Vintage Vespa Forum

New member with a question

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daolski Olly S
London, London, UK   GBR
Hello everyone,

Spent the last couple of years building a small frame - first scooter for me.

the bike is almost finished. I'm just having trouble with the engine. Its a pk xl case with a DR130 kit, mildly ported, with an SHBC 20 20 Carb with Ram air filter and 48 idle and 90 main jet, and Sip Road Banana exhaust.

I have installed a VMC variotronic ignition, and have timed it to 24 BTDC at idle which should retard to 18 degrees at high RPM.

It ran very well when I first installed in the bike. Decided I would try a different expansion exhaust and couldn't get it to work. It then kept hesitating a low revs and just sounded like it was running rough. I also didnt like the straight cut primary gear whine as it was really loud, so took the opportunity to split the cases and change that out for helical cut and check everything out. Since its been back together, it is still hesitating at low revs, maybe 1/8 to 1/4 throttle, and then will rev out fine. This happens when warm. On cold start up the throttle response is quite crisp. Then once warm, it just seems to start bogging at idle and if you blip the throttle 1/4, it will struggle and return to idle and then die. There also seems to be quite a bit of smoke out of the exhaust after this, which will clear if you rev the bike high a few times. This happens on the stand and in the bike, though in the bike it is really boggy and just doesn't feel happy moving off.

With this problem I have done a lot of reading and problems solving: here's what I have done so far:

-Learned what the mixture screw does and how to adjust it.
- Tried 45, 48 and 50 idle jets. No difference
- Compression tested the head - all fine 125psi
- Leak tested the engine - again no leaks, holds pressure warm or cold
- Leak down test on sealing pad - all good 2T oil was still there 30 mins later
- changed spark plug.

I now think that this is ignition related and the perhaps somehow I have a weak spark at idle and part throttle. I did notice the inside of my flywheel one of the magnets sits a little proud of the others and has signs of rubbing. There are also signs of rubbing on the ignition coil.

Would it be worth changing the flywheel as a next step. I emailed Avotecnica in Italy who make the VMC ignition kit with the pictures of rubbing but am yet to have a response.

Anyhow I would be grateful if anyone has any bright ideas?


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