Modern Vespa

Thanks for the brain dump, 108! Lots of food for thought in there.

With my jetting, I'm running an unholy union of Lermarxon's slider & mixer tube, and (Jack's advice) a small air corrector (so as to enrich the top end of the main stack), and also an idle jet that's so fat that when it gets its shoes shined, it's got to take their word.

This is where it is right now:

130AC/X234/130MJ
67-160 Idle
Standard air filter, drilled heart holes

1.2mm squish
Timing 17.5 degrees VAPE Road (static) @ 4000 RPM
Float bowl drilled to 2.5mm
Fast flow tap

That huge idle jet is playing with the main stack way better than I'd anticipated. The 55/100 Jack recommended was undriveable. But I'm thinking of drilling out my 52/140 idle to make a 55/140 (the 52 was too lean) and seeing how that goes. Or maybe a 57/140. And then on the main, trying a 140 air corrector and working the main down again, where I think it'll land at 132. And with the ignition, advancing it a degree and a half, so it's nuts-on 18 degrees at 6000. (That's where I was taking the thread, to see about fine-tuning the timing.) I've got a hunch that there's too much retard with that timing. My CHT temps rarely touch 120 degrees C / 250 F.

About the idle: right now I can take the bike out for a rip, burn like half a tank, get the engine totally soaked, and then at the stoplight it plunges to idle in just a second or two and holds it steady. There's zero indication of leanness in the idle circuit / mix screw.

The only leanness I'm struggling with is when I'm ripping along pretty quick in 4th, and I roll off to barely open throttle for some coasting, and it stutters. Never had that stutter with a regular slide with the corner notch. I really suspect it's cuz the Lemarxon slide has no notch at all, and the bypass orifice remains covered. But I guess there's only one way to find out for sure.

Yeah your weird pilot doesn't surprise me with the vape road.

They say static, but there's a curve it has, just a different shape, no easier to deal with.

Just double checking, the idle/fuel mix screw is 2.5 turns out (fine thread pitch screw) give or take 1/4 turn..? Just so there's a baseline to your jetting.

So 17.5deg @4k is around 16deg @1700rpm…

I went with 15deg on the malossi with vape road and my own smallie engine. Maybe give it a try on a rainy day, at least you have a setting which works for you that you can go back to.

Yeah that stutter you mention is typical 1/8-1/4 throttle lean or rich problem, could be either… that's where the si carb typically has problems and your weird pilot has helped out with.

I doubt you have wheelie/front wheel lifting power…? that's a bit of a big assumption, but hearing you mention the stuttering rolling off the throttle…

Interesting. Wondering how that works with the Stella reed vs rotary. My biggest problem has been heat.

When jetting is done, if it ends and there is a single annoying yet significant error (stuttering on rundown, gets hot at one position, low torque, idle increase/poor start when hot, long list....), depending on where the error was, it often means the whole set up was incorrect from the start. I think your air corrector at AC130 is too big, which is a stage zero decision.

108 Yeah, the VAPE Road curve is... well, it's a curve, but barely.
3 degrees variation up and down, from idle to the top end of usable RPM.

Question though -- where's this 1700RPM magic number come from?

And you're... recommending I retard the ignition a little bit further? Remembering that I've got the bigger-than-recommended squish. Thought I'd try nudging it in the opposite direction.

As for the mixture screw, it's at 2 1/4 turns right now.

As for that stutter, maybe I'm not explaining it well: when accelerating from a stop, all is fine. When rolling around slowly in 2nd and 3rd, 1/8th throttle is plenty rich too, even four-stroking a little. The problem is after a fast cruise, and I roll off the throttle to anything under 1/8th. The stuttering begins. It's not rich -- I'm pretty certain it's lean. As I understand it, at this point all the fuel is going through the main jet and into the idle jet and then into the engine by way of the mixture screw. And with the Lemarxon slide, the bypass hole that normally augments the mixture screw is blocked.

Wheelie-ing power does exist, meanwhile. It's not hard to make the front lift by goosing it when moving in 2nd.

Before doing more permanent things, try a smaller numerator on the pilot jet and play with (turn out) the mixture screw on the road. More emulsion at closed throttle. That notch-fill gives you more control using the screw. If you cut it you'll lose that.

Your squish is bigger than what is mentioned in the manual, but that's what I was referring to before, is that you don't see anyone using those numbers on on the engine thats built for longevity. You do see 0.8mm with people who build race engines though.

1.3mm squish seems to be recommended by a fair few builders. So 1.2mm is actually tighter than what I would aim for. And hence reducing the timing to compensate for it… if your squish was more than 1.35mm maybe 16deg would be worth a try. That's what I would try with your situation.

Where did 1.2mm come from?

But what's to say what's worked in the past for me works for you?

For me, it's just a case of narrowing down what works with a process of elimination, with directions from certain folks who have had success. Some of it works, some of it doesn't. If I've tried it a dozen times myself, and I ask the same questions of builders who have built quite a few engines in the past year or so, so if it's worked for them, I test it again, and if it works I'll consider that as best practice. I'll go through those motions before I try anything else.

1700rpm comes from instructions in kits and builders. I used to time the ignition from idle, then rev the bike... of course the bike starts, but ended up chasing my tail with jetting till I sorted out the ignition timing. Got you, makes me wonder if the pilot is too big and the slide is too lean, helping out with acceleration from a stop but from cruising to rolling off the throttle its become lean.

Good you get wheelie power though… there's hope.

Apologies to Leon, totally Hijacked your thread...

What 108 said -- apologies, Leon, for hijacking your thread, and please bear with me while I hijack it just a bit more. I guess it's been mildly on-topic in that we're still talking about jetting and ignition timing on a not-so-dissimilar engine. Thanks for the hot tip, Ray8. I'll give that a try and see if any number of additional quarter-turns out on the mix screw get rid of that stuttering.

From 108 again: It came from me aiming at 1.3mm and arriving at 1.2mm with the spacers I had, and calling it good enough for gub'ment work. Plus, with this solder crushing technique, how accurate could it really be? Maybe it's 1.3mm, who knows. That sounds like a possibility, and as soon as we get a few warm days here I'll start mucking around with that. Will start with just the mixture screw by itself. I do know that at the throttle position I'm talking about, which is well below Chandlerman's quarter-throttle "Bermuda Triangle of Carb Turning", is just *barely* cracked. More like 1/12th throttle. And looking at this slide and carb here, at that throttle position there's only the first 2mm of the underside cutout exposed to the intake. So there's *some* leaning going on, but not a lot.

+1 on 108's post.

The optimal squish band is determined by the diameter of the cylinder. I'm too lazy to go dig up the calculation, but Michael Forrest, aka Dragonfly75, has a youtube channel and tools if you really want to know:

I run my 187's and my Smallie at .8, but go up on my 210 to 1.2, to keep the temps low since it's more of a tourer and grocery-getter than a racer.

Solder taped right across the piston is usually pretty accurate. 3 measurements and take the average is a good practice, might as well when you've gone to the trouble.

Measuring the solder with feeler gauges on a flat piece of glass works for me. With vernier calipers i risk the chance of squishing it with the caliper or inaccurate measurement to avoid crushing it.

But understand the 1.2mm, sometimes gaskets just arent the right thickness and you just roll with it...

Dragonfly75's heads are a little more exotic than anything that comes with a Vespa kit. Did you notice his squish angle is zero? This is the number the big boys use. Very dangerous angle for use on the road. Most kit supplied heads are a plus few degrees on the squish angle. This means much he says is not relevant to us. A few degrees squish angle drops the velocity by a long way, allowing us to do what we like.
If anyone wants to measure their squish angle. Put in a fatter piece of solder to measure the edge clearance. Measure the solder at the end as usual but also on the inside of the squish band. Subtract one from the other, bit of trigonometry. Voila, squish angle. If more than 2 degrees it's a mild head.

A practical application of how any of this applies to us, on our shopping bikes, is a bigger squish edge clearance, gives more torque but less power. A tighter squish edge clearance, gives more power but less torque.
With this set and the ignition timing adjusted dynamically, power can be maximized. If that's your thing.

Sorry Leon, never thought Michael Forrest's content would grace your thread…

I cringed a little at his baffle cone creation video… actually was more worried he would hit his fingers with the hammer…

Chandlerman, have you used his jetting calculator? I bought a copy but never got it to do anything useful.

I have not. He produces a lot of interesting info, much of it goes further into the math than a lot of people care to, but it's definitely educational.

Hi jack,
So have had a morning of it with your suggestions,
(with a 55/100 idle) started MJ's at 140 and went down 138 135 132 130 128 125 122
(140 to 130 just would NOT rev out)
some where about 122/125 seem to be it, maybe 128, but its a bit hard to gauge, because of the of the bottom

Its really shit house at off throttle up to half throttle. To get the rev's up and get it fanging on the main stack you've got to hold it back in 3rd and flip flip flip flip flip the throttle to kick the revs up, to get it past coasting (which its not good) to fanging, then away it goes, change to 4th and again it struggles till you get those rev's up again and away it goes,then if you take off the throttle a bit it dies and struggles again, its gutless, it just wont take off. you give it the throttle and it just struggles to pick up.
The idle screw is almost fully wound down (say half a turn before it is fully in) to get it to idle, & the mixer screw is fully wound in, if its out a bit the scoot wont idle. This seems odd to me

Apologies 108, for my ignorance, but I don't even know who Michael Forrest is.

