Vespa PX200 - Malossi - Blown engine

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Hi everyone,

My 1988 Vespa PX200 was modified last summer with a cut crank (Serie Pro 57 mm stroke), Malossi Sport 210 kit, Malossi PHBH 30 (125 main, don't know the pilot unfortunately). It was ported and polished. The ignition was set at 19 degrees. I also had a BGM Bigbox installed as well. I was pretty careful breaking it in and I was pretty happy for a while. I also added a SIP speedo so I could se the cylinder head temperature as well. The spark plug was NGK B7ES.

About a month and a half ago, I decided to see what it would do. I warmed it throughly and gave it the beans. The first run was pretty OK, and I checked the temperature and saw 127 C so I thought everything's fine and did another WOT on the return leg. The temperature was looking fine and I kept going. The whole run was actually quite short, only 6 kms when I suddenly lost power. I pulled the clutch in immediately and noticed the temp gauge which I hadn't looked at for about a minute at most read 210 C. I couldn't restart the bike so I called someone to pick the scooter and had it transported to the shop where it was modified. You can see the results in the pictures (please insert "dumb f..." comments here )

https://flic.kr/p/R2DxEM

https://flic.kr/p/PKhBXu

https://flic.kr/p/R2DxC2

https://flic.kr/p/QsuHfE

https://flic.kr/p/QRbKDM

https://flic.kr/p/QsuHcU

The cylinder, piston and piston ring are destroyed. The mechanic says that the cylinder and the piston were oiled sufficiently so it's probably a case of poor mixture. I shall be replacing these and I don't want the same to happen again obviously. A larger main jet seems to be needed, what else should I be paying attention to? Was I wrong in going at top speed for a prolonged time? May be I should've backed off as soon as I hit the top speed (128 km/h according to the GPS). My electrical system is standard (except for the addition of a battery for the SIP speedo) and I felt like it wasn't putting out good juice as the blinkers and the headlight weren't as bright as usual. Should I also modify the electrical system (spark magnifier, variable ignition etc)?

I'm not after top speed but reliable, clean and strong running at real world speeds. Your comments will be much appreciated.

Murat

[GeorgeS]

Looks like a 4-corner type seize, which I think is more likely to be weak mixture related rather than giving it too much from cold as you say it was nicely warmed up already.

High running temps could also be related to an airleak perhaps (again, results in weak mixture).

[Jack221]

You could get away with putting that piston back in. If you have a new ring in your garage, then fit it but that would have to be way worse for me to waste my money on a new piston.
Spark plug will be ruined so you will need a new one of those for sure.
The colour of the piston crown says it all....way too weak for a flat out run. Take more time to set the carb up properly and it will go fast and run for ever. The main jet seems a bit small but may be ok and the carb might not have the right atomiser or needle fitted to give enough gap when wide open, you will need to check carefully before doing it again....just saying.

[Rsport220]

Piston top looks very grey! If it was mine I would repair / replace the damage and then try the following; measure the fuel flow through carb into float bowl, change the plug for a 8 or 9 heat rating, change the main to 135/138, knock the ignition timing back to 16 deg. Also like the previous post mentions, I would check the needle tip / attomiser sizes are not restricting the main jet, there is a calculation in the dellorto manual for this. Put some fresh fuel in the tank.
One thing that you don`t mention is the compression ratio, are you using the malossi sport kit with their own heads? Malossi state something like 12.5:1 which seems quite high for pump fuel.

[drunkmunkey6969]

Looks like a combination of weak jetting, wrong plug and incorrect ignition setting.

Jetting a carb involves more than just a main jet:

Air/mix screw
Pilot jet
Main jet
Atomiser
Needle
Clip setting
Float/tap valve

Plug should be BR8ES round town and BR9ES on long runs.

Ignition should be circa 17 or 16 degrees static.




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[Adam_Winstone]

Yep, I didn't get past, "The ignition was set at 19 degrees.", without thinking that this was likely to be a big factor. Indeed, I was looking for it as I've had little 'dabbling' with Vespa engines but my first intro to Malossi 210s was having to reset the static timing on a mate's 210 that had just holed a piston... a couple of hundred miles after having the jetting set up on a respected tuner's dyno (so carburation was not going to be the issue).

I did wonder if this was coming into the realms of fuel starvation and needing a fuel pump, however, I reset it to 16 and it never had an issue again, regardless of being held at WOT for long periods.

Adam

[arrow]

...One thing that you don`t mention is the compression ratio, are you using the malossi sport kit with their own heads? Malossi state something like 12.5:1 which seems quite high for pump fuel.

The head is Malossi's as well. I wouldn't expect it to have such a high compression rate, I thought it was mellower than the race kit by some margin. Should I be adding a gasket to reduce the compression rate then?

[mick1]

I'd be checking the piston to bore clearance as well.

[Rsport220]

If you add a gasket as the squish will also change (not always a problem but worth noting), machine the bowl bigger if you need to but you should really measure the compression you`ve actually got before changing anything. I`m sure malossi know what they`re doing and if its not been pinking due to high compression then I wouldn`t worry about it.

[225worb]

I've been running malossi's for a good few years now from the early spec kit to now the MHR...
Running a one off reed set up on an old group 6 race engine I started using it on the road, 30mm dellorto, 18 degrees withlightened prange flywheel reworked standard head, blah blah...fast and very reliable and putting out high bhp for its time... used for sprinting too...
After a few changes over the yearsadding a fuel pump and broken cases... GULP!!
I've not had any seizures or holed pistons only major engine(case) failure
So I'd look towards fueling...
As I say 18 degrees has always worked for me...
Good luck