Modern Vespa

I have jetting problems.
First with a 221 Polini, now with a Sip 200 (208) aluminium cylinder and the same Bgm 60mm crank, Bgm big box, ignition at 19 deg. Have had a piston melted.
Same symphtoms on both cylinders.

It cant take moore fuel than 55/160, BE5, 160ac and 125mj.
But it needs more at wot.
It vill four stroke and bludder at low/mid throttle. Not fun to ride in town.

Could it be something with the crank or porting?

Is it possible to give it more at wot but not in low/mid without the main jet interfering with low/mid?

1) if you have not drilled the float passage on your carb, you need to do that. Otherwise, the carb cannot flow more than a 125 main jet, but will give you fuel starvation at sustained WOT with larger mains

2) You need a 140 or 120 AC to go with that drilled float passage

3) Your timing is too advanced. Take it back to 17 BTDC.

4) Get a temperature gauge

5) Have you confirmed you don't have an air leak? The combination of four stroking at low/mid throttle, but seizing when you're on the main sounds like a probable air leak to me.

We can discuss port & intake timings, but that's not your problem yet.

The float passage is drilled 2mm.

The Sip 200 cylinder is not far from a original cylinder so I tought 19deg and not so radical jetting would be enough.

I will try to reset timing.

Maybe I have to make a leak test.

Based on your other answers, I strongly suspect an air leak at this point.

The Bgm big box cant go all the way up on the exhaust flange on the cylinder because of the centre stand.
It leaks there. Is that a problem?

That's not the reason it won't dial in, but you should clean that up and seal it with RTV just the same

I made a leak test, 5psi 45 mins no leak.

The carb box is for a autolube. If there is a leak it must be there since I had it removed and blocked off when I made the test. Although I could not se any obvious leak spot when I took the carb box off.

Any suggestions what next?

Good to hear you're airtight.

Did you adjust timing to 17 BTDC?

Just so I'm clear, when you say "melted piston," do you mean damage to the sides/skirt/ring lands due to a seize, or damage to the piston crown?

Pictures of the damage would help a lot here. Actually, pictures (and video, if feasible) generally make everything easier when diagnosing issues

What head & base spacers are you using? What's your squish? Do you know your port timings? (Timings are probably not the cause of your seize, but if your squish is off, we can fix both at the same time)

What were you doing, riding-wise, when when you blew up the top end? What throttle positions? i.e. Sustained, high speed touring at 1/4-1/2 throttle? WOT runs? Stoplight racing? Rolling off the throttle after a long run? etc. Was the motor warmed up? Do you have any temp gauges (CHT or EGT)? If so, what were they showing before and during the seize? How are the sensors mounted (in-head, under-plug, 2" from piston, etc.)?

Lots of questions to try and fill in the blanks on this one.

I suspect your carb drilling may be too small - or not all the way through.

Not sure how you know it "won't take more than a 160 AC and 125 MJ" ? Can you clarify how you know that?

Couple things you could try.
1. Swap to a 120 AC. this will move your main just down, or allow the existing main jet to work properly at WOT.
2. pull the carb. Pull out the main jet stack. Put your 2mm drill bit down the float bowl hole and shine a light down the main kart stack. Can you see the FULL tip of the drill bit?

in any event, I would drill this slightly larger - 2.1mm -2.3mm range.

3. All the above. (Suggested)

The smaller AC will put you into a smaller range for your main jets. Any other restrictions may benefit from this - so it's always my preference for WOT abuse.

Can you post up pics of the issue?

Wot for a few minutes. Will check carburetor drilling.
Anything that gives moore fuel than my setup 55/160, BE5, 160ac, 125 mj will make it strutter/four stroke.
I have tried everything from 55/100, BE4, 120ac, 162mj. Have ALL jets in between.

Could lowering the ignition from 19 to 17 deg help?

Dont hava a temperature gauge.

Holing a piston like that is usually too much timing advance.

I'd make sure your timing marks are correct (find & visually verify TDC), then set it to no more than 17 BTDC, as measured at 4-5,000 RPM's.

