Modern Vespa

Hello,

I expect the answer to that question is yes - so here is another.

I am looking for advice as to what I should be aiming for in terms of port timings.

I have a p125x and which I have put back to 125cc with a cyclinder. I got one online, supplier says its identical to the original P125x, but its a Goetze.

Now, the engine cases were modified to enlarge the inlet port from the carb. So it opens before and closes after.

Piston is pretty much flush with the top of the cylinder at tdc.

57 stroke, 105 CR length

Assuming tdc is 0-deg
Exhaust open@ 105.5 deg (38.5mm from top of cylinder)
Transfers open@125 deg (50mm from top)
Other middle port opens@ 128 deg closes at 232 deg
Transfers close at 236 deg
Exhaust closes at 255 deg

Rotary Inlet starts to open @246 deg
Rotary inlet fully closed @ 46 deg

Would anyone be able to shed some light on exactly what I should be looking for in terms of the above values?

One of my principle questions is whether the lengthened rotary inlet will be causing me problems to tune the carb.

many thanks
Chief

No issue.

While you're in there it could be made slightly faster. The new cylinder likely needs the rough edges cleaned anyway.

Your timings are absurdly low right now. Based on your measurements (and assuming a flush deck height), you're at:

154 ED / 94 TD / BD 30

That's going to have you running out of port timing almost before you get into the power curve.

You'd need to raise the cylinder like 4mm to have something usable (171 ED / 118 TD / 26 BD) .

If you dry fit the cylinder, how does the piston sit relative to the ports at BDC? Is it above, below, or flush?

Can you re-measure deck height and get a dead-on accurate number, because you're going to need to raise the cylinder, now we're trying to figure out how much.

You're also most likely going to need to grind down the head to get a useful squish, so I hope you have a belt sander you can use, because it's going to need to come down a lot.

chandlerman,

Appreciate your taking the time to help.

for info - The cyclinder is on and it runs. But, there is no location pin present and the cylinder looks a bit loose so im not sure its locating centrally.

Now to answer your questions:


When you say deck height. Which dimension exactly are we talking about? At TDC the piston edge is pretty much spot on with the edge of the cyclinder.

At BDC the piston does not fully open the ports. I would say there is around 3-4mm of the port still remaining at BDC so the exhaust port never fully opens.

If you are saying that the cylinder needs to come up by 4mm, going on where the piston is with relation to the port, this does not surprise me. However, how can this even be? I am trying to fit a "stock* cyclinder to the engine, how could it be needing so much work? Is something else a miss here?

I have a belt sander btw, but the head does not have 4mm material to grind. I would say lucky to have 1mm.

I will take some pics tomorrow and we can see how this all looks.
Thanks again
Chief

Thanks for the advice. I did bevel off the port edges, but Im wondering now whether this cylinder ic actually right for a p125x if its so wildly out.

The 125/150's don't have a locating pin like the 200, so don't worry about that. Or don't worry a lot, anyway. There are solutions, but those are out-of-scope for the moment. Deck height is the distance from the piston crown to the top of the cylinder at the point where the piston touches the cylinder wall.

I'll go get a picture of how I measure it once I wrap up my work day. Getting REALLY accurate measures for all of this makes a big difference in calculating port timing, because a millimeter can make a material difference in the measurements. This is actually GOOD, because it means that we can raise the cylinder without needing to worry about opening the intakes. The only repop parts that are built to anything approaching an actual spec are performance parts, which a stock cylinder replacement most definitely isn't, so it's more likely a copy-of-a-copy-of-a-copy-of-a-copy, with tolerances slipping accordingly.

If the cylinder is effectively 4mm too short and the ports are also 4mm too low, then all you need to do is stick a thick base spacer on it and figure out your squish. This is fairly Best Case, to be honest. Stock heads usually have like a 3mm rim, but we can tackle that problem when we come to it.

If your deck height measurement is off, any variation from zero on the final measurement will reduce the need for the spacer height. Roger that.

Hello again,

So I've been thinking. Maybe it was a mistake to use this cylinder in the first place the exhaust doesn't fit that well either now?

I've looked at the old cylinder which is a piaggio one. The exhaust lined up better with that one, so maybe it would be worth getting that one back on or at least trying it if I am going be playing with spacers.

Added some photos of that one and did some measuring. Exhaust opens 40mm from top. closes at 62mm. The transfers all seem to be at same level. open at 48mm close at 59.

I seem to remember that the piston never completely cleared the exhause port on that one either. Can't remember how much was left though.

I made a short video of the idling today. it's at the following link. A fee rev ups towards the end.

?si=NUs40kqTA43NQpSz

Yesterday i took her up a really steep ride. parked at the top, and the exhaust was a bit smoky.

So now, what do I do? keep this current cylinder on, or put back the piaggio and see how it goes with some spacers.

Still don't really know where i need to get to with the timings. Please help me through this journey:-)

Chief

The real question here is "what are you trying to accomplish?"

If you want more power & top end, any of the modern performance boxes combined with a modern 177 top end will give you significant improvements in performance.

You'll need to adjust your jetting, but we can guess up front to about 90% accurate what that will need to be (140AC/BE5/116MJ).

Otherwise, take the "if it ain't broke, don't fix it" approach and just run what you've got.

This was helpful for me, once upon-a, I was looking for easy and fast, was too impatient for all that math-

https://www.scootering.com/scootering-classics-vespa-tech-part-2/

Hi Chandlerman,

Thanks for coming back again.
So just to recap - I have been putting back together a 1981 P125x which had the cylinder and case buggered about with.

As the port locations looked a bit odd and the case inlet had been extended, I was looking for information on what would be optimal in terms of port timings and inlet timings.

While i can measure/calculate where things open and when they close, I have no idea of what would be optimal. for example (i)What blowdown should i be looking for, when should the rotary inlet open and close with relation to the ports etc.

Are there any guidelines for this? Is my inlet open too long? what would this mean for the functioning etc etc. I can find, many resources of how to map this stuff, but not really any straight forward guide. For instance, in your first post you referred to my not even reaching the power stroke. How can I know where this is and when it will be reached etc.

I don't want a 177 upgrade, just want to know that my 125 cylinder is working optimally.

Hello Mr Mod Eric and thanks for your advice.

I did came across this article and it does give some figures:

"The standard machine had port timings of 112° of transfer and 153° of exhaust and 20.5° of blowdown:

A CONSERVATIVE figure for a low-power/mass-produced/off-the shelf touring machine might be 122° transfer, 168° exhaust and 23° blowdown.

So again, the standard factory figures were way out. However, by fitting the 1.5mm packer (instead of the 0.2mm item) we now have 119° transfer, 158° exhaust and 19.5° blowdown. So although this has slightly reduced the blowdown timing, both the exhaust and transfer are raised in one simple manoeuvre."

So this guy found the standard machine to be less than optimal, and has increased the 112 to 119, the 153 to 158 and blowdown was reduced.

This gives me some useful data as it implies a certain improvement. Did you make similar changes? any practical learnings'

I would definitely be happy to do some maths, if it will help me understand what is going on

Cheers
Chief

Here is a chart I found that gives a rough idea of different cylinder timings and what they accomplish. How you set it up depends on what you are trying to achieve.

The factory timings are conservative and favor low down torque with a predictable and forgiving power band. It also allows for a fair amount of manufacturing sloppiness, use of inexpensive materials and user abuse and neglect. Stock engines seem to have a fair amount of tuning potential, but at a certain point these changes get closer to exceeding the limits of the materials and design.

Usually, increasing performance by modifying stock parts is trickier than installing a cylinder kit. It all depends on what you want to accomplish .