Modern Vespa

Dremeling the bowl on a stock 200 head and closing the squish clearance is something people have been doing with varying success since the 80s.

I'm not aware of any of todays shop brought heads for Vespa's that have a potential MSV issue.

I'd say on this scooter if the jetting was correct, it would still be running today.

Lots of things going on here. Slapping on parts and waiting to work is not easy. You mentioned a BGM long stroke crank. Why did you go for a 60mm crank?? What are your port timings, inlet duration, blowdown, etc.?? What were you aiming to achieve??

Built by previous owner. I have asked but he does not remember witch Bgm crank it is.

Johan,

Can you clarify "modification" to the head? I read that before and wondered, but now understanding you did this to both heads?

Are you grinding inside the head combustion chamber?

This area, if not smooth, would create all kinds of "hot spots" that would explain detonation.

When you say "modify", do you mean just the edge so it sits lower, or inside?

If your spark plug threads have been exposed, this too would explain the issue.

Post a picture here so we can see?
Have a spark plug installed.
We might otherwise be chasing ghosts.

Just the original head, not the Polini so thats not what that was about.

On the original: I extended the "sparkplug" area not up where the sparkplug sits but to the sides a little bit inside the combustion area.
Sorry have no pic.

I'm with team "hole in the piston is about ignition, not jetting".

Get a strobe on there and see if it all goes to hell when you get high in RPM. something is very fishy on the stator/cdi side.

I Second. Bad timing, and lean fuel mix = piston melter.

Its strobed at high rpm. Went from 18 to 17 deg when I reved up. Thats it.

It makes a strange sound like it gargles when it jerks/fourstrokes at low throttle.
Did that with the Polini to.

Have tried another carb, cdi, stator plate all kinds of jets.

Maybe you'll have better luck than I did asking these questions.

Like I said I did not buy or build the engine. The crank was already there and the engine was fresh rebuilt.
The only thing I did was the thing with the dremel.

I will change the head and put in a 1,5mm foot spacer.

I think you are on the right track. From what I've read, the 60mm crank with a 1.5mm spacer should give you a distance from the cylinder top to the piston edge at TDC (a/k/a "piston before top" or "PBT") of about 1.8mm. The squish band in the stock head is also recessed something like 2mm. So, when you first put things together and measure squish, you are going to get a big number. Not to worry. The head can be machined to eliminate as much of that recess as you like (can also be done by hand using sandpaper on a plate of glass).

I am not a P200 guy, so I will defer to others on the best amount of squish. If I had to guess, I would say the "built-in" 1.8mm squish is about right and you want to get rid of all the recess in the head.

It is no longer available, but this link shows an MMW head that I think will give you an idea what you should be aiming for: https://www.sip-scootershop.com/en/product/cylinder-head-mmw-px200-for-200cc-standard-for-vespa-200-rally-p200e-px200-e-lusso-98-my-cosa-200_13013930

Kowalski, thanks for the link, that's the head I was talking about.

Johan, you seem like a competent guy, maybe it's time to split this engine and see if something is out of place. I'll give you an example of a poor running engine I had to deal with: Customer built an engine but forgot to install the flywheel bearing sleeve onto the crank. Funny one to diagnose but that is the kind of stuff that can get you stumped. I know you tested crank case integrity, but is there fuel in your gear oil? Is there wetness around flywheel seal? Is there rubbing marks on the flywheel magnets?

What is your measured squish clearance? If the SIP 200 is an aluminum analogue to cast iron 200 cylinder, then with 60mm crank installed, and standard 0.25 base gasket you should get about 0 deck, meaning piston is in line with cylinder top at TDC. Stock head lip is around 2mm? You installed a 0.75 head gasket, but did you modify the head lip? You're getting 150psi, it's not danger zone, but with stock head it can be if you have an air leak. You're using an injector air box, is the oil passage sealed?

If you switched carbs, electronics and melted two completely different pistons fitted with different heads then the only common thing present is the bottom end - crank, seals, bearings and air box. Start looking there.

Well, you've managed to get a lot of opinions…

As you move forward deciphering/ selecting your options, keep a close eye on you plug, with each change.
That's a constant no matter which opinion you folllow.

