At My Wits End - 2002 RM125 Stutter

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9/23/2017 8:02 AM

So I'm working on my co-workers 2002 RM125 that he picked up last summer for $500 - down the molehill we go.

He asked me to fix it up on his dime, so I did all the normal stuff to the chassis, new tubes, replaced some spokes, new cables, repacked the silencer...blah blah etc.

Now the problem - The engine has a mean stutter when riding it when it tries to come on the powerband - wont clean out and almost sounds like a misfire. Will clean out an run well when warm on the stand.

Here's what I've done and checked: Nothing has worked. (FSM = factory service manual)

-Head re-cut back to stock specs by Williams Moto Werx
-New piston ring
-Piston has 2~hrs on it
-Cylinder measured in spec (End of spec it has a steel sleeve) on a CMM
-Cleaned PV
-New Boyesen Reeds
-Cleaned carb
-Jetted per FMF spec
-Reset float height per FSM
-New Plug
-Verified PV is functioning
-Re-set PV spring tension per FSM
-New Airfilter
-Brand new FMF Factory Fatty
-Verified Electrical components (Coil, stator, kill switch) with multimeter and FSM

I'm leaning towards a bad CDI box, or maybe slight chance of left side crank seal leak (All thats left to check lol).

Anyone have any insights, sanity check, or a 2001-2007 RM125 CDI box laying around I can borrow to check bikes performance? Or that they want to sell?

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9/23/2017 8:14 AM

At first I was going to say PV but it sounds like you've got that covered. Now I'm definitely leaning towards electrical. You've already thrown enough of your buddies money at it, might as well get this one for $50 to test it. Can always resell it if you don't need it http://www.ebay.com/itm/2001-Suzuki-Rm125-Ecu-Computer-Controller-Unit-Black-Box-Ecm-Cdi-/182780983396?fits=Year%3A2001%7CMake%3ASuzuki%7CModel%3ARM125&hash=item2a8e987864:g:g~IAAOSwC11ZxBKR&vxp=mtr

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Positively, absolutely 110% obsessed with anything MOTO.

9/24/2017 4:49 PM

Does it smoke when it comes on the pipe? A transmission side crank seal will do that.

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Tomac and/or Anderson for 2020.....

9/25/2017 6:08 PM

I'd double check that the powervalve isn't catching on the steel liner. I fixed a CR250 for someone recently with the same complaint, just ground the liner back to allow the powervalve to clear it and fully open.

If you need a CDI, I have a spare one from a 2005 (I fitted a factory ignition). Will need to work out getting it over to the States as it's back in the UK, so will take a few weeks.

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9/26/2017 5:46 AM

When I had it apart the PV seemed fine, I triple checked it and it it was nice and free after I cleaned it. I'll think about the CDI, thanks for the offer though! I'm hurrying on this bike to get it done before the snow flies (Gah!)

Bob - my only experience with a bad right side crank seal was that it would smoke a ton all the time on my old YZ250. It does smoke when it tries to come on the pipe, I have one more leaner main jet coming, if that doesnt work I will have to figure out a leak down test.

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10/1/2017 5:59 PM

Took her out again yesterday - even more confused.

Except 1st/2nd it runs great. 3rd-6th on the road it runs perfect. Came home, gapped a new plug, same thing. The kicker is the electronics on this bike are dumb enough it doesn't know what gear its in...

I'm going to replace crank seals on a whim and see if that does anything, otherwise I'm stumped.

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10/1/2017 6:33 PM

I had a heck of a time figuring out why my son's YZ125 would run great for a short while, then would bog, stutter, and cause great gnashing of the teeth. A home-made leakdown tester and some soapy water revealed what nothing else would- a very minor leak at the reed block.

The other issue you may run into is with the TMX carb. There's a tiny gasket (about 3/8" square) that goes between the carb body and where the slide bottoms out. There are two screws under the float that allow you to remove the piece; I think it's called the needle jet holder, but it doesn't hold anything. If that gasket gets old and cracks- from today's terrible ethanol fuels- it's not available from Mikuni, and it'll never run just right. Unless someone starts making that gasket, it's carb replacement time.

