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99 PX125e
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Hi everyone... first post from me (newbie), so apologies in advance for jumping in with a request for assistance right off the bat!

I've recently acquired a 1999 PX 125 Disc (absolutely love it!), with a Polini 177 cylinder kit and SIP Road 2 exhaust.

It revs cleanly enough and without hesitation, however when the clutch lever is pulled-in to upchange, the engine revs fail to drop-off, only doing so once a higher gear has been selected and the clutch lever released. Reading through various threads on modernvespa.com, it appears that the issue is attributable to either a carburettor or air leak problem.

Taking the carburettor first - there's a very useful SIP guide to carburettor adjustment on the forum which I plan to follow - unless anyone can advise of a better method? I have a Dellorto SI 20.20 with 105 Main and 52-140 Pilot jets to compensate for the cylinder kit and exhaust change.

Secondly, if the issue is down to an air leak, where/what would be the most likely causes for the symptoms described?

Thanks in advance for any assistance/guidance offered.

Riff
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Welcome to NSM. A nice spec scooter and from the pinnacle of PX evolution.

From the jetting it would seem like the previous owner didn't have a scooby about jetting. A 52/120 has no business in your set up, or anyone elses. Would be surprised if you can get anywhere on one tank.

Need to know a few things first. The main jet has 3 parts. All 3 put together are actually the main jet but mostly we just call the tip the main jet and the whole thing the stack. Anyway, the number of the top bit, air corrector and middle part, atomiser are as important.
What are the numbers of yours? Possibly AC140 and BE5.

Any issues starting? Has a nice slow tickover? Plenty of oil in the gearbox?

Picture of your spark plug tip would help a lot too.
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Isn't that a pretty lean main jet for that set up? I would assume it would fall in the 110 to 115 range.
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Pilot is too fat and main is too lean
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Are you sure the throttle cable is not catching and is adjusted correctly? Also, is the slide returning to the correct position (idle) when you release the throttle? It may just be sticking from time to time. A 52/140 is, if anything, too rich. I'll generally use a 45/140 or 55/160 in a 20/20 with a DR or Pinasco 177 kit. I've not tried Polini yet.

As said above, you'll most likely want c.110-112 as a main jet.
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Wow, thanks for the info and suggestions, everyone.

I'll certainly check the carb slide and throttle cable/action for sticking, or fouling. In terms of jetting, I'm pretty confused; I've found an article printed in Scootering Magazine from a while back which relates to the Polini 177 bolt-on cylinder upgrade for the PX 125, the project undertaken for the mag at Readspeed in Worcester.

In this article they refer to changing the standard MAIN of 96 for a 102-105 (mine has a 105), and the standard PILOT of 45-140 for a 52-140 (which mine has), stating "This helps to remove the PX's low-throttle hesitation".

Looking at the consensus from you guys, I should be looking at a MAIN of 112'ish and PILOT of 45-140. What do I do... help?

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Jet Eye Master
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52/140 is still too big. I read 52/120 but blame everything on my age these days.

Still need to know your air corrector and atomiser sizes.

The main jet number doesn't mean anything without this info.

Jetting from other peoples projects is a very good starting point but I expect the problems you are having are from 'cooking' the spark plug. If you post a picture of it, it will look not so great.

And answers to these.
Any issues starting? Has a nice slow tickover? Plenty of oil in the gearbox?

If you have an air leak at least one of these will be an issue
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Thanks Jack... I'll remove the main and advise of the numbers/spec shortly. The plug is brand new, in good shape, with the expected amount of 'wetness' around its base - nothing untoward or indicative of over-heating.

That said, the cylinder kit looks pretty new, so I've only ridden gently, keeping everything civil for the time being.

Thanks again.
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OK, so here goes...

From MAIN jet: 140 / BE5 / 105

Starts easily (e-start or 2nd kick), idles evenly and transmission oil correctly filled and flushed/replaced by me, last week.

PILOT jet as previously specified.

Thanks again
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All would seem pretty good then.

Jetting won't be so far wrong with a 140AC but needs to be increased to a 112MJ at least. The pilot jet will need to be smaller but we wont know how much until the main jet is done.

A plug photo would confirm it.
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Disassembled carb and cleaned thoroughly, checked operation of slide mechanism and spring to throttle connection... nothing untoward or obvious to be found. Airbox and pipework appears sound, too.

Have ordered the jet combo mentioned previously, with a 112 MAIN and will keep fingers crossed.

Thanks again... I'll keep posting progress updates until issue is resolved.

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Not familiar
Not familiar with that model. Does the timing have a centrifugal advance?

On autos, this behavior was often the result of a centrifugal advance sticking at full advance.

If not, check the timing to see that it is correct.

Does it vary depending on the position of the handlebars? If so, cable mis-routed, too short, or hung up on something.
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Again, thanks for the suggestions and pointers.

I stripped and cleaned the carburettor (standard aerosol carb cleaner), installed a 112 MAIN and 140-45 PILOT and cleaned/checked operation of the slide and throttle cable connection. Additionally, I drained the fuel tank of super unleaded, replaced with standard unleaded and... it appears that one, or a combination of these, has resolved the problem.

OK, so it's slightly more 'lumpy' whilst pulling-away/at lower revs (guess down to the 140-45 PJ), but once up-and-away, things feel far more 'liberated' in the power department, with almost no over-revving/revving-on between gears.

With regular use, running-in of the new cylinder kit and occasional adjustment of the clutch/throttle cable and carburettor, things should settle-down nicely.

Thanks again everyone.

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