New product: SIP GS150 cylinder kit | First impressions

Over the years pretty much every model of 2-stroke Vespa has earned a widely-available performance kit. One of the exceptions was the Vespa GS150; but that is shortly to be resolved. SIP are putting their kit for the ‘sporty’ Vespa into production, with the aim to release it before Christmas.

Sticky was fortunate enough to borrow this stunning GS150 VS5 from Matthew Philips of Douglas Vespa Parts Finder, in order to give SIP’s new kit an early appraisal. Here are his findings…

What’s so special about the GS150?

In short; a lot of things. In 1955 the GS VS1 introduced a great many technical and aesthetic features that left the competing Lambretta LD of the time feeling VERY dated.

The GS150 VS1 was the first production Vespa to feature:

• A 4-speed gearbox
• A piston-port engine with a downdraft carburettor sitting atop the cylinder
• 10-inch wheels as standard

A claimed performance of 8hp @ 7,500rpm made the contemporary 5.4hp VL1 Vespa 150 look frumpier than a bean and sprout salad.

In truth the GS150 was a Wideframe engine, but very different technically; given the design’s origins in Piaggio’s ‘Sei Giorni’ endurance racer. The gearbox layout and carburettor position are all unique to the GS series.

What’s wrong with the GS150?

In short; a few major things. 

The biggest problem with the GS150 is their reputation for heat-seizing if ridden hard. Piaggio tried various methods to prevent this: 5% 2-stroke oil ratio, and increasingly rich jetting on later models to name just two. These solutions worked to a point, but still the GS was never renowned as a scooter that could be held flat-out for long periods on the open road.

What are the origins of the SIP kit?

 The obvious solution to this overheating problem is to make the cylinder out of aluminium; a material which naturally dissipates heat far more effectively than the original cast iron cylinder.

SIP’s GS150 kit is based on a short-run project started by Ralph Bollag from Switzerland. He employed a famous kart tuner to redesign the GS cylinder as an aluminium kit. Bollag only made a few of these kits and then sold the project to SIP; who have pushed it through to mass production.

What is the kit specification?

The aluminium kit has the same basic port layout as the original cylinder: just one exhaust port, one inlet controlled by the piston skirt, and two thoroughly redesigned transfer ports. 

Included in the kit sent to me were:

• A 2-ring lightweight cast alloy piston
• A Nicasil-lined aluminium kit with a 2-stud bolt-on exhaust stub
• An aluminium cylinder head
• A base gasket
• A steel exhaust stub

What do I think GS riders want?

Everyone wants more power, right? From my brief experience with GS riders, I picture a potential client for the SIP kit as someone who wants to swap the fewest parts, in order to keep the GS looking and sounding as standard as possible. 

GS150s are valuable scooters in standard condition, so I envisage most owners being willing to try a different sort of box exhaust but not go very far beyond that. Performance is likely to always remain of secondary importance to style.

The combustion chamber is raised and located inside the bore to centralise the head

How does the SIP kit meet these demands?

There is no phrase more hated in the scooterist lexicon than ‘plug-and-play’; with perhaps the exception of ‘last orders at the bar’.

However, the GS kit is actually fairly ‘plug and play’

• You can fit the kit onto a totally standard engine fitted with an original bronze-bush small-end in the crankshaft. I say ‘you can’, but it is much better to upgrade the crankshaft to one with a needle-roller small end bearing. Thankfully Matt’s GS already had such a crank fitted
• The transfer port entrances of the cylinder are reduced down to almost match the transfer ports in the casing. There is a lot of extra area to be gained if you open them out, but there’s no need to do it if you want to leave your casing standard. Our test scooter has completely unmodified engine casings
• The inlet port has been designed so that you can fit a standard inlet manifold and carburettor. Alternatively, this kit can be adapted to accept a reedvalve manifold and larger carb; like that of the Quattrini M1XL kit

Which things should I watch out for?

• The inlet port shape is actually quite a poor match to the manifold that SIP supplied; so there is room to improve things by grinding and filling with some sort of ethanol-resistant epoxy.
• Like the standard GS kit, there is a tendency for the piston ring ends to catch in the inlet port as the barrel is fitted. You must push the rings into the groove with a screwdriver to allow the barrel to fit.
• The cylinder fin design around the exhaust port contacts the strap part of the cylinder cowling. We also used a different-length spacer nut under the cowling to improve the fit.

