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post #1 of 14 Old 08-12-2003, 11:22 AM Thread Starter
 
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Re: two-stroke help needed

I sent this note out to a 125 list I'm on, but I thought I'd copy it here as well:

I can't get my '95 rs125 (with airbox mod) to run properly. After much inspection I think I've found/fixed several problems of my own making but I'm still no closer to getting the bike to work. My latest theory is that the problem is electrical - but I'm an idiot and usually wrong. I'll describe the symptoms in case I'm way off with my electrical theory, but we did measure the resistance and it was off, I'll get to that...

Anyways, when you rev the bike while it's warming up I get a crisp, clean throttle response. Once I get the bike warmed (> 50 degrees) up the lower revs are still clean but if I try and rev it out w/full throttle it sounds like it's stumbling and it won't rev out past 11 or so. I can rev it out pretty high (12,000 or so) as long as I don't use full throttle but more of a 'blipping.' Same thing when I get it on the track, I can get an occasional run-out to redline and the bike HAULS - but most of the time it just bogs and the revs won't move up or even drop and I bring it back in after half a lap.

Why I don't think it's a caurberation issue: The carb is clean, set properly which confirmed from the previous owner's notes. I've changed the main jets several sizes in each direction and it doesn't help the problem. I've inspected my reeds which are fine, the head and piston have been replaced with new rings, gaskets etc... also the bike makes good compression and no ugly sounds.

I finally put a meter on it yesterday at the track and noticed that all the values were roughly showing twice the resistance in Ohms that they should be. So this is where I'm going to start looking next (I wrote down the exact values but I won't have access to them tonight) but they were all about double like I said. I've already replaced the plug, plug cap, and coil to no avail. Any suggestions on where I should look next? In that every value that the manual tell you the check was off I'm thinking it's a component that it shared between all the electrical systems or the harness has a bad ground or something. Does this sound familiar to anyone or does anyone have a suggestion on where I should look next?

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post #2 of 14 Old 08-12-2003, 11:23 AM
 
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Re: two-stroke help needed

Sounds like the main jet is too rich or the choke is stuck, also I'd double check the float bowl for debris, loose debris can clog a jet, then fall out later, and it'll run fine again, until it clogs again



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post #3 of 14 Old 08-12-2003, 11:29 AM
 
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Re: two-stroke help needed

Yep, sounds fat for some reason. *

You may have the same set-up but parts wear out. *Needles and seats can go bad. *A little piece of junk can keep the float needle from working properly. *How are all the little seals and o-rings in the carb?

As a general rule, if an engine runs good cold, but bad warm, it's running fat. *If it runs like $hit cold, and then good warm, it's lean.

On a different note: a bad plug can have the same symptoms, unfortunately, as it takes more spark up top. Does the manual have procedures to go through to check the ignition system?








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post #4 of 14 Old 08-12-2003, 11:30 AM Thread Starter
 
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Re: two-stroke help needed

it's clean enough to eat off of... I've replaced the float needle too.I haven't yet but will replace the o-ring for the carb.


Thanks guys!

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post #5 of 14 Old 08-12-2003, 1:38 PM
 
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Re: two-stroke help needed

I can hook you up with a guy who races 250's if you like. He just had a problem with his recently around the electrical system. I think he had a bad connection on his battery which was causing the problem. His bike can run with a battery or without it (constant loss system), but with the half baked connection he had going it was totally messing up the electronics and the bike ran like crap.

Just PM me offline if you want his email and I will shoot it to you later tonight (don't have it on me at the moment).

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post #6 of 14 Old 08-12-2003, 2:18 PM
 
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Re: two-stroke help needed

I've owned, run and blown up a few 2-strokes in my day. First impression I got from your descriptio was carburation.

2-strokes don't react the same way a 4-strokes do to changes. Fuel mix, fule type, oil type and ratio ALL factor into how those things run. If you changed any of that it's a new ballgame

When you tuning it make sure you buy the same gas, the same oil and mix it the same way. If it has it's own mixing jet then make sure the tank, linkes and jet are not obstructed.

