High rpms = 4 stroking

DanielMaia

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Jun 24, 2009
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Hi

I did some mods, repaired a broken thread with a heli coil, i did some port matching in the intake, i think it's great.

But i have one problem: until i get 34 or 35km/h, the bike have the typic sound of 2 stroke. Up than 35km/h starts to 4 stroking :(

I have a NGK B7HS, stock carb, base gasket is aluminium papper (0.05mm or so), head gasket stock, and a little skirt mod in the piston.

Can be the carburetor that restricts the air and starts the 4 stroking?



Just for note:

The 4 stroking in high rpm is caused by too much fuel in the mixture (as Joe, Norman and Riding rich said), so you should lean it out. Smaller main jet, or moving the c-clip to the first or second position

Problem solved
 
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Riding Rich

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Also if you have a gasket between the intake port and tube make sure it matches.
On my Grubee if there is any blockage it acts that way.

And i would remove the stock head gasket if you used a thin base gasket.

That'll get the compression back up.

Mine runs much better with super thin head gasket.
 

Riding Rich

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Yep thatll lean it out.

mine actuall like to run super rich.

My clip is all the way down to the last notch.

Try all the positions and use the one that runs the best.
 

Riding Rich

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Did you start with a new plug after the modifications?

I would still try adjusting the clip to diff positions.

theres nothing to loose.
 

Riding Rich

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Also how much did you take off the piston skirt?
That actually changes the Port Timing.
So dose the base gasket you have.

My motor became reall touchy after i modded it to death.

Literally to death.
My bottom bearing is very very tired.
Can't wait to get it apart and see the carnage.
 

Bikeguy Joe

Godfather of Motorized Bicycles
Jan 8, 2008
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Did you start with a new plug after the modifications?

I would still try adjusting the clip to diff positions.

theres nothing to loose.

Start with a new plug, and your clip where it is. Then go from there. Some run fine with the C clip at the second from the top, some will require a different position.

DON'T JUST MAKE IT A COUPLE OF POSITIONS LEANER without doing the above first, or you could ruin your engine with an over lean run!

Still it is most likely four stroking for the main jet being too large.
 
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DanielMaia

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Jun 24, 2009
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I was trying the sbp expansion chamber, really dont know what happen, but the piston rings broken my piston :(



**** happens :(
 

Norman

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did a pin come out ? A pin that keeps the rings from rotating. Looks broke about where a pin should go.
Looks like the piston had some blow by on the skirt among other problems it has. A little hard to see the piston well as the picture is dark.Did you clean the pistons top or is that how it came out looking that clean on top? Also looks like there is a bit of a seized spot looks like it got very hot too. Where it looked like it seized could be scrubbed metal from all the parts flinging around
Was the engine at high rpm when it let go?
How bad of a hit did the cylinder take?
Was this a needle bearing wrist pin engine if so was the needles on the wrist pin still all in place?
when looking at the pistons ring grooves looks like the top side of the lower rings land has been broken for a while because its dark kind of looks like carbon on the broken area and lots of carbon below the broken area it could of been broke in that area for a while then came unglued and took out the top ring land aka. top side of the piston.
Did it sound funny or knock before it let go?
I do not think the pipe caused the failure.
Like you said stuff happens
time for a new engine I feel for ya that looks nasty.
Bottom line did you have fun with this while it lasted?
 
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Riding Rich

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Seeing how the fracture is on the side of the piston where one of the main ports are at i would think a ring got caught in it.
If the little pin came out of the piston ring groove it probably spun around and ended up with the end of the ring in the port.

And if the Horse shoe clip was missing on that side it got jammed up.
 

DanielMaia

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Jun 24, 2009
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Hi norman and riding rich(nice nick :D)

I'll take more pictures to you see how the piston was before this happens

this is how the piston was before that:


The engine was in really low rpm qhen the piston broked, i think that if was in high rpm it blow the all cylinder..the cylinder at the moment is good...it broke when the rings passed in the transfer ports (hope that you understand, my english is quite bad :\ )




Yes, it wasn't the fault of expansion chamber, it was the crapping piston rings that killed my little piston :(
 

Riding Rich

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In the number 2 pic i see the ring end gaps are allmost aligned.
They should be one ring a little left the other a little to the right.

You mentioned skimming the piston skirt.
Are you positive you put the piston in the same way it was taken out?
The keeper pins in the ring groove should be on the intake side of the motor.
Otherwise if you reverse it i think a ring end might end up in a port just waiting to get caught.

Also the g-clips or horseshoe clips shoul have the Tab pointing up on down.
I see in picture 2 you have it pionting to the side.

Sorry the 1 picture is a little blurry but you can see a #5 and an arrow marked on the piston crown.

The arrow is on the intake side pointing toward the exhaust port.
The piston has to be put in the motor this way.
Or you will break a ring.:-||
 

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DanielMaia

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Jun 24, 2009
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Yes, i know :p

I put the rings in the right position, otherwise the cylinder dont fit ;)

And the piston was in the right position too ;)

I will buy a new kit soon :\
 

DanielMaia

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But im from portugal...the shipping costs more than a motor kit :(

I will buy from a guy here, but i order 3x 66cc motor kits, to re-sell here in Portugal :)

thanks
 

Norman

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once you install the c clip/wrist pin retainer the direction it points to don't matter as long as it will spin in the groove with some force, not like a top or easy you do want the bent part to point away from the wrist pin if the bent part is angled off to one side.
From what I've seen the piston will fit either direction as all the ports don't come close to the ends of either ring. I've ran the piston both ways although some piston/ports may be different.
I really think your piston cracked at the lower ring pin retainer then let completely go. Not much you can do about it.
To bad your not in the US someone here probably has one for sale cheap. I hope you can find one where you live you might get lucky there as I've heard you guys have more scooters there than here
thanks for showing us that and I hope you have good luck getting it fixed.
 

DanielMaia

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Jun 24, 2009
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I think that i mount correctly the piston rings...but i know that the piston rings have to direct to the intake..

Soon i'll buy a new kit, and this motor is going to be opened to a big maintenance :)

Thanks norman ;)