Modern Vespa

I've been working on a '80 P125X. After I got it going, it really pulled. After a few miles, eventually when I really gave it throttle it would completely lose power and eventually stall. However, it idles fine. It will not idle with the choke on as expected, but if quickly throw on the choke and hit the throttle I can rev it up and get plenty of power.

I cleaned the carburetor out, replaced the gaskets, and torqued it down properly. Issue remains. Then I did the timing. Issue remains.

The jets are stock: 98 main, BE3 mixer tube, the idle jet says "48 160" so I'm assuming that's the standard 160 idle jet.

The gearbox oil looked good to me, it perhaps smelled a bit like 2-stroke oil, but maybe it was just a special formula of gearbox oil the last guy put in. The oil didn't have a tell-tale smell of gasoline like the last vespa I worked on...

So it sounds like a fuel starvation issue to me (running extremely lean when the throttle is really opened). But where and how? Is it possible that one of the seals in the engine is causing an air leak under load that just completely leans out the mixture? Or is this obviously a carb issue?

Thanks!

Have you checked the fuel tap? It's a common cause of fuel starvation on Vespas, in my experience.

Put some air into the fuel line from the carb end and it may unblock it. You can try doing that on both the standard and reserve positions - did the same on my PX before I got it running. Failing that, you made need to replace the tap.

Also check the fuel filter at the carb end to see if it's clear.

Hope that helps...

air fuel mix crew is too far out and thus too rich which will cause a low rev flat spot..turn it in an eighth at a time until its crisp

I have a late '70s p125x. This may not be directly related to your issue at all, but, I wonder about this 'stock' slow running jet of 48/160. It seems too lean to me, and it's not allways listed as stock. Haynes states 48/100 but i'm guessing that's a typo? I've tried richer slow running jets than the 48/160 and things run better imo. See this too -> Throttle issue

also, what exhaust do you have?

Thanks for all the replies!!

Beats- I used compressed air to blow out the fuel line (air inserted at the carb side of the line, watching bubbles come up from the fuel tank) both for the "ON" and "Reserve" petcock positions. A good technique to have in my pocket, thanks! However, no change.

The carb's fuel filter is clear.

Bluecati- I messed with the air-mixture screw every which way- starting point being 1.5 turns out, I messed with it until I went all the way in and then until 2.5 turns out. These definitely affected the way the bike idled, however, little to no change on the bogging while accelerating under load.

I've had a little over a gallon of gas in the whole time. I got some more gas and filled the tank completely up thinking this was a fuel pressure thing. No change.

I should note this major bogging happens when under load. I can mostly rev it up when the bike is just sitting (though not as well as other bikes I've seen).

I tried to simulate what happens while filming the carb and trying to get it bog out. I couldn't really do it, though I approached a "bog" and the carb spit back fuel. I feel like this is happening while under load- fuel gets spit back and not put INTO the cylinder. But maybe I'm witnessing an Effect and not a Cause.

In the video the air filter and air box cover is off, and I realize that it will not run right like that, but I've been testing it with the air filter/box cover on. It doesn't seem to make a difference at the moment.

If I am very strategic, I can get a good rev going, throw the choke on and accelerate powerfully and even rev up to a decent rpm until I hear it start to 4stroking, at which point I can strategically open the choke up a bit to get maybe a few more rpms, a very clumsy and dangerous dance I do

Oliver- I looked at that link, and it appears that I have the version with the idle jet with a hole in the top, and that particular breathing hole on the carb appears to be properly plugged on my bike.

I am reluctant to mess with jetting because these are apparently "stock" (maybe they're not) and when I woke the beast up, the first test drive around the block ran a lot better than it does now... What the heck did I do The timing adjustment I made was minor, only about 1 degree advancement.

Scooterhelp lists the following specs for P125x:
venturi size: 20 mm
main jet: 98/100
slow running jet: 160/100
throttle valve: 6823.01
mixer tube: BE 3
atomiser: 280/100
starter jet: 60/100

I assume those numbers are written "out of 100" and that my 'combo' idle jet is "AND". So, 48 AND 160 is my idle jet, meaning 48 starter jet and 160 slow running jet. Not sure if that's the correct convention- I'm just spitballing! I have no idea what my atomizer is, but my main jet is 98, and its holder (mixer tube) is BE 3.

Thanks again for the help. Any other ideas?

electrical issues do feel like fuel / carb issues
try looking for wires that are stretched or old looking... maybe a poor ground @ the CDI (or HT coil if it was a points ignition).

I've had erratic running due to:
- spark plug cap is old, worn down on inside contacts = loose (replaced it)
- gas flow issue due to vacuum (gas cap vents improved)
- stator harness was shorting out where it exits the case and goes to the junction box (electrical tape, case threw hole was smoothed up and a grommet was installed to protect the wires)
- jet/s were clogged (replaced or cleaned)
- carb box base gasket was crap (replaced)
- bellow was split and sucking air (replaced)
- on a points stator scoot years ago a wire was touching the inside of the flywheel causing all power to go to ground. The strange thing was it only touched above 3000rpm.

good luck

Before you dismiss fuel issues you should also check that breathers are functioning correctly on carb and fuel tank.

Firstly check that when you undo the fuel cap there is no inrush of air then, if possible, try running with no fuel cap .

I don't have specific knowledge of this particular carb but most have a means of equalizing pressure in the float chamber. This is sometimes just a drilling but often its a tube which also acts as an overflow from the float chamber to outside atmosphere and has a drain hose attached. This must not be blocked or pinched.

I'm back. I'm fairly certain the problem is not electrical. I tried several spark plugs, the condenser has been recently replaced, cleaned up all connections, timed and timed again, ensured points were in good order, and rewired the stator (this fixed the headlight, but it runs exactly the same).

