That 'hole' in the piston looks awfully round and centered. Take a close inspection of your piston.
theuplift's piston
Posters_Piston_Hole.JPG (43.91 KiB) Viewed 2823 times
Brand new piston
I've seen brand new pistons with a blind hole. Zooming in on the hole, it looks that it could have been used as part of the manufacturing process. The hole is flared, and there's a hint of a raised edge - like a tapered center was pressed into the hole. Maybe a post piston casting operation - finish sizing(?) uses the hole as a reference.
NewPiston_Cavity.JPG (37.05 KiB) Viewed 2823 times
motormike wrote:How could a piston with a hole through the top have compression?....I'm not a scooter tuner
Mouse "I've seen brand new pistons with a blind hole."
-- Blind hole --
"A blind hole refers to a hole that is reamed, drilled, or milled to a specified depth, thus without breaking through to the other side of the workpiece. The etymology is that it is not possible to see through a blind hole. Other types of holes also include through holes, and clearance holes. In this instance blind may also refer to any feature that is taken to a specific depth. More specifically referring to internally threaded holes (tapped holes)."
Bottom view of mouse's piston with the 'hole' centered in the dome
There is no compression loss in the NOS piston used for this example
excellent,.....machinists' lingo 101. I am not a machinist, nor have I played one on television.
I have a "blind" hole on my belly....with lint at the bottom.....
RideRed <(2)Gyro S><Helix><Sym Wolf> "Live Large - Ride Small"
So I finally got my new top end kit and installed it. I tested compression to be about 130PSI and then rode it around the block for a couple minutes...it is now reading just under 90PSI. After a couple minutes around the block the motor backfired and wouldnt start again. I have narrowed it down to a fuel delivery issue and think that the petcock isnt allowing fuel to flow into the cylinder effectively. Also, the intake manifold got really cold during my short ride. Is this normal? I am running out of things to troubleshoot...
yes the larger manifold gets cold like that Did you just install the kit and take off down the road or did you do proper heat cycles before you rode it? If you installed the kit and just decided to go for a spin and tried to go full throttle you may have damaged your cylinder or piston...........hope not but these scoots rarely backfire
1986 Spree/ Eve af16 swapped 65cc Prodigy vari 8g 9:1 Ruima gears
21mm OKO 42/100 32:1 45mph kymco zx exhaust
1994 Elite S 17.5mm Arreche, Daytona reeds, SEF ramp, pump plugged
1987 Elite SE50 af16 swapped
1989 Elite E SB50p
2003 Yamaha Zuma
A 40-lb. Loss of compression? I fear you've found your trouble.
Did you increase the jet size with the big-bore? Break-in Legends abound, but the main thing is not to overheat with small jetting, to high compression, or too hot a plug.
I recommend you pull it apart for a close look at the cylinder and piston.
Wheelman-111
Most of my money is spent on scooterparts. The rest is just wasted.
"ISO": '03 Vespa ET4 Malossi187 74MPH
Flash 9: 2001 Elite SR Contesta 72 ZX Tran, 9:1 Gears, Stock Airbox/Carb/Pipe 58.8 MPH
Punkin: 2010 Vespa/Malossi S78, 61MPH
I definitely lubed the piston and cylinder walls upon installation. I installed a brand new 21mm Carb kit from Taz at the same time as the top end install. I did not change the jet sizing as I assumed that this carb was much bigger than the stock carb. I also ran the plug that VT Steve recommended and sent with his big bore kit. What is the normal range of compression for this setup??
On Pump Premium fuel, you're taking a chance if you run much higher than 125 PSI. The gauges are anything-but-guaranteed accurate, so test several times. Use two different gauges if possible. Open the throttle wide or remove the carb altogether to get most accurate reading.
Wheelman-111
Most of my money is spent on scooterparts. The rest is just wasted.
"ISO": '03 Vespa ET4 Malossi187 74MPH
Flash 9: 2001 Elite SR Contesta 72 ZX Tran, 9:1 Gears, Stock Airbox/Carb/Pipe 58.8 MPH
Punkin: 2010 Vespa/Malossi S78, 61MPH
from 130 psi to 90psi = i would check the crank lateral play cuz your crank seals could be leaking, i was told not to install a bore kit on an old crank, there are other factors involved
maddog wrote:from 130 psi to 90psi = i would check the crank lateral play cuz your crank seals could be leaking, i was told not to install a bore kit on an old crank, there are other factors involved
In less than an hour of running? You really are insane.
Bear 45/70
'83 Aero 80 X 3
'84 Aero 80 X 3 '85 Aero 80 '84 Aero 125 X 2
'84 Aero 125 '84 Aero 125 X 2 '85 Aero 50
'85 Spree
Depending on how much oil was in the cylinder and the accuracy of the guage, could be that 90 is a good number after the oil has been removed or moved out the tail pipe.
Pete M wrote:Depending on how much oil was in the cylinder and the accuracy of the guage, could be that 90 is a good number after the oil has been removed or moved out the tail pipe.
90 is never a good number. Somewhere between 70 and 80 the engine will not even start. I have an engine that is at 90 and it starts and runs but then it is an 80 cc engine.
Bear 45/70
'83 Aero 80 X 3
'84 Aero 80 X 3 '85 Aero 80 '84 Aero 125 X 2
'84 Aero 125 '84 Aero 125 X 2 '85 Aero 50
'85 Spree
Did you just install the kit and take off down the road or did you do proper heat cycles before you rode it? If you installed the kit and just decided to go for a spin and tried to go full throttle you may have damaged your cylinder or piston...........hope not but these scoots rarely backfire
