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Vespa PX125
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Hi Guys,

I'm new to the forum, although I have been popping on here for the last few years and it has always been a massive help!

My PX 125 broke down yesterday after a run of about 30 miles. I fitted a Simonini exhaust and up-jetted the carb to a 100 a few days ago and it had been running fine. After about 30 miles of doing 30mph I opened it up to 50 and it lost power and cut out. I have checked the plug and it is sparking fine, it's a good colour so it shouldn't have been running to lean. Prior to this I have had trouble getting it started, but closing the fuel tap when left standing seemed to rectify this. I has also had a slight issue with lurching when trying to hold a consistent speed, could this be a fuel line issue, or maybe flooding?

I am also aware there could be some damage to the piston and cylinder barrel as the compression seems to be poor and it won't kick over at all now.

Although this is my second PX (first one was stolen!) my mechanical knowledge isn't the best, and the thought of doing any work on the barrel and piston is a bit daunting!

Anyway any help of advice would be greatly appreciated.

Cheers,

Tim.
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The Hornet (GT200, aka Love Bug) and 'Dimples' - a GTS 300
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Moved to where you should get the best answers.
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The lurching you describe is most like the cruciform/gears being so worn the cruciform pops out of place.
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Let's have everyone decide right now that "won't kick over" DOES NOT mean "will not start." If it won't kick over that means the crankshaft won't turn, for whatever reason. But I think you really meant to say "engine won't start."

Yeah, if you installed a Crapponini exhaust and didn't upjet sufficently, you could have slightly smeared up some aluminum from piston into the ring channels, gumming up the rings and reducing your compression to nearly nothing. You'll need to pull off the cylinder's head to take a peek.
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Yeah your jetting of 100 is way too low, 118-124 w/ the simonini
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I agree. Pull the head and take a look. It will take 10 minutes and you'll have your answer. Steady state running is were things tend to go bad. The lower compression is a bad sign.... sorry.

Chuck
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Hi guys,

thanks so much for the reply's. Apologies for not using your correct lingo, I guess you knew what I meant though!

I pulled the head off this afternoon and took a peek inside, I think you are right when you say it has gummed up slightly as this is what I found (below).

If the jetting is the issue will adding a larger jet resolve this? I put the 100 in as that's is what many people advised.The scratches on the barrel aren't deep, so I hoping it won't need honing?

I had a polini pipe on my last PX witch was a piece of junk and ended up with a 4inch hole in it after riding two-up down to Brighton. I also had a sterling left hand which was nice, but I Simonini beats them both hands down!
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- muriatic acid should be used to remove the aluminum off the barrel walls. If you don't then your only increasing the chances of the happening again
- don't breath this stuff...!!
- a light hone might not be needed but if the grooves catch in your finger nail = then YES a quick home IS needed
- your gear "jumping" issue could be due to the cruciform being worn out... if so that entails a case split
- might just be that your gear cables are a bit too tight and are"fighting" each other... loose one @ the adjuster and see if that helps at all. If it does then your lucky.
- jetting is tricky and any advice given is hard due to all the factor involved. Get a main jet set from BGM (Jet200.com in the vespa section and / or call Mark for advice).
- your seize looks due to a loose head / air leak... also could ave been a exhaust leak @ the barrel or at the slip joint = get some Ultra Copper Hight Temp goo and use it

if the rings are not shot then reuse them but it would be better for you to get new ones and check their ring end gap is correct along with a timing check. The rings need to be free in the grooves and whole.

good luck
find a club in your area
that would be the best plan
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This is all amazing advice, thanks very much.

You're right that the head was loose, loose enough for a lovely oil/petrol mix to spurt all over the head and exhaust. I cleaned it up and will make sure it's nice and tight. The exhaust was an absolute mare to get on, it was incredibly tight so I don't think any air would be getting in there, especially as it would have been hot after the long run before it broke down.

I'll look in to removing the aluminum and finding out which jet is best, but here comes the million dollar question, any ideas why it won't start!? Ta.
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I would replace the rings and hone the cylinder. Make sure you torque the head down properly. Install a head temp sensor too. This way you can monitor your temp so you don't seize again.
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Yeah I think that is going to be my next course of action, well I say me, I think honing a cylinder is a bit beyond me so I'll leave it to the experts (what a cop-out, I know!). I'll get my self some more jets and work down and see where it's running best.