Also, make sure that it's firing consistently and not jumping around.

I assume you're running static timing, right?

A 125 main is believable for a moderately tuned 210, especially on a 140 or 120 air corrector.

Yes static timing. TDC and timing is set with degree wheel and strobe. Can I do something wrong with the degree wheel?
Do I have to grind up the holes in the stator plate to get less than 18 deg?

If the stator won't turn past 18 BTDC, then you'd need to extend the slots in the stator, yes. I've had to do it before. It's not big deal.

And you strobed the timing at 4-5K RPM's?

I'd really like to see a video of the bike running, assuming it's back in one piece.

Yep.
Confirm your TDC mark is actually TDC by either shining a flashlight down the spark plug hole or sticking a screwdriver into it.

I'm curious what head you're using with that cylinder. Also, a photo of your spark plug might be informative.

I confirmed as far as its possible with a screwdriver that TDC is on my mark. Then I extended the slots in the stator and adjusted it acording to picture.
Swaped in a 140ac, BE3, 125mj. Strobed it at 4-5k (i think, reved it high) 17 deg.

It actually runs better, just a little strutter/four stroke. Really hope adjusting the ignition was what made the difference.
Its cold here, 5 deg celcius when I made a test ride. Dont know if that helped accept the 140ac without fourstrokeing.

Should I extend the sluts even more an lower the ignition to 15 or 16 deg?

The SIP comments regarding this cylinder are 23deg timing and stock carb. They don't post jetting specs, but looking at the dyno graph the power increase is 1.5Nm - only - so jetting increase should be very minimal. At best you could easily run stock jetting with a smaller AC. The Cosa 200 cylinder used a 95 main jet with a smaller AC (can't remember the number) and it made more power. My guess is that SIP's cylinder is loosely based on the Cosa 200 anyway.

Timing is not your problem, you can leave it at 23. Drilling the carb is not your problem or necessary based on my 30 years of tuning vespas. This whole notion came from a German forum from people who cake every part of the engine with RTV sealant so I don't care to take them seriously - and based on experience it is not needed. Overly large main jets are not necessary! If you start to get over 120 main just start reducing the AC and you will get more fuel. The purpose of the AC is to slow down/regulate fuel flow at high rpm. This is why the AC is referred to as a 'choke' by the people who made this carb. By reducing the 'choke' you increase fuel flow, that simple. I also want to add that the drilling in which the main jet stack resides is a 'fuel well' where the fuel level is expected to rise and fall depending on air/fuel demand. It is not expected to remain full at all time and in level with fuel float.

Last comment regarding carbs is that you should do what the engine wants not what you think it needs. If you get low speed blubbering then jet down or AC up. Even if you had a small air leak you would end up compensating for this with jetting, without even being aware of the air leak.

One thing that can lead to inconsistent carb operation, and I've seen a lot of this lately due to ethanol in fuel, is swollen float needle tips that get stuck and prevent fuel supply for WOT but enough for slow speed. Also, even when not stuck they increase their length, which alters float height and causes low fuel level, ie lean fuel jetting. Float should be level with carb top (when flipped upside down).

If you had a massive air leak the piston would seize on the skirt before holing the crown, and you confirmed you don't have an air leak. What you have is the top being blasted off by detonation.

What is your compression? High compression with low exhaust duration would lead to very high combustion pressure and temperature that puts holes in pistons. Add to this a faulty fuel float/level and you have a perfect storm.

This is the compression. The carburetor is new, two months.

Ok, 150psi, that's not dangerous provided that you have a good sized exhaust port and head design.

What head are you using?

I would still check the float level. It doesn't matter how old it is, quality of parts used today sucks. I'm providing a picture of a float needle that swelled up and altered float height.

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Johan,
As this is a self help forum - conflicting advise can happen... 🙂
Diagnosing is a process of elimination - if everyone knew the answer before you started, we would call "fixing" instead.

