Modern Vespa

Hey all,

I have two p200s. Both set up the same.
Both stock set up. Stock jetting. Both with sip road 2 exhausts. The red one has a 125 mj a d the blue one has a sip road 2 sport with a 122 mj.

The red one is a stock p2.
The blue one is a px125e with p200 motor.

Both run well.
However the red one is just faster. Smoother engine, better top speed, lives at 80kph with room in the throttle to pass a car if I wanted to.

The blue one. Not so much. Idles well. Takes off well. But top speed maybe 80-90kph if I really push it. The engine is relatively newly built vs the read one has 12000 uninterrupted kms.

My question is why doesn't blue run the same as red?

Trying to deduce here: the blue one has an American style cylinder head with a squish band of 3.6mm.

Red one I'm unsure of squish or cylinder head.

The exhaust is different blue has standard sr2 and the blue has the sr2 sport.

Could I be running out of gear? I never counted the gear teeth when I reassembled the engine. Maybe it's that----but doesn't feel like it.


Any speculations?
Or do I really start digging? Where would I start?

Thank you everyone.

N

I guess there might be a number of reasons, different heads perhaps, compression different, timing not the same, quality of spark and so on. There was a 10hp and 12hp P cylinder mentioned on SIP, not sure if this was world wide or just Germany.
The SIP road sport, it has 2 pipes right? I've seen dynos where it has an extra 1-2 hp from memory.
You could try swapping stuff between them to see if anything makes a difference, but I would start with compression. Then check and compare heads.

yup, first things first is to check compression. The do a "wet test" compression check to see how much the results change.

Wasn´t that smaller carb for US market too?

That does ring a bell.

Thank you for your responses everyone.

Yes---this engine---lots to unpack here.

More to the story that we can dissect. I bought the engine in pieces off a fellow scooterist locally. I believe the cases are that of a rally 200---or so the vin suggests so.

The carb is 24mm spaco carb. The cylinder was bought off another scooterist and bored to the 3rd oversize.

So the engine is a kind of a bitza.

Timing is appropriate.
Jetting is stock.
spark is strong.

I will check compression.

What is wet compression?
If I have low compression, would that affect the top speed?

I read that some US market p2s were designed with "emissions" in mind so that they sold them with a larger squish band and a 20mm carb.

What would symptoms of wrong gearing be? Because I don't want to rule that out either.

Thanks everyone. This helps me to process and have some kind of direction.

Even engines with identical parts can vary from one another in performance. As you probably know, swapping out the stock 4th for a shorter gear is a popular choice. Neither choice is technically wrong, but you would notice a stock 4th by the need to rev higher in 3rd before shifting.

You measure the volumetric compression ratio, not the actual pressure.
When the difference is in the combustion chamber in the head you can simply measure how much fluid it holds = the volume of the combustion chamber.
Since you know bore x stroke the rest is simples.

Wet test is just squirting a little bit of oil down the spark plug hole. It creates a better seal for the piston rings. So let's say you have a 90psi reading on your compression tester (for a standard dry comp test), squirt a bit of oil in the combustion chamber. If the comp reading increases quite a bit, then that's usually indicative of worn out piston rings.

Learned something again!

Nice one for when you go buy a scoot.

Ordered a kit. Dunno what happened to the previous set.

or a car

Yes but.... modern cars can be a challenge.

A little off topic maybe, but I don't think 3.2mm actually counts as squish! Not squishy enough from what I've read.

Seems to be 2 different 200 heads. Not sure about compression differences as no one measured the volume in the thread, but the raised portion is quite different in these two photos (from here P200 'USA' cylinder head mystery

Weird as on a 2-stroke it is an easy thing to take the hat off and measure. Would have thought that a major fact in the enigma.

The c.r. ; volume of the combustin chamber, is directly related to the mean piston pressure = power. This is the only reason why running on ethanol/methanol is a thing. The drop in caloric value is less than the increase in pressure possible without detonation.

Hence shaving the cil. mating surface was(is) a Stage 2. tuning thing in 4-stroke bikes.

The smaller carb diameter obviously has a performance effect too but the effect is not as one would expect and often results in bétter performance at lower revs.

For top speed though; both carb and c.r.

The mystery deepens w/ another look at that head that came on top of their exclusive Cosa II nicasil lined aluminum cylinder. Can somebody please try to explain???
Cosa head is on the right