Soft seizure on Polini 177, PX150,?

Hi, I have a '05 PX with a Polini 177 kit, 24mm carb, polini pipe and a racing crank (cut) Mazzuchelli), I matched the ports and the timing was left original (18°), I installed first a 26mm carb (24 modified by SIP), but with a 125 main jet, it was too lean so since I don’t have bigger jets in stock, I reinstalled the 24mm with a 125 main jet. I rode it for 100km when I had a soft seaizure at around 100 km-h…I removed the piston and cylindre, cleaned with a file and sand paper the glazed and hardened spots, and resinstalled that. the mixture looks fine, but today, at less than 95 km-h, on a flat road, I had another seizure…
now, what’s your suggestion? I found a 130, I didn’t try it yet, but before to ruin anything else…what’s your suggestions?


Hi, I run a Polini 177 with 28 mil dellorto,muzz crank,cosa re-enforced cluth RZ pipe etc. I had the very same problem until a friend of mine told me what they do on motorcrossers. When you remove the cylinder fron the engine and inspect the piston you will see some glazing around the areas of the seize. drill one or two very small hloes through the skirt then countersink slightly. You can put a light line from one hole to the other in close proximity.Once this has been completed lightly sand with fine wet and dry the glazed marks on the piston side. Dont go mad though! Before re-fiting check the piston for small cracks,as mine had one around the gudgeon pin housing. If this is the case renew the piston. If you need pictures of the piston please contact me at [email protected]


Hoe this helps as ive had no problems since.