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Freak 2 stroke problem

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Has anyone ever had this happen:

Trying to start up my 250cc yamaha (1985), its rich (problems with needle posslibly), i finally pull the plug, spray some brake cleaner in there to dry it out, it finally starts up and idles.

I adjust the idle to medium/high idle.

Go over to the throttle (its a 250cc Kart), and blip the throttle a few times, and rev it good about 4 times.

Then the revs stay up!

Didn't sound like MAX rpm, but it was still way up there.

I hit the kill switch, and it stays pegged. FFFFFFFFFuuuuuuuu..........

pull the plug wire off, and it stays pegged! ARRGH, begin chicken dance awaiting rod to fly out....

I ripped off the cone air filter and cover the carb with my hand. It boggs out and finally comes to a stop.

It was basically dieseling I think.

I've had this happen once before on my 90hp outboard engine when I was choking the crap out of it trying to get it to run one day (carb issues).

Now I'm freaked out and don't know what to do. I suspect the needle clip position was too low, but what are the odds of this happening to anyone but me?

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Yep had that happen once on an old yamaha rz 350 street bike. (yes also a 2-stroke). What you had happen was the engine was running off the burning carbon. You probly helped it by putting a cleaner in the cyl witch loosened it up. You need to pull the head clean the carbon out and fix your carb.( Jetting) Also try a diff premix oil and ratio.

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I think you may have an air leak. The boot between the carb and the cylinder may have tiny cracks in it. You may not be able to see them, but if you start it, and it starts screamin, spray WD40 generously on that boot, and I bet it settles back down to a normal idle. Replace the boot(manifold, or whatever they call it)

Good luck

Oh yeah, be ready to shut it down if it's something else. Other than that, stuck throttle slide maybe

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If it was an air leak that would cause it to run at a higher rpm but when you hit the kill button it will shut off. His stays running with no spark.

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The boot is in great shape and reeds look perfect.

Its like it is caused by an overly rich condition, that lights off the plug and just makes it diesel.

Slide in carb is perfect and wasn't sticking.

I just don't understand how it could rev so high with a closed throttle.

I'm stumped.

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Sounded like the throttle hung up just that one time. It can happen if your cable has a fray in it. Also it's a overly lean symptom not rich.

Did you electricute yourself when you grabbed the spark cap? Ha Ha!

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I've had a throttle cable stick on the pulley in the throttle housing. It was dieseling (kill switch wouldn't stop it) Make sure your cable isn't loose and catching on something.

There couldnt' have been enough carbon buildup removed from the cleaner to cause it to run @ high RPM that long. It had to be getting some fuel and potentially lots of air.

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rebuild carb and use a sonic tank....go back to stock settings...running rich is better then running lean....if you have that much compression to "diesel" well i dunno..that has to be one hell of a high compression motor you have...like others said excessive carbon on top of the piston could do that for sure so may be pull the head and follow the manufactures guidlines for removal of the carbon......air leak will cause instant lean condition and from your post that sounds like a very remote possibility...stuck carb slide can cause runaway throttle which you will have eliminated after a thourough carb clean/rebuild....i use a 12 guage/10guage cotton bore swap on slide housing to polish to prevent just such a problem as a sticking slide.....use a little remington oil or marvel mystery oil to lube during reinstallation...your bike is also now 25 years old which means modern premix oils are a thousand times better....i go 32:1 in all my vintage bikes now with yamalube R or golden spectro oil..yeah i know the damn sticker says 20:1 and vegetable oil but thats then and this is now....many are racing with a 40:1 ratio but thats sounds a little lean to me....go throrough your bike, everything back to stock settings...32:1 ratio (run 20:1 after motor rebuild for a tank or two0 and i suspect you will be just fine....but..check your plug often for color if its bright white (way too lean) or jet black (way to rich) you now have the info to check for air leaks and play with jetting...i would also do the rebuild with stock jetting as after 25 years who knows who did what to your carb

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It has moderate compression.

To diesel, all it needs is a red hot spark plug. Kinda like an RC car engine running on nitro with a glow plug.

The only air leak that was happening is the exhaust pipe was bouncing on the exhaust port about 3/4 of an inch. the springs kept hammering it back to the head.

I need to add exhaust support bracing in spots still.

this is on a 250cc super kart. Dirtbike powered gokart, aka shifter kart. 2004 haase racing chassis...etc....

