2 stroke issue (street)

Motodave15
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Motodave15
Edited Date/Time 11/16/2020 3:33pm
Does anybody have a idea why a two stroke would cut off in 4th gear at 50 mph and lock up the rear wheel... Then start up again like nothing happened and ride fine for another 10 to 15 miles then do the same thing... Its such a weird issue.

Plug is not fouled
Carb has correct jets and fuel is flowing good
Air filter is clean
Clutch basket and cable are solid.

This is not my bike, its my buddies. And were brainstorming on what the issue could be.
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BenG
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BenG 11/16/2020 2:54am
11/16/2020 2:54am
I completely rebuilt my 125 and it would lock up hard. I thought it was the top end so took it apart and it was fine. Chased electric and nothing, turns out the rear brake would randomly lock up then right away would work fine. Rebuilt the caliper and it's been great.
FGR01
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FGR01 11/16/2020 7:38am
11/16/2020 7:38am
Sounds like it's seizing. Are we talking an MX bike here that you're cruising on the street? They are not made for extended high-speed, constant throttle type riding. On an MX track, you are doing short bursts corner to corner. On the street, the turnover of fuel/oil through the engine is too constant and you run hot and dry on oil. It would need to be richened up to compensate for this.
Motodave15
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Motodave15 11/16/2020 9:49am
11/16/2020 9:49am
FGR01 wrote:
Sounds like it's seizing. Are we talking an MX bike here that you're cruising on the street? They are not made for extended high-speed, constant throttle...
Sounds like it's seizing. Are we talking an MX bike here that you're cruising on the street? They are not made for extended high-speed, constant throttle type riding. On an MX track, you are doing short bursts corner to corner. On the street, the turnover of fuel/oil through the engine is too constant and you run hot and dry on oil. It would need to be richened up to compensate for this.
Its actually a Vespa p200.
FGR01
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FGR01 11/16/2020 12:06pm
11/16/2020 12:06pm
As a scooter, I assume it has 2 stroke oil injection? Maybe the injection is on the fritz.

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JMCR250
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JMCR250 11/16/2020 12:06pm
11/16/2020 12:06pm
Agree it sounds like it's seizing. Check for a cooling or lubrication issue or the piston/cylinder is out of spec.
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seth505
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seth505 11/16/2020 12:58pm
11/16/2020 12:58pm
I have a little 50cc italian scooter I hopped up to a 70cc for fun with bigger carb and all kinds of stuff. I had an issue with it cutting out at full throttle and it ended up being a fueling issue. I had to run new fuel line that would actually supply it properly and it worked out awesome. If that thing is actually locking up the rear wheel then that's another story though.
Falcon
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Falcon 11/16/2020 2:15pm
11/16/2020 2:15pm
Heat seizing, as stated above. Look for signs of excess heat: the first thing to check is the exhaust, right where it meets the cylinder. Is it blued? Remove the exhaust and look up the port at the piston. Is it blued, scored, or marked?

Scooters are a strange breed. Does yours have a clutch, like some Vespas? Try turning off the run switch every now and then to get a blast of fuel/air mix through the cylinder when you ride. (Leave the throttle partially open.) When you turn it back on, the engine will bump start and you'll go back to normal, but with a cooler engine. Wink
Motodave15
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Motodave15 11/16/2020 2:43pm
11/16/2020 2:43pm
Falcon wrote:
Heat seizing, as stated above. Look for signs of excess heat: the first thing to check is the exhaust, right where it meets the cylinder. Is...
Heat seizing, as stated above. Look for signs of excess heat: the first thing to check is the exhaust, right where it meets the cylinder. Is it blued? Remove the exhaust and look up the port at the piston. Is it blued, scored, or marked?

Scooters are a strange breed. Does yours have a clutch, like some Vespas? Try turning off the run switch every now and then to get a blast of fuel/air mix through the cylinder when you ride. (Leave the throttle partially open.) When you turn it back on, the engine will bump start and you'll go back to normal, but with a cooler engine. Wink
Yeah it does have a clutch, with a cable. We will check the port at the piston. I will get back to you
Motodave15
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Motodave15 11/16/2020 3:13pm
11/16/2020 3:13pm
Just for further information, here is everything that was updated/fixed from the mechanic. We're just trying to troubleshoot before calling him back to narrow it down.

Paul_Pitzonka
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Paul_Pitzonka 11/16/2020 3:33pm
11/16/2020 3:33pm
Agree with above sounds like it’s partially/ heat seizing... It appears to me he rejetted it as well as changed the ignition system... both of these could be where your problem lies...Unfortunately, as is the case with most seizures, the bore and piston have probably seen better days...

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