'05 RM125: off-idle tuning

This is my old Suzuki, which I purchased last spring as a sort of quarantine project/fun bike to ride around my dad's farm. I used to ride as a kid about 17 years ago, and am just now getting back to riding off road. So far I've done easy maintenance stuff: chain adjustment, radiator repair, wheel bearings etc. The motor seems to pull pretty strong in the mid range, though it doesn't seem to make additional power way on top of the rev range like my old Yamahas did - I assume this is just how the engine was tuned from the factory. The bike also idles well, but sometimes bogs or coughs a little when you first crack the throttle. Which part of the carb should I look to adjust for crisp throttle response down low (like with the clutch pulled in before I'm about to start rolling) ? Is this an air screw issue? I've considered cheating a little and just raising the idle speed, but first I should try and do things the right way. The carb is the factory Mikuni TMX38SS. I'm excited to get the carb where it needs to be, repack the overly loud silencer, and then work on the bike's most glaring issue: my riding abilities. 

I don't have any specific experience with that bike aside from a short stint on an MX track, but most of the 125s I've ridden do not run particularly cleanly at small throttle openings. When they are tuned to run well (as a race bike, on the pipe) they usually are rich on tip in and will "four stroke" if you hold the engine there. If it bogs (the hollow waaahhhh sound) then it is probably going lean, which you really don't want.

I think the one I rode was an '06? It was a top end only kind of bike. No bottom end, so I kept it up on the pipe the whole time. It was also kind of worn out from being a race bike for 10 years, but even so it was a fun bike to ride.

Step one on an older bike for me is always to clean clean the carb out really well, check what jets and needle/needle position are, and set the float height. Also make sure the cap over the slide is actually sealed - if it leaks, it shows up like a vacuum leak and can cause some weird stuff. Check to make sure that the side isn't worn or scored - if it is, it can cause an inconsistent leak path around the slide. I always set them up with stock settings to start with for a baseline.

It's a 125 2 stroke - how does compression feel? If you pull the pipe and look at the piston skirt, how does that look? Pistons don't live long in these if ridden hard, and low compression can show up as carburetion problems.

Repacking the muffler may also change how it runs, so you may want to do that before doing any tuning. Also, make sure the pipe seals to the exhaust port properly and isn't leaking,

Once you know you've got a solid baseline, that specific part of carb operation is pilot jet, slide cutaway, air screw, and a little bit of the needle. If you make a change, ride a bit before changing again. 2 strokes can take a bit to clean out and don't always react quickly when you're tuning the bottom end. My suggestion would be to either go down half a step or step on the needle (clip position, raising the needle) or go in a quarter turn or half turn on the air screw and see what it does. Raising the needle may cause it to be too rich and start 4 stroking at part throttle/loading up though.

I would not suggest turning the idle speed up to mask the problem on a 2 stroke. The RM125 I rode was set up to not idle at all - you want as little air and as much vacuum (fuel/oil) getting sucked in on overrun as you can. A high idle can leave you with a dry engine if you're coasting, which is no good.

In reply to gearheadE30 :

Good advice, thanks! I was told the engine was completely rebuilt prior to my purchase, I haven't pulled it apart to verify but I can see that all the gaskets appear brand new where they stick out a bit from the jug/case. Also, I sold my '94 KX125 to buy this bike, and this one feels a lot faster/more powerful despite the KX having a brand new top end I did myself. That could be 11 years of R&D showing though. I'll have a look at the piston skirt when I repack the muffler, and go from there. Is there an easy online reference to find out what the float height should be ? Before I gave the carb a good cleaning, it would spill fuel out the overflow tube, and the bike stopped running due to the float I assume being stuck..

In reply to dannyp84 :

10 years of improvements go a long way, but coming from a KX125 tells me you know what to expect from a 125 2 stroke - you aren't jumping off an XR400 or something totally different. So the engine/compression is likely not your issue then, and a recent rebuild would hopefully rule out crank seals.

I bet you can find a manual or something for it online; I don't know how much the RM changed year to year. I'm a KTM guy myself though, so not sure about Suzuki-specific sources. I did find this, though: http://www.crservice.dk/Suzuki%20cross/RM250/07RM_HOPUP_English.pdf

That document suggests that you can get different needle jets for that carb, which is another low-throttle tuning tool. Not something I've personally messed with, but it did remind me to check the needle and needle valve to make sure they aren't worn. I've had a few older bikes come through with one side of the needle worn flat, or the needle jet ovaled out, which causes all kinds of strange symptoms.

