Vespa PX200 - Malossi - Blown engine

[Adam_Winstone]

^... 18 ... don't get me wrong, I'm not saying that 18 can't work, far from it, but all else must be appropriate, i.e....

"for its time", when fuel was better and didn't contain ethanol.

"old group 6 race engine", race spec' so I assume that the exhaust duration was raised, dropping the corrected compression ratio.

"reworked standard head", and I have no doubt that you got the corrected compression right, rather than just bolted on any old head?

"adding a fuel pump", so ensuring things stay rich and cool.

... the above are all factors that will influence the ignition requirement and show that you know what you're doing, rather than just keeping your fingers crossed. Indeed, that 1 degree from 19 (original post) to 18 can make all the difference too... and shows the right direction that the original poster wants to go with any ignition change.

Adam

[Jack221]

For the record mine has been on 18 degrees for years. No issue. Piston still un-dented.
Its usually bad jetting or too big squish clearance causing all the problems.

[Rsport220]

With many opinions it would seem an adjustable ignition like the Kepher would be just the ticket. Change the timing with a pot twiddle, lap top, phone or even make your own curve, so much easier than physically moving it. I`ll find out soon if its worth the money!!!!

[225worb]

Do you need rings??
I have some low milage oned If that's any help....

[holty]

i think rssport220 comment the piston is very grey, hits the nail on the head, the crown is very light coloured which indicates a weak mixture, as everyone knows knows the fuel coming in cools the engine down, as for cause, main jet would be my guess,too small, been there and done that, or an air leak, did you do a leakdown test ? and adams recomendation of 19 degrres could be a factor as well, 17 degrees is the more reliable setting these days with the poor fuel we have, i fitted an egt gauge to my lammy after melting a piston, i think you get a more instant reading compared to cht, you do have the chance to pull the choke on if its getting warm, providing you look at the gauge of course, good luck with getting it fixed and set up ,
holty

[Adam_Winstone]

"and adams recomendation of 19 degrres", just to avoid any confusion, I wasn't recommending 19, rather was pointing out that the original poster says that his was set to 19, whereas 225worb mentioned his being set to 18. I agree that 19 could be a factor and that 17 or 16 might even be a better option for keeping it cool.

Adam

[Jack221]

The Kheper ignition is a great idea. Being able to set the timing by phone means no wasted power. Is there anywhere to buy one? Seems like they are out of stock. I did have a Kheper version 1 but it blew up. I then had a Kheper version 2, that lasted much longer and blew up two of those. I am a little cautious about the Kheper version 3 but will give it a go if I can find one. To dare is to do, so they say.

[arrow]

For the record mine has been on 18 degrees for years. No issue. Piston still un-dented.
Its usually bad jetting or too big squish clearance causing all the problems.

Thanks a lot, much appreciated. I have ordered the replacement piston, rings and cylinder already. I will keep it in mind though, if I blow the engine next time (God forbid ).

I will raise the main jet size, purchase an electronic ignition. I haven't done a leakdown test yet, I will definitely get it done before I attempt to run it again.

One more question; Are modified Vespa's good only for sprinting? Even if I had the jetting and ignition right, should I refrain from long top speed runs in the future? I'm not planning on it, just curious...

Thank you for the replies so far; you all have been great help.

[holty]

"and adams recomendation of 19 degrres", just to avoid any confusion, I wasn't recommending 19, rather was pointing out that the original poster says that his was set to 19, whereas 225worb mentioned his being set to 18. I agree that 19 could be a factor and that 17 or 16 might even be a better option for keeping it cool.

Adam

sorry for the confusion adam i didnt word that very well, i know you wouldnt recommend 19 degrees and neither would i, we both know that a reliable road going scooter must have a certain amount safety built into the way the engine is set up, not too weak, not too much retard on the ignition, not too much compression, all things to take into account when building an engine, can you do long high power runs when you have rebuilt your engine? it will depend on all the above factors, i do also think that holding the throttle fully open for a long time is probably going to do more harm than good, unless of course you can monitor egt temps and can tell if your engine temp is normal or if its getting too hot, and you could use your choke to cool it down and prevent your engine being trashed again , there are other factors to consider, quality of fuel, which seems to cause quite a few problems, ambient air temperature, and air pressure all affect mixture, best power will be made when your engine is close to detonation, but i try to run richer mixtures than that to give more reliability with the sacrifice of max bhp , engine set-up can make or break an engine.

[Adam_Winstone]

^... good advice about considering all elements and trying to get a 'balance' of the sum of all parts, rather than thinking along the lines of any one aspect.