Marking-up the inlet manifold. It took a bit of grinding to match the SIP cylinder to the inlet manifold

One inlet stud is a little long and if fully fitted will protrude through and partially block the cylinder stud. The best solution is to shorten the stud before fitting.

Ease of fitting

Overall, fitting this kit is not difficult as you can remove the cylinder studs (double-nut method) and take off the old barrel with the engine still in the frame. It helps to have a gudgeon pin puller to remove the old piston.

The barrel came with a single base gasket. There is no head gasket and the raised combustion chamber of the head spigots inside the bore of the barrel. Our engine set up with 1.7mm squish, 166 degrees exhaust timing and 120 degrees transfer timing.

The GS engine design uses 4 x M8 studs in a square pattern, so the head fit is quite secure.

Using the cigarette paper method to check the points opening position after re-setting the ignition timing to 18 degrees static (See Complete Spanner’s Wideframe Manual for timing methods).

Exhaust choice

The exhaust stub bolts on to provide a similar round outlet to the Vespa PX125. Exhausts can either be spring-fitted or clamped on at the barrel end.

We tested the kit with two different SIP exhausts: A SIP Road 2 in the style of a GS standard exhaust. This wasn’t a great fit because the tailpipe would have hit the bodywork under suspension travel, but Dan from Total Revamp modified it for us to tuck the tailpipe closer to the wheel.

The second exhaust was a Vespa Largeframe version of the SIP Road 3, complete with Vespa Wideframe exhaust mount adapter. Not only did this fit better but it also made better power.

The SIP Road 2 version of the GS exhaust didn’t work or fit as well as the Road 3 largeframe exhaust with adapter bracket

Performance: standard

Matt’s GS150 is a VS5; which is the last variation in the series. By this stage the engine performance had been restricted slightly by richer jetting and various small changes. The original 1955 GS boasted 8hp and the 1959 VS5 claimed only 7.5hp.

To put this in a slightly more modern context; a 1990s PX125 would claim around 9hp at the crank. 

The standard GS150 may have been sporty when it was released; but only in comparison to other 1950s scooters. 

The standard VS5 performs as you’d expect from the figures. It rides like a worn-out PX125; humming through the four widely-spaced gears and eventually reaching 50mph. It’ll go a little faster with the aid of a tailwind or a hill, but faced with an incline then you will soon be hunting for a lower gear.

As we’ve always found, these ‘claimed’ power outputs measure considerably less at the wheel, once the transmission and frictional losses are taken into account. A standard PX125 measured 6.6hp compared to 6.3hp for the GS150; nothing to get too excited about for sure.

GS150 vs PX125

Standard GS150 (blue), vs standard PX125 (red). Below 6,000 rpm the PX125 is significantly better.

Upgrading

Matt wanted what I expect most GS riders to want: a performance upgrade with the minimum of changes. As such, we elected to run on his standard contact-breaker (points) ignition, just with the timing set to SIP’s recommendation of 18 degrees for a static ignition. Matt also runs a Lambretta HT coil and external condenser to improve reliability on the otherwise original battery-powered ignition system.

The supplied standard-style inlet manifold was very roughly carved out to better match the cylinder, but really it wouldn’t do any harm to flow the two perfectly.

We decided to stick with the standard downdraft carb and air filter system but removed the filter mesh and replaced Matt’s terribly worn throttle slide with a new one. Changing away from the standard GS carb set-up is possible but more complicated than you expect because of the choke system. 

The starting choke on a GS150 does not work by opening an extra fuel jet in the carb. It operates by closing a flap inside the frame to restrict the amount of air going through the air hose to the carb. As such, the hose and airbox must all be in good condition if you want the engine to start. 

SIP Kit (pink) at 11.7hp, vs GS150 standard (red) at 6.2hp. The SIP kit pushes peak power 1,000 higher up the rev range

Performance: SIP kit

In the configuration we tested, using a SIP Road 3 exhaust, power output is nearly doubled; from 6.3hp (at 5,700 rpm) to 11.7hp (at 6,500 rpm). Considering the SIP kit is the same 145cc capacity as standard, I think that’s pretty good going.

The SIP kit pushes peak power a little higher up the rev range; which has its pros and cons. Below 4,700 revs the engine makes less power than standard but once past that point it is all gains. The advantage of making the engine rev a little harder is that you can travel faster on the road without raising the gearing. On the flat, sat bolt upright, Matt’s GS would now hit a real 60mph (98 km/h). It would also go faster downhill and climb moderate hills in 4^th gear if you kept the revs up. 

