Modern Vespa

OK, a few weeks ago I took my rebuilt 74 Vespa 90, kitted to 130cc out for a hard ride. Clipping along at 45mph the puppy made a strange sound and started slowing down fast. The engine died and couldn't be restarted. In fact, I couldn't turn it over at all. Waited for it to cool and tried to kick start it and it turned over hard so I figured I wouldn't screw it up worse so we trucked it home. Today it rained so no scooting and I decided to pull the cylinder and piston out and look. Came apart easily until I got to the pulling the cylinder and it was a bit stuck. Hit it with my hand and a few wiggles and it came off. Everything looks good No scoring in the cylinder and the piston and rings looked almost new. There was a bit of carbon on top of the piston with a little place that looked like some carbon may have come off.

Here's the question? Has anyone had this happen to them? Do you think a bit of carbon got down between the piston and cylinder causing it to lock up ? Any Idea's before I put it back together ?

Does the crank turn freely? Does the rear wheel turn freely? Something locked up on your bike, and you should find it before it happens again at high speed or in a turn.

Hi Mate, a wee bit of carbon is not going to hurt your piston

What condition is your plug in? White as snow? Coffee and cream, black wet sooty?

How many miles are on your kit? How did you set your up timing etc.

Whilst squeezing in the clutch to biting point how does this affect your compression. Was the clutch part of your rebuild.

Lots to ask you mate. Just like the previous post says don't ride it until you check all the usual, crank etc.

Cheers

did you smoke a bearing or twist the crank webs out?
check to be sure you didn't suck a wrist pin clip, too.

if you were on the gas, chances are you sucked a clutch side seal...

pull the clutch cover and check to be sure you didn't cough the clutch retainer clip or explode the clutch...

sounds like it's time for a rebuild...

best,
-greasy

Sounds like it was "just" rebuilt

Papa, Check all the things mentioned above. I can talk you through it if you want to call me.

It is quicker than punching the keyboard

Cheers.

Agree on the timing bit. Also, did you mic your kit for cylinder clearances before install? I am constantly suprised at how many "decent" kits have too tight a clearance right out of the box.

Good call Tony.

Come on papa, we are waiting

What does 'mic your kit' mean? I'm rebuilding a smallframe motor with the same kit and would prefer not to make a similar mistake.
thanks
-chris

It's short for micrometer, as in, 'did you measure it with some gauge that can measure sub mm distances?'. Most of the better kits come within tollerances.

If there was a loss in power before lock up it'd likely either be a seize or some bearing going south. Other things to look at would be the woodruff keys at the crank gear and the clutch. But in general, you should probably take it apart or at least take apart the top end, primary and clutch to see if it's all spinning properly. If you aren't well experienced with looking at seized cylinder, get a second opinion. There could be something you'd missed.

Best way to do it is to take it to a machine (either motorcycle or racing engine) shop. I know that Hot Rod Al also does this service if you are willing to ship. Ensure they they measure several different places - i.e. thruout the bore from top to bottom, and several different places around the bore. Have them do the same with the cylinder. (The following is my advice - I would advise that you also do your own research) I have found that most cylinders have a recommended fit range of a couple mm's. (say, .02 to .06) I have found that most kits out of the box are either right at the low point, or a bit under (.019 to .02ish) If it were me, and the preceeding were my cylinder, I would take it to the machine shop and get it properly machined towards the looser side - say .05 This has upsides and downsides, however. Upside is that it is ready to run hard pretty much out of the box - I don't really do a break in period other than a couple minutes of idling. Downside is that they will loose compression a bit sooner than a cylinder that has started out at .02ish. However, if you are riding it as hard as I do, the differences in long term usage is negliabile - i'll find some way or another to either blow it up or change out parts to play around with it.

Edit: The numbers above were just ones pulled from my head. Be sure to check your recommened specs before you do anything.

We just took 8 cylinder kits to the machine shop to be re-bored because they were too tight. We like them at .003" and they were at .0018".
Not cool.

What brand were they?

WHAT?

Mate, what make?

could also be the ring gap was too tight.

Also I've found that the cooling fins on the smallframe are not quite sufficient to cool it for sustained WOT. Running 1-2 jet sizes larger than ideal will keep it to a safe temperature. I change jets depending on the type of riding I expect to be doing.

Are those kits the Polini kits we've all been waiting for?

They were Indain 2-port 150cc, 3-port 150cc, and 200cc kits.

Thanks you have just won me $100.00. I bet a friend, YES! I win.

Take notes kids.

Cheers

Same thing happened to a buddy of mine who had just rebuilt a Stella with a big bore kit. He had to rebuild the top end again! Something about those Big Bore kits. I think he didn't properly break it in. He was out running it at 50 MPH when the new engine seized.
I think proper breakin is important.

Break in is usually overdone. An hour or so of riding should be sufficient. I do a few 20 min heating/cooling cycles but that's my voodoo. The condition of the engine that the cylinder is going on, cylinder preparation, installation and final set up are much more important. Not using poor quality parts (garbage in garbage out), checking the ring gaps and cleaning up the port edges with a small file would probably solve 4 of 5 new top end problems.

Ive had the piston and cylinder inspected and it looks like maybe I over rev'ed it a bit. No damage so Im just going to hone the cylinder a bit and reassemble. Will let you know how it works out

Looks like that's what I'm doing with my smallframe's old knackered top end.

I'm pretty sure we have oversize pistons if that will cure your polini 130 woes.