I have a 92 yz250 problems. Please help.?

Question

This bike starts first kick without choke. It idles fine but, in first gear she appears completely fine, but in second it hits the power band and a surge of power follows but for a very short time, it is then quite flat and when the rpms increase just sounds like its bogging down. A friend of mine suggested probably using a hot plug but when I checked it is a cold plug. I am going to try a new plug but does anyone have any other ideas as to what could be the cause. Thank you

Answer 1

Running out of gas>>fuel delivery prob.

Starts easy/idles fine/fine thru low gear,,,
that absolutely rules out any Compression ~seizure issues

Your Symptoms are OPPOSITE of low compression,,and air leaks,and primary compression faults such as leaky main seals.

Clogged air cleaner or exhaust both manifest themselves as overly Rich/Cold running.
Neither are characterized as "Flat",,"Bogging".
Nor does their effect occur after torque peak.
Such problems will be sensed well Before in the RPM range.

Your symptoms are Opposite of being overly rich due to any cause.
It's going LEAN,not rich

Reeds ,,at high speed can be practically REMOVED.
Inertial air flow becomes so high the reeds basically Hang Open.
Yamaha themselves were initially reluctant to put reeds on their production road-race engines,,,even after they introduced them on consumer market products.
Because at Torque Peak they constitute a restriction to air flow.
PRIOR to torque peak range( "power band") is where the bulk of Reed's benefits occur.
And that's where any probs will manifest themselves most distinctly.
Your symptoms are OPPOSITE of any reed valve fault.


Spark Plug,
Spark plugs can run bad,weird,etc in all sorts of ways.
But in some ways they're very reliable in their behaviour and characteristics.

Any Plug,,,good or bad,,,has a Exponentially harder time firing as combustion chamber Pressure Increases.
Higher the psi Rises,,,the more difficult it becomes for the plug to fire.

Engines produce LOW combustion chamber pressures at low speeds/small throttle openings.
Everything you Feel about your engine's pulling & power characteristics is Purely PRESSURE related.

A 250 makes 2~3 ft lbs of torque at idle & just off idle,,
almost NONE.
At Torque peak,,a YZ makes probly about 18 ft lbs.
The ONLY reason Torque Rises nearly 10 X is because PRESSURE has risen

If a Spark Plug works OK at Idle,,,that proves Nothing
Nor does it at low speed.
Why not??...cuz it's only working under very low pressure.

As engine revs up,, Spark Plugs' 3 worst enemies grow Bigger and Badder.
*Pressure
*Temperature
*Frequency

PSI and TEMP both PEAK at Torque Peak.
That's where the Torque Peak Comes from.
And it's always at a relatively Hi Frequency.

If a spark plug is Weak or bad,,,,it WILL act it's WORSE at that point.

"Bad Plug" might start and idle fine,,,might rev up OK with only an imperceptible ,occasional misfire.
But if it has any shortcomings at all,,,,it WILL show them when engine starts pulling hard towards it's torque peak.

That's Not "mechanics",,it's PHYSICS.

If a Plug doesn't break down when engine power surges,,,there's nothing wrong with the plug itself.

Plugs can be either The Problem,,,or A Symptom.

Supposedly "Bad Plugs" can run fine all day,,,then foul out at low speeds.

Is that a Bad Plug,,,
or is it a Symptom of another Fault which is damaging the spark plug?
See the difference?

There's no valid reason to NOT arbitrarily just install a new plug,,,they're cheap and easy.
A Fresh plug never hurts.

But doing so arbitrarily is just RITUAL,,
it's NOT trouble shooting.

So if you can feel a Power Surge ,,
without the Plug breaking down into a miss-firing spasm,,,
there's Nothing wrong with the Spark Plug.

At worst,,,any misbehaviour of the plug is merely as a SYMPTOM.
..........................................................................................................
Check
*Fuel Tank Venk/Gas Cap
*Float Level,
*fuel filter/petcock
*fuel line deterioration
*Any build-up in float valve that might restrict flow
*Needle Jet Nozzle---even though that item is primarlily for mid range...it does participate in main jet function
..................................................................................
Test ride to verify Leanness:
Cruise in 2nd at 1/2 throtlle for a few seconds to let Fuel Bowl refill,,,then nail it and let it wind out.
Shift to 3rd,,,,,,let it go till it Bogs/Flattens.

Back Off Throttle at that point,,,and cruise a few seconds to let carb re-fill,,,Nail it again .

IF:
if it responds favorably,,,after cruising at part throttle...
The Fuel Flow is obviously restricted.
"Carb is filling up Too Slow"
In effect,,,you're "running outa gas" temporarily


Try to repeat the partial throttle acceleration,,,,"nurse it up thru the gears".
If it responds favorably,,,it's starving for fuel
If it DONT,,it's too lean in some part of hi-speed delivery circuit.>>main jet,or needle jet orifices.

If that makes no Difference,,,then try pulling Choke on while it's revved up and bogging.
Normally,,that would create an instant fit of Stuttering,Blubbering,4-stroking

You are FORCING it Rich by using choke,,,
just pouring extra fuel at it.