Perhaps the biggest improvement is that the engine – even fresh out of the box with zero running in – felt happy at speed. I’ve always felt that 60mph cruising is the minimum for actually going places on a dual carriageway or motorway, and this kit gets very close. This additional performance is sufficient to change the GS150 from an occasional weekend-use machine into something practical for riding distances on par with a PX200 or T5.

For fun, we compared the output of the SIP GS150 kit with Road 3 exhaust (pink) to a standard T5 with Sito+ exhaust (blue) and the graphs are remarkably similar. The GS is actually slightly better on peak power

GS150 vs T5 Sito Plus

On the road the big differences between the GS and T5 are the gearbox and flywheel weight. On a T5, with a lightweight flywheel, you can quite quickly swap between gears. On the GS with a heavier flywheel and a weaker gearbox design, you must take your time over gearshifts.

Another advantage of an alloy kit, if you combine it with an upgraded needle-roller bearing crankshaft, is that you can run a reduced percentage of 2-stroke oil. I suspect that 3% of quality 2-stroke oil should be sufficient.

Strange Carb Behaviour

One of the odd features of the GS on the dyno was very inconsistent fuelling. The first run would be lean. The second run would be ok and by the third run the air-fuel mixture would read very rich.

This behaviour matched the road, where often the engine would require the choke; even to start when hot.

Eventually, I realised that the problem stemmed from the carburettor, and in particular its position directly above the cylinder on a short manifold. When you stop a hot GS engine, heat transfers from the manifold to the carburettor. The carb quickly becomes so hot that it’s uncomfortable to touch. This leads to accelerated fuel evaporation from the carb and a lean start-up fuel mixture that changes relatively slowly as the carburettor cools. It might take a minute or two for cooling to happen: prior to that you may need a little choke to keep the motor running.  

We eventually settled on a 110 main jet (standard is 103) running the stock airbox and hose but with all the air filter mesh removed. SIP suggest running the airbox without any filter material inside because they found that this set-up gave the most consistent runs. 

GS150 SIP kit all gear vs speed

A graph of all four gears overlaid against speed. This is useful to explain the speeds at which you should change gear (where one curve crosses the next). Top speed on the flat is around 60mph (98kph) but by this speed you are already in the over-rev so it will only go faster in very favourable conditions.

Summary

Personally I think the SIP GS kit hits the necessary target for most people. It almost doubles peak power when fitted with a modern box exhaust, it revs on harder so you can go faster without changing the gearing and it certainly cools better than an iron barrel. I don’t think you can ask a great deal more as a bolt-on of the same capacity as the original engine.

From my perspective the downsides are that the high rpm gain comes at a cost of low rpm power; even compared to the standard cylinder. This lack of mid-range is not a great problem on a 4-speed engine, but it does mean that you may have to work the gearbox when faced with hills or headwinds.

For most GS riders who want to keep their scooter as standard-looking and sounding as possible, then this is a worthwhile, invisible upgrade.

Going further

There’s room to get a little more low-rpm power with an advance-retard electronic ignition kit. A longer stroke crankshaft also has the potential to add some power; particularly at low revs. Both of these could be added almost invisibly.

SIP have also provided additional stud holes so you can bolt on an external reedvalve like that of the Quattrini M1XL and a bigger carb. You can also massively enlarge the transfer feeds in the casings to suit the capacious ducts in the cylinder. 

Personally I think the SIP kit is best reserved as an easy bolt-on upgrade rather than trying to turn it into a warp-factor missile. That’s an easy conversion because the standard gearbox and a lightly upgraded clutch can cope.

VIDEO: Watch Sticky’s review here

4-stud inlet port

When and how much?

SIP are expecting this kit to go on sale before the end of 2024 and to retail for less than €500. You can now pre-order it on the SIP website using this link. On top of that you may need a new inlet manifold, some carburettor jets and a suitable box exhaust like the Sip Road 3 for GS. In theory, you can then almost double your power output for less than €1,000; which isn’t bad for any Vespa and isn’t even possible for most vehicles.

Cheers

Sticky 

Thanks to Diablo Moto for use of the dyno, Dan at Total Revamp for exhaust mods, Matthew Philips (Douglas Vespa Parts Finder) for loan of the GS and SIP for help with parts.