Modern Vespa

Got a 1978 P125x. Getting it up and above 35mph it cuts out and if I hold it, the power surges. I have been running 2% premix in it but this particular tank may be a bit "Fat", having added oil for 2 gallons but only putting in 1.75 gallons (previous tank was correct at 2%). One guy said that its getting too much air in it, another said its trying to soft seize. I am completely stumped. it almost acts like a vapor lock, or its not getting enough air or enough fuel. Any ideas?

Check to make sure your fuel cap holes are not plugged up.Check your carb.bolts are tight.Make sure your mix screw is at lest 1.5 turns out from all the way in. Hit that thing with some 90+octane . Is it smoking? Whats you plug look like? Soft Shezit! is Bad make sure you pull that clutch right away or bye bye lump...!

Check to make sure your fuel cap holes are not plugged up.Check your carb.bolts are tight.Make sure your mix screw is at lest 1.5 turns out from all the way in. Hit that thing with some 90+octane . Is it smoking? Whats you plug look like? Soft Shezit! is Bad make sure you pull that clutch right away or bye bye lump...!

That was my first thought, and I ran a mall drill bit through the holes to make certain they were good, and even added two more holes. I always run 90 octane (habit from my 4T scoots) and since its not a dellorto carb (something with an "S"???) I thkn its 3 turns out for the mixture.

Swap out your coil for one you know works.

Coils can short under load (high cyl compression and revs), then resume their composure when compression / revs drop, giving you a "surging" that is a bit like bumping up against a rev limiter, only with a longer period (lower frequency).

Good luck!

- Eric

Spaco - yeah the threading is finer than in a Dellorto, so a good starting point is 2.5 as opposed to 1.5

I have a spaco 24/24 runs perfect 1.5 turns out.
Coil sounds like a good cause too.

By coil, do you mean one of the coils in the stator? I can understand revs shorting it out as more current is forced through as the flywheel spins faster, but how would high cylinder compression affect it? I know compression obviously affects performance and power, but I thought it had to do strictly with atmospheric pressure within the chamber (that is, the space above the piston within the cylinder, held by the rings and cylinder head). Clearly this affects how responsive it is, which might lead to easer revving, and I can see it perhaps having some sort of effect on the exposed part of the plug, but how would it affect a coil?

I'm learning every day, so any reply would be appreciated.

i mean...if you have a spare HT coil it wouldn't hurt to swap it just because it's an easy thing to test.

But i think you have a carb dialing-in issue. You're getting too much gas at certain points... and holding the throttle lets it burn off some of that excess gas that's in the mix...but it takes a minute to overcome it.


That's my thought.

You may have a clutch side oil seal leak. As the increase in revs causes more suction (which in normal circumstances draws more fuel/air mix into the engine through the carb), some of the gear oil is sucked in.

Both my bike and my girlfriend's bike had the same symptoms when our clutch-side seals started to go. In both cases it eventually progressed to the extremely high revs at idle that are pathognomonic for an airleak.

With the bike idling at a regular speed, remove your gear oil fill plug and put your finger over it. If you feel suction with each revolution, it's time to change your seals. If you want to confirm it, check if your used gear oil smells like gas. Simple and easy thing to check. For your sake, I hope it's the spark plug lead like has been suggested.

Eldorado: You said pathognomonic.
Consider yourself outed. Phaetn: I may be right or wrong, and others' suggestions are also good ones, and should be considered and checked out. Internet diagnosis is notoriously difficult.

To answer your question, I mean the ignition coil that transforms 6 or 12 volts AC or pulsed DC (depending on the bike) into about 15,000 volts at the spark plug.

Electricity behaves a lot like water, but without the wetness, especially at high voltages.

There is a certain voltage (which is similar to pressure) required to "push" the electrons (or current, measured in amps) through a given substance under certain conditions. The quality of resisting this flow of electrons is called resistance and is measured in ohms (in its simplest form).

A fair amount of "pressure" is required to push the spark through the air across the points of the spark plug. The pressure required increases with actual combustion pressure and temperature.

At idle, the throttle plate is closed, and the piston is essentially sucking air in through a tiny opening (like breathing through a straw), causing the air to enter the combustion chamber well below atmospheric pressure (at a sort of a vacuum). It is then compressed to very little above atmospheric pressure before it is ignited and burned. Under wide open throttle, the engine is sucking in air against no vacuum, at atmospheric pressure, and compressing it to pretty high pressures of, say, 125 to 150psi. The air-fuel mixture is also hotter the harder the engine is working.

It is harder for the spark to jump across the spark plug gap under conditions of high pressure and temperature than under conditions of low pressure and temperature, and so requires more electrical "pressure," or a higher voltage to do the job.

What this means is that when the coil is given a pulse of electricity by the points (or electronic module), it's output voltage will rise as high as it needs to in order to get the spark to jump the gap, but will not rise significantly further, as the current is now being bled off across the spark plug gap. The higher the pressure and temperature, the higher the voltage will rise before the spark occurs.

What the heck is my point, and is this the Smuttynose stout talking?

Well, the insulation between the bazillions of windings of the coil may be imperfect in places, but may hold up quite well under lower voltages.
Higher voltages, though, may be enough to cause the voltage to drain off through an internal (or external, ie: spark plug wire insulation) short.
In this case, the motor will run fine until the spark voltage rises above the threshold allowed by the insulation fault, then bleed off, allowing operation under low-load conditions, but not under high-load conditions.

Make sense?

Didn't think so.

Good night.

- Eric

Outed as what? I work in health care, and teach part-time. It's a word that's in my general vocabulary, and I couldn't think of a good synonym off the top of my head.

Awesome, thanks for the tutelage. It makes total sense. To be honest initially I hadn't considered the HT coil and was only thinking a stator coil; nevertheless, your post was still really informative for me. Thanks!

Only people in academic medicine actually use those words, plus, your response to my offhanded joke sounds defensive, ergo, you're either academic or in training.

Maybe I'm the only one who thought that was funny...

Guess I've been out in the country too long. Sorry.

It was a combination of things (being a 31 year old bike was only the beginning):

- HT Coil was bad

- Carb Jet was incorrect (150/EB5) changed to 160/EB3

- Head was a tiny bit loose.

I did a plug chop and found that the fireball appeared to be misshapen (leaving a nice white streak on the plug) and when I removed the head, there was fresh evidence of blow-by.

Never fear though, y Malossi 166cc kit is ready from being polished and ported, and should slap right in with no problems.