Advice required

[cruyff]

Bought my Px200 last year and just before bad weather set in I had a heat seizure at about 70mph. Have new piston ( 1st oversize) and barrel just been honed, coming this week. The scoot was standard apart from a simonini exhaust. Checked the carb today and the main jet is a 116, which is the standard main jet, but would it run to lean with the exhaust and thus cause the seizure? With new piston and a sip road 2 exhaust what would be the best main jet to start with ?

[henri]

with a sip road 2 i'd start at 120 an probably end on a 118 ,if auto-lube that is . if pre-mixing i'd start at 122 an have a 120 standing by . H

[cruyff]

Thanks Henri it is autolube any tips on how I should run it in ?

[Robjack]

I've got exactly the same set up and 120 main wasn't big enough, 122 is spot on for me. I'd go with that while you run in then do plug checks afterwards to see if you need to drop a size or two.

When I had my rebore and oversize piston, I kept the throttle below three quarters for 5/600 miles then start opening up in short bursts for the next few hundred. Don't let the engine labour on hills, better to be at higher revs than do that. You will get 10 different ways to do it if you ask 10 different people, but this worked for me.

[henri]

as above ,+ use mineral oil for first 500 miles ,engine will bed-in faster on that , then switch to semi-synth .H

[cruyff]

Thanks guys will heed all advice given , will bet more questions will follow

[pxguru]

Firstly a heat seize has to be a very bad one to need a re-bore. Several big scuffs on the piston is no problem at all for a PX200. Won't affect performance any. As long as the bore is not worn oval or deep scored above the exhaust port it would be fine.

Another point which is a bit late for you but might help others reading. The original standard piston rings on a 200 are not that good at sealing (all the black burn marks below the rings that you always see). Grand Sport (GS) do thin ring piston kits which are direct replacements that work better. Beedspeed also do a thin ring 68mm piston which works better and will rebore a standard barrel to 210cc. Either of these make slightly more power than original.

A good starting point for running in is a jet 10 numbers over what you think. So in your case a 130. If it splutters way too much reduce it one jet at a time until its ok enough to ride for the running in period. Don't try to jet it in exactly until it is actually run in.

[cruyff]

Pxguru, thanks for the reply. After the heat sezeiure I lost confidence in the scoot as a lock up on the motorway at 70mph is well scary!!!!!!!.
When the piston was pulled it was badly marked with one particular deep score the length of the piston. I could not find out if the previous owner had this problem as well. The guy helping me who has decent knowledge to be fair said the piston could possibly go back in but as it will be me riding it I don't care what the cost is. In your opinion could the 116 jet with the simonini pipe be the likeliest cause of it. While the scoot is off road I am taking the time to replace all cables and a couple of minor things

[henri]

i'd say a 116 n simmoni + 70mph thrash on motorway=seize every time .H

[cruyff]

Thanks H , is it better to sustain a speed of about sixty if you are covering distance?

[henri]

i dont go by the speedo ,but were-ever the engine seems most comfortable between 2/3rds an 3/4 throttle .leaving me something in reserve for hills/overtaking etc . H

[cruyff]

That seems like sensible advice

[pxguru]

If required, a standard engine correctly set up PX200 should be able to cruise flat out all day long. Not very efficiently though.

As money is no object, if you haven't run your re-bore yet you might be able to exchange the piston for the modern GS one, instead of the one with the 1960's technology. The 1st oversize is exactly the same size so the re-bore doesn't need to be re done.

www.sip-scootershop.com/en/products/piston+grandsport+200cc+_12091000
www.sip-scootershop.com/en/products/~/products/piston+meteor+for+vespa+200_15629200

As H said, 116 jet (even with autolube working) and a non standard exhaust will heat seize every time! 2 stroke engines will not run flat out for ever with a weak mixture. Rich jetting runs fine but performance gets worse, anything from slightly under jetted they can heat seize, slightly more under jetted and if you ignore all the signs, will eventually melt a hole in the piston!