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10/2/2017 9:18 AM

I'll build a homemade leak tester and try that too.

Interesting comment on the carb, only other person I found who had this similar issue went through the same stuff I did, replaced the carb and that fixed it. I'll tear it back off and check that too

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10/2/2017 9:32 AM

Either use a untra sound cleaner or a very good carb soak. If the bike set for a good period with old gas in it , there will be clear deposits in it that you may or mat not get out of the very small passages in the carb.

Paw Paw

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10/2/2017 10:52 AM

Matt Fisher wrote:

I had a heck of a time figuring out why my son's YZ125 would run great for a short while, then would bog, stutter, and cause great gnashing of the teeth. A home-made leakdown tester and some soapy water revealed what nothing else would- a very minor leak at the reed block.

The other issue you may run into is with the TMX carb. There's a tiny gasket (about 3/8" square) that goes between the carb body and where the slide bottoms out. There are two screws under the float that allow you to remove the piece; I think it's called the needle jet holder, but it doesn't hold anything. If that gasket gets old and cracks- from today's terrible ethanol fuels- it's not available from Mikuni, and it'll never run just right. Unless someone starts making that gasket, it's carb replacement time.

The metering block gasket leaked on my YZ125 as well.....had a heck of a time trying to get it jetted. Sent the carb off to Ron Black (RB Designs) and he worked his carb mod and noted in his findings that the metering block gasket was leaking. He replaced the gasket and modded the carb..... Solid work from RB Designs.

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10/2/2017 11:09 AM

Hmm this is all starting to make sense. For all I know this bike could have sat for a long time.

FWIW here's a link to another thread (With Video), guy has same issue and replacing carb solved it. https://thumpertalk.com/forums/topic/945491-2003-rm-125-hesitation-video/

I will shoot Ron an email asking about this metering block gasket.

Thanks!!

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10/2/2017 12:37 PM

Well, RB is closed until the end of October, and I have searched high and low, and you guys were right - this gasket does not exist.

Now the question is...Do I pull the trigger on an eBay carb.

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10/2/2017 3:11 PM

Matt Fisher wrote:

I had a heck of a time figuring out why my son's YZ125 would run great for a short while, then would bog, stutter, and cause great gnashing of the teeth. A home-made leakdown tester and some soapy water revealed what nothing else would- a very minor leak at the reed block.

The other issue you may run into is with the TMX carb. There's a tiny gasket (about 3/8" square) that goes between the carb body and where the slide bottoms out. There are two screws under the float that allow you to remove the piece; I think it's called the needle jet holder, but it doesn't hold anything. If that gasket gets old and cracks- from today's terrible ethanol fuels- it's not available from Mikuni, and it'll never run just right. Unless someone starts making that gasket, it's carb replacement time.

Everything that you are describing in your original post can be caused by a leaking venture gasket(Mikuni). This gasket is still available, but not where most people would be looking. It is available through Ski-Doo. They used the same Mikuni TMX-x carbs as the bikes. The gasket you are looking for is #2 on the parts fiche.

http://shop.discounthondaparts.com/fiche_section_detail.asp?section=8456941&category=Snowmobiles&make=Ski-Doo&year=2007&fveh=295107

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10/3/2017 8:28 AM

Matt Fisher wrote:

I had a heck of a time figuring out why my son's YZ125 would run great for a short while, then would bog, stutter, and cause great gnashing of the teeth. A home-made leakdown tester and some soapy water revealed what nothing else would- a very minor leak at the reed block.