It will tell you if you are getting detonation.
A new plug and an eye on those silver balls will be your guide - or canary in a coal mine.

Tackle the easiest to address first.
Don't assume your gurgling or 4 stroking down low is necessarily the cause of your melted piston.

You've confirmed timing is ok.
I'd personally then keep an eye on fuel/ squish/ right plug length (no threads showing from plug or from head when plug is screwed in) as timing seems to be conservatively set.

Good luck and keep us posted!
We are all curious.

just to add to CM's post - holing a piston takes detonation. you can hear that. so keep your ears open.

Had a little time tonight to tare it apart. I dont remember doing sutch an ugly job with the had. It feels kind off embarrassing to post the pics but I think I owe you that.
Cant see any leaks.
The hole for the autolube shaft is plugged, and I have a non autolobe carb so I dont have to plug anything special in the carb box, right?
The black sooot on the piston looks strange? Could my uggly dremel job have made swirls in the combustion chambre?

Ooo.
I love when I win a bet.
🙂

2 things.
1. I assume the answer to "what are your goals" is: I want more power.
Of all the people commenting and posting here, all of whom have answered that question highly successfully with many a build, none of them did any grinding inside the chamber. If I made a list of 10 ways to get more power, this would be number 79.

2. You can polish that out to baby butt smooth everywhere, or just buy yourself a nice head for not so many krona. Yo to you.

Clean the top of the piston off and start again.
You will likely have a much better time jetting it in, as you are no longer trying to compensate for detonation caused by hot spots exploding your fuel before the right point of your stroke.


Luckily you caught before another piston.
Maybe you will be able to add back some advance.

Others here commenting are more knowledgeable/experienced than I am on that front.

What CM2 said.

Also, fix your head leak, then pressure test the motor, then you can start to sort out your ignition timing and jetting.

What head leak? 🤔

My gole was less power since I replaced a Polini kit.
I had heard from different sources that I should have the spacer in the top. So i went for a 0.25 foot gasket and a 0.75 copper head gasket. The compression was to high so I lowered it by grinding the combustion chambre out a bit 😅 wont happen again.

Honestly, I dont think it will preform any different when I have it back together with the 1.5 foot spacer and new head.

Had the same issues with the Polini kit, but its worth trying. I will post here when its done.

Thanks again for all advice 😀

The thing that lurks in this thread is the part about dremeling the head. Unless I missed it, you haven't really explained what you did or your reasoning for doing it. I know in general why it is done, just not in this instance.

I can't tell from your photo whether the plug threads are protruding into the combustion chamber.

Edit: Right, to lower compression caused by a really tight squish, caused by your thin foot spacer etc.

Is that not from the head leaking?

It looks like the gasket sealer was pushed off the edge of the deck and oil has run out.

And I just noticed this, but are you running a stock P200 head on a 208 cylinder?

Yes a Sip 200 alu cylinder with 60mm crank (gives 208, right?) and an original Piaggio px200 cylinder head.
Is that wrong and if so what cylinder head would you recomend?

I think it was me who made it look like a leak there when I took it appart, but I will look into it.

I also think that is gearbox oil, it happened to me too with a new clutch side Malossi oil seal. Same burned oil on the piston head

A stock cylinder head would work. Measuring your port timings and squish would let you know whether a head shim or spacer would be needed to get proper squish or whether you needed a zero squish head, etc.

So its back in one piece again with new parts and a bit lower compression.
Its snow here now so I cant do a test run.

Sip 200 aluminum original style cylinder, original untuched head, Bgm 60mm crank, 1.5mm cylinder foot spacer, no head gasket, gives original px200 huge squish (1.2 cyl + 1.5 head = 2.7mm), 130 psi compression, Bgm 2300g flywheel, Bgm big box, NGK B7ES, timing at 18 deg.

Should timing be more advanced?

Is 7 a too hot sparkplug, and just to be sure B7ES long thread 19mm is the right length for a stock PX200 cylinder head?

You probably want to drop the head space to a .5mm, which'll put you at 1.7 squish. That should be a good compromise since we don't know your port timings.