I cleaned the carb too many times to count, and all those gaskets and mating surfaces are clean and fresh and the carb is properly torqued. The gas tank seems to be venting properly, though I wasn't certain how to check if the carb is venting properly. Gas flows like a river out of the fuel line if I let it.

I'm assuming the clutch side doesn't have a leak because the gear oil seems fine, and there's no dark smoke from the exhaust- only healthy 2t smoke when revving.

A friend thinks it is a symptom of an air leak on the flywheel side. There is always a little oil at the bottom of the flywheel housing so I replaced that seal. It did nothing, still runs exactly the same (It bogs out completely when I accelerate, runs amazingly well with the choke on, but I have to un-choke it to idle). Oil still comes out there. There is a bit of play in the crankshaft. Is this my culprit? Air leak on the flyside due to this play? See the video:

I also forgot to mention earlier that it's the stock exhaust.

a pal of mine with a v100 has a crank with a "loose/worn out" bearing on flywheel side and he thinks that is why his spark is funky...

if the flywheel wobbles then that could cause it to rub on coils (not good) and / or mess up the alignment of the pickup....

he is going to replace that bearing only and lap the 2 tapers to match better when the crank is out of the engine since there is damage to the cranks mating surface from sheared woodruff years ago.

Ok I think I found the problem. I noticed some two stroke oil leaking out of where the carb mounts. If stuff can leak out, air can leak in. I found the spot where it starts to bog hard on the throttle and quickly squirted starting fluid at that spot and sure enough it surged right up.

Thing is- it's a new gasket, and I've tried torquing the carb down light, spec, and a little tighter than spec- all give me the same air leak.

Can I seal this carb gasket with something like Indian Head?

I use this on all my gaskets, sealing surfaces and also as a "lock tight" on exhaust studs (works great).

the trick is to
- apply goo with a small paint brush via a hobby shop... not too much not too little
- let it air dry for 5 mins so it can get tacky
- hand tighten the bolts that hold carb down
- wait anther 10 mins
- then torque the carb down evenly to spec using a torque wrench

no air leaks
no further problems for the summer

Awesome, thanks for the tips! Is there any reason for your preference of ultra copper? I have Red RTV already just sitting there on my shelf

I like copper
holds up to heat very well for a long time

I use the red goo on the exhaust slip joint

use what you want
Ultra copper for me

You probably warped the carb base and need to lap it, but the sealer should work too. I'm super-careful when I put them on, use my torque wrench, do a quarter-turn at at time on each bolt, etc. I've still warped carbs and and to lap them.

Ohya I'm sure it's warped... Is it the carb that typically warps, or the airbox that it sits on? I can easily lap the carb but the airbox I'm not so sure...

Anywho, turns out I had copper RTV sitting around- just coated the gasket in it and torqued it down nice and evenly- to about 12 ft-lbs.

Do I really have to wait 24 hours for this stuff to cure!?

if the front flange bolt is warping the slide grooves you can fix that also...
small flat file

happened to me over and over again

then I went modern go fast carbs / premix and those issues are but a memory

Alright, I sealed those 3 gaskets at the carb/airbox really well I think, and no change (tried twice).

I believe the air leak is at where the case halves meet. See this video:

What to do at this juncture- split the casing with the engine in the bike put a new gasket in there and seal it up? Do a proper re-build? Try simply re-torquing those case bolts?

im not convinced

Am I doing it wrong?

it is hard to tell
your spraying in an area that shouldn't effect the top end mixture

when people use the spray trick they spray up near the top end or carb mount area and the rpms go UP due to the extra fuel

up your idle a little anyway = it is low

spray up around the barrel, base of carb, exhaust flange

if the spray is brake cleaner DO NOT BREATH = you shall DIE

only spray carb cleaner in a well ventilated outdoor area

B SAFE

I am watching this with interest because I am having similar problems.

I have a Malossi 166 kit, P2 carb, ported, 126 main jet

Scooter runs fine around town but within a few minutes of taking it onto the open road and opening the throttle the scooter will cut out. Only seems to cut out at full throttle though, and I think that it might just be under load when going uphill.

Suspecting that it is a fuel issue I have so far ..

replaced the fuel hose (previous one went hard and looked suspect)
replaced the fual cap
cleaned the tank - filled with fresh fuel
replaced the fuel tap
dropped the jet size (suspecting that I might be flooding)

None of these seem to have resolved the problem. I had thought that the jet was too high and I might have been flooding the engine, but it still did it after dropping the jet size down.

I have still to try replacing fuel banjo and float and needle (although not sure if it's likely to resolve the problem)

Does anyone have any other suggestions?

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Sounds more like fuel starvation than flooding to me.

Have you pulled the fuel line and confirmed that you have adequate fuel flow? Let it run into a measuring cup and see how much fuel you get in a minute. I think it's supposed to be a cup per minute or thereabouts.

Did you check the fuel filter in the carb for debris?

Also, an obligatory reminder that if you've leaned up your main jet, you're now potentially putting yourself at risk of seizing if you were tuned properly before.

Have you checked for loose wires from the stator to the junction box to the coils and on to the spark plug? It could be the vibration is causing something to short out and lose spark. This is the easiest thing to troubleshoot since it's just making sure everything is soundly connected.

Lastly, it could be an old wire has split inside the insulation and is no longer making good contact. How old is the wiring? It wears out with time/corrosion and vibration.

I thought fuel starvation, but after replacing everything so far I wondered about flooding with a high main jet.

I did have an issue in the junction box (connector coming loose and loss of lights) but I have since reconnected and taped up to stop it coming apart again. I could replace connector but is this not quite a big job?