If you get a Surge in power/rpm soon as you apply choke,,,
It's Obviously too lean.
It LIKED the extra gas....at least for an instant.

Ultimately it Will get way too rich within a few seconds.
Unless the choke drains the carb down too far.

Mikuni chokes are Greater cold-starters,,,but "Bad" for metering tests.
Because they are either ON or OFF.
You can't "throttle" them to adjust hi speed mixture.

But they're Great for Bowl Starvation tests,,,because they suck Fuel FAST and from a very low level in the carb.
Choke Plates dont deliver Fuel like Mikunis do,,they only block air & raise vacuum in the jets.

If fuel level is insufficient,,,the Choke will deplete it it quickly.

So leave the choke on for several seconds with throttle wide open,,,,and see if any Choke induced Richness disappears or not.
If it seems to finally "run outa gas" and begins bogging again,,,,that's bound to be fuel supply deficit.

.................................
If your engine is starting First Kick with No Choke and then Idling with Broken Rings,Seized Piston,Low Compression,Fouled Plug,Clogged Exhaust and Air Filter,,
Then it's the probly the ONLY 2 stroke in history thats ever done that.

Especially those damned freaked out YZ's.
EVERYTHING about them suggests hard start & poor idle,and weak low end power.
They were probably the most extreme/exotic-spec'd engine
put in a production bike at the time.

NO Slack allowed,,,,to Run right everything about them must BE right

...........................
I can easily be wrong,,,maybe CDI is crapping out?

But if it
starts easy,
idles good,
revs ups ok,
pulls throught the lower gears ok,
pulls strongs as it comes into it's powerband,,
power surges,,,
Then goes "BLAAAAaaaahhh" and acts and sounds like it has run outa gas,,,,

I 'bleive That's the first thing I'd check & make sure it's getting the fuel it needs

Good Luck with it

Answer 2

First off, a cooler/hotter plug will not cause the problem you are describing. For that bike though, stay with an 8 with an R in it. BR8EG, etc. However, your problem is most likely plug related- it's probably fouled. Replace it with a new one and see if that does the trick. If you are comfortable pulling the carb, look at the reeds for frayed or broken signs. But its doubtful that's the problem either. A top end may be necessary too. However, both of these problems would still let the bike run on the pipe ok, just without the full power. It would still get up on the pipe and run ok. Let us know if the plug replacement doesn't work.

Answer 3

There are a few possibilities:
1. The powervavle is sticking shut.
2. The crank seals are bad
3. The top-end needs replacing
4. The spark plug is bad

The powervalve is the most likely culprit. Take the expansion chamber off, and spray in the exhaust port with some WD-40 and try to wiggle the powervalve open and shut a bunch of times. This should free it up.

Answer 4

sounds like it could be a couple of things but I suspect it's getting time to put a top end (piston, ring, etc.) in her. this is a common occurance and you really start noticing it more and more in the higher gears as it is working the engine harder the more gear ratio pressure is applied.

first thing is run a compression check and see if you are within tolerance (not sure on a '92 YZ250, but guessing in the 125-175 psi range) ... check in a shop manual to get specific pressure your bike should be.

second thing it could be a plugged (or partially plugged) pipe/muffler. when was the last time your repacked your muffler (or spark arrestor)??? once the packing gets somewhat saturated with the unburned oil, the air doesn't flow out well, causing additional back pressure, causing loss of power.

several other things but those two are more than likely your problem.

Answer 5

go here & learn.. how to fix just about everything on your bike..
http://www.dansmc.com/MC_repaircourse.htm

but sounds like you need to clean the pipes & air filter..
here is a quote from the above site.."One problem with two stroke exhausts is that they can get plugged up with carbon from the oil used to lubricate the engine. The bike will start easily but just will not rev up. This is especially true of quite exhausts. If the exhaust has removable baffles or is fiberglass packed, you can easily clean them and replace the fiberglass packing."

Answer 6

Sounds like fuel or Air starvation to me, clean out the carb & air filters with an airline if possible, " be careful not to lose any of the small parts". I dont think its a compression problem " but a check would'nt hurt", it could be electrical but be methodical and check one thing at a time.

Good Luck
Paul

Answer 7

I agree with pmk about needing a top end.
If you don't have a compression tester -
Remove the exhaust pipe and look in the exhaust port with a flashlight and mirror.
An scoring of the piston and cylinder wall, or discoloration below the rings, the top end will need to be rebuilt.

Answer 8

Sounds like you may need the top end rebuilt. I always use Golden Spectro and mix it 32-3. If smoke and oil are blowing it's not your carb or plugs. It's either overfilled with oil or needs a top end rebuild.

Answer 9

sounds like it's not breathing right, clean the air filter, then clean the exhaust pipe out, also clean the power valve and replace the spark plug. if it still does it check the compression to see if it's in spec.

Answer 10

1

Answer 11

I had a bike like this, it was water in the carb, which also caused the plug to foul. check the carb before you try a new plug in case it fouls another.