I'd go with a B8 plug, but if you generally ride around town, B7 is good.

Timing is fine for now at 18 BTDC.

Agreed with CM.
Plug thread: easy to check visually.
others may know answer from experience.

Then my compression would rise and will that worke with the original cylinder head?

Is it better to look for a factory made high compression head with zero squish if will find one?

Why a 1,5mm base packer?? Stock cylinders (and probably SIP's stockish aluminum) run better on a 60mm crank, no base packer needed. Piston should reach TDC without the base packer. You need to measure your timings before anything. I had an O-Tuned stock engine with a 60mm crank and an MMW head with 1,2mm squish. Along with a Polini box it was a torque monster going uphills on 3rd gear…

This suggestion has been made a number of times. I'm not sure what the issue is.

Yes, the 1,5 foot spacer is what Sip wants me to use.
And with that and an original head and I get 130psi and 2,7mm squish. If I would take down squish to 1,5mm the compression would be really high. Or am I missing something?

The part you are missing is the port timings. It's the other half of the equation. How you set up the port timings changes where in the rpm band the engine makes its power. You want to know those timings so that you can set them up so that the your engine works well for the kind of riding you do as well as for the exhaust and carb you are running.

The thickness of the spacer under the cylinder changes this and of course it changes your squish. You can use this to your advantage. Using a different thickness base shim might get you closer to the squish you want so you won't have to change the size of the combustion chamber volume or machine the head, etc. It's kind of a balancing act. Sometimes it's a win-win and you end up with better timings AND squish where you want it just by adjusting the spacer and or head gasket.

You seem to be focused on just the compression ratio and squish. In the past, you have adjusted these variables by increasing the head chamber volume, which is a harder thing to get right. If you looked at your port timings, you would likely find some wiggle room to get decent timings and squish without having to resort to fooling with the combustion chamber. It's way easier and works well for most people.

I'd say it's the other 3/4 on a tuned bike. Maybe the other 85%. No amount of carb setup can save you from bad parts matching or port timing.

Without those specific data points, we can't give specific guidance. We're working with what you've given us, which are basically irrelevant compared to port timings and some guidance on how you intend to ride.

Squish and compression are things you worry about AFTER port timings. Compression will naturally settle out as a result of all the other decisions and can fall into a very wide range of acceptable values. It's not normally something you have to explicitly tune for, even on highly tuned motors.

<lecture mode="annoyed">
You have knowledgeable people all asking you the same questions, and you keep not answering them.

At this point, you need to either accept that you have homework to get us port timing measurements, or your need to get used to disappointment, whether performance or reliability.

It's not "impossible to jet," but at this point, you're refusing to do the work to get it there and instead hoping someone will swoop in and magically give you the answer you want to hear.
</lecture>

Well said. That sums it up nicely.

You may already know this, but what is being asked is an easy thing to do. You've already measured PBT at 1.2mm. Now you just need to measure the distance from the top of the cylinder to the top of the exhaust port, and from the top of the cylinder to the top of the highest intake port. If you know those three measurements, there are several online calculators that can figure your timing durations for you.

My prediction is your port timing is good and you just need to adjust squish by milling the base of the cylinder head.

I wouldn't hesitate to put an electric fuel pump on it. Or design a pressure system for the tank itself,

I felt this way halfway down page 1

From what I understand, my connectingrod is 110mm and the piston protrusion is -1,2mm (down in the cylinder)

then I get the results as in the pictures.

Two different calculators and two different results.

Rod should be 110mm. Measurements should be taken without a base packer...

Try this calculator...

https://www.ddog.at/stz/rechnen.php

I see that BGM does a 60mm crank with a 128mm rod for a Quattrini 244. If that's the crank you have, I don't think it's suitable for your setup...

https://www.scooter-center.com/en/crankshaft-bgm-pro-rotary-valve-60mm-stroke-128mm-connecting-rod-piston-pin-oe16mm-vespa-px200-rally200-ducati-cosa200-motovespa-tx200-suitable-for-quattrini-m244/vmc244-cylinder-bgm031128g