Modern Vespa

Fit 0.8mm on the base and nothing on the head. 1.0mm squish is better on smaller cc. Timings are still on the low side, even with that.

Thanks! I'm including calls for doing it both ways, just to check my work:

#1 PBT= .4mm, .5mm base spacer, .5mm head spacer. Head=.35mm, Squish = 1.25

#2 PBT=.7mm, .8mm base spacer, no head spacer. Head=.35, Squish=1.05

The spec sheet for the kit listed low timings: Intake = 114*, Exhaust = 173*. FMP also had low timings in his video: Intake= 111*, Exhaust = 173*.
I always worry about running hot if the squish is too tight.

I don't think your numbers are correct, looks like you raised cylinder and lowered timings.

I think you're right. I am raising the cylinder by going from a .5 gasket to .8 gasket. I rechecked the measurements.

With a .5 gasket, I get a measurement of .55 piston top BELOW the top of the cylinder.

Adding a .8 gasket (.3+.5 gaskets) gives a measurement of .78 piston top BELOW the top of the cylinder.

When I put it in the calculator, I get lower timings when the cylinder is raised higher, which doesn't make sense.

I think the numbers should be entered as negative numbers, but the calculator won't compute them in that field. It used to.

Try this calculator

https://lambretta-images.com/tuningh/port-timing-calculators/

The ddog calculator works.

If you're below deck… the value should be minus/negative (-0.55mm)…

Thanks! That looks more like it. How are the inlet open/closed numbers obtained?

Edit: Ok, got it. The Lambretta calculator explains it and the numbers work out the same.

The transfer numbers were random.

I left it as I'm assuming you're running the reeds.

Apparently different calculators use different formulas.

Again, don't rely only on the calculator. Confirm all numbers by spinning the engine with a degree wheel and taking actual measurements. It only takes a few minutes and it gives you a real picture of what's actually happening at any given time in the cycle.

Will do. I printed out a degree wheel and will be using it to check TDC and timing marks. It will be easy to check what everything else is doing.

That was instructive:

I got T=120, E=179 using the degree wheel and T=122.79, E=178.69 with the calculator (PBT=.78).

That seems reasonably close.

FWIW, I fed your numbers from the last screenshot into my own port timing calculator and got the same numbers it did.

What's most interesting to me about this is that it's the same calculation for exhaust port as transfer duration, so given that the exhaust number matches, I'd be interested to see if you get the same result if you recheck your degree wheel transfer measure.

I think my measurement is accurate, but eyeballing the transfers and the piston with the degree wheel, less so, as they are harder to see and the edges are less even compared to the 1 exhaust port.

It is within a couple degrees. Is it close enough?

I've got some time today, so I think I'm just going to finish putting it together. There isn't much more that I could adjust. Squish is at 1.3mm measuring the squished solder with a micrometer. My HF digital caliper is playing up.

Yup. Get it buttoned up. Your numbers should be adequate.

Why the 1.3 squish? It will run fine but on the big side for a 190. The bigger squish makes the timing more sensitive ie. Slower overall, (if not set up to self destruct when far from home).

Just playing it safe and going by the tech sheet…at least for squish. 1.2 being the safe number for tuning newbies, with .8 being reserved for the real playas.

Chandlerman, I do see where I get the discrepancy for the transfers. If I got a bright light in there and wiggled the piston just so while watching the transfer in the exact place I measured it…

There is also room to lower the exhaust port to where the piston top sits at BDC, but maybe another time. I have been off the road for awhile and anticipate a long slog of jetting. When I get the 200 engined buttoned up, maybe I'll throw it in and take another crack at the Stella engine.

Once I'm certain on my timing, I'll match the bottom of the exhaust to BDC, but not until then. It needs to be flush with the port, neither above nor below. Usually, though, it's pretty much dead level to begin with unless I'm doing something that's not really a match for the cylinder's design, and even then, it's probably not more than a millimeter or two.

Firstly; there is no safe. Increasing the squish lowers the compression, safe. Also increases the chance of detonation (2 stroke engine destroying supernova), not safe, if the timing is slightly over advanced or jetting too lean.

Out the box kit compression is usually pretty low, even on the big boys stuff, so tighter squish makes it safer for making mistakes. Opposite of what seems obvious.

Thanks for explaining that. Safe squish is one of those depends who you ask kind of topics. 1.2 mm is a number I have come across a few times, .8 is another. Thought I was doing better. The Polini kit was 1.8.

The VMC kit definitely gets full marks for looking fast…Pressure testing and installing reed block, carb, etc.

To bad it's covered up, need a clear cowl.

I was trying to pressure test with a plate with some rubbery material glued to it under the carb. Turned out the reed block was leaking.

Resealed the reed block and the thin plate thing leaked slightly. I need to make a thicker block off plate.

I think the best way to do this is:

Seal reed block and test.

Add and seal carb box with block off plate and test.

The Stella reed block is hit and miss for sealing. There is also no place for that extra screw that secures the carb box to the case on P series bikes. The box just kind of flops around until you put the carb on.

I'm sealing this stuff all up while it's on the bench. I'm also going to check my timing marks with the degree wheel. I think it helps to let the sealant cure before it gets hit with fuel.

When I knew that I was in for a top end replacement, I immediately dropped the engine. I have broken more stuff working on the engine in the bike than riding it. Amazing that folks can dremel ports and everything with the engine in. My vision has been falling off a cliff since age 11.

