Grubee SkyHawk 66cc/80cc Cutting Out

Maskot · Jul 3, 2015

Finally, after two days of fixing it up like new, I took my bike out and gave it full throttle. Got up to around 30mph when the engine decided to magically cut out, it consistently did this every time. I have no idea why it's happening or what's causing it. I have a stock NC carb, checked the fuel filter, everything runs smooth at lower speeds, spark plug looks perfect, connection looks and feels good. I've had this thing for almost 4 years, not sure if that could be a problem either.

Thanks... :-||

Davezilla · Jul 3, 2015

Check for junk in the float bowl or at the needle and seat valve... Otherwise, check for spark and see if it's ignition related... All these things need in order to run are fuel, ignition, and compression... Start with the easy stuff and see if you got spark, then while the plug is out, run a compression check if you got a compression tester, then if you got both spark and compression, pull the carb and check that the main jet or needle and seat valve are not blocked off by debris...

Maskot · Jul 3, 2015

Davezilla said:
Check for junk in the float bowl or at the needle and seat valve... Otherwise, check for spark and see if it's ignition related... All these things need in order to run are fuel, ignition, and compression... Start with the easy stuff and see if you got spark, then while the plug is out, run a compression check if you got a compression tester, then if you got both spark and compression, pull the carb and check that the main jet or needle and seat valve are not blocked off by debris...

The carb is only a few months old, the float bowl is clean and the jet is clear. The engine runs well, it just loses power at higher speeds, as if a governor is limiting the rpm. It's very strange.

Davezilla · Jul 3, 2015

If the bike was quicker before then I would check to see if it's losing compression either from a blown head gasket or by blowing it past the rings. You can check with a compression tester if you have one, otherwise you could take the head off and look for signs of head gasket leakage on both sides of the gasket, this starts out as a black or dark colored mark on the head gasket where the leak is if it has one. If it's nice and silver all the way around the mating surfaces then look down the cylinder with the piston at bdc, any scratches in the cylinder bore can let enough compression slip by to cause a power loss, and if the inside of the cylinder wall looks really shiny with no signs of the cross hatch pattern left it's possible the cylinder glazed before the rings fully seated and can cause a blow by. Re honing the cylinder with a 240 grit flex hone and a new set of rings will usually fix this problem as long as there aren't any deep scratches down the bore.
if there are any scratches down the bore or pieces of the nikasil plating flaked off, the only fix there is a new cylinder and set of rings, and the piston too if it's scratched too badly.

Maskot · Jul 4, 2015

Davezilla said:
If the bike was quicker before then I would check to see if it's losing compression either from a blown head gasket or by blowing it past the rings. You can check with a compression tester if you have one, otherwise you could take the head off and look for signs of head gasket leakage on both sides of the gasket, this starts out as a black or dark colored mark on the head gasket where the leak is if it has one. If it's nice and silver all the way around the mating surfaces then look down the cylinder with the piston at bdc, any scratches in the cylinder bore can let enough compression slip by to cause a power loss, and if the inside of the cylinder wall looks really shiny with no signs of the cross hatch pattern left it's possible the cylinder glazed before the rings fully seated and can cause a blow by. Re honing the cylinder with a 240 grit flex hone and a new set of rings will usually fix this problem as long as there aren't any deep scratches down the bore.
if there are any scratches down the bore or pieces of the nikasil plating flaked off, the only fix there is a new cylinder and set of rings, and the piston too if it's scratched too badly.

I took some photos of the cylinder, oh boy...

maniac57 · Jul 4, 2015

Looks okay to me..you DO know a new top end needs to be broken-in just like a totally new kit right?
Most chinadolls run like crap until the rings seat.
Maybe give it a few miles before you worry too much.
Check the seals while you have it apart.

Maskot · Jul 4, 2015

maniac57 said:
Looks okay to me..you DO know a new top end needs to be broken-in just like a totally new kit right?
Most chinadolls run like crap until the rings seat.
Maybe give it a few miles before you worry too much.
Check the seals while you have it apart.

This is a FOUR YEAR old engine, it's well broken in, has almost 200 miles on it.

Daement · Jul 5, 2015

Maskot said:
This is a FOUR YEAR old engine, it's well broken in, has almost 200 miles on it.

200 miles is barely broken in. 4 years 200 miles, she has self confidence issues.

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Grubee SkyHawk 66cc/80cc Cutting Out

Maskot

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Davezilla

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Maskot

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Davezilla

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Maskot

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maniac57

Old, Fat, and still faster than you

Maskot

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Daement

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