For most carbs, set the float height so that the float seam, bottom, or whatever other horizontal reference they have, is parallel or slightly below parallel to the float bowl sealing surface.

I'm a trials rider and we live at that end of the carb and I think your issue is air screw or pilot jet related ... Pretty good advice from gearhead there with (IMHO) one exception - the bit about not idling to preserve oiling on the overrun. 

Definitely give the carb a thorough clean out, check all the gaskets and O-rings and the float level carefully. Check the right jets and things are installed (the link above has the settings which are probably correct). Set the air screw 1+1/2 turns out and get the bike running and warmed up. At idle shift the air screw a 1/4 turn at a time and see if the idle rises or falls, you are aiming to maximize the idle speed. Adjust the idle speed to keep it where you want it. Then try riding around and feeling for the blubber - rich or lean both produce similar effects just try 1/8  turn either way on the air screw to improve it. 

FWIW conventional wisdom is that off idle is controlled by the air screw, 1/8th to 1/4 throttle by the cutaway on the slide (very rare to fiddle with slides !) 1/4 to 3/4 by the needle (you have the  clip to raise, richer or lower, leaner and you can get different tapers but unlikely to be needed), and full throttle by the main jet. Naturally they overlap a bit to make life harder and the only real way for grassroots tuners to check mixture is with plug chops and reading two stroke plugs really is a dark art ...

Good luck and enjoy

R

In reply to RichardNZ :

Will rich vs lean condition make a distinctly different sound when you crack the throttle? This all gives me a good place to start, now I just need time to work on it

In reply to dannyp84 :

Ummmm, not sure. Trials Bikes, at least modern ones, are ridiculously well muffled (and I'm getting old ) so generally do it under load by feeling for the least stumble.

Usually severely lean is a hollow "waaahhhhh" sound, and rich will still pop and bang and stutter but it won't pull and it won't fire consistently, like it's trying to clear its throat. It will smoke a lot, too. Eventually it will foul the plug and stop running. You can also throw a new plug in it, run in the area that is causing you problems, immediately shut the engine off, and read the plug. In very general terms, white is lean, black and wet is rich, and a tan/brown color is good.

So this week I finally had some time to mess around, went to ride the bike to refresh my mind on what it does wrong, and after starting only for a brief moment, it wouldn't start again. I took it home, pulled the float bowl off and gave everything a quick clean, put it back together and the bike fired up and immediately went to high rpm for some reason before dying again. Would not start after that. Since then I've reset the air screw to factory settings, and a friend who knows a good bit more about what to look for is supposed to stop over tonight to help me troubleshoot. I'm thinking something fell out of adjustment or there's a nasty vacuum leak somewhere, because last year when I got the bike it ran fairly well and when it does run it pulls strong for a 125. 

Crank seals...

Wide open with no throttle is a tell tale sign.

maj75 (Forum Supporter) said:

Crank seals...

Wide open with no throttle is a tell tale sign.

Slide sitting on the anti twist pin will do it every time... 

Mag side crank seal sucks air, but not much depending on sealing and venting. Causes all manner of lean issues but I've never had a revver.

Drive side crank seal is the opposite - mostly oil hence extreme richness and huge amounts of smoke.

Yeah, I've never seen a crank seal bad enough to suddenly be a WOT revver. The carbs I've worked on don't let the slide go up enough to pass the anti twist pin either, and that bike probably has a D slide anyway.

Is there fuel in the carb? Lay the bike over with the petcock on and see if gas drains out the vents. If it doesn't, you've got a stuck float needle and the engine was just burning whatever was left in the crankcase before dying.

Pull the plug - is it wet or dry? The rev tells me it is probably lean and isn't really going to WOT, just idling super high, but if it's a real WOT straight up to overrev with a ton of smoke, I suppose it could be really rich and have a giant intake leak somewhere. That doesn't usually happen out of nowhere though.

So I'm about a month behind on updating this but I replaced the reeds and the bike then started briefly then died. I went back to checking for spark and noticed it was good but not every time I kicked the bike, very inconsistent. Traced the wiring to look for damage and found a plug that was connected but not all the way :/ ... clicked that back together and since then the bike starts, runs and idles very reliably. Off-idle response is still not perfectly snappy but it is completely rideable now for a change.. sometimes it's the little things that get you .