Modern Vespa

Hello all

I have a fully restored 1971 Vespa Sprint 150. Currently has a pinasco 177 kit installed. Was running great until a few weeks ago when my flywheel (6V, original) sheared off on the threaded hub.

My mechanic managed to lend me a used one so I could have the bike running. We installed it and got the vespa running. Ran great for a about 2/3 more weeks when it started behaving. Now, when I'm picking up speed (after stopping at a light, for example) it stutters a bit and then picks up the revs. It's like the engine found a pit of no power. Almost like no combustion for an instance. The thing is, it's quite random. And it happens mostly at low throttle. At WOT it's not as noticeable. But had it happen as well. Also it is running quite rough, the vibration is not "smooth" and it's almost as if the idle is not as steady as it should. The difference is subtle, but when you notice it, you can really feel it. On the road it's running like sh*t, because when I need to speed up it suddenly does not respond and then picks up the revs again.

I checked everything I could:
- fuel hose, was bigger by 4cm/1.5 inches. Cut it to the correct length;
- made sure fuel got to the carb (removed the banjo and saw fuel spilling through);
- cleaned the jets in petrol and made sure they were not clogged;
- noticed the flat terminal from the pickup to the external ignition coil was shearing off, so crimped and soldered a new terminal.

I cannot wrap my head around what it is. Is it bad timing? It starts at 1st/2nd kick with choke open. Fuel mixture? The spark plug is not fowled (it's new) and it's got a nice brown colour. Ignition/points going out? It's random and does not always happen at any rev range. Either if it is ignition/points going out or bad timing why start behaving so long after replacing the flywheel?

Can anyone with a bit more experience give their opinion? I'm at a loss here. A trip to the mechanic is scheduled but not for another week. I ordered a 12V kit when the flywheel sheared off to replace the original 6V points ignition, but I'm not sure the kit will correct the current behaviour.

As a bonus, here's a pic of it!

Thank you all.

That's a great looking bike! Post up some more pictures once it's running strong again.

My best guess here is that when the flywheel came loose, it sheared off the woodruff key, which then smeared some metal onto the mating surfaces, either (or both) the crank and the flywheel.

When you re-install it, even if the keyway was not damaged, you can wind up with the flywheel not sitting onto the crank's taper correctly.

On a points bike, the sensor is a cam on the inside of the flywheel which physically opens the points. If it's out of alignment by even a slight amount due to metal on those mating surfaces, it could be causing the bike to misfire like you're describing.

This also means that the flywheel is effectively off balance, which will result in vastly accelerated engine wear, so don't ride it if you can avoid it until you can look at it

What you can do to check this is pull the flywheel, paint the crank cone and inside of the flywheel's mating surface with machinist's blue/marking dye or, lacking that, a permanent marker. Then, spin the flywheel on the crank (remove the woodruff key first, obviously) and see if you have high points on either surface where the dye rubs off first. If so, you need to very carefully file those high spots down.

Also, while I don't think I'd recommend it in your case, given that this is a pretty mildly tuned motor, but you can also get some valve grinding compound and put a very small amount on the cone, then spin the flywheel on the cone to lap the surfaces together. If you do this, be sure you protect the seal (put some tape around the base of the crankshaft) and clean it off very thoroughly to ensure no compound gets on the seal and ruins everything.

For reference... On an electronic ignition bike, this same issue produces misalignment of the flywheel magnets with the ignition trigger on the stator, which produces the exact same behavior you're talking about (I just deal with this myself, as a matter of fact) and can even cause the ignition to fail entirely.

The flywheel sheared on the stub itself, near the rivets. So only the outer part was loose. The part that contacts with the crankshaft was still in its place. and held by the flywheel nut.
I will take note of that to check the woodruff key and its housing.
I just hope the crankshaft isn't bent.

However, my question remains: why start behaving so long after replacing the part? It ran fine for 2 weeks, then started behaving.
I see movement on the points, and when checking I see spark on the sparkplug.
Could it be the external ignition coil going out with the heat? Not that we're having abnormal temperatures, but it's summer.
I haven't been using it because I don't want to risk damaging anything else (because I don't know what's wrong) nor risk calling the tow (bad experience when the flywheel sheared off), but will have to drive it to the shop in 2 days (about 22km/13miles from where I live).
Hope everything goes for the best.

Hmm...if the flywheel was rattling around against the stator, something could easily have been damaged and just taken a little while to manifest.

Unfortunately, I don't think we have enough information to effectively troubleshoot it right now. I'll still add some common failures that often come with having the flywheel and stator banging around together.

Unless you have an emotional attachment to this stator, it's probably cheaper to just replace it than pay someone to troubleshoot it--which could ultimately still result in a diagnosis of "it needs replacement."

It could be something annoying and difficult to track down. such as a wire got pushed out of place and is now rubbing intermittently, but not grounding out the ignition until it wore through the insulation (I've had that, too).

You're on a replacement flywheel. Was it a Known Good part, or just a spare your mechanic had lying around in his parts pile?

As you noted, it could be the ignition coil failing, that the points are messed up, or the condenser is failing (that usually causes backfires, not cut-out's, though).

This spring, again on an electronic ignition, I had the opposite happen recently where the stator plate broke and the stator was rattling around inside the flywheel. That damaged things enough I had to replace both the stator (physical damage) and flywheel (damaged magnets).

Thank you for your replies.
Yes, I suspect the flywheel may have damaged something in the stator. I removed the rubber plug on the flywheel and tested each screw I could find. The stator screws are tightened as whell as the screws that secure the coils.
When installing the replacement flywheel we had to slightly adjust one of the coils (and the two adjacent magnets) because the flywheel was rubbing on it.

I have no love for this part it's over 50 years. I want to have a reliable bike that does not give me an anxiety attack when I start it because I don't know whether it's going to break down halfway through the journey
The replacement flywheel was his "testing flywheel" that he had kept around to use when discarding flywheel problems. Had a little superfitial rust on the stub but was in working condition according to what I know. I have a 12V kit ordered and will install it once the mechanic is available.

That flywheel is heavy, and when it sheared, likely knocked, vibrated or bumped something out of alignment.

Had the exact same thing happen with my brown Super, which also has a Veloce engine and 6v original flywheel. Flywheel went wiggly and cracked at the hub on a long downhill speed run.

- Check that the connection from the stator to the HT coil is still good.

- Check the ignition coil for signs of rubbing.

- Pull the stator and make sure the ignition wiring is still intact and not grounding.

This is The Way.

Thank you for your answer.
What I'm finding interesting is that both answers so far lead to an ignition problem. So I can rest assured my engine is in good condition.
I was afraid it could be carb tuning or a failing fuel tap but my suspitions lay correct: its a problem with the 6V ignition.

Agreed. As someone on here famously wrote once upon a time, "90% of carb issues are electrical."

Carb failures do happen, but they're relatively rare and usually can be resolved by cleaning the jets well and/or replacing the float needle.