2003 PX125 Disc chronic misfire at speed

[Pompeydave]

Hi all,

I've got a 2003 PX125 Disc thats all standard apart from a Gianelli exhaust. At speed in third and mainly fourth gear I'm getting a chronic intermittent misfire that slows me down from an indicated 60 mph to around 40. If I back off the revs and ride at 45-50 it seems ok, but if I gun it again in third or top it misfires again immedately. It never stops, just slows down until I back the throttle off and try again. It doesn't overheat, but is running a rich as the old plug was well sooty. I haven't weakened the mixture yet, but can't believe this would cause such a bad misfire?

I've so far changed the jets in the carb back to standard size, changed the CDI for a new one, changed the HT lead, cap and plug for new items, and changed the Low Tension coil on the stator, all to no avail.

An old lammie trick to isolate the switch from the equation by disconnecting the green wire from the switch to the cdi, would this idea work on a PX Vespa? Any other ideas please? Can't believe the fuel tap is blocked as it's only done 3500 miles from new and is in pristine condition overall. Any known faults out there that affected this model around 2003 manufacture? Thanks in advance.

[rosscla]

Yes, removing the green from the switch to the CDi has the same effect, it's essentially the same ignition system.

I can't think of a reason why the switch would be an issue at a particular speed or rev range though.

It sounds to me like you're describing it bogging down at the top end.

[Pompeydave]

No, it's not just bogging down, it's a complete ignition failure or fuel starvation. 9 times out of 10 I've found these type of fault to be ignition related, but unless I've got a dodgy pickup I'm starting to think it might be a lack of fuel for some reason?

[bazman]

Sounds like your pick up box to me.

[paul d]

I'd try all of the simple stuff first. Run with the fuel cap off just to make sure the breather isn't blocked. Make sure the fuel line isn't too long from the tank to the carb. Check that the fuel line isn't kinked. Run without the air filter, see if it makes a difference. Check the fuel filter on the carb. Do you have a standard exhaust or could you get a loan of one? Check that the choke is fully off. Drain the fuel and give it a fresh refill. If none of the above works I'd try and borrow a stator that works. At least you'd know it was a stator problem if it ran right with the change. Best of luck with it.

[Pompeydave]

Ok thanks, I will give all the above a go and report back. I have it running on a Giannelli exhaust currently, but have a different aftermarket pipe I can try. I was told the other night that occasionally the flywheel can be faulty, even though it looks almost new in many cases, so that may be something else to think about.

[Pompeydave]

Update time. I've run it without the green wire connected- bingo! Or so I thought . It was initially free of all misfiring and ran sweet as a nut. So I tested the green wire up to the switch and had a good connection even when wiggling it around in different ways. Ok, must be the switch I thought. Bought a new switch from Allstyles, fitted it, went out again- misfire has retured, and is there whether the green wire is connected or not. Always at high rpm's in top gear. So, next I cleaned out the fuel system, stripped the carb, found it to be clean as a whistle, all jets are good too. No crap in the tank and a very good flow from the tap. I'm on my fourth or fifth tank of fuel since this started so it's not bad fuel either.

So, to recap, the CDI, HT lead, HT cap, plug, Ignition switch and LT coil have all been replaced to no avail. The regulator connections are all good as is the green wire to the ignition switch. Which by my reckoning leaves the stator pickup, the stator itself and the flywheel. There's nothing left to change after those. Pickup next then. Will report back with findings...

[Pompeydave]

Update time- this problem has been on the back burner for six plus months due to me playing with a Lammie engine rebuild. Anyway, I'm back on the case now, and I've changed the pickup. Made no difference, but I have now noticed that this problem seems to ocurr more often when the tank is low on fuel. I'm wondering if its the fuel tap? Bought a fast flow SIP one from Allstyles + the tool to get in there and undo it. Will change it over next week and report back...

[acko]

Sounds like stator plate to me.
Can you borrow one off a mate or get yours tested?

[B-Race Tuning]

I've only just picked up this thread so sorry for delay in a response, and you seem to have covered most things to date. Have you checked all three wires going into the CDI from the stator? Just take them off and hold the end and about 1.5" along the wire and gently pull apart. You're looking for stretch, indicating the insulation is good but the wire core has broken. A very common cause of your symptoms.(Obviously wouldn't be fuel related?). Simon.