Continuing saga of the 180 Polini

Looking for you good guys advice again…

Been running my PX150 with Polini 180 and PM Evo24 pipe on a 102 main jet , as it’s the only one that was not making it stutter and stop.

The original jet was 98.

Tonight the stuttering was back… big style.

Took plug out - quite black - so went back down again to 98 main jet - seemed hunky dory again. But then on a further test drive - got up to 63mph then POP engine dies altogether !

Sh1t - I think - must have blown the cylinder. After gliding down to about 15mph to let traffic past , tried to start her and off she went again , and took me home at 50mph easy.

All I’ve done recently is switched to Castrol TTS fully synth , and stopped the 1% extra in the tank.

Do you guys think something major is wrong , or is this a easy fix job with jetting, mixture etc?

BTW - I ran the kit in for about 360 miles without going over 45mph , and easy acceleration.

Thanx folks.

I think a 102 jet is way too small. Havn’t fitted a polini for ages but as an example the smallest i’d fit for a malossi 166 is 108 and thats with a simonnini.

No the jetting advice we are giving is for the 20/20 carb. Idf ti was 24mm I think we would be starting at 116 std

I completely agree , especially with the pedigree of info on hand here.

But if I can’t fault the air intake in anyway…
And I get a very wet & black plug with 102 and above , what else can I do… I’m terrified about melting this thing…

Ran fine with the 180 kit and standard pipe on the 98.

If the Polini blow’s up this man will carry your sorry arse Niffy.

Just to update you all… put the 102 in and a B8 NGK platinum plug and the scoot runs brill now.

Did around 450 miles on the Scottish Cannonball at varying speeds (mostly slow through the mountains) and not one hiccup.

Thanks for all the info guys.

Did you ween it off or just come straight off?That wouldn’t help.

Carburation’s a dark art(that I’m not that clued up on admittedley),but several factor’s may come into play here.Have you changed you mixture screw?Have you changed the atomiser?Is the carb seated properly after you re-jetted and do you have the correct 'plug and gap along with timing?

Brand new kit - well run in.

Spoke to PM today and explained the story.

I bought the kit and PM pipe from them at the same time , and asked them to supply jets to suit. So they sent me a 108 , 105 & 102.

Tried 108 first , then 105 then on 102 it ran very well.
At first.

Then the problems as mentioned above started.

Today they said to try the 100 , but I may have created a heat spot on the cylinder. They also advised going back to puting an extra 1% oil oin the tank.

There’s no air blockage that I can see, and after last night with the 102 jet , the plug was wet & black.

AAMOI , PM say that fitting the 125/150/180 Evo pipe uses a 98 main jet as standard.

Thanks guys , I’ll let you know how I get on.

Had a good run tonight with the 105 in , didn’t go much above 60mph , but did Dunbar to Edinburgh and back and it all ran very well indeeed.

I thought they were too small at first as well. Running on 98 just now , but can’t won’t go over 50mph.

Is the Air Screw setting crucial - and if so… where is it ?

Each bike is different and we often get bikes in which shouldn’t run but do on lower jets. Stick to the basics ie. plug colour,listening for pinking etc. and you will be fine. One thing I will say is that polini kits are pretty badly made and there could be a mix match between the piston cut outs and the ports in the barrel. This could be restricting your bike.

Nice one Niffy you had a better weekend than me then

I would agree with Curare and Diablo that you should be in the 108-110 range at least with that set-up,there must be a blockage or other problem.

First off remove your carb and give it a though clean,make sure ALL the jets are free.Also give your air filter a blow out.

As I said,jetting can be a black art,and 2 bikes are rarely exactley the same,but it seems you have another problem here.

I won’t normally argue with a tuner but I do find PM’s advice „dubious“ at times.Try 'phoning Taffspeed and see what they suggest.

No just came right off the extra 1%.

Put the mixture screw to the „full in then 2.5 turns out“ as recommended in the PM instructions.
BUT… it does say „Air Screw“ is this the top one or the one on the side?
I’ve only faffed about with the large one sticking out vertyically.

Didn’t remove the carb to re-jet . just took it’s lid off.
I’m using the same plug from standard - Champ L82C , did wonder about that.


98 main with a Polini is too small, so I would not run with this jet any more.

The air screw is not an essential setting but crucial in the transition from idle to the first 1/8 of throttle opening.

You want this transition to be fast and smooth so you should set the air screw considering how your engine behaves.
If it is lazy in the transition means you have to lean the mix.
If you feel like a small collaps you should rich the mix.

With the Polini you should use NGK 9, or Denso 27, or Bosch 3 grades sparks. It is important. Also you have to set the timing well.
Get a strobo a stay at 18. ( the IT mark)
You could go to 19, but only after your carb is properly set and you are using the right spark grade…

Another thing:

if your engine is requiring that small jet to function properly, it could mean something blocking the air filter.

On my PX125E 83 -all standard exept for the pm exhaust I use 108 mainjet, it works very well ;D
With a Polini 177 I think you should start with at least 110 the jets they sent you sounds to small to me, JuanK, Diabolo and Curare knows their stuff!!! ;D


As a wee follow up (ho hum) just been for a 5 mile jaunt - sat at 50 and no probs whatso-ever. Will order a 100 jet tomorrow.

But do you guys think the oil mixture could affect this ?

Here’s a thought , could it be smaller jets because the 125/150 's use smaller carbs than the 200’s as standard ?

Bearing in mind the factory fit jet in this was a 98.

Also Juan is right for a shop that puts out such fast scoots they don’t arf give some strange advice. Personally I don’t like them for various reasons going back to the time they told me it wasn’t their exhaust that was peaky but it was me fitting my cylinder head wrong. This despite the fact I hadn’t touched it!