Searching for elusive air leaks

Checking for air leak

Stacey has a problem. On occasion she will over rev. I mean really over rev, like a vampire banshee! (My children are into vampire dramas......so I know these things........sadly). Anyway, it happens when we come to road junctions and slow to a stop. The kill switch seems ineffective and the only thing that brings her revs down is changing gear, letting the clutch out and stalling her. She is then right as rain when you start her up again. Anyway, my tentative enquiries suggest Stacey has an air leak somewhere. It would seem that there can be several places where this may occur. In no particular order of importance, causes could be

- spark plug too tight

- fuel pipe too long

- carb felt washer on manifold pipe not sufficiently greased up

- loose carb where it attaches to inlet manifold

- idling screw not properly adjusted

- flywheel oil seal blown

- main crank, clutch side, oil seal blown

- leak between engine casings

- or leak where cylinder barrel joins main engine block

I have discovered with a torch dangling down in the car area that there seems to be quite a bit of fuel in the plastic tray at the bottom of the carb which suggests a carb air leak problem.     Whatever, we have some detective work to do and so I have come up with a plan. Now I am a non-engineer and so I need to get this plan checked by the boys wot know a thing or two about vespas......namely the proboards vespa smallframes forum guys and the gents over at the vespa club of Great Britain. I'm pretty sure, based on previous advice from them, that there isn't anything they don't know about vespas.  So here we go. Here is my, in the words of "balderick", my "cunning plan!"  For non- UK readers, Balderick was a much loved character in the "black adder' comedy series).

1. Check  the carb is bolted down properly and that the gasket is intact underneath it. Make sure they have spring washers on and are not tightened down too much. I'll clean all the carb area as well at the same time. 

2.  make sure that the joint between the carb and intake is sealing up good;that the felt gasket is in the sleeve right and that it is well greased up. Packing that felt O ring with grease and making sure  it goes on the intake side between the inner and outer flange and gets pushed in by the intake side of the carb is critical and to be honest we remember putting it in but NOT greasing it up. Oops!

3. confirm that the carburettor and its box are bolted on correctly by grabbing the carburettor box and making sure it doesn't wiggle. 

4. Check that intake and exhaust manifolds are tight and gaskets used are secure and functioning

5. We could take out the idle jet, clean it and blow out that idle circuit with carb spray and compressed air 

6. then it may be a blown oil seal; so we will have to drain a little of the gear oil and give it a sniff - if we smell petrol then the main seal's gone and will need replacing... ....or maybe we could check the........

7. cylinder head; that there isn't a big oily patch under the cylinder head, and that the head nuts are all tightened properly. We'll check the seal at the cylinder head to the barrel and the barrel to the engine looking for leaks underneath the engine as well where the engine case halves join.

8. If our worst fears are confirmed and it is an oil seal leak then we will have to dismantle and rebuild the engine. Someone told me that a really simple check for crank seals is to remove the clutch cover breather and put finger over the hole. 9 times out of 10 this will bring the revs down straight away - on really badly blown seals you will actually hear the air escaping from the hole. I can't say I've heard any hissing air but there again I haven't been consciously listening out for it. It could also be the flywheel side seal and so we will need to check for oil runs behind the flywheel.  When we take the flywheel off, we are looking for a oil mark from the seal to the lip on the engine where the botton of the flywheel sits. There may also be a small pool of oil on the lip. If there's no sign of oil there then it's probably not the flyside seal although you never know. The seal is cheap and easy to change so it's worth a try.

(Someone also told me  If we need to use the scooter before we can do the work, put an extra splash of oil in the petrol and only go up to 3rd gear. Oil to keep the piston from seizing and if we don't go above 3rd gear, when it happens again changing gear stops it revving to banshee level again. But get it seen too ASAP. Not good for the piston or piston rings, worth checking them out at the same time).

9. We could also check all nut torque specs / looseness on the cylinder head, the carb manifold / box, the exhaust too

10. spark plug tight - better check it too

11.  Fuel line too long ....lift out tank and the fuel tap should be same plane as frame top