Cheers again for all your help.
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Its not that hard at all. You can buy the hone at any auto parts store and all you need is a regular drill.
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It a lot easier to hone a four stroke cylinder than a two stroke. No ports to deal with and catch the honing tool. Most shops that work on dirt bikes will do it for a small fee.
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Tierney wrote:
It a lot easier to hone a four stroke cylinder than a two stroke. No ports to deal with and catch the honing tool. Most shops that work on dirt bikes will do it for a small fee.
Very true, that's why I use this style:

http://www.harborfreight.com/4-inch-engine-cylinder-hone-97164.html

Either way I think many would agree a hone needs to be done. I dont think it's all that difficult to do yourself though.
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That is definitely the one I would use as well to deglaze the cylinder walls. But if I want a good honing job, I'll take it to a machine shop or cycle shop that works on two strokes. Like I said before, it is not that expensive, and it's money well spent if you want the rings to break in properly. In this case he may just get by with a deglazing, I don't know because it is not right in front of me. And I don't trust pictures too much. And it's not my scoot.
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if you take it to machinist bring your piston with you and have him spec out the piston to bore clearance. Have him me sure the piston @ the grudgen pin or just lower than it. as you know the piston does have the shape of a perfect cylinder when cold... as it heats up the head of the piston expands making it unfold from base to crown.

** most tuners using cast iron barrels with no liners... **
.09 mm to .1 mm = .00354 " to .004 "

math is...

mm spec x .03937 = thousandths of an inch
.08 mm = .0031 "
.09 mm = .0035 "
.1 mm = .0039 " = almost 4 thousandths of an inch

go for 3.5 thousandths of an inch or just over that spec so you don't have a 4 corner seize due to piston expansion...!!
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Spiderwebb, gave you a thumbs up. I have been looking for this real world info for a while. Thanks
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Spiderwebb why do you recommend such a small piston/cylinder clearance? The Piaggio spec is over 0.2mm for all P's, only the very early Vespa's had clearances at around 0.1mm or smaller

I am not disagreeing with you....I know WAY too little, I am just wondering what the reasoning behind that is and maybe get to hold the same view

Here are the Piaggio specs of pistons and matching cylinder sizes - you just need to do the math between cylinder and piston sizes (granted: it should be stock pistons and cylinders):
P series
P series
Older Vespa's - sorry about poor pic quality
Older Vespa's - sorry about poor pic quality
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At 0.2mm that will be one hell of a clackity-ratchety top end.
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carelgrundlingh wrote:
Spiderwebb why do you recommend such a small piston/cylinder clearance? The Piaggio spec is over 0.2mm for all P's, only the very early Vespa's had clearances at around 0.1mm or smaller
.2 mm = almost 8 thousands of an inch
way too loose

piston would rock in the bore and age fast
rings would age faster too

the new lined nik barrels run tighter specs due to being higher performance kits with thinner rings.

the tighter it is setup to = the slower the breaking period has to be
⚠️ Last edited by spiderwebb on UTC; edited 1 time
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Molto Verboso
71' Sprint Veloce , 05' Vespa PX150, 1978 P200E
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JoeP wrote:
I would replace the rings and hone the cylinder. Make sure you torque the head down properly. Install a head temp sensor too. This way you can monitor your temp so you don't seize again.
I am not saying that a temp sensor is a bad idea but I don't think that the manufacturer ever released any specs on temps. What I am trying to say is , OK, you go and buy a temp sensor and install it but then what are you comparing it to? the manufacturer never gave any specs (as far as I know). I don't think you could go for any other two stroke scooter/motorcycle references because each motor is built differently.

I think that as long as a good spark plug, good full synthetic oil is use you should get a steady temp reading all the time, specially when the engine is also air cooled

Is just my .2c
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scooterist wrote:
JoeP wrote:
I would replace the rings and hone the cylinder. Make sure you torque the head down properly. Install a head temp sensor too. This way you can monitor your temp so you don't seize again.
I am not saying that a temp sensor is a bad idea but I don't think that the manufacturer ever released any specs on temps. What I am trying to say is , OK, you go and buy a temp sensor and install it but then what are you comparing it to? the manufacturer never gave any specs (as far as I know). I don't think you could go for any other two stroke scooter/motorcycle references because each motor is built differently.

I think that as long as a good spark plug, good full synthetic oil is use you should get a steady temp reading all the time, specially when the engine is also air cooled

Is just my .2c
No. The temperature that aluminum on a Vespa melts is the same temperature regardless if it is on a Honda. So I would argue you are wrong. There is something to be said for the differences in the composition of the metals, but for the most part I would aluminum melting is aluminum melting and it would happen at the same temperature.
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I always let off on the gas or up jet when my CHT gets above 400 F and if my EGT goes above 1250 F I know that there is a lean location in my mixture
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vespa mallossi 166
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Hi just read youre blog and it sounds like i have the same problems on my mallossi 166 ? well neally the same . So cheers i can now check the info out myself and will let you know how it turns out.
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deano1234 wrote:
Hi just read you're blog and it sounds like i have the same problems on my mallossi 166...??
the stock crank has to be cut to increase the intake timing for this kit... once this is done the performance and overall rid ability of this kit goes way up. A pal in my club was running his 166 for 10 years+ and then did the crank web cut. He told me it was night and day.
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