Piston seizure at full rev..help!

Hi All,

My initial setup on my PX125 disc was Polini177 (no porting) Sito plus exhaust,20/20 carb, 108 main jet and BH7s spark plug, built by MSC and ran without problems, done about 8000 miles.

I then upgraded the carb to 26/26 and upjetted main jet to 120 and same again ran fine even at top speed.

I have now went for Simonini and upjetted to 125 main jet but on my first run out at full throttle and roughly 70 mph the piston siezed and I pulled the cluth before I was thrown of the scoot, I started her back up 5 mins later and all was OK, and ran her for another half hour to see if any damage and she ran fine but I did not go up to full throttle and I was shitting it. I still had the BH7s in but I have just changed it to a Bosch WCA3 which is equivilant to BH8S, could this have caused the seizure?

Is the main jet to lean?

Should I go back to Sito plus, the Simonini seems a big improvement I must say but do not want to ride my scoot every day with my fingers on the clutch feeling nervous, I would like to keep the Simonini if anyone has any suggestions

I recently decoked the Cylinder and did not use a torque rench to tighten the head back on, could this cause piston seizye? help please…

Regards,

Brian.

that is very strange as by what you are telling us it seems that setup would be really rich. I run a 125 main on my p208 fully ported cut crank. ported intake light flywheel jl exhaust etc. and thats a vortex 24 carb.

Curare,

[mark=yellow]as far as for the main jet it seams it can’t be too lean to cause the problem you had, but it is always a good thing to check it with the plug chop technique.[/mark]

Have checked plug and it looks good, maybe a bit to good, plenty of carbon fouling.

[mark=yellow]The ignition advance for your polini has to be set at 18[/mark]

Should be, I am pretty sure MSC set this up at 18 degrees.

[mark=yellow]So the grade of the spark could have cause it expecially with a timing set a little too advanced.[/mark]

This crossed my my, as per previous conversations on this topic.

[mark=yellow]The prob. is that now even with the right set up your polini could tend to seize.[/mark]

Strange, I used to travel motorway every day with the Sito, Polini etc with no worries, never any pinking, now I have added the Simo it the piston nipped immediately…I thought Simo ans Sito were not that much different, may have to go backwards to the Sito again.

Curare…do you think torque settings may have caused problem?

Also Tipwise, I think you are reffering to the polini 208 when you speek of re honing?

Regards,

Brian.

Curare,

I will check timing to be sure, I had a look at your webpage, very nice, I see you are a Polini man yourself…have you experienced any nipping of the piston?

Brian.

hello,

as far as for the main jet it seams it can’t be too lean to cause the problem you had, but it is always a good thing to check it with the plug chop technique.

The ignition advance for your polini has to be set at 18

the spark plug grade you were using (a 7 grade on NGK scale ) is too hot.
The Bosch W3 you are using now is correct.
If you go to NGK get a 9 grade.

So the grade of the spark could have cause it expecially with a timing set a little too advanced.

The prob. is that now even with the right set up your polini could tend to seize.

I totally forgot. If you didn’t have the barrel re-honed then that is your problem guaranteed. Polini’s come way too tight out of the box. You need to re-hone it to around 4 thousandths clearance to be safe. Take it off measure the clearance and take it to a good machine shop to re-hone it.

no, torque setting didn’t cause the problem…
but when you use a hotter spark grade then you should, with an exhaust as the Sito is one thing, but when you start getting out from your engine some more power and rpm. using an expansion, then it is a problem.

So spark plug for your engine should be:
NGK 9
Bosch 3

and check again timing with a strobo just to be sure…