Two Stroke Motocross

Two Stroke Motocross Forum => Vintage Two Strokes => Topic started by: motoman00019 on July 04, 2013, 04:45:13 PM

Title: 83 it250 sluggish
Post by: motoman00019 on July 04, 2013, 04:45:13 PM

I rebuilt an 83 it250, had the cylinder redone with nicasil, bike idles ok, and is very strong when the power band kicks in. in neutral it has great response, as soon as I get to second gear its boggs bad unless I work the clutch to let it hit the powerband. it has a new carb with factory jetting for this bike.
I get black oil build up leaking from the exhuast at the cylinder, has fresh 93 octane gas @ 40:1 mix compression is at 125 psi,
any ideas?
thanks

Title: 83 it250 sluggish
Post by: _X_ on July 04, 2013, 05:12:50 PM

try turning the air screw out a quarter turn at a time till it cleans up, could be too rich down low.

Title: 83 it250 sluggish
Post by: TMKIWI on July 04, 2013, 10:44:10 PM

Older motors normally have a large squish clearence.
If you tighten it up to 1.2mm it will improve the midrange.
Do you know what yours is set at ?.
Is probably running a bit rich as well. Try lowering the needle.

Title: 83 it250 sluggish
Post by: motoman00019 on July 05, 2013, 04:08:30 AM

never checked my squish clearance, can I do it without removing the head? or should i just remove the base gasket and use 3 bond and check for interference?
I can tell you that anything past 1/4 throttle under load she just bogs until it dies, unless I clutch it a bit than it takes off like wildfire.

Title: 83 it250 sluggish
Post by: citabjockey on July 05, 2013, 05:15:22 AM

could it be bad reed valves?

Quote from: motoman00019 on July 05, 2013, 11:08:30 AM

never checked my squish clearance, can I do it without removing the head? or should i just remove the base gasket and use 3 bond and check for interference?
I can tell you that anything past 1/4 throttle under load she just bogs until it dies, unless I clutch it a bit than it takes off like wildfire.

Title: 83 it250 sluggish
Post by: TMKIWI on July 05, 2013, 05:49:32 AM

Quote from: motoman00019 on July 05, 2013, 11:08:30 AM

never checked my squish clearance, can I do it without removing the head? 

Yep. Remove the sparkplug and get 2 pieces of electrical soldering wire about 2mm thick.
Cut the 2 pieces about 125mm long and give them a slight bend into a L shape.
Turn motor over slowly until the piston is about 20mm from TDC. You can hold a screw driver or similar down the spark plug hole and get a feel for when it is just before TDC.
Next , feed the 2 pieces of solder into spark plug hole ( don't drop them in ) and push them 1 at a time until the bent end or L is touching the cylinder wall.Have them opposite each other in line with the crank.
Slowly push down on kick starter until you feel the piston contact the solder and then push past.
Remove the solder and measure with a micrometer or vernier.
The measurement is your squish clearence.

Title: 83 it250 sluggish
Post by: SachsGS on July 05, 2013, 03:51:10 PM

Compression is low, you should see a lot more then 125PSI with a fresh engine.

Title: 83 it250 sluggish
Post by: motoman00019 on July 06, 2013, 02:44:52 AM

what should the compression be ? and whats causing the black oily crap leaking out the exhaust at the exhaust port? these are tough to get a good seal from expansion chamber pipe to cylinder...... thanks for all the help 

Title: 83 it250 sluggish
Post by: citabjockey on July 06, 2013, 04:17:23 AM

But 125 should be enough to have it run close to ok. Also, squish bands may not have been optimal back then but it sure doesn't explain his symptoms. Reeds, on the other hand, if leaking backwards, will cause the airflow in the carb to reverse which double loads it with fuel at low rpms/throttle settings. Hence massive spooge, stalling, crappy running. Then it cleans up as the revs increase to and more "back pressure" is put onto the reeds forcing them to seal. Take out the reed block and see if you can see daylight between the block and the ends of the reed petals and for cracking on the seal surfaces.

Quote from: SachsGS on July 05, 2013, 10:51:10 PM

Compression is low, you should see a lot more then 125PSI with a fresh engine.

Title: 83 it250 sluggish
Post by: SachsGS on July 07, 2013, 02:58:23 PM

Should be at least 150 PSI. Below 120 PSI motor will start to run lousy.

Title: 83 it250 sluggish
Post by: TMKIWI on July 08, 2013, 08:00:56 AM

I have seen older motors that with the head gasket installed , the compression is way too low.
I had to machine 60 thou of my 125 head just to get the squish and compression right. That was with an OEM piston as well.
Thats why I suggested to check the squish. It will let you know if any machining needs doing.
The OP said the motor is rebuilt so it won't be a ring problem.