Modern Vespa

I have a '62 Vespa 125 - I recently rebuilt parts of the ignition system: New points, condenser, etc. I checked the timing to make sure it was on after replacing the points.
What happens is, intermittently, the engine starts making loud knocking right after being started, the flywheel key shears off and the flywheel comes launching off the engine, and bangs around inside the shroud. After this happened the first time, I took a careful look through everything to try and find the cause. There was a loose wire on the coil, which I soldered back into place.... I'm pretty sure that wasn't the issue, though, because after running fine for a week after the going-through, it started doing it again.. I've run out of ideas - I imagine what is happening is the ignition is firing at the wrong time - While the piston is coming up - and that is shocking the flywheel and snapping the keys in half..


Has anyone else had this happen?

Something interesting : It hasn't happened while the Vespa is in motion - only after just being started..

Thanks!

Luke B

wow that is some seriously wrong shit. seriously.

i would say your timing is still very wrong. dont start it again until you do some more homework and inspection of the engine.

your crankshaft and top end are in serious peril. serious peril man. [/img]

The key on the crank is there to locate the flywheel to the crank, not to keep it from spinning. It's the matched taper of the crank and flywheel and the correct torque that keeps it in place. If the flywheel was not tightened properly or if the tapers are not clean or are deformed then it will not be held in place properly.

At this point it's likely that the area around the key way is quite deformed. This can sometimes be corrected with a file to the high spots and then some lapping compound. This is best done outside the engine as any lapping compound that finds its way to the seal will kill it rather quickly, but I have done it without pulling the crank from the engine. The "right" answer is to replace the crank depending on how bad it is.

Have any pictures?

I will post pictures soon. While I agree that the key isn't necessarily there to keep the flywheel from rotating on the shaft, what is happening here is obviously the flywheel being shocked in the opposite rotational direction by some kind of a misfire - what is causing the misfire is the mystery...

I can't imagine any kind of misfire knocking the flywheel off of the crank. Maybe a piston ring hanging up on a port?

And you have the flywheel nut in place? And torqued? A properly fitting flywheel is hard to get moving, there is no way it should rattle in the fan cover when bolted down...

The more likely order of events is:

1. The flywheel, not being properly seated/tightened, shifts and shears the key.
2. The timing is now WAY off and fires at some really odd point in the rotation.
3. The mass of the flywheel is not firmly attached to the crank at this point so when the misfire happens in step 2 it sets the flywheel spinning but the crank comes to a stop.

ya, re-reading this:

"and the flywheel comes launching off the engine, and bangs around inside the shroud."

I am thinking there is no flywheel nut in place. How could it if it was bolted down, even without a key in place?

On my 150 super I had the key shear once. Before that happened there were signs that the flywheel was shifting around but it being my first bike I didn't see them. One time when I started the bike the kickstarter kicked back really hard but the engine started. So I hop on, put it in first and as I start to let the clutch out the bike started moving backward. Hit the kill switch, kicked it again and this time it was running forward.

I had this happen after a backfire! The bike ran backwards for about 20 feet and then died when I let out the clutch. When I started it back up it was fine. I talked about it for years as my "I saw Bigfoot" moment.

did someone remove the circlip from the flywheel?

are you properly torquing down your flywheel nut?

are you using a lock washer and torquing the flywheel down correctly??

The flywheel has a nut and lock washer, I torqued the nut to the correct specs, and the flywheel has the circlip.. What's so strange is that I had the flywheel off to check the timing - I put it back on (torqued), and the bike ran great. When I brought the Vespa out a few days later, it ran very poorly after being started, and sheared the key almost immediately....

Quote:
"The more likely order of events is:

1. The flywheel, not being properly seated/tightened, shifts and shears the key.
2. The timing is now WAY off and fires at some really odd point in the rotation.
3. The mass of the flywheel is not firmly attached to the crank at this point so when the misfire happens in step 2 it sets the flywheel spinning but the crank comes to a stop."

What I disagree about this explanation is that I don't see why, even if I hadn't tightened the nut, would the key shear off with out some kind of counter force. And the engine can't misfire due to timing if the key is still in one piece... Just my thoughts. I'll get around to pictures one of these days.

pictures would help a lot.

can you maybe get a detailed one of the crank with the flywheel off? and the flywheel itself

I agree with this. If the shaft is all scored as well as the cone on the flywheel, it's a loosing battle. Can be cleaned up with some sand paper, start coarse end up with 600 at least. That should help.

Then tightend the s*&t out of it..

Vespa update - Well, I think the problem is solved. It really was a combination of the area around the key being damaged slightly so it wasn't seating quite right, and me not tightening the s^*# out of it hard enough. I was totally wrong about the misfire - oppsclunkthud had the correct diagnoses.

What I should have realized at the beginning was if there was a misfire, the nut would have tightened due to the thread direction, not loosened! The only explanation for the flywheel coming off was improper seating/tightening. The problem was I was just so sure I was right!

After cleaning up the shaft and tightening it as hard as I could, I was able to test ride it before the snow hit - all seems well.

Thanks for all of the helpful responses!

should be 55-65 ft lbs on the flywheel nut

make sure the threads aren't ruined on it

and use a NEW lock washer

cotter pin on the flywheel? was this meant for the bodge thread ;)

nope, sorry -- wrote that while I was half-awake via my phone.

you'd want to use a cotter pin on the rear hub.

I meant to say new lock washer.

Don't think ya wanna use a lock washer either.

A 'wave washer' is needed.

right. but a new one. they get flatted out after they've been torqued down in one spot.

maybe not 100% correct but this is what I used:

get the huge ones. the small ones you have to change all the time.

I still use the one that came with my bike

I use these Nordloc washers and they've worked great.

That could be part of the problem - I just reused the original washer.. Definitely going to buy some new ones...