Couple observations.
- It could be fuel flow as Pull my finger shows, but I'm skeptical due to the symptoms.
- Having seen the piston, I would definitely keep timing high on the list of possibilities. More on that in a moment.
- Drilling the carb has real effects - though it might not be your root cause. Your opening post suggests you have not done it successfully - even if it's not the root cause. Here's why I say that: . If you can run a 140+ MJ and get it to rev past about 5K RPM - your carb float hole is constricting fuel flow rather than the jet hole. It's just simple physics. In fact, if you can go up 6-10 points higher jet and it doesn't really effect rev out, then your main jet simply can't be doing it's job as the restriction bottle neck - and fuel flow through the carb must be the limiting factor.

Side note: lean WOT jetting is just as likely to cause an overheat and seize rather than a holing. Which is why I suspect your issue is more likely to be timing.

Now - about that timing... if you retard it too much, it can create a different issue. Your combustion will start to occur in the exhaust port. This will create a super hot spot and your rings will get exposed as they pass. Failure mode is often seize and/or broken rings.

So your current settings look reasonable - but don't over due the retarding or you might drive the issue into the exhaust port.

Given all that - how do you figure it out what's actually happening?

Those holes didn't happen all at once.
First you were having nasty detonation in the combustion chamber, and little hot balls of aluminum were melting from the top of the piston.
In your case - right from the middle - right under the spark plug.

This would have been evident if you pulled your spark plug after a WOT run, but before you had burned an entire hole in the piston top.
You would have seen those balls of molten piston all over the spark plug.

You can now use this to determine if the timing change fixed things.
Put a NEW spark plug in, or a used one that has no signs of melted tiny balls on the insulator or inside (see pic below).

Keep an eye on this - checking it after rides - and make sure you don't have the issue any more.

Note the ash gray electrode on this spark plug. They are running way too lean. Yours should be the color of hot chocolate. Everyone loves hot chocolate.

Go back and shoot a picture of your spark plug and post it - it will tell us more - and you can get used to seeing what to look for.

sorry if I ask you an obvious question, have you checked if the flow of fuel reaching the carburetor is constant and sufficient?

charlieman22, the reason I was thinking about the float needle being problematic is because Johan mentioned that he can't increase fuel flow past 125Mj, although I'm not actually sure what he means. This SIP 200 cylinder seems to make 10% power more than stock cylinder, and primarily around 4000rpm so jetting should not be that drastic that it would require large jets or drilling of passages to accommodate flow. I won't get into the drilling part anyway since it is not relevant to the problem. This cylinder requires modest jetting increase. Additionally, by increasing stroke (as in this case), all else being equal, you effectively reduce transfer and exhaust timing which is detrimental to power, so I doubt this engine can rev high enough anyway.

Last time I saw a hole like that in a stock cylinder it was when a standard head was taken to 1mm squish spec, which is fine but since the stock head has such a wide squish band the MSV number goes north of 45! Recently I was experimenting with a 200 engine and at 160psi and stock head detonation was immediate. You have to use a reprofiled head to lower MSV. On my 'performance' engine I can easily run 160psi and 23-25 degree advance because MSV is set correctly.

Let's wait and see what kind of head is used on this project. There is no reason this engine should not take 23deg advance other than if the MSV is too high.

Interesting perspective PMF.
Perhaps it's the issue.

OP, we will all be curious to know:
- what do you mean by "won't take more than a 125 MJ"?
- was it the same head on the prior cylinder when you holed the first piston?
- pic of the spark plug that was used at failure.
Post some pics for us so we can see if you get a chance …

Question PMF - what's MSV?

MSV - Maximum Squish Velocity. This a measure of how fast the squished charge moves between the piston and squish band toward the center of head (at the top of the stroke). Normal number is 20-25 Meters/second. Tests show that below this number the combustion process is slow and if the number is high then combustion is violent and erratic, leading to engine knock, detonation etc.

Thanks for the explanation PMF. So how's an average Joe supposed to figure that number? Or better yet, reprofile the head to an appropriate MSV number? Hmmm.....

