Tech Torque

As out­lined last month, we have a full com­ple­ment of tun­ing and up­grades com­ing for all sorts of Vespa ma­chines, and we started the jour­ney with a SIP Road 2.0 pipe up­grade for the PX125 as our first item. With the re­sults in from that, we now move on to

Scootering - - Contents - Words & Pho­to­graphs: Dan Clare Thanks to SIP Scoot­er­shop and BGM – Scooter Cen­tre for their sup­port on this ar­ti­cle.

Dan con­tin­ues the new and on­go­ing Vespa tun­ing and up­grades sec­tion – this month look­ing at cheap cylin­der and head tricks to gain more power.

A quick re­cap

Last month we es­tab­lished a base­line fig­ure of 6.3bhp for a 2015 PX125 fit­ted with cat ex­haust. The power peaked at 6000rpm and of­fered no over-rev. Thus it took around 29 sec­onds to ac­cel­er­ate from a stand­ing start up to 50mph on the flat. The stan­dard ma­chine would wheeze out 45mph up­hill, 50mph on the flat and only 52mph downhill. Af­ter fit­ting a SIP Road 2.0 pipe, we gained 1.5bhp, nearly 1000rpm of over rev, dropped nine sec­onds off the 0-50mph, and al­though up­hill and on the flat speeds only changed by 2mph, the top speed downhill gained 5mph due to over-rev now be­ing avail­able.

What’s next?

Okay, not ev­ery­one wants to (or can af­ford to) fit cylin­der kits and other ex­otic items… so be­fore I go skip­ping joy­ously through the play­ing fields of Nikasil kits and long stroke cranks, let’s take an­other look at what could be done with the stan­dard 125cc unit.

Step 1: Good head

Mea­sure the squish (gap be­tween cylin­der head and pis­ton crown when pis­ton is at top of its stroke) on a new PX125, chances are it will be huge; our model came in at around 2.6mm. A bet­ter squish mea­sure­ment which works well is 2% of stroke, which would be 1.14mm on this scooter, so the fac­tory fig­ure is miles out and ac­tu­ally won’t be very ef­fi­cient at all. Look­ing at the cylin­der head on a PX125 though, you will see that it has an in­ner re­cess of around 2.6mm which is roughly the same size as the huge squish. Then, look­ing be­neath the cylin­der, the stan­dard base gas­ket is a metal item at 0.2mm thick, so with these items in mind, here is the so­lu­tion: post the cylin­der to the lo­cal ma­chine shop or scooter dealer (we used Chis­el­speed) and ask them to ‘zero deck’ the head, thus re­mov­ing the 2.6mm re­cess. Re­move the four cylin­der studs, pop the cylin­der off and re­move the old 0.2mm metal gas­ket (do not dis­card it yet though, you’ll need it later).

While we waited for the head to come back from the shop, we or­dered a 1.5mm base gas­ket from SIP. It’s ac­tu­ally 1.55mm when mea­sured, but will com­press to 1.5 when fit­ted. Us­ing the old gas­ket we traced out the line for the trans­fer ports and used a craft knife to cut these out, the gas­ket then matched the cas­ing prop­erly. Once the head came back we sim­ply popped it all back to­gether, now dis­card­ing the old base gas­ket. Upon re­fit­ting ev­ery­thing we now had a squish of 1.3mm and the cylin­der ports were all sit­ting 1.3mm higher than be­fore, which helped in­crease their rel­a­tively low tim­ings.

Port mea­sure­ments

The stan­dard ma­chine had port tim­ings of 112º of trans­fer and 153º of ex­haust and 20.5º of blow­down (the du­ra­tion of ex­haust be­fore trans­fers open). A CON­SER­VA­TIVE fig­ure for a low-power/ mass-pro­duced/off-the shelf tour­ing ma­chine might be 122º trans­fer, 168º ex­haust and 23º blow­down. So again, the

stan­dard fac­tory fig­ures were way out. How­ever, by fit­ting the 1.5mm packer (in­stead of the 0.2mm item) we now have 119º trans­fer, 158º ex­haust and 19.5º blow­down. So al­though this has slightly re­duced the blow­down tim­ing, both the ex­haust and trans­fer are raised in one sim­ple ma­noeu­vre.

