Modern Vespa

So a PK75, motovespa, SHBC L 20mm carb has had engine cases split for a good old refurb of seals, gaskets, bearings. All put back together, carb refurbed and reassembled and the fuel tap on tank was shot, leaking. New fuel tap put on as it was leaking and stiff, but a fast flow tap. Fuel line put back on, scooter started 2nd kick, then dialled in the carb, all good. Kicked it over again, all good, then again and again.

The fuel tap has a reserve on it. As soon as i put the tap on to reserve scooter stops running then refuses to kick over at all. I mean WTF is this all about. So leave it for a week, fuel turned off as ran out of time. Kicked over, it started, blah blah. Turns the fuel tap to off or to reserve or normal position, same thing, no starting. So left another week as felt like i was going to have an angry moment.

Decided to reinvestigate the carb. Couple of things, washer missing on the fuel intake cover and the choke valve brass locking nut at top of carb damaged. OK, ive got a brand new genuine delly SHBC 20L that i had spare to hand, jetted it 2 pts up for exhaust, put it on, dialled in, all good, tickover good, no smoking. Soon as i put the fuel tap to reserve, the bastard thing did exactly the same, conked out and refuses to start. I cant try push starting it as ive got a damaged back.

Now what the frig is going on here? Is there an issue with a fast flow tap on a small frame? Is there too much fuel pressure, gravity say for the carb? Is this causing issue? Is my fuel line too short, its 12 inches long and new piaggio fuel line. Its absolutely doing my head in, any clues people? And yes, ive removed fuel cap to see if it would run and nope, it doesn't.

Any ideas as to what is going on folks? Thank you.

Which fast flow fuel tap is it?

If I remember correctly… SIP is just on/off.

I'd take off the tap and triple check the rubber seal and holes to make sure there's nothing blocking it.

Tap lever installed upside down?

Edit: scratch that, there is reserve, but I've never used it… lol

Have you taken the tank out to see if you get any flow with the tap in the reserve position. If you have, did you change the fuel hose, if so, is it the same length as the old one.

There is flow with the tap, as soon as you put the tap on you can see the fuel running down line to carb and quickly. I have the tank fuelled to full too to rule out gravity issues. I have tried raising the tank up a few inches to see if anything changes, no. The tank is not bolted in as this is an ongoing issue i cannot seem to sort. I thought to myself that the brand new out of box carb would possibly give me some clues. It started first kick, thought oh joy ive sorted then exactly the same thing when tap lever changed to reserve. Fuel tap is correct position, first thing i did was check the tank whilst out and turning fuel on, all good.

This is not an electrical issue. Its had new stator, flywheel, CDI, plug, seals, gaskets, bearings, genuine new delorto 20 carb. Everything sparks strongly, its some issue with the fuel delivery. Why would turning tap to reserve instantly kill the running, as if you have pulled choke leaver out when scoots hot. It will not start despite turning fuel tap off etc, its as if it gets some sort of air block. Choke valve is not stuck open either. Might increasing the fuel line length be of use if the carb has issues with a fast flow tap??? Even if you turn your fuel off, on a small frame you would expect scoot to run for a minute on the residual fuel left in the fuel line. This is confusing the crap out of me that turning to reserve kills the running, then it wont bloody start for love or money. Fuel line is 6mm bore which is stock for the carb fuel inlet and tap, like most vespas and lammys.

If it stops instantly, that sort of points more to electrical. I was thinking it was the general 'stops gradually like it does when it runs out of petrol'

I know you say its not electrical but have you checked for a spark when its starting. I'm thinking along the lines of somehow the tap in the reserve position is making an earth connection similar to the green wire for ignition. Is it the same with the fuel gauge sender unplugged.

You could try a make shift fuel reservoir from an oil bottle. You can see if the fuel getting sucked into the carb properly.

Then you'll know if it's the tap or not.

12" fuel line will have no impact on your running problem. Just as long as the carb is below the tank.

Yes, good spark to engine casing, lights etc come on when kicking over. Fuel tap lever is rubber grommeted through frame, its not coming into contact with any metal. Its a freaking odd one for sure.

Having edited this, just had a thought! What if, the fuel sender unit is causing a short???? I dont think ive tried running the scoot with the fuel sender detatched! I can disconnect the sender unit and still leave it in tank. It shouldnt cause an issue detaching it should it? Still really doesnt explain why a fuel tap turn would cause issue with the sender unit though does it. Its worth a go though.

The fuel line is a nice direct straight line to carb, yes, carb well below tank. 12 inches is pretty much the shortest length you can fit on a smallie that gives you enough play to tighten fuel line to fuel tap with jubilee clip. Thx for the tip.

Disconnecting it just puts the fuel gauge on zero and brings the low fuel light on.

As you say though, you have a spark.