I just gotta say CM22 - I appreciate your thorough explanations and viewpoint on troubleshooting issues. You have helped me many a time without even knowing it! Bravo!

I will start by saying thank you to all of you that participates here!

I have used the sparkplug after this issue so I cant post a picture. But it looked sandblasted, grey, absolutely dry.

Before you judge me with the cylinder head I have now and what I have done to it, the exact same thing happend with the Polini kit (cylinder and head)
Sluttering/fourstrokeing and a hole in the piston.
It was in the same jetting area on this setup where the fourstroke starts.

On the Sip 200 alu cylinder I have now is a original cylinder head
Bgm 60mm crank, 0,25mm cylinder foot gasket.
Dremeled out the cumbustion chambre (not in the squish area)
Extended the area where the plug sits until I had the compression in the picture with a Polini 0,75mm copper head gasket.

If this is absolutely wrong I would be happy if you can give me better cylinder head ideas.
Im looking for a nice powerful (given the conditions) engine.
Id like to keep the 24t clutch that the previous owner had installed with the Polini setup.

Should I aim for a stock untuched head and this 0,75 gasket witch will give me a little moore compression?

Sip says 1,5mm foot spacer but I tried to keep it low. (0,25mm foot and 0,75mm head)

Piston shows running extreme lean at higher rpm.
I would say CM had it on his first answer

Yeah, but getting it on the first diagnosis would violate CM2's observation

And all of CM2's observations are spot-on here. At this point, I think it's "C) All of the above" for what's going on.

Yes, it's too lean, which is causing seizures and also softening up the head & piston crown, which the advanced timing then hammers through.

If the timing wasn't holing the piston, it would just seize on its own or degrade the ring lands until it lost compression.

Now getting to your latest update... That's seizure lean, all right. Now, like I said above, we're basically just discussing how you're killing it.

It sounds like you have your timing back in a good place, so I'm going to assume that's no longer an issue so we can get to the root cause.

You never reported your squish, but it wants to be about 1.4-1.5mm. Not less than 1.2mm in your motor. Given your compression is 150 PSI, I suspect it's going to be less than that.

Your motor only wants to be in the 120-130 PSI compression range, so you can fix that by getting your spacers sorted.

Unless you're running an OLD rubber float needle (like 20+ years old), that's not the issue. And if you are, you'd be much more likely to see the float needle leaking than swelling. (can you even BUY rubber float needles any more? I only see Viton options, which are ethanol-proof). If you can confirm that the needle is new(er) and Viton, we can be done with that diagnostic branch.

(Another tangent, but a badly leaking float needle can cause un-tunable richness at low throttle, so this allows us to shut off that path, too)

Personally, when I've had my main jet outpacing my float passage, the motor would start to surge pretty violently at WOT. It was pretty obvious.

See at 28 seconds in this video for an example. I didn't keep it up for more than a couple seconds because while I'm all about doing it for science, I'm not blowing up a top end for clicks.

No judgement by me.

I'll keep hammering with questions until you answer them (e.g. squish), but there's nothing exotic enough about this motor that it should be inherently difficult to tune. You don't need to start machining combustion chambers until other options have been exhausted, and you have barely begun to look at other options.

First off, even if you hadn't told us your compression was 150 PSI, since you only 1mm of total gaskets with a 60mm crank, I could tell you that your compression is going to be too high, which increases the likelihood of detonation--and which your pictures confirm. You don't need to do anything with your cylinder head except fix the base and head spacers and jetting.

WHY did you want to keep the foot spacer low? What was your thinking? Did you measure port timings?

For next steps, can you:
1) Confirm that the float passage is drilled to at least 2mm (or, if not, go ahead and drill it--see my video above if you want to see how I did one)
2) Confirm the new(er) viton float needle so we can quit discussing it.
3) Measure squish and report the measurement back
4) Order a 1.5mm base gasket if you don't have one on hand. (You're probably going to need to order a few different gaskets

You should listen to the advice from SIP on gaskets. A 1.5mm base gasket will give you relatively balanced port timings, which should give you a solid mild tune across the entire power curve. You can then use head spacers to get your squish to 1.5mm.

If you really want to get into the port timing side of this, get the measurements and we can do that, but quite frankly, I don't think you're ready for those conversations yet.

Once you've sorted the carb float & passage; we've figured out your gasket order; and you've installed them, we can get back to the carb.

Lastly, your ignition timing should be good if you're 17 BTDC while the motor is revved up. You probably saw that the strobe was jumpy and also probably a few degrees advanced until the RPM's were up--hence why you don't check timing at idle.

I have tried to listen to your tips about jetting i a lot richer Jack221. Unfortenately it made it very jerky att low throttle.

Can the tip from CM to lower ignition to 17 deg sort out the jerky fourstrokeing when jetting richer?

Nope. Fixing your port timings and squish so that the carb can be set up correctly will, though.

Fixing ignition timing helps with detonation holing the piston when you're too lean.

What seems strange if squish is the problem is that I had exactly the same issue with the Polini 221. Polini cylinder head and 0,25 gasket or the copper Polini 0,75, or my currently setup with another cylinder head and gasket.
Jerky/fourstrokeing if richer jets on all three different setups.

I have confirmed that the float chambre passage is drilled. it again with a 2.1mm drill and I could se the tip.

The squish is what you get with an original cylinder head and a 0.75mm gasket. The piston is in top of the cylinder and has zero sqish. Maybe you can tell me if that is more than 1.5mm.

So I need to order some cylinder foot spacers. 1,5mm, 1mm?

I know we have some people here who belong to the "there is nothing I won't Dremel" school but, to me, the idea that some bloke wielding a Dremel in his shed is going to improve on the shape of a factory machined cylinder head is just … improbable. I would start over with a new P200 head, treat that as sacrosanct, and focus on the other variables being discussed here.

I'm in the "Dremel anything" school, and even I don't muck around with cylinder heads for exactly this reason. And even if I knew better, I can't get the necessary levels of precision to do the work, so it's irrelevant anyway.

You measure the squish by empirically testing it. Crush a piece of solder pressed against the cylinder wall between the head and piston. Searching on here (or google, which will probably link you to here--we're good like that ) will provide examples.

That number will inform next steps.

I will get a 1,5mm foot spacer and a new cylinder head.

Again, I had the exact same problem with the Polini kit so there is something else or something more going on here than the modyfied head.

Thank you all for helping!

Johan, did you use a Polini head with your previous cylinder or your modified stock head?

Chandlerman, 150psi is 10.20:1 compression, which is not a big deal as long as the motor is set properly. Max you can do on air cooled engines is 12.5:1 safely. A stock 200 head set to 1mm clearance is over 160psi, I think? I can't find my notes to be sure. Problem is that since the squish band is over 55% and bowl volume low the MSV goes too high and detonates. If I install a standard head (160psi) at 1mm detonation is immediate and if I install a 10.5:1 MMW head (155psi) at same 1mm on same stock engine back to back as I have done, there is no detonation. The reason is that on the MMW head MSV is kept low by enlarging combustion chamber and narrowing the squish band. Stock heads are 120psi and that is junk because you are not taking advantage of high compression benefits. Compression is free HP, take advantage of it.

I can't see anyone selling a modified MMW head with o-ring for stock 200 engine, at the moment, but I got one from SIP and it works excellent. That would be my recommendation to anyone who wants to run higher compression without putting a hole in their piston. There are also old Pinasco 213 heads with narrow squish band that work excellent even at 165psi. I used one on a motor that would top out at 155km/h without a hint of detonation (23deg timing). Incidentally, after 24 years that Polini 208 cylinder finally wore out and a new piston is going in.

I modified a stock head on the last setup with the Sip cylinder.

Polini head on the first Polini cylinder setup.

Had a hole in the piston and
"early" jetting fourstroke on both setups.