First off, if you don't like to rear, I tend to make a novel of what I'm dealing with. Everything is in the details and I'm just trying to list everything out at first to show what I'm dealing with.


So lately I've been having a problem trying to get my bike to start. Well, ongoing issue more so.

Last time I took my bike for a ride was early/mid March.

I tried starting it up two weeks later and nothing, no matter what. Found my carb kept overflowing so I think I've got that addressed since readjust my float level (Mikuni TM24).

Since it wasn't starting with the carb, I pulled that off the intake and sprayed some starter fluid through the plug hole, put the brand new plug back in and cranked it over and nothing.

I pulled the spark plug and put it up next to the frame and found that yes I do have spark. I never put it next to the cylinder head because the frame was right there. I then found out that the ground was mislocated. It had been attached to the bolts that hold down the reed cage. Since correcting this issue, I have not tried to start the bike. Instead I started to rule out other issues just in case.

Compression: Upon first crank, I get about 60psi and the 2nd or 3rd pump brings it up to 90-92 psi. The only thing I found for what the compression should be is that around 100psi is ok. Now given that my bike has a big bore kit to it, I don't know what the psi should be but after talking to a local scooter shop, they said 120 is low and should be around 140psi.

With that information, I decided to pull the top end off and check for ring wear. I then removed the head, piston and the single ring for inspection. Checked the ring gap and it came to 0.013" (0.33mm for you metric folk). From what I can find, new should be between 0.15-0.30mm with a wear at 1.0mm. So according to specs like those, the ring should be good, right?

Piston size: It's a 49mm (measured size of 48.8mm). So apparently I have a 75cc kit.

After looking at some bore kits, they specify that a longer crank is needed to fully utilize the bore size. I have no idea if mine came with an aftermarket crank or not.

This is such an odd starting problem that's got me confused. I'm about ready to pull the trigger on a new top end, possibly a Polini 80cc kit just so I know that everything on the top end is new. It would eliminate any confusion(a given for good compression) and narrow it down to the carb since I can verify that I am getting spark.

I have also looked into getting a 50mm Malossi piston and having the head bored out more but due to the tight fit from the cap to the head, I'm uncertain if that would even be a possibility.

Any advice? Recommendations on bigger bore kits in the 70-80cc range?

I'm just using this bike for putting around town as my main toy is Ducati 916.

The top end:


The bore, no scratches or marks of any kind:


The piston: