I do a lot of work on scooters in the garage. Mainly I try to squeeze an hour or two of garage time in on weeknights if I'm not too tired. Sometimes, if I'm really lucky, I can get a solid block of several hours during a weekend day... but that is a rarity. I'm going to try to document the work I do in the "Garage Time" segment. So without further ado, here's the first entry, motor work on a '67 Sears Primavera VMA1.
This scooter came in with the classic symptoms of an air leak. Those are high reving motor, which settles down when you pull out the choke, and a chirping sound when it is running. The chirping sound is often a symtom of something not tightened down properly and air getting sucked into the combustion chamber. An air leak on a two stroke is not only a bad thing, it is downright deadly. If left unrepaired, an air leak creates a lean air/fuel mixture condition, extreme heat in the combustion chamber, which then results in a melted piston...
In this case, I was very meticulous and ruled out the easy things first, and then moved on. Since this is someone else's scooter, I always try to work on the easy and cheap possibilities, thus hopefully have a cheap fix. That isn't always the case, as this scooter proved.
So first, I double checked the fuel system. Air leak symptoms are very similar to those of fuel starvation, which also causes a lean condition. So since the carb and fuel tap are easy to get to on a Primavera, I tackled them first to mark them off the list of possible problems. I cleaned and rebuilt the carb with new gaskets and confirmed that the fuel tap was flowing well. Once that was done, the problem didn't go away.
Next, I drained the oil, and checked it. If the clutch-side seal goes out, it allows fuel from the cranshaft area to leak into the crankcase. Draining the oil is an easy way to check for the tell-tail of gas in the oil. It is a simple diagnostic, which in this case, let nowhere. The oil was fine. I checked off a bad clutch-side seal.
The next thing to check is the flywheel side seal. This requires removing the fan cover and then the flywheel. Removing the flywheel on these older Vespas can be a problem, because they are held on by a tight compression fit, and come off with pressure from the flywheel nut on a small circlip in the flywheel. Often, if it has been a long time since they were removed, the circlip simply bends, and the flywheel stays firmly on the crankshaft end. This scooter was one of the many I have worked on where it was a bear to remove the flywheel. I got a new circlip, and blasted the flywheel area near the crankshaft with high heat. After some heating, and a judicious whack on the nut with my impact wrench, the flywheel finally came free.
Inpecting the area around the seal with the flywheel off revealed some weeping of fluid from the seal. Could this be the source of the chirping, and ultimately the air leak? The only way to find out was to replace the seal. Luckily, the Primavera flywheel seal can be replaced from the outside, without the necessity to take the motor apart. The largeframe Vespas (except for the P's) require a full tear down to get to the seal! With the seal replaced and the flywheel back on, I fired the scooter up.... the problem was not solved!
Uggh... so the next possibility is a loose cylinder head. On a largeframe, this would have been among the first things I checked. However, on a smallframe, getting to the cylinder studs is a bit of a chore, so I left it to now. I pulled the carb and rear wheel, and dropped the motor down for access to the top end. The cylinder stud nuts didn't seem to be too loose, but I tightened them anyway.
While I was inspecting everything, I noticed that the intake manifold appeared to be loose. BINGO! That would certainly cause an air leak