Modern Vespa

Comments on my ET4 200 cc project

There is a significant difference between the ET4 and LX150 cylinder head when considering the use of a Piaggio Liberty 200 cylinder. There is an index hole milled in the head gasket sealing surface on the ET4 head that reduces the effectiveness of the head gasket. This issue will cause early failure of the head gasket. I've attached a photo of each head with a 200 cc head gasket, p/n 843204.
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Using the LX150 cylinder head on an ET4 will require plugging the SAS port with a 3/8-16 set screw. This is easily accomplished with a 5/16 bit and a tapered tap. Using a tapered tap will cause the threads to taper near the bottom and lock the screw in place. Permanent locktite is recommended.

The Liberty 200 engine has a heavier crankshaft than the ET4 200. To reduce the imbalance, use the piston pin from the ET4 150 cc piston:

Piston, Rings and Pin - 150cc 170gr
Piston, Rings and Pin - 200cc 220gr (25% heavier)

Piston, Rings 200cc, Pin 150cc 207gr 19.60% heavier

The Piaggio Liberty 200 cylinder head is shorter than the ET4 150 cylinder. I found it to be .030 shorter. Therefore, in order to obtain the correct squish and compression ratio, stacking of available base gaskets is required. Not adjusting the base gasket thickness to the proper value will risk the piston hitting the cylinder head. Example:
Measured piston height below deck: .010
Stack base gaskets to obtain desired height: .8 + .6 + .4

These settings are for use with an internally gutted stock muffler and modified airbox. Carburettor model CM129213

Main Jet drilled with #59 bit
Pilot Jet drilled with # 79 bit
Needle shimmed w/ 0.031 washer (see needle drawing for dimensions)

Nice write up. Good info for the community archive. Assume you used the LX head. Did you consider welding the hole and milling the ET head?

I did consider welding the ET4 index hole and then milling. While I'm an expert TIG welder, the hole is deep enough to require substantial preheating of the cylinder head to produce the desired weld. There is a degree of risk in warping the head and there isn't much material to mill the head flat. Further, I would need to fabricate a jig to hold the irregular shaped head for machining on my bridgeport mill. The simplest solution was obtaining an LX150 head and valve cover, which I happened to have from an engine with broken cases.

Long-term ET4 200cc Update: Reliability

The project has been very reliable for 7 years now. I've added a manual fuel petcock to allow running the engine out of fuel for storage. The valve was sourced from ScooterPartsCo.com. Description: Manual Fuel Tap for Vespa, 90 degree outlet, PU10001

Performance Feedback:

I have two ET4 scooters. One is bone stock for my wife and mine has the modifications. Acceleration is much better compared to the stock engine. Top speed is only a few mph faster when compared to my wife of 120 lbs and my 180 lbs.

Cost Benefit:

On a scale of 1 to 5, with 5 being very cost effective: 2.5
The effort and cost for the performance gained is fair to medium. However, I much prefer to ride the 200cc ET4 than the 150cc ET4. It seems to me the performance is limited by the small valves and ports. Perhaps that is the reason for the Piaggio 3V engine . . .

Would I recommend it? Probably not. Better to purchase a GTS 250 instead.

Thanks for the follow up, great info