SIP Road exhaust for a 2012 Vespa PX150

The stock bike comes with a 45/140 idle jet and main jet stack is 140/BE5/96. Stock bike bogs badly when moving off from idle because the idle jet is too rich. See Vespa page for full note.

This project covers replacing the stock catalytic converter exhaust with an SIP Road exhaust on a 2012 Vespa PX150 for use away from public roads. It also covers drilling the original air filter.

Initially I tried a 50/160 idle jet. It runs well but tickover is uneven so changed idle jet to 48/160. Ticks over much better.

Currently runnning 160/BE3/107 main jet stack (104 main jet with original air filter; 107 main jet with drilled air filter). See plug pictures below.

Stage 1: removing the original exhaust

Remove the side covers, remove the spare wheel, lift the rear wheel off the ground, remove the rear wheel.

Remove the exhaust re-circulator (held by two small bolts) and the exhaust (held by two bolts: loosen the clamp bolt and remove the main bolt shown below).

Location of main bolt before wheel removed.

The cable clip can be refitted using the small bolt from the exhaust re-circulator assembly.

The original exhaust and the SIP Road.

Stage 2: fitting the SIP Road exhaust

This was the hardest part because the mountings on the SIP Road do not line up properly. The large bolt did not locate properly in the fixed nut and instead pushed the nut assembly out of line.

Solution: bend the nut assembly out of the way (or remove it) and put the nut on by hand. Use a magnetic bolt retriever tool to hold it in place and gently screw the bolt in until it engages with the nut. There is just enough room to get a 17mm spanner in to tighten the nut up.

Stage 3: changing the jets

Remove the air filter cover - two bolts.

Remove the air filter - two bolts and a rubber grommet. Note which way it fits.

Put a cloth over the air inlet to stop things dropping into the cyliner.

Replace the idle jet and the main jet. When replacing the jets, do not over tighten them. Snug fit only.

The main jet stack is assembled by push fitting the three parts together.

Result of test drive: a huge improvement! Better acceleration, higher top speed in each gear, stronger tick over, no bogging. Exhaust is a little noisier but not offensive.

Stage 4: tuning the carb with different jets


Good acceleration up to nearly WOT. Bogs at WOT. Plug shows too rich. It is lighter in the photo than it really is.

Test with new NGK BR7HS at WOT for 1km. Hard to learn anything from this.


Good acceleration right through to WOT.


Good acceleration right through to WOT.

NGK BR8HS new plug after WOT for 1km in third gear up hill. The plug is too cool. Not the correct equivalent of RL82C

Scooter seems to be running a bit too lean with 103 so will stick with 104.

Stage 5: drilling the air filter

Drilling 7mm and 5mm holes in the heart shape of the air filter assists air flow.

107 main jet fitted to cope with the extra air flow.

Here is another link to a site showing how to fit an SIP Road and replace the jets:

2005 PX150 SIP Road exhaust

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