How do I find out what my Main Jet is?

Hi All,

I have a PX125 with Polini177 and Sito plus exhaust…how do I find out what my Main Jet is?? I bought the scooter from MSC so the work was done for me.

The reason I ask is I want to change my exhaust to a polini so will I have to change the jet or anything else or is it just a straight swapover??

Cheers,

BP.

Guys,

I got my SI 26mm today and checked out the jets that came with it.

It was a 160/BE3/120 on my 20/20 carb it was 107 main jet, be5 140.

so di I keep 160 or 140…be5 or be3? to go with Daniel’s suggestion of 132-118 main jet.

Regards,

Brian.

hello,

vespadoctor already said you everything about jet.
I think you have the standard carb since you didn’t specified anything else.
You need a bigger carb. I tried Polini 177. The 20, or 24 are too small for what he can do.

…as far as concern the Polini exhaust… I had it and I through it away litterally in the carbage!
Yes you have more power but is way too narrow spread. Low rpm nothing!!!..and LOUD as hell!!!

If you whant to stay right hand by RZ, much better and much more expensive. For more torque you have to go left hand like SIP, Scorpion, JL , but you have to sacrifice the spare tire. All these left hand exhaust are also less loud then RZ right hand.

PS: if somebody need it I have the Kit Polini 28 carb never used, and a Mazzucchelli race crankshaft never used. I sell it half price that i bought. (I’m using bigger things since I opted for reed)
if interested : [email protected]

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Hi , yes start with a 130 main jet and work down til it gets right. Retard the timing as well and just take easy til you get the right mix, or book some dyno time at a scooter tuners and get it set in one afternoon, just keep notes on the best settings for future reference.

I’LL let BEERACE, SLIDINGDOG or the DOC answer that Brian, they are the engine specialists, i am more of a designer/fabricator/engineer than an engine man (I can do engines but I am not the best to answer technical questions) I don’t think it is hard to match the cases, you can do it with a dremel type tool, but you would need to split the cases to do it right and make sure it is all cleaned out before you put them back together, use a base gasket to mark where the ports are then gently trim back the cases to match. MB DEVELOPMENTS website show something that might help.

hello,

the Polini kit as the Malossi have multiple transfer that use the crancase to transfer mix to the cylinder insted only one transfer as standard. This means you should port the cases to let them perfom the way the were suppose to. Anyway the Polini is already ok even without cases work but you loose about 3-4 hp.
As far as concern the carb for sure I would not use the standard or 24si with the Polini. It will cool better perform better and last longer with the 28 but as I said before you loose autolube if you have it.

Your question: will I have enough power for the SIP ?
I don’t understand well what you mean.
Maybe your concern is about if you have enough gas flow to profit of this exhaust performance?

Of course the more work you do to your engine the more evident will be the presence of a good exhaust as the SIP one is.
But if your economy allows I can’t see restriction of any kind.
I just think that for a simple bolt on kit tuning this exhaust is a little expensive, but boy if it is beautifull!

                                    [:roll:]

Hi Adrian,

Thanks for your advice, Depending on how the scooter runs I may get the ports matched but if the scoot runs OK with this setup I will not bother,

On the other hand I am getting right into this tuning stuff, I would like to port the cases myself, is it hard? are there any resources/instructions available on the web.

Regards,

Brian.

Ahh!!!

forget that then…I will wait anyway to see performance with my new gear.

Brian.

Hi Vespadoctor,

It’s a 107 main jet, be5 140. I say Polini but I would love to fit a polished SIP performance pipe but I don’t think I can do this with just having a Polini177 cylinder kit.