The other issue you may run into is with the TMX carb. There's a tiny gasket (about 3/8" square) that goes between the carb body and where the slide bottoms out. There are two screws under the float that allow you to remove the piece; I think it's called the needle jet holder, but it doesn't hold anything. If that gasket gets old and cracks- from today's terrible ethanol fuels- it's not available from Mikuni, and it'll never run just right. Unless someone starts making that gasket, it's carb replacement time.

digger wrote:

Everything that you are describing in your original post can be caused by a leaking venture gasket(Mikuni). This gasket is still available, but not where most people would be looking. It is available through Ski-Doo. They used the same Mikuni TMX-x carbs as the bikes. The gasket you are looking for is #2 on the parts fiche.

http://shop.discounthondaparts.com/fiche_section_detail.asp?section=8456941&category=Snowmobiles&make=Ski-Doo&year=2007&fveh=295107

digger-
You.
Are.
Awesome.

Thank you!

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10/3/2017 9:21 AM

Matt Fisher wrote:

digger-
You.
Are.
Awesome.

Thank you!

You aint kidding!

Got the gasket on order at the local Ski Doo shop, should be here by the end of the week and I will post how it goes.

Digger - I'm originally from central NY (Finger lakes-ish), next time I'm in the area I owe you a beer.

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10/3/2017 10:11 AM

Good luck.Hopefully that does the trick. If not, I have family visiting at the end of the month and I can get them to bring that CDI over for you to try.

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10/3/2017 4:03 PM

I think the ignition is different from 2002 vs 05 with the power jet carb

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10/4/2017 2:57 PM

digger wrote:

Everything that you are describing in your original post can be caused by a leaking venture gasket(Mikuni). This gasket is still available, but not where most people would be looking. It is available through Ski-Doo. They used the same Mikuni TMX-x carbs as the bikes. The gasket you are looking for is #2 on the parts fiche.

http://shop.discounthondaparts.com/fiche_section_detail.asp?section=8456941&category=Snowmobiles&make=Ski-Doo&year=2007&fveh=295107

Matt Fisher wrote:

digger-
You.
Are.
Awesome.

Thank you!

Markopolo400 wrote:

You aint kidding!

Got the gasket on order at the local Ski Doo shop, should be here by the end of the week and I will post how it goes.

Digger - I'm originally from central NY (Finger lakes-ish), next time I'm in the area I owe you a beer.

I'm glad that you guys found that info helpful.

Marko, I hope that solves the problems you are having with your co-workers bike. I feel your frustration. BTW, I live on the eastern side of NY, just above Albany, but am very familiar with central NY. I have been racing HS in WNYOA for over 20 years, and love it out there.

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10/5/2017 5:16 PM

Well, got the secret gasket, and installed it. On the stand, the bike runs PERFECT. Super crisp and clean.

But it is still stuttering just a hair...I'm baffled at this point. Not sure what to do honestly. Stupid RM!

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10/6/2017 1:31 PM

My wife's 04 RM125 came with FMF jetting and it was horrible, went to the jetting specs pro circuit recommend and it made a heap of difference.

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10/6/2017 2:13 PM

Ignition timing? Ive had old bikes have small hairline cracks in airboots. Have you had the clutch side cover off and made sure the gear that opperates the pv is functioning?

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10/6/2017 5:13 PM

I've been reading your progress on this project.

1. While riding and stuttering is happening can you slightly pull choke open and see if it changes or stops.
2. While riding and stuttering is happening can you put two rags over air filter and see if it changes or stops.

I am trying to figure out if you're lean or rich, or is the piston/spark plug is causing this issue. I need test results first.
Lastly, pull plug cap off spark plug and set in on the plug lightly, but enough so it won't fall off when you test it, this will slightly change the spark and might give you another clue.

I've seen the stutter when someone accidentally used a piston that was 2mm lower from TDC then the correct one.
You checked the piston to head clearance I suppose after this head was cut?

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10/7/2017 8:58 AM

I had the cover off the PV on the side of the cylinder and made sure the PV was opening while the bike was running, I;m going to build a home made leak tested hopefully this weekend and see if that can tell me anything.

Speed_racer, I opened the choke once while testing it, bike bogged right down like it was super rich, I did not double check the squish after the head was cut, but I could do that tomorrow as well.

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