Definitely no need for apologies, definitely not missing anything out.

Michael is semi-well known on FB amassing a huge wealth of data on 2strokes, from all corners of the internet but dismissing anyone challenging his thoughts on 2stroke engines. He sells excel sheets with the data as calculators for people to map jetting, port timing etc.

He has some interesting insights, which may or may not be valid, hard to say as I don't think anyone has tested it themselves (or even him for a matter of fact). Which personally I have no problems with, but the lack of being open to other people's ideas, especially people who have vastly much more experience, when he himself has only ridden less than a handful of bikes is somewhat ignorant in my opinion.

Holding up your hand and saying you don't know I can understand…

Apologies for the bash…

But back to normal programming…

As I understand from what you're saying, 130MJ doesn't rev out at full throttle, 128 nearly there and 125 is clean. So 125 will do for now. Main stack can stay at AC120/BE3/125 for now.

Any air filter fitted anywhere?

Now we're back to the idle issue you had before. On this engine, as it is now with the new cylinder, it's never run right at idle? And you tried a different carb and jets and was just the same? No fuel leak? How about a different reed block?

One thing that does do this kind of thing is an upside down piston. If you look into the exhaust stub, it's easy to see.

That's right, 130 upwards were all shit, give it throttle and it just bog not pick up and accelerate.

no air filter or lid cos I was racing out of the back yard down around the block and back again changing pieces as I went so it was easier to just leave any filter off (i still have a venturi its off as well)

No fuel leaks, and regarding the piston, Ive already had that thought a week a go, and droped the exhaust to have a look and make sure it was in the right way It was exhaust ports match, and the inlet ports are not hindered.

Once you fix this issue, you'll need to confirm the MJ again.

Something is very wrong. Did you pressure test after fitting the new cylinder? With all the overheating on the old cylinder, the seals could be damaged.

Did you try another reed block?

What does the spark plug look like after idling for a while?

I agree, there seems to be something very odd.

The reed block is new, (I do have another), I checked the petals the other day when I was swapping carbies over, they appeared to be ok
Did a pressure test when I put it together held good pressure at 250mmhg took more than an hour to lose 50mmhg. no leaks around the crankshaft seals or cylinder then.
Just pulled the plug, well that looks way too lean for my liking, I'll check it after idling for a while tomorrow.
I reckon I'll do another test tomorrow as well.

Way too lean from what you are saying. Hopefully that's all it is.
Open the mix screw to 4 turns and idle screw fully in. If somewhere near correct the pilot jet it will be idling at about 1600rpm.

This is sounding more and more like the issues I've had jetting an Si carb with a stock Stella reed block on a 187 engine.

spark plug wet, dark & carbon'y after sitting idling for a time.

So Mix screw 4 turns out and idle screw fully in 1300 to 1380rpm about
Mix screw 0 turns out and idle screw fully in 1500 to 1550rpm about

interesting, this was going really well with the reed block as a 177 for a few years, then when I decided to rebuilt it with the rx190 same, non of these symptoms weird

Bye the way any consensus on the issue of the cylinder, being the same for both the Pinasco Granturismo 177 & the 190 kits, and just replacing the spacer/gasket, & head, just out of interest

At least it's idling now. What's the idle jet? Needs to be a few sizes smaller.
To be accurate, it's going to need the filter and box top on, every jet change.

55/100

It always idled before,but needed either or both the idle screw wound to almost fully down, and the mixer screw wound nearly right in

It just weird that it takes the idle screw to almost all the way down to get it to idle, in the past it was never like that, if I had it screwed down that much before it would be roaring its head off.

With 1/2 to 1 turn left before it bottoms out, there's not much room left for adjustment.

Jet needs to be slightly smaller. 50/100 is easily available and might be ok.
With all the jets as is, hold the throttle at 1/4. How high does the rpm go?

I'll weigh in here (and not take things O/T for once) and say that the 55/100 is likely the culprit. I'd tried that superfat pilot on my otherwise kinda-sorta-running-ok setup and the idle was a spluttery mess, no matter how I compensated with the mixture and idle setting. The only way it would reliably idle was, like you, with the idle screw turned all the way in.