Brian.

hello,

yes, no doubt your Polini will perform better with a bigger carb.
You can use dell’orto 28 as in the Polini carb kit with Polini manifold or malossi manifold that will cover the oil metering device hole if your Px is autolube.(If it has autolube you won’t be able to autolube with carbs as phbh)
Your kit will run cooler too since it will get more fresh air, and that’s a good thing since the Polini kit as the DR tends to size for the material it is made of.
This means you have to be carefull the way you set your carburetor. Start with a main jet that will be obvioussly big and then reduce.
It will work well with a SIP exhaust. You will have a lot of torque!
But…at this point you have enough power to upgear…and the upgear option depend on the year your PX is and the clutch it uses.

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the main jet should be around the 102-108 size.with the sito plus being a slightly larger than standard chamber roughly the same as a polini in size the main jet you have should be ok for what you want.give the scoot a good hard run,nice and hot and look at the colour of the spark plug,should be chocolate brown to a dark brown.fit the polini pipe and give the scoot a thrashing same as before then check the plug again.if the colour is same ok but if its going greyer then change the jet.if you remove the airfilter from the top of the carb and look down on the top of the carb there are two round brass things one bigger than the other with a flat screwdriver slot in the top unscrew the bigger one and right at the bottom of the tube you will see the jet size engraved into it.maybe 96,98,102,106,108 any of these are likely.if its a 102 try a 104 if its a 108 try a 110.its not rocket science its suck it and see.each motor behaves differently so they all want different jet sizes .hope this has helped,if you need more help you know where to find it

stroud valleys(midnight hour)sc est 1984

Hi Curare,

Thank you for your kind info,

I was checking the SIP catalogue and they sell a SI customized 26mm carb…I would like to go for this What do you think? Will my Polini and SIP exhaust run happy with this?

Regards,

Brian.

Hi Brian, I run a 26mm SI carb on my LML 150 (as mentioned elswhere on the board) and have had no trouble with setting it up, so you should be fine if you put one on a 177.

Hi Curare,

Can I ask some questions:

[mark=red]I tried Polini 177. The 20, or 24 are too small for what he can do.[/mark]
Will my vespa go faster if I add bigger carb and what would you recommend.

Will my vespa run OK if I have Polini177, Big Carb and SIP polished performance exhaust.

Regards,

Brian.

Cureare,

Please be aware that I have not had the transfers matched/milled…will I still have enough power to run with the SIP

Regards,

BP.

It would be better if you had the engine cases matched to the barrel ports as well, and the carb inlet matched.

hello,

about porting engine cases it is like everything. It is not that difficult after you learn! If not you can easily ruin your cases especially with the polini 177 the shape are tricky if you don’t know what you are douing you have 99% possibility of making a hole. There is not much material to play with. Many weld some alu on the clutch side, but if you know the right way you could do without welding.
I really descourage you porting your cases if you have never done it before.
Also to enlarge the compression plate as Polini suggests 4mm on each side of the length can come up in a disaster since if you make a mistake and you make it larger on the side, the inlet port will remain always open because it will be larger of the cranckshaft that seals it.
good luck…

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hello,

why don’t you try the way it is so you have a starting experience point?
Like this is difficult to say.
I think that a BE5 will be too much, maybe a BE4 is OK.
You have BE5 and BE3 if one is too rich and the other is too lean then you know you nee a BE4.(BE jet most influence from 1/8 to 3/4 trottle)
Main jet 120 maybe a little small but you have a kit just bolt on no cranckcase porting and cranckshaft modification and maybe is enough. (main jet most influence from 3/4 to full trottle)
You have to try.

See the way your engine respond from idle too 1/8, than from 1/8 to 1/4, then from 1/4 to 3/4, and from 3/4 to full open trottle.

you have to get all this transition right smooth no hole and esitations in the response.

roll:

Adrian,

You run a SI 26mm…I am ready to buy the SIP exhaust and I have already bought the SI 26mm carb for £50 on ebay which was a good price.

I was told by Daniel at SIP to use 118 - 130 main jet working my way down when I told him my config of Polini 177, SIP exhaust but SI 24mm.

I am wondering if I should use the same jetting technique if using a SI 26mm.

What do you think.

Regards,

Brian.