Modern Vespa

Thank you. No i dont think it is that crank so its most likely a 60mm crank with a 110mm rod.

I EDITED THE POST WITH PICS ON CALCULATIONS WITH THE RIGHT CONNECTING ROD LENGTH (110mm)

(Dont know for sure witch crank since previous owner and builder dont remember that)

The einlass auf and einlass zu how do I messure that, and do I have to take the carburetor box off?

So should I deduct the thickness of the packer when I calculate?

This is what I get using your 1.5mm spacer and 1.2mm PBT. Note that with this calculator PBT is only negative if the piston is sticking up out of the cylinder.

As stated before. I'm not a P200 guy, so I will let those who are address the numbers. As a 177 guy the exhaust duration seems good, but the transfer duration seems too high, which makes blow down too low.

Thank you Kowalski. So what does it menan and what can I do about that transfer duration seems high and blow dow too low?

It may mean the engine will have a somewhat narrow power band, and you can adjust that by using a smaller spacer under the cylinder, but we should wait for someone with more P200 expertise before we start down that road.

Your 1.2 PBT means with no spacer PBT would be -.3. In other words, the piston would be coming out of the cylinder by .3mm. Also you are only getting a 10mm difference between the exhaust and transfer port heights. Both of those things seem a little weird to me, but could be perfectly normal for this cylinder.

I get 125/168. If the piston edge is flush with the top of the cylinder. This is absolutely fine if you want a torquey mid range and to occasionally do 70mph.
Set the squish to 1.5 or 2.0 at the most. Timing 19 degrees static. Go enjoy.

As discussed many times, at length, jetting is everything. It's not a 4 stroke engine. Just because it starts up, rides and doesn't blow up, means not so much. 2 strokes have a wide operating range of jetting.
In the early stages it has to be rich enough, so that wide open max rpm is obviously reduced by an over rich jet stack.

Sounds like Im back with a .25mm base gasket and an original cylinder head then. That will give 1.5mm squish and high compression.

I have a black paper base gasket .25mm. Are they any good or do I have to order a new in aluminum?

To remove the cylinder with the engine sitting in the bike (1966 Sprint) I have to remove the carb and box to lower it in the back. Is there a better way to do it?

Paper gasket is fine.

You have two other options for removing the cylinder. First is to leave it in place and remove the studs, at which point you can rotate the cylinder and remove it.

Second is to get a hammer and just bash the frame back so the airbox clears it. You'll still need to disconnect the throttle, choke, and fuel line, though.

Thanks for the tip to remove the studs. I have put it together again with a .25mm base gasket and no head gasket. Piston is about zero to top of cylinder. Should be around 1,4-1,5 squish. Took a compression test. Am I in the danger zone with an original head and is 19 deg still good for timing?

Well, all you did was move the cylinder down a bit, so I'd say you are in the same zone as before. 160psi on stock head is a lot of compression, too much maybe?

160 is okay, IMO.

I'd pressure test it and run it. Get it warmed up, get a feel for where it's doing well (and not-so-well), then pull the plug and inspect the plug as well as get a look at the piston crown.

You won't have much data, but you will at least know if it's safe to continue tuning, starting with getting a boggy main and walking it down.

Good to hear, I will do that. I got a little snow here but I hope it will disappear a day or two next week. Could be hard to find a good day for a test run for a while thoug.

Chandlerman, in your earlier post you claimed that this engine 'wants about 120-130psi'. Now you agree that 160psi is acceptable. That is a 23-33% difference of opinion. Care to explain the spread?

The compression gauges are notoriously inaccurate as an absolute measure.

But They r decent at relative measurements.

Personally, I wouldn't get to worked up about the actual number - but rather how it changes with time.
More a focus on relative than absolute.

The issue the PO has most likely been fighting are hot spots and unwanted combustion related to those. With that resolved, it's a good chance to see how it goes now.

(A reasonable squish and the removal of the hot spots will more than likely resolve the issues and provide some joyful riding. )

I think the advice to keep an eye on the spark plug and piston top will provide the needed feedback to know if he's on the right track. If it still creates melted balls and ashen gray spark plug - then some additional investigation might be in order - but I wouldn't chase making changes based on a compression reading alone.
My $.02 thrown in to the mix.

Id like to try out this setup but IF a reduction of the compression would be necessary and to keep the cylinder timing with .25mm base gasket:

I have a .25 Polini head gasket.
It should give about 1,75mm squish.
Would it make any usefull differece and is it possible to use this 68,5mm gasket with a 66,5mm cylinder?

charlieman, you said 'with that resolved', but I ask what was actually resolved? Johan made a vertical shift on a very conservative cylinder. There is so much talk here about timing specs, people want to see timing etc... this is not some one-off cylinder, it's a stock cylinder on very conservative steroids - aspirin at best. People want to know what rod is installed... well is the piston protruding 5 or 10mm? No?, then nothing to see there. Timing specs have been posted since the cylinder came out. Dyno pulls were done and posted for all to see. The only mystery here is why the same engine melted two entirely different cylinders - Polini and SIP. Those two cylinders are so vastly different that it should warrant completely different results and issues.

In my opinion nothing was actually resolved other than installing yet another cylinder and fingers were crossed.

We can wax about compression in relative or absolute terms when it suits, but Johan is using the same tool for all tests - with all its faults and error - so in his world it shows absolute readings while relative to my compression tester. What I see is that he gained 10 psi with new set up and I'm curious how the engine is going to handle this, given that nothing else was actually 'resolved'. I'll just add here that this compression test is dynamic, while plugging numbers into a computer is static. And between two schools of though - Japanese vs European - dynamic compression is more relevant, real world number because it takes into account exhaust port height. So a small volume head is not dangerous on a cylinder that has 205 degree exhaust duration, wile it is dangerous on a Vespa with 165 degree exhaust duration. And anyone who has ever noticed charring inside transfer ports, on crank webs and case surface should understand why compression and exhaust area are a very important relationship. Meaning, if Johan's exhaust started to open at say 77.5 deg ATDC (205 deg duration example) his compression reading would be magnitude lower, likely in 130psi range and no-consequential.

I will reiterate on more time, this SIP cylinder is nothing impressive - based on dyno chart - and it doesn't require massive amounts of massaging. The jetting increase should be minimal. There is no need for drilling carbs, installing huge jets and all that stuff. There is no need for retarded timing since the cylinder is made FOR STANDARD 23 DEGREE TIMING. All this info is available to anyone who bothers to read. If there is a need for timing deviation it indicates a problem.

Johan, instead of getting dozens of answers from strangers why not contact SIP for help. They made the cylinder, they tested it, they know it. From what I can see your engine spec is same as what SIP used for their development. To me it's obvious that you have a problem in the bottom end if you keep getting same results with different cylinders.

The famous artist George Bush once said: Fool me once, shame on — shame on you. Fool me — you can't get fooled again. Johan, you should't keep doing same thing and expect different results. Rebuild the whole engine and contact SIP.

The compression does still seem high. And it shouldn't be on such a mellow kit. At first I thought the piston holing might have had something to do with dremeling the cylinder head into a butthole. But that doesn't seem to be it either. I do agree with pulling the whole thing apart and having a look.

Perhaps I'm missing something.
Has it been run since the head fiasco was fixed?

My new builds have read 150 on my compression gauge - but if I use my other gauge they are 20+ lbs lower.

Two stroke stuffing did an episode where he showed the same. I wouldn't put too much stock in the specific number shown.

My outlook is, OP had detonation issues.
Head had detonation causing issues.
Both pistons melted when being used on motors with modified heads.

Johan - can you confirm if that's correct or mistaken?

If yes, then with m that issue "resolved", I'd test it and use the plug and piston to get a read - before considering tearing down.

charlieman, the Polini cylinder came with dedicated Polini head.

You bring up detonation, well what causes detonation? Lean at WOT? or???

In a two stroke you can easily get detonation at high rev part throttle as well. You can design you engine to doom if the exhaust port is wrong size relative to transfers as most standard vespa cylinders are. Except that since their output is so low that they generate so little heat they can't get into trouble. But the moment you increase output you get in trouble because, and this is very important, because a small exhaust port can't equalize cylinder pressure so excess heat and hot gasses are forced into transfers and bottom case area. When this happens you start to super heat the fresh charge and in extreme cases this charge starts to pre-ignite which leads to detonation and holes in pistons. So high compression does matter in a cylinder with small exhaust ports and detonation does happen at part throttle condition and not merely WOT, where it is expected the most. What SIP accomplished with their new cylinder is provide larger transfer area for all this excess pressure to dump into the case.

Again, I will reiterate the case for compression testing if same equipment is used consistently. Johan uses the same tester on his engine so if we see an increase from 150 to 160psi it is a true increase on his tester, regardless of relative to other more expensive testers.

Only the second time with Sip cylinder the Piaggio dremeled head was used. The first time it was the Polini kit head as it is out of the box.

I have only made hard WOT runs twice and both with "melted" pistons.

Sip says 1,5mm base gasket but I dont know what head would workeout then to get 1,5mm squish, the lower timing from the calculators and not give too mutch compression? It is an impossible equation.
Exept if the combustion chambre is expanded.

The downfall of aluminum/nicasill cylinders is that you can't adjust exhaust port height as needed. The material is very had to cut, it ends up chipping, flaking and causing massive damage to piston rings.

Maybe it would be best to start from beginning and rebuild the engine with a 57mm crank to spec that SIP designed the cylinder. It will also give you the opportunity to inspect seals and look for leaks, rather just replace the seals. It will be an opportunity to install a crank with known durations etc. Personally I would have torn down the engine after first melt down. That event forced aluminum dust into to case, think about what that abrasive environment does to seals and bearings. But you actually did this twice, you twice pressurized the case with abrasive aluminum dust but expect the crank, bearings and seals to be as new. It's time to strip this engine, stop throwing good money and parts after a problem in your bottom end. This is a plug and play cylinder with minimal effort required. You have a problem in the bottom you refuse to face.

Nicasil is scary the first time. But with the right tools, hard or difficult it isn't. The major issue is, once the additional performance is felt, from just a little bit of Dremel work, it gets very addictive and until followed by expensive.

160psi is nice, means its great. 180psi is questionable but ok. 200psi is taking the p.

OP. Jet it in properly and then enjoy it.

From the beginning the guy said he keeps putting holes in pistons no matter how fat he goes on MJ, to the point where the engine can't take any more fuel. How does a motor, that is so over fuelled that it can barely clear through the rev range put a hole in the piston?

Perhaps the next cylinder should be on MV members who keep encouraging this carnage.

Piston skirt

All the info is on the web page. This sip 200 runs a stock 200 head. And that's a stock 200 euro head, which is slightly higher compression than the USA stock head. The sip kit can also run a 60mm crank with a packer to set the squish. Same head. Any issue is not going to be the kit.

His previous 2 pistons were running jetting that wouldn't make a bone stock P200 splutter. My guess is he was confusing stutter with splutter.

Jack, it is my understanding that there were never USA (North America) and European heads. All markets got same heads. There were early U.S.A. casting heads, then second iteration U.S.A. cast heads with 4 corners added and last casting dropped the U.S.A. lettering. Ofcourse, COSA 200 head is basically a PX200 head with rubber dampers added and U.S.A. lettering.

Anyway, I found some MMW heads on eBay, I personally use the o-ring variant with slightly higher compression but the non o-ring head with 10.5:1 compression would be perfect choice for this cylinder, in conjunction with the 1.5mm spacer as the head doesn't have a lip.

https://www.ebay.ca/itm/274698000487

https://www.ebay.ca/itm/276684949579

The pictures of those pistons show lean seizure on skirts. I still think that jetting is adequate, in fact since it 4strokes through the midrange (as pointed out in very first post) I think you can easily drop the pilot to 50 safely, possibly even less. I still think your problem was the head squish band width and an air leak that are causing the sudden pressure and heat that is causing piston holes and unwanted piston growth. I've had lean seizures before and worked on many engines with air leaks that would seize and there were never piston holes. Simply the piston seizes before you punch holes. That is the thing about lean jetting on standard compression, the piston just seizes. But add high compression and now you get detonation.

On my stock 200 this is the jetting (with plug picture):

1.2mm squish
sip road 2 exhaust
Std timing/ignition 23
24/24G carb
160-BE5-125
38-120
Velocity stack

The velocity stack changes air flow so much that jetting changes are drastic. Again, this is a stock, unsorted cylinder. But I want to show that you don't need such a big pilot jet on a similar set up as mine.

PMF,

Have you ever thought about wearing gloves when you work on stuff?

Sure.

two stroke motors can run successfully anywhere from about 90PSI on the low end to up in the 160-180+ range on the high end.

As I said, in the absence of specific information, people get generic advice.

My mildly tuned motors are in the 120-130 range. Compression doesn't increase significantly because stroke is the same, squish is moderate, and the rings are fresh. My more highly tuned motors are in the 160+ range because all of those factors are pretty well maxed out.

Most motors are in the 120-130 range because they lack some or all of the above optimizations.

Tuned motors go higher as stroke increases and squish decreases, which is one part of why they produce more power (more mixture == more explosion == more power (that doesn't blow past the rings)).

Chandlerman, don't ever call combustion explosion because the difference between the two is about 7000 meters/second.

Compression is not derived from stroke but combustion chamber volume and to some extent exhaust port height. ie, for two engines with same combustion chamber volume you will get different compression output if exhaust ports are different height, same on 4strokes if you change cam duration.

You can take the same 57 stroke 200, install 3 different heads, all at 1mm squish, and get entirely different compression depending on head dimensions - squash area %, bowl volume and height etc. Same would be true if you installed a 60mm crank with 3 head scenario. Issue with stock 200 head has to do with squish area % that makes MSV go too wild, hence the need for wide squish gap.

Last thing I'd like to comment is that there is no mild squish or performance squish. There is proper squish based on stroke and then sloppy factory squish that we are stuck with and need correcting.

PMF, what's your thinking on the specific cylinder vs timing thing? I guess it's pretty clear in this case, as you said if the manufacturer recommends 23 degrees, then that's what it is.

As an example of what I'm asking, what if you lift the exhaust 1.5mm on a Polini 177 with a static Vape?
Or, the Quattrini 244 instructions say "Vespatronic advance 26 @ 1500 rpm" - and you're using original ignition?

What is it that tells you where the timing ought to be if manufacturer's specs are unavailable or unsuitable?

Ultimately the timing will depend on loads of things such as fuel octane, cylinder head design, exhaust etc and testing will determine what you end up with. However, there is a constant and that is that combustion takes roughly same amount of time on all engines. On efficient race engines it is 50 degrees and on standard vespas 55 at best, maybe even 60 degrees on engines with massive squish clearances. In my younger days when I was into engine modelling programs I would plug 55 degree value and it would give me realistic cylinder pressure and power. We also know that peak mechanical efficiency on a two stroke engine is when peak combustion pressure occurs at about 20-30 degrees after top dead centre. This means that for peak mechanical efficiency - pressure acting on piston as it descends - combustion should be reaching its peak around 20-30 degrees ATDC. So you can do your math where to start if on vespas combustion is roughly 55 degrees (my opinion) - 25 degrees before top dead center is when combustion should start.

This is where combustion efficiency becomes important. Starting spark at 18 degrees BTDC is wasting potential because you will never burn peak at desired degrees ATDC. But because vespas have inefficient air cooling, head retains heat and is prone to to detonation so late ignition is what we are stuck with. The only way you can make the most of this lousy situation is to improve combustion as much as possible. This means minimum squish, highest possible combustion, good ignition. You just have to speed up combustion time OR slow down piston speed at TDC.

On racing two strokes (125) ignition is 27-30 degrees BTDC up to 10000 rpm, and just then drops off to 8-10 deg around 12000 rpm. But those engines run slow burning fuel and are calculated to burn peak at that 20 deg ATDC.

It's hard to suggest what timing works on modified engines. Smaller displacement 177 engines have smaller area to cover for their burn so it's easier than on larger displacement bikes. Don't be afraid to try 20+ degrees BTDC. I've ran 26 degrees on my polini 208 and that gave me best power all the way up to red line. Long time ago I had a German pen pal who devised an adjustable plate (worm gear moved stator plate while on dyno) and he also found 26 degrees worked best on his Malossi 210 engine. Key is a good cylinder head with minimum squish.

doesn't the rule apply that the more efficient the combustion, the less advance is needed? Advance is the solution to slow combustion, giving a lot of advance can help at low engine speed when turbulence is disordered, but at high speed it serves to slow down the piston, knocking and finally create holes on its upper part

Yes, generally the more efficient the combustion the faster it will burn so less advance is needed. It's hard to be specific when we are generalizing and trying to cover so many different Vespa cylinder configurations. And bringing other 2strokes into conversation makes things even more vague.

I think that ultimately one sets their ignition based on what their engine wants, not what I or the internet says. From my experience Vespas respond well to more advance and 7500rpm peak power engine set ups, given our transmission and cooling handicaps. For that rpm range we don't have to shift ignition too much, if at all.

Here is another thing, a tuned vespa peaks where other modern 2strokes just start to wake up. So what is high speed???? What is mid range???? To me modern high speed is 12000. Back in the 60s it was 18000. Imagine trying to move a 4 speed Vespa in those rpm ranges? Wouldn't happen. Modern 2strokes are still full ignition advance in the same range that most Vespas are peaking out. The only reason high efficiency 2stroke engines cut advance to 5-8 over 12000 is to dump late burning fuel into the pipe in order to keep resonance alive for another bit of over rev. If you dump heat into a pipe it will start to act like a short pipe and speed up resonance. Conversely, if you inject water into your pipe it will act like a longer pipe because you cooled it. So, you can see how manipulating heat that enters the pipe changes its characteristics and ignition timing is part of that.

Anyway, combustion takes certain amount of time (in seconds) in a very specific amount of degrees of rotation to make peak pressure just ATDC. That peak needs to happen 25-30 deg ATDC before the flame ends up chasing a descending piston. There are many ways to skin that cat, as long as peak occurs when it makes most work.

So Im back. The engine is rebuilt with new gaskets, seals and bearings. New original Tamien 57mm crank.
All spluttering and fourstrokeing disapeared. Runns great through the whole register.

Just to throw something else into the mix, I recently had a px200 engine which exhibited similar traits and it turned out to be the pickup rubbing on the flywheel, it was strobed at 17 degrees at tickover but once out on a ride the engine was running very hot and idle was hanging when rolling off the throttle so timing must have have been all over the place at higher revs

Flywheel pic..

Interestig, I will look in to that!
What was the problem, the stator or the flywheel? Or really bad crankcase bearings?

In this case it was the stator, I changed it over to a spare I had lying around and after that it was perfect! A badly worn flywheel bearing would definitely expedite this sort of issue.