polini 187 port timings

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johnd
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Wed Aug 22, 2018 2:26 pm

Ive not built a PX 125 engine for my own use since the late eighties, but since selling my Lambrettas a few years ago and now trying to get back into scooters ive found myself priced out of the market as far as Innocenti stuff goes so ive been reduced to tuning up an LML125 rotary motor for my latest project.
Ive opted for an Iron Polini 177 kit, MMW head for long stroke, Mazz 60mm race crank, 24/24 carb and a SIP road 2 pipe. Bike is for commuting and occasional tootling up the coast road on a nice day, but it has to be able to outrun the local chinese "vespa" clones too.
My question is: will the polini 177 and 60mm compliment each other? Ive measured the port distance from the barrel deck and im getting 36mm for the exhaust and 48mm for the transfer ports.
This is working out as approx: 175.69 degrees of exhaust timing, 123.9 degrees of transfer timing and 26 degrees of blowdown timing.
I'm after a reasonably usable motor, have i just pushed the timings into "peaky" territory with the long stroke crank? Bearing in mind that i'll also be using a box exhaust?
any Px125 tuners out there??

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johnd
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Wed Aug 22, 2018 2:31 pm

I should add that i'm getting these aproximate figures using a theoretical piston to deck height of 0.5mm as i've not put the motor together just yet .It will also be port matched and blue printed.

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johnd
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Sun Sep 30, 2018 1:57 pm

Just thought i'd report back to this thread as i've built the engine now and taken a few rough measurements with a Vernier caliper and a port timing calculator.
Longstroke Polini 187 (Iron cylinder)
I used the gasket that came with the kit and an MMW malossi head for polini pistons and although i ended up with a negative deck height of 0.5mm i got a squish of 1.55 straight out of the box.
Port timings as i measured them with a Vernier caliper so there is a small margin of error.
Exhaust: 171.8
Transfer: 118.8
Blowdown: 26.5

Running a SIP road2 pipe the engine feels very torquey with no "hole" in the power that i was afraid of by lengthening the timings. It feels like there is enough power there to pull 23/68 easily enough and probably 23/65 if you are light enough.
Would like to try an expansion but i'm not sure which Vespa pipe would suit those timings. Thoughts?

Jack221
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Tue Oct 02, 2018 1:43 pm

Good effort but your porting is a long way from exciting. 172/119 are good solid numbers that give broad low power. I'm sure she goes fine.

To go faster you'll need more than just a decent exhaust ;) If you want to beat those Chinese twist and go nutters, you're going to need way more duration.
Save your money and keep the gearbox where it is. Just add some more rpm.

If you can, get a machine shop to take exactly 2.50mm of the top of the cylinder.
Then grind the centre 15mm of the exhaust port up by 1.00mm
Fit a 2.5mm base packer to get your piston flush at TDC again.

Porting will then be 183/129/27
Still a fair way from the limit but much more interesting.
Ok with the SI carb but will need a major up jet from now.

Obviously you'll need a disc brake, proper shocks and tubeless rims or you'll kill yourself.

If you know what I'm talking about and are up for it, I'll talk you through the rest of set up later on.
Vespa est ieiunas, quia semper operaturi

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johnd
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Wed Oct 03, 2018 10:09 am

Jack221 wrote:
Tue Oct 02, 2018 1:43 pm
Good effort but your porting is a long way from exciting. 172/119 are good solid numbers that give broad low power. I'm sure she goes fine.

To go faster you'll need more than just a decent exhaust ;) If you want to beat those Chinese twist and go nutters, you're going to need way more duration.
Save your money and keep the gearbox where it is. Just add some more rpm.

If you can, get a machine shop to take exactly 2.50mm of the top of the cylinder.
Then grind the centre 15mm of the exhaust port up by 1.00mm
Fit a 2.5mm base packer to get your piston flush at TDC again.

Porting will then be 183/129/27
Still a fair way from the limit but much more interesting.
Ok with the SI carb but will need a major up jet from now.

Obviously you'll need a disc brake, proper shocks and tubeless rims or you'll kill yourself.

If you know what I'm talking about and are up for it, I'll talk you through the rest of set up later on.
Great reply, and I know exactly where you are going with those timings, I've been there with scooters before and I'm staying away this time. 👍😂
This project has a target, and it was chosen because a good second hand PX200 engine is getting harder to find and too expensive to buy. The target was to build a px125/150 engine that would pull a bit better than a standard 200, and outrun the guy who lives locally who rides a Chinese Vespa clone covered in Two Tone stickers and targets because the standard LML125 engine couldn't. 😂
The challenge was that I've been into fast lambrettas for most of my scootering career and if I rode a Vespa it was a P200, tuning the px125 was something I've never thought about for long or at all. I know next to nothing about Vespa expansions, cylinder kits and all the new stuff that's come out since I last read scootering a few years ago.
The timings I've ended up with I can live with. The engine feels like it has a useable road tune for what it is and the box pipe even has a surprising "feel" as you let it rev on, it doesn't feel like it's strangling the kit.
I suppose the question I was asking was: What Vespa expansion chamber would best compliment those timings?
And more importantly, what is regarded as a the most reliable/value for money clutch for a PX these days? as that always used to be the weak link and it's the one thing I've not upgraded yet. (Scooter not being used just yet)
Disc brake and tubeless rims were added before the engine got tuned 👍

Jack221
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Fri Oct 05, 2018 11:52 am

Can see where you are coming from. Tuning Lambrettas is quite a minefield and not surprised to see you have been left traumatised. Vespas are more predictable but still have their issues. Even the most tuning on an LC Vespa with 4 gears is really not that advanced when compared with racing motorcycles.

Most tuning though is not the fault of the tuning itself but more how and by who it is set up. Setting up by copying jets or timing from a book, website, or someone else’s scooter, is a good place to start but not where any tuned engine becomes reliable.

At your level of porting it doesn’t matter so much which exhaust you pick. The difference between them is so slight its better just to pick the one you like for look or sound.

On largeframe Vespas the clutch is an issue. Always has been. My current favorite is the Superstrong basket with Honda plates, XXXL springs and a bearing centre. This is quite expensive but can take the power. On yours a banded basket, thick circlip, XXL springs and Honda plates will last a long time.
Vespa est ieiunas, quia semper operaturi

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freakmoped
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Tue Dec 11, 2018 5:36 pm

118-120/174-176 with si and box or big box xombined with 200-210 intake timing (rotary valve tolerance MUST be 0,05mm) exhaust port 65%. use gs piston rings.

Makes fun and sense :)
www.freakmoped.at
Utube channel

Wer nur in die Fussstapfen anderer tritt, hinterlässt keine eigenen Spuren....

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johnd
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Wed Dec 12, 2018 2:52 pm

freakmoped wrote:
Tue Dec 11, 2018 5:36 pm
118-120/174-176 with si and box or big box xombined with 200-210 intake timing (rotary valve tolerance MUST be 0,05mm) exhaust port 65%. use gs piston rings.

Makes fun and sense :)
I took quite a bit of inspiration from your YouTube channel 👍

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