Sucking thing about a 2-stroke is then often run best right before they blow... Good luck and remember, it's better to be a little on the fat side than the lean side.

BTW, when was the last time the bike ran right? What changed since then (think of everything). Something as simple as different brand (cheapo->expensive->cheapo) head gasket could affect squish band and REALLY hose up the way the motor runs.

YMMV. Good luck and let us know what you find out.

OH, and I had once had a bad plug out of the box. Change that.. it's cheap and easy to do and it will at least take that off the possiblities list.
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post #7 of 14 Old 08-12-2003, 11:46 PM
 
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Re: two-stroke help needed

Defintaly sounds like too big a main jet, thats what my 125 does when its too fat, carburates horably around 7-9k and either shuts off like a light switch at 10k, like hitting the kill button, or it 'catches' and pulls clean to redline. But its only at wide open throttle, back it back about 1/8' and it will pull cleanly all the way to redline. Assuming you have checked, cleaned everything else, I could only suggest leaning it down. Check you mid range as well, perhaps the needle is wrong and loading the plug in the midrange? Unless the previously owner is the same weight, same 'throttle' style, same track and altitude, his jetting may not be optimum for you. Do you have a detonation counter or pyro? If not, just watch the plug as you lean it down so you don't melt it down.

Mike
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post #8 of 14 Old 08-13-2003, 12:57 AM
 
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Re: two-stroke help needed

just to come out of left field how is the silencer clean dirty packed unpacked? A restrictive can will make the bottom and midrange pull hard and spudder at high rpm. The soundwave is reflecting faster off the dense gasses creating a hard initial pull but chokes it as it still needs to get the exhaust out.
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post #9 of 14 Old 08-13-2003, 1:49 AM
 
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Re: two-stroke help needed

The quickest, easiest way to isolate the problem is to do a practice day, and start swapping your parts with known good ones. Since you've done this with the plug/cap and coil, the stator is next. They aren't as big an issue on 95s and later as they were on the 94s and earlier, but they can still cause headaches. Heat makes them schitzoid.

Have you checked the spark generator, which resides under the right side engine cover? Sometimes these go bad as well, but they are more the all or nothing types. I will assume the reeds are new or in very good shape.

I have been fortunate not to have endured an electrical gremlin on my 125 so far, but I know the rainy day is coming....... That's what my spares kit is for.

Do a leak down test of the crankcase as well. There may be a leak that only rears its head at higher rpms.
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post #10 of 14 Old 08-13-2003, 8:53 AM Thread Starter
 
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Re: two-stroke help needed

thanks for the great suggestions guys! I did replace the can with a different one and didn't think about that. Also I'll start ordering the other electrical parts before the coil - I'll get to replacing that next. I guess the good thing is if they're not broken now they will be soon... Have any of you 125 owners ever messed with your timing - that was another suggestion I received. It looks like it might be a 1/2 degree advanced from 2 degrees which I believe is the stock setting?



p.s.
I also sent a note out to the 125 list but I'm getting way better answers over here, this place rocks!




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post #11 of 14 Old 08-13-2003, 9:20 AM
 
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Re: two-stroke help needed

sounds like the problem i was having at jennings with the F3. turned out to be my rectifier/regulator that was doing it

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post #12 of 14 Old 08-13-2003, 9:41 AM
 
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Re: two-stroke help needed

I would replace the plug wire while I was at it. If the resistance is high in the wire, it will kick up the resistance all the way back through the system.* Also, the timing should be set to the basic stock setting (at least until you can get this sorted). *

Is the base gasket seal good? *What do you have the squish set at? *

The symptoms you have could also be the result of too much above the piston compression which yields great midrange, but kills the top end. *I would try a conservative thickness base gasket and make certain it isn't cracked. *I usually torque down the cylinder with a new gasket, then remove it and measure the thickness, as they flatten out under torque and don't always yield the expected measurement.

Also, I would assume you are using 110+ octane right? *Anything lower and you'll never get anywhere (or seize before you get there).

Also, don't believe a simple plug reading. *The only real way to know what the burn is like is to pop the head and look at the piston. *What oil are you using? *Some of them require that you blast the plug with contact cleaner to actually read them.