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It has moderate compression.

To diesel, all it needs is a red hot spark plug. Kinda like an RC car engine running on nitro with a glow plug.

+1 saw it happen to a friends blaster, was running to lean, made the plug redhead, when he hit the kill switch it just kept running, had to shut the fuel off and wait 20-30 seconds for it to run out of fuel!

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It has moderate compression.

To diesel, all it needs is a red hot spark plug. Kinda like an RC car engine running on nitro with a glow plug.

The only air leak that was happening is the exhaust pipe was bouncing on the exhaust port about 3/4 of an inch. the springs kept hammering it back to the head.

I need to add exhaust support bracing in spots still.

this is on a 250cc super kart. Dirtbike powered gokart, aka shifter kart. 2004 haase racing chassis...etc....

to diesel, all it needs is a hot spot, and a piece of carbon could do that...however, it doesn't explain the continued fuel delivery unless it just went incredibly lean. The slide being stuck could do it. Needle may have wedged in the jet orifice. What's the plug look like? Got pics?

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I do believe it went lean at that high rpm because I don't remember seeing a huge cloud of smoke, as if it were getting proper amounts of fuel/oil at that rpm.

The rpm could be attributed to the dieseling effect screwing up the ignition timing of the air/fuel charge, and the metering rod being out of the hole too far could cause the needed fuel (since the plug was soaked when trying to start it). But these are just guesses.

After it happened, I was so shaken from it, I quit for the day, pushed it back into the garage and drank heavily in a dark corner.

Whatever color the plug 'was', it ain't no more since I killed the motor by flooding it with fuel.

definitely scary though

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have you pulled the plug? Taken off the head? I fear you may now have a molten exhaust side of your piston. If I remember right, there is a septum in that port that gets hot and causes problems. Ya might take the header off and look up the exhaust port. I don't remember that particular design, so pics would be valuable. How long had it been since this motor had run?

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have you pulled the plug? Taken off the head? I fear you may now have a molten exhaust side of your piston. If I remember right, there is a septum in that port that gets hot and causes problems. Ya might take the header off and look up the exhaust port. I don't remember that particular design, so pics would be valuable. How long had it been since this motor had run?

It does have one of those rectum things in the exhaust. Its a two piece rectum with pins in the center that allow both parts to rotate at the same time.

I had it out cause I just redid the waterpump shaft, the cover had to come off, taking that exhaust baffle out with it. It all went back together perfectly.

The motor was apart for 1.5 months waiting on waterpump stuff.

I haven't pulled the plug since it shut off this weekend.

I pulled the carb slide out, checked it, and raised the clip position on the needle

haven't started up yet.

I can take pics

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ARRGH, begin chicken dance awaiting rod to fly out....

That's hilarious. Only cause I've done it before.

I rebuilt a 2 stroke boat motor a few years ago and it revved to the moon when I first fired it up and it did want to stop running. Pulled the dead mans switch, yanked the plug wires.... nothing. I finally disconnected the fuel supply and that did the trick.

Culprit? Timing was way advanced.

Backed it down to an appropriate level and she purred like a kitten.

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That's hilarious. Only cause I've done it before.

I rebuilt a 2 stroke boat motor a few years ago and it revved to the moon when I first fired it up and it did want to stop running. Pulled the dead mans switch, yanked the plug wires.... nothing. I finally disconnected the fuel supply and brought it back.

Culprit? Timing was way advanced.

Backed it down to an appropriate level and she purred like a kitten.

Ahh, maybe I should check timing too? looks like someone messed with it at some point.

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Please, if someone could find the time to look it up in their manual, I need the specs for a 1984 yamaha yz250 stator.

There should be two readings I believe.

I'm getting 1437 on one and 247 on the other.

if i'm testing it correctly, that is.

i'm testing red and the white/red wire = 247

and for the red and the brown wire = 1437

thanks in advance!

I'm not sure how many people own a manual for this old of a bike, but i'm still searching for ohm specs for the coil.

I tried purchasing new/replacement coils from 2 different places and it was the wrong one both times.

I'd really like to know if they're at fault or not.

FYI: The motor hasn't ran since that day. It'll only backfire every now and then when kicking it over, but something's wrong with the ignition.

I put a timing light on it while kicking it over, and the timing is about 30 degrees retarded.

Something's wrong..... pls help

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