Unpredictable power loss during cruise

[milonakess]

I have a Genuine Roughhouse R50, 2015, 7500 miles, stock except rollers and clutch springs, de-restricted.
It's a 2 stroke 50cc minarelli horizontal for those who are unfamiliar.

I've been getting this problem since last month, when I cruise,sometimes the engine seems to lose all power suddenly, it goes down to near idle and it will not rev when I give it gas, the sound changes, and I slow down to a crawl.
It is not what they call vapor/vacuum lock because I ran it with the gas cap half on and it still happened.
It's not predictable, last night it happened once on a 35 mile cruise.

And when it happens, I shut her off, fire it up right again and I can take off with normal full power as normal as if nothing is wrong.

Things I have done so far after this started to happen:
removed airbox and inspected air filter
new fuel filter
Checked for blockage in fuel lines, applied vacuum to the petcock and fuel flow is fine.
cleaned carb twice(wasn't dirty)
checked all wiring
removed and inspected exhaust and cleaned off some carbon
Cleaned and sanded all grounds, electrical connections
Inspected reed valves, no cracking or warping, good condition.
replaced spark plug (gap 0.6-0.7mm per manual)
Replaced the stator recently, suspected it was that, but it wasn't.

Still getting it sometimes. I use the scooter everyday so it's not old gas.

Any idea what this could be??
The scooter has no trouble starting/idling.
Video proof of the symptoms:
Here is it, running bad during a ride, literally crawled to a stop and filmed.
I'm giving it full throttle there, and that's as far as it will rev.
www.youtube.com/watch?v=EL078Al0e9I&feature=youtu.be
www.youtube.com/watch?v=3fwpjRqzEcc&feature=youtu.be


And here is is, after I shut it down, then fire it back up immediately:
www.youtube.com/watch?v=oK08hFL4npk&feature=youtu.be

as you can see, running and revving happily, running perfectly afterwards.

Sometimes it fixes itself when I back off the throttle and roll back into it, or if it idles for 20-30 sec. But most of the time I have to shut it down.

The fact that I can shut it down, fire it back up immediately and it's fine makes it very confusing to troubleshoot.
I can ride the scooter no problem using 1/2 to 3/4 throttle  for long periods of time and it rides fine, no issues.
It's when I go WOT after a while (sometimes, it's unpredictable) or as i'm rolling into the throttle after coasting or slowing down that it happens.

Any help is appreciated

[tiny]

It sounds like an electrical issue. Start at plug and work your way forward/back making sure everthing is snug nothing loose or wires cut.

[ThomasTPFL]

I don't see anything about cleaning the carb? And reeds do wear out.

[milonakess]

Nov 6, 2017 2:52:02 GMT -5 ThomasTPFL said:

I don't see anything about cleaning the carb? And reeds do wear out.

Cleaned the carb multiple times, nothings changed.
The Reed valves have no sign of wear that I know of. They are not warped/cracked and still seal well as far as I can tell. I flipped them just to be sure but no difference. Wouldn't worn reeds would show different symptoms like hard starting,backfiring and overall down on power?
I have none of those except the down on power part when I experience this issue. but the fact that I can shut down and fire it up back immediately, and it's perfectly fine power wise does not point directly to a bad reed.

[geoffh]

Sometimes it,s the little things,check the acc cable is free and adjusted properly and the throttle grip is secure on the bar,check the slide,needle and clip are all correct.

geoff

[ThomasTPFL]

If I'm reading correctly this is an intermittent issue?

What everyone else has said plus...

What are the vacuum lines like? Sometimes things get old and get micro cracks that only leak when the conditions are right.

Same with the plug cable and such.

Did you try a new CDI? Never seen it on a Genuine buy I've seen a Vespa exhibit similar symptoms from an overheating CDI box.

Is your float or float needle sticking? The little rubber tip can get gummy and want to stick closed.

Something flapping around in the air path? I had a bike with a stock air box driving me insane because a bit of rubber seal was randomly flapping itself over in front of the carb tube.

[ThomasTPFL]

Have you seen this guy lurking around?

[lostforawhile]

This is what mine was doing then this morning, started it up, 20 seconds later it's dead, my new pickup coil is now open circuit ed and dead, apparently this happens a lot, the only explanation I can come with, is the Chinese are known to use p*** poor brittle copper wire. the winding probably broke due to vibration

[kkzxxx]

Could be worn crank seals, or a cdi maybe.

[uglo]

Check to make sure your fuel and vac lines are properly secured after you're engine is hot.. I had an issue where the vacuum from intake manifold to fuel pump was getting too hot.. expanding.. and letting a bunch of air in. Scoot ran like complete garbage until I figured it out!

[gooddog]

if you carry some starter fluid with you and the bike goes to idle give it a quick shot of starter fluid and if it revs you know it is a fuel delivery issue/carb issue somewhere.

[milonakess]

Allright so Quick Update:

I replaced the CDI and Coil and plug wire. Didn't help with the symptom unfortunately.

SO I took off the cooling covers and noticed some oil and sludge on the bottom right corner of the cylinder head.
It's on the bottom right cylinder head stud/nut and around the spark plug area.
The headgasket and the crankcase gasket don't look to be leaking since it's clean and not soaked in oil/gas anything.

I made a video, here's the link.
www.youtube.com/watch?v=OKTfWluBHu8&feature=youtu.be

I'm thinking this is definitely the issue now. But what could be causing the leakage? And from where? the head stud?
They're all tight to spec.
BTW, no signs of any leak anywhere else on the engine, just this small corner.
Thoughts?

[lostforawhile]

just for a thought, look at the pins in the CDI connector, see if they have backed off, what about your fuel petcock? it could be failing and randomly causing fuel starvation, then once the vacuum stops, it works right again. check your intake manifold gaskets. Also your filter could have crap in it, moving around, is this a gravity feed system, or a pump?

[milonakess]

Dec 15, 2017 19:16:05 GMT -5 lostforawhile said:

just for a thought, look at the pins in the CDI connector, see if they have backed off, what about your fuel petcock? it could be failing and randomly causing fuel starvation, then once the vacuum stops, it works right again. check your intake manifold gaskets. Also your filter could have crap in it, moving around, is this a gravity feed system, or a pump?

CDI Pins are good.
Fuel petcock is working fine as far as I can test it, I applied vacuum to it for a good minute, and it emptied half a tank.
I already replaced the fuel filter too. It is a gravity system.
I don't think it's a gasket, because the problem resets when I shut off/restart the engine. A gasket would cause repeated/consistent issues. this is more random. It sometimes doesn't happen for days.

[lostforawhile]

the fuel petcock can test ok, but still be bad, the opening that lets fuel through is extremely tiny, and even a little piece of dirt in the tank, could be randomly blocking it