Simonini exhaust jetting?

Hi everyone,

I’m going to put on a Simonini exhaust to my Sprint with PX150 engine. Do I need to change the jets around on the carb? I’m using a standard 20/20 with the autolube shut off.


hi yes my 26mm carb is actually 25.5mm when measured but it’s close enough, i have heard that some are only 25mm so it must depend on who is working on them, or if it is a friday afternoon special. it is a big improvement on the 20mm carb but would be little different to a 24mm carb so if you are tuning a 200 it would be better getting a 28 or 30mm big carb and or reed.

yes you will need to upjet (but can’t say how much) non-autolubes need bigger main jets than lube engines have as it has to allow for oil as well as fuel, where lube engines only have jets for fuel, and you will benefit from retarding the timing (between 1 and 3 degrees as a guide) my LML 150 has a SIMONINI with a 26mm SI carb with a 120 main jet and pulls like a train, you should only need to change the main jet before you start changing everything else. my experience with yamaha lc’s is to buy the next 5 highest jets from your main jet, fit the biggest, then work down, as it is better to run it with too much fuel and go down to the right mixture than have it too lean which might damage the motor. look in a haynes manual and compare lube & non-lube carb settings, the only difference is the main jet usually, the atomisers and other jets are the same. also whip your carb off & you will see all the holes are slightly off line so do a little trimming to line up: carb to airbox, all the gaskets and match the bottom of the air box to the inlet on the engine (you will see the gaskets cover part of the inlet holes so you will gain some improvement by smoothing the flow, you don’t need to alter the actual hole on the engine, just carb to box to gaskets, but to get more power you will need to do it eventually,) let the tuning bug bite!

cos the indian one was crap and i wanted to improve my scooter as much as possible whilst still using standard parts, i was going to use a 24mm px200 carb but the 26mm had just come out and was on special offer, so i thought why not, i did not want extreme tuning like a big carb, just better.


the Simonini is the best affordable exhaust, it is better then Polini, and Leo Vinci.
Yes, as Adrian told you, you have to up jet. Go 10 higher so it will be sure too rich and then go down by two size each step.

As far as regard the timing try to retard 1, if more, power will suffer. Put a colder spark then the original you are using.

As far as regard carb.
Si is ok only if you want to keep autolube. If not go for a normal one you can fine tune much better with neddle jets and jet needles.


Thanks Adrian, I will take your advice and experiment with bigger jets. Hadn’t thought about the point that non autolube needs bigger jets because of oil + gas.

By the way, why did you choose 26mm carb instead of a bigger one?


Hey Adrian,

I’ve been reliably informed that the 26mm’s are actually only 25mm’s and are being pushed by dealers with a lot on their hands. Not so long ago I was looking at a 28 or 30mm as a step up from a 24mm but a reputable and usually reliable dealer spent a good 15 minutes attempting to convince me to get one of these.
It sounded like a load of old fluff at the time to be honest.

I realise you went up from a 20mm but any thoughts anyway?