And the re­sults

For the cost of hav­ing the cylin­der head ma­chined and fit­ting a base packer we have gained 1bhp, go­ing from 7.8bhp up to 8.8bhp. Given that on my dyno a fac­tory PX200 puts out around 9bhp we are not do­ing too badly with the lit­tle PX125. On the road the ex­tra 1bhp is just de­tectable, the en­gine seems hap­pier to rev, the 0-50mph time has dropped an­other sec­ond on av­er­age (to 19 sec­onds), and the top speed on the flat and downhill has gained an­other 1mph (go­ing from 52mph to 53mph on the flat and from 57mph to 58mph downhill.) It’s all small in­cre­men­tal gains, but they are cheap and easy mod­i­fi­ca­tions, the re­sults of which can ac­tu­ally be de­tected on the road.

Step 2: Use Grinder

No not the ‘so­cial’ app… a port­ing tool. The sec­ond you men­tion port­ing some peo­ple seem to phase out, as if it’s an art or skill be­yond them. Well let me tell you, there are lots of sit­u­a­tions sur­round­ing tun­ing/port­ing where in-depth skill and knowl­edge mat­ter; this isn’t one of them. Quite sim­ply you can widen, and raise, the whole PX125 ex­haust port by 1mm with a £10 long-shaft tung­sten car­bide burr. The burr will fit in a home hand drill, or an ex­pen­sive grind­ing/port­ing tool alike.

I wouldn’t ad­vise a home-tuner to get in­volved in tun­ing ex­pen­sive Nikasil-lined kits, one chip of the liner or one slip of the port­ing tool and it can spell dis­as­ter. But these cast iron Vespa kits are cheap, and tough! Plus there’s no Nikasil liner to chip. Sim­ply use some mask­ing tape to mark out the rough 1mm bound­ary that you don’t want to go past at the top and sides of the port, and then gen­tly ream the port out un­til you have added 1mm all the way round (not the bot­tom of the port). Af­ter that, just get a round hand file, and hand-cham­fer the edge of the port, and then fin­ish by hand with some fine grade wet n’ dry. Job done. Once the cylin­der is off and in hand for the head/ gas­ket pro­ce­dure, the port­ing job only takes an ex­tra 20 min­utes all in… go on, try it… you know you want to.

More re­sults

Guess what, you’ve now added an­other 1bhp, tak­ing you from 8.8 up to 9.8, the 0-50mph has dropped to 18 sec­onds, the scooter will sit at 54/55mph on the flat all day long, and once up to that speed… it holds it far more com­fort­ably when you hit a head­wind or slight in­cline. The mo­tor is now 55% more pow­er­ful than stan­dard and just has the edge over a stan­dard PX200 on the dyno… plus, the downhill ‘over-rev’ speed just hit 60mph. Not bad.

More port tim­ings

The packer and cylin­der head mod saw us go from 112º trans­fer, 153º ex­haust and 20.5º blow­down, to 119º trans­fer, 158º ex­haust and 19.5º blow­down. This lat­est port­ing mod leaves the trans­fers where they are (which is fine on this 125cc) and ups the ex­haust fig­ure to 163º, thus rais­ing blow­down to 22º. Ev­ery­thing is go­ing in the right di­rec­tion.

More home tun­ing?

At this stage I con­sid­ered how far I could take the stan­dard cast iron kit and other com­po­nents. But it was look­ing a bit un­re­al­is­tic in terms of cost vs time vs ben­e­fit. Quite sim­ply, I al­ways look for bang for buck when I’m tun­ing, and the sim­ple changes high­lighted in parts 1&2 of this fea­ture of­fer that. I con­sid­ered send­ing the cylin­der away for a re­bore and to be fit­ted with a GranS­port pis­ton to in­crease the cylin­der to 177cc. But once you re­alise a Malossi 166 kit can be bought for around £170, then the cost of GranS­port pis­ton, plus re­bore, ma­chin­ing to head, postage each way etc. starts to look like a close call. I also didn’t want to bog this down in tech­ni­cal de­tail. So with that in mind, next time we’ll com­pare the BGM ex­haust to the SIP ex­haust, and then bolt on the BGM 177 kit and see what that can bring to the party. Plus, now we’re faster, per­haps it’s time we started look­ing at tyres, brakes and han­dling? The SIP shock­ers look like a good place to start. That’s all for now, stay tuned.

PX125 – ripe for tun­ing.

1: Stan­dard PX unit. 2: Re­move cowl and head, be­fore ‘dou­ble-nut­ting’ studs and wind­ing out. You can then re­move the cylin­der and old gas­ket. 3: Use the old metal gas­ket to trace and cut the new 1.5mm ver­sion. 4: Per­fect match. 5: Stan­dard head with...







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