So yeah. What happens when you go back up to 50/100 or 50/120 or beyond?

Ive got a 48/100
it's Sunday night tea time, So Ill get back with some answers stay tuned

As said, your AC is too big and is drowning your pilot. SI carbs work differently to almost every other carb.

Reading this is confusing. (Language is a funny thing)
So he's running 130AC,
meaning
"too big" too lean/rich?
"and is drowning" too much fuel/air?

Is it "too big, and therefore drowning your pilot with more air?

As I understand; lean (more air)-185 160 150 140 130 120-(less air)rich
Sorry, just trying to clarify/understand, for myself, what is actually being said.

Ok. Guess it didn't make much sense without more words. If the AC is too big; too much air. The MJ has to be bigger too, for the mid upwards rpm to be nearer correct (which Jim's is or it would have blown up long ago). With the bigger MJ the progressive element of the pilot is wiped out. Smaller AC, smaller MJ, normality is restored.
A 55/100 is not too big for a modern 177 kit.

jack Leon -- I looked at my notes. The overly rich 55/100 was least bad when I'd had the main stack on a 120AC / 132 main (with this 221 Malossi Sport). But still bad. Too rich at idle, even with the mix screw turned in to 1.00. Plug was wet.

I'm reluctant to face any unpleasant truths about what this means, i.e. something being fundamentally wrong with the engine. Maybe I just didn't get the main down far enough, and it really belonged at 130 or 128. (My splutter point on the 120AC was the 135.) Anyhoo, at that point I wasn't quite twigging to how wholly dependent the pilot jet is on the main stack. Nor did I get how much the AC can really reduce the necessary MJ size.

I'll revisit the giant idle jets in March, for jettting round IV.

Further up this thread, I think Jack mentioned that if set to 4 turns out, and with the idle screw all the way in, the bike should idle at 1600rpm. Apart from that, and the obvious other things (i.e. revs settle quickly with no hunting, takeoff from idle is crisp), what are some other tells for an idle that's near where it should be? For example, if you let the bike idle for 10 minutes, what *should* the plug look like?

When I went down this path the conclusion was that the stock Stella reed block was too limiting and the suggestion was go with 4 pedal reed and a side draft carb.

Heat wasn't a problem when I was running a 177 Polini kit, but was when I went with a 60mm crank and VMC Super G kit.

As I am also using my scooter for touring, I'm really interested to see whether you can get your kit dialed in using the Si carb. If so, I would duplicate your whole engine build.

Why some kits run hot and some kits don't shouldn't be a mystery. There was a whole thread awhile back about getting highway performance out of an LML engine with stock reed. It went on and on with the owner eventually going with a side draft carb and selling the bike.

Touring builds seem to be looked down on, but sustained highway speed riding is actually a very demanding application for an air cooled engine, especially in hot climates.

Maybe the stock Stella reed with a 37cc increase in displacement is just an unworkable combo that you can't jet your way out of. At least for touring. I would love to be proven wrong.

The Hot Reed might be an option in some cases. I used one with an LML reed block that had the bridge cut out. It worked decently on a 175cc (Malossi 166 with a 60mm Mazzuchelli stroker). Admittedly, the Stella still had a 20mm SI clone on it and a Road 2.0 exhaust.

Back at it,
So jack & all, heres an an update 120 be3 128, 48/100 now with the venturi & lid on, idle screw 1/2 turn from fully down, mixer screw 1/2 turn out.

idle rpm@1150
1/4 throttle rpm@2260

still stuggling to take off from standing still, and up around the 3000 odd rpm's, its shithouse, but once I get (struggle to eventually) the rev's up to around 4500 -5000rpm, it will take off no worries.
(it may take a bigger MJ once its all sorted)

sitting for 10 minutes at idle, plug looks very wet
once back from test ride, plug looks dark grey/dry

Open the mix screw to 2 turns. Keeping everything else as it is, reduce the main jet until the 4000 rpm cleans up. I want to see what number fixes the bottom end.
Don't ride like a maniac on weak jetting, this is just a test.

Hi jack
so after work today had a crack, 125 fairly similar to the 128, 122 120 118 all progressively got more struggle y at the 3 to 4000 rpm's. I could eventually get the main stack to fire up by flip blip the throttle to get it to hit 4500/5000 rpm's
And when the main stack did fire up, progressively less go with each lower jet
It wasn't cleaning up at all as I went down the jet sizes (I was going to go down 115 110 but the end of the throttle cable in the headset busted off so that stop that)