

I need to check float level but I don't have the right tool to do it with. That little measuring deal that threads in the bowl. Is there another way? I may try bending up the tab a little to see what happens.
Title: carb sputter...at a loss here.
Post by: SachsGS on June 04, 2012, 03:42:35 PM: Flip the carb upside down.The tab that supports the floats should be parallel to the float bowl base.Make sure that the spring loaded pin in the float needle is free and not stuck.

The fact that the position of the choke has no effect on the engine running would have me check that circuit very carefully.
Title: carb sputter...at a loss here.
Post by: Sapper on June 04, 2012, 03:49:15 PM: Yep, gonna pull it off tonight after I paint the bathroom

But I found this. http://forums.mxtrax.co.uk/showthread.php?282219-kx-125-slightly-boggy

I have a manual too.
Just didn't look at it that well. Being an 03 and having alot of time on it, I'm probably gonna need a new needle seat. This sucks cause I just put an order into Motosport about an hour ago for some parts.
Title: carb sputter...at a loss here.
Post by: SachsGS on June 04, 2012, 03:58:11 PM: Sometimes if I'm chasing a phantom carb problem I'll lean the affected circuits right out and then slowly enrichen them step by step.This way you will often stumble across a seemingly unrelated problem that turns out to be the root cause.I've pinpointed defective carb bodies this way.
Title: carb sputter...at a loss here.
Post by: Sapper on June 05, 2012, 12:42:01 PM: OK, here's what I figured out last night. The float level according to the manuals setting was 10mm too deep. So I adjusted it according to the book. It didn't even allow fuel in. I was kicking for a min or 2, pulled sediment plug and it was dry. Went back, double checked measurement.

So then, I noticed that when the carb is upside down, the weight of the floats will compress the little pin in the needleseat. Not sure if that's supposed to happen. Weak spring in there??

So I easily tilted carb to the side, just enough for pressure to relieve off of little pin. set that to 9.5mm. Hooked up fuel line with carb level and got some fuel to feed bowl (I kept sediment plug out to see if fuel came out).

My dilemma here is, if I have float level too lean, I stand a good chance of leaning it out and melting my new topend. Too rich I just get the bog again. At least it's well lubricated.

The only other option I have is to try and track down, buy....whatever... the actual gauge used to set them.
This is Killing me. I want a Holley, DAMMIT!.

Update:
I found out after extensive interenet research, the small pin on the needel seat cannot be depressed. You actually have to hold carb at an angle to where the floats are slightly released from pin but tang is still touching. So this means I'm about 2mm off. So, I'll be back at tonite after we get get back from a practice on the 250.
Title: carb sputter...at a loss here.
Post by: TMKIWI on June 06, 2012, 04:08:10 AM: Don't hold the carb upside down when cheacking float level. If the needle has a spring in it the weight of the float will depress it. ( just like you found).
Hold the carb at 90 deg to how it normally sits with the float pivot at the top. Slowly rotate the carb upside down ( only a couple of deg ) till the float touches the needle. That will give you an actual reading.
As a rule of thumb most floats are set level with the carb along the seam where the bowl sits. Normally the top of the float is flat and that is what you should be looking at. Not the rounded bottom.
Hope you get it sorted.
Title: carb sputter...at a loss here.
Post by: Sapper on June 06, 2012, 12:35:57 PM: TMKIWI, Thanks for for clarifying it. I wasn't sure. I haven't gotten it completely set yet. I got it to where it'll get fuel but runs out. So it'll come off again tonite for more R&D. What really ticks me off is the sputter is still there.

Then the kicker....Another guy showed up at the track, my son was practicing at, last night with a ...03 KX125...EXACT same problem. But he just bought the bike so he has no clue what jets are in it, etc.

After doing alot of internet searches, this is very common with the 03. Everyone talks about it and everyone recommends different solutions, but, NO ONE posts up what they did when / if they got it fixed.
Title: carb sputter...at a loss here.
Post by: SachsGS on June 06, 2012, 03:17:36 PM: Hold the carb upside down and sight the needle.The float assy. should be parallel to the float bowl contact surface prior to compressing the float needle pin.When the float needle pin is compressed the floats should NOT contact anything in the carb.As carbs wear,and Keihins are the worst,you have to hold the carb upside down to make sure you are removing all of the wear related slop in the float/float pin/float needle and seat assy otherwise you won't "see it".