This is why it's best to leak test an si setup with an old carb body. If you're not testing with every component in place, it's an incomplete test.

1.8mm squish is massive on anything with performance. Could be the root cause of the polini ring break.
Iron rings can't deal with any detonation.

I'll have a look at the piston to see if there is any evidence of detonation. Good to know.

What do you recommend for squish?

No right answer to squish clearance. Typically on an average area and turbulence head, it should be between 1 and 2 percent of stroke. On yours that's 0.6 to1.2mm. The variation depends on the angle between the piston crown and head, against the power delivery requirements.
This all only matters for the perfect balanced set up. Can pretty much bolt anything together and it runs ok.......for a good while at least.

Thank you. I will calculate it next time through. It might be a bit tighter now that everything is torqued.

Pressure tested with the carb box. Lost under a pound in a couple hours. It's the carb box, but the best I can do. It doesn't lose any pressure when tested without it.

Checked the TDC and timing with the dial indicator. Usually I can't do it this way, but it worked because of the center spark plug and the wide opening in the head.

Got the engine in the bike. Didn't take long at all. Getting better at some things at least.

Hoping to get things buttoned up this week. I have been off the road for 3 weeks and am itching to ride again, but don't want to rush it and make mistakes.

So jetting: Thinking of starting with 140/BE5/128/50-120. 128 is the biggest jet I have.

How do I do the WOT 2nd gear splutter test while breaking in the cylinder?

As for break in. I usually prime the autolube and run a tank of 1% premix through as well.

I also accelerate/decelerate about 10 times to seat the rings, then ride normally at varying speeds but not full throttle, high revs or long rides at a constant speed.

After a few tanks, I would consider it broken in, but will avoid beating it for 500 miles or so. Hoping this box will at least last me the season.

The cylinder is lined with a super hard surface of nicasil,
so there isn't much breaking in for it.

The rings will do the breaking in.
I'm a run it through the gears about 10-20 times guy.

I don't mind letting it wind right up in revs - once good and warm.
Others wait longer.

If you want something more scientific, perhaps you could check compression before you start - and when you get a change in readings that stops moving - assume it's done.

3rd is better than second - because it's got a bit more load on it - so you will get a slightly better read.

But if you don't have space, second will do for preliminary - and you can repeat once you get a bit more mileage/ opportunity.

*if I've got new bearings and crank in, I will do same as above but avoid taking revs right up for the first few heat cycles - to ensure everything is spinning and playing nicely with ea other.

Thanks. It's an iron cylinder, btw.

Oops.
Clunk.
Thud.

So legit ring and cylinder break in.
Less experience there…
Will leave to other's (myriad) of opinions on heat cycles before letting loose.

I go with the Malossi 166 cast iron break-in instructions: Three full heat cycles of gentle riding, then ride it like you stole it.

The official breakin is 300k, no more than 5000rpm


https://www.scooter-center.com/media/pdf/6d/55/c5/VMC-SUPERG-istruzioni.pdf

Do they have one of those for the Stelvio? I looked around but couldn't find anything on their site

I coudn't either, maybe an email to vmc or scooter centre might yield results

Thank you. I didn't find one for the Stelvio either.

I will just do the usual break in. The speed limit on the back roads is 45 mph with lots of hills. I will just enjoy them sensibly with rich jetting until I've run a few tanks through.

When I dropped my engine a few weeks back, I found that the coiled fuel hose had shrunk and it was even shorter yesterday. The blue hose I had was hard as a rock. It's like 6 months old!!

A hillbilly farm fix was in order. Had some 1/4 " brake line kicking around. I will sleeve it with some tubing to keep the clamps from rubbing on the frame. I'm keeping it like this. The brake line is pushed an inch and a half into the fuel hose going into the frame and almost an inch into the blue line. I warmed the ends up with a heat gun so the line is molded to the bit of brake line.

I'm starting with 140/BE5/128. 50-120 idle jet. I'm pretty comfortable setting up the main, but the idle circuit was impossible the last time through. It would run rich no matter what jet I put in and the mixture screw did not seem to do much until it was nearly closed.

When I put on the Si26, things seemed more promising. It would rev out at 1/4 throttle on the stand (it wouldn't before) and the mixture screw seemed to actually do something. Worst case scenario, I don't get quarter throttle any better than before. If that turns out to be the case, I can live with it.

Just saw this. The other school of thought that says; Ride it. Drive it. If it's gonna pop, it's gonna pop, no matter watcha do. I'm inclined to agree.

Parallels to the VW world, which has a very fussy break in. Cam lobes lubed with designer goo. A case of Brad Penn racing oil. Run the engine 20 min in the driveway at 2500 rpm…a freakin eternity. Oil change. 100 more miles. Oil change. 1,000 more careful miles. Oil change. Another 2,000 before it really feels good. Lots of people overthinking it.

I worry about break-in for the gearbox, not the top end. Historically, break-in was necessary to overcome shortcomings in the finishing stages of the machining process. Basically, *drive* (not just run) the vehicle enough that all the little bits of swarf, rough edges, etc. could work themselves out into the oil. Then change the oil and drive it however you were going to drive it.

I find that there's always some shimmer in the oil on freshly-built motors after a couple tanks of gas on motors where I've swapped out gears, shafts, etc.

But for the top end, it's three heat cycles, check the torque on the head nuts, and go.