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post #13 of 14 Old 08-13-2003, 10:31 AM Thread Starter
 
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Re: two-stroke help needed

I would replace the plug wire while I was at it. If the resistance is high in the wire, it will kick up the resistance all the way back through the system.
isn't the plug wire part of the coil? If you're talking about the wire between the plug cap and the coil (I think you are) - it's all connected as one piece to the coil

Also, the timing should be set to the basic stock setting (at least until you can get this sorted).
done - the manual doesn't explicitly list the stock setting, but where they show you how to adjust it (after telling you not to) it's at 2 degrees - anyone know if that's the factory setting?

Is the base gasket seal good? What do you have the squish set at?
we replaced the base gasket with a new one just to be sure, and we also put a the old head back on (slightly pitted but not too bad) trying to (more like pissing in the dark) make sure the squish was OK. Squish is still unknown though as I don't own a set of calipers (I know, I'll pick one up today along with a new volt meter) and of course I did replace the pistion. Using the idiot eyeball method the piston head at TDC is sticking up a MM or two.

The symptoms you have could also be the result of too much above the piston compression which yields great midrange, but kills the top end. I would try a conservative thickness base gasket and make certain it isn't cracked. I usually torque down the cylinder with a new gasket, then remove it and measure the thickness, as they flatten out under torque and don't always yield the expected measurement.
I've been trying to stay as close to the previous owners state of tune so I've always been trying to stick with the same gaskets - I don't know what size is in there now except for was the same thickness as the one that was in there - I'll give that a shot when I know my squish

Also, I would assume you are using 110+ octane right? Anything lower and you'll never get anywhere (or seize before you get there).
previous owner ran VP C12 - there is a performance engine shop by me that stocks Sunoco - so far I've used both their 107 and 110 motor octane-rated fuels, and the last 10 gallons I got was their 110 brew.

Also, don't believe a simple plug reading. The only real way to know what the burn is like is to pop the head and look at the piston.
when we put the old head back on over the weekend we took a look at the pistion - was in good shape and the ring seemed to be doing it's job.

What oil are you using? Some of them require that you blast the plug with contact cleaner to actually read them.
motul two-stoke 800 synth - when we did a plug chop the plug looked good - just a hair on the rich side but with lots of golden tint to it.

If this ends up being the squish anyone (especially Mark) should feel free to smack me in the head whenever they want Thanks again for the help!




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post #14 of 14 Old 08-13-2003, 10:37 AM
 
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Re: two-stroke help needed

Quote:
abtech : I would replace the plug wire while I was at it. *If the resistance is high in the wire, it will kick up the resistance all the way back through the system.* Also, the timing should be set to the basic stock setting (at least until you can get this sorted). *

Is the base gasket seal good? *What do you have the squish set at? *

The symptoms you have could also be the result of too much above the piston compression which yields great midrange, but kills the top end. *I would try a conservative thickness base gasket and make certain it isn't cracked. *I usually torque down the cylinder with a new gasket, then remove it and measure the thickness, as they flatten out under torque and don't always yield the expected measurement.

Also, I would assume you are using 110+ octane right? *Anything lower and you'll never get anywhere (or seize before you get there).

Also, don't believe a simple plug reading. *The only real way to know what the burn is like is to pop the head and look at the piston. *What oil are you using? *Some of them require that you blast the plug with contact cleaner to actually read them.
Plug wire was replaced when he did the coil/cap I believe.

He's running 110leaded(sunoco?) with motul premix oil

We've got a good seal on the base gasket and cyl head O-rings, currently using a .5 base gasket. I believe the deckheight was +2mm and he's using the 10.2cc head (setup per previous owner). I suppose we could try a thicker base gasket, but the bike ran fine in this configuration until it went to hell...

Topend looks ok, piston and plug have that light chocolate brown with a little golden tan coloring. I did note that the intake side of the pistons skirt is slightly scuffed(one of the used pistons from darren showed a similar wear pattern). I don't think its the main jet as we've tried leaning it way down with zero improvement.
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