On Keihin carbs the pin upon which the floats pivot is a sliding fit and will simply fall out if held in the 90 degree position.
Title: carb sputter...at a loss here.
Post by: TMKIWI on June 07, 2012, 07:38:31 AM: The other 90 deg sachs.
Not side ways.
Title: carb sputter...at a loss here.
Post by: SachsGS on June 07, 2012, 02:46:41 PM: Since you guys are all upside down,your upside down is my right side up and we are all talking about the same thing,correct?
Title: carb sputter...at a loss here.
Post by: TMKIWI on June 08, 2012, 06:21:40 AM: Yep sure are.
Title: carb sputter...at a loss here.
Post by: ford832 on June 08, 2012, 01:48:43 PM: Quote from: TMKIWI on June 06, 2012, 11:08:10 AM
Don't hold the carb upside down when cheacking float level. If the needle has a spring in it the weight of the float will depress it. ( just like you found).
Hold the carb at 90 deg to how it normally sits with the float pivot at the top. Slowly rotate the carb upside down ( only a couple of deg ) till the float touches the needle. That will give you an actual reading.
As a rule of thumb most floats are set level with the carb along the seam where the bowl sits. Normally the top of the float is flat and that is what you should be looking at. Not the rounded bottom.
Hope you get it sorted.

I do mine like this but always set the float a bit higher than level.This won't effect jetting but will stop a lot of your fuel from going on the ground from the overflow(mileage and range is everything when you're trying to make it to the end of a race without refueling ) or from wanting to leak out when you've got it leaned against a tree.Over time it changes anyway so it's good to keep an eye on it.
Title: carb sputter...at a loss here.
Post by: Sapper on June 10, 2012, 01:27:13 PM: UHHHGGGGGG!!!!!

latest update:
Set everything Friday night. The best I can get it to run with just a very slight sputter, no go lean and just totally rip when on the pipe, the floats were set to 8mm. Pulled it up in the trailer, put it up on the stand, checked it all over. Looked great, ran great. Came out Sat. morning...there's a nice big wet spot on my blacktop driveway. A half tank of fuel worth that ate right into it.

Dropped the ramp and it hit me. That beautiful smell of race fuel. But not how I wanted to smell it...soaked into my trailer floor.

So, here's heres what I've encountered so far:
9.5mm-no fuel, 9mm- very little fuel, 8.5-runs lean/out of fuel, 8mm runs good = leaks fuel.

So now what??
Title: carb sputter...at a loss here.
Post by: VintageBlueSmoke on June 10, 2012, 02:08:00 PM: Not trying to be a smart ass but...

That is what the petcock on the tank is for.

Sorry. I'll shut up now.
Title: carb sputter...at a loss here.
Post by: SachsGS on June 10, 2012, 02:40:09 PM: Is your needle and seat assy. o.k.?
Title: carb sputter...at a loss here.
Post by: teampryor on June 10, 2012, 02:59:51 PM: Our experience with superminis and 125s that a bike that was previously jetted correctly its usually worn/frayed reeds that causes sputter. Have you inspected the reeds for wear or chipped or seperated edges?
We find the need to replace our v-force reed petals around 20 hours. The corners chip and the edges swell then do not seal as well resulting in the bike sputtering.
Hope this helps. Good luck
Title: carb sputter...at a loss here.
Post by: Sapper on June 10, 2012, 07:16:38 PM: Vintage, You...a smartass??? Noooo. LOL
I've never, ever had to shut off the fuel with a bike just sitting there. Now when it's being transported, yes. But even then I only lose just a few drops in the trailer, truck, etc on any of my bikes if we forget to shut off the fuel.

Sachs, yes, I checked it very closely.

TP, We ave V-force as well and I checked the reeds as we were putting it back together. They looked new and still fit tight to the box. Either way, I still need to get these floats right now that I moved them around. I think I'm gonna fab up my own level gauge from the bowl. It's supposed to be 10mm above the bowl mating surface.
Title: carb sputter...at a loss here.
Post by: SachsGS on June 11, 2012, 12:50:44 AM: Somehow your needle and seat assy. is leaking,either it is worn,dirty,defective seat O-ring,leaking floats or subject to an incorrect float height but you should not have "a half a tank of fuel on the floor".