Modern Vespa

I had her purring like a kitten yesterday and a little while ago I found a small puddle of fuel under the motor and now the bike will not start. I pulled the carb apart and she is spotless. The jets are all clear and there's gas pumping out of the petcock when I try to start her up. I don't know what else to do. Can anyone point me in the right direction? She was running great yesterday and she even started up for me early this morning. Now all she does is crank. Please help! Thanks!

Leaking fuel suggests a fuel petcock that is stuck open (especially if there's a lot of fuel), and a needle valve in the carb that is not sealing. That could be due to a spot of gunk caught in the needle, a failure of the rubbery coating on the needle tip, or a bit of extra friction in the float assembly that isn't allowing the valve to move smoothly.

My guess is the fuel system was not perfectly cleaned and you have some gunk stuck in there. When you take the carb off, put some vacuum on the petcock and persuade some fuel to fill a small glass jar from the hose that goes into the carb, and check it for absolute cleanliness. If the fuel spills from the hose without vacuum, you need a new petcock.

I already checked the petcock and fuel does not flow unless I apply vacuum to it and then it flows out fine. There's definitely fuel in the bowl, so I know it's getting to the carb. The only thing I didn't check was the needle valve. I'll pull it apart again and have a look. There wasn't a huge amount of gas on the ground.

Assuming the fuel is coming from the carb…yes, you knew fuel was getting into the carb! If it's just a little fuel, then my theory is that the needle valve isn't seating, the fuel between the petcock and the carb is seeping around the needle valve, overfilling the bowl, flooding the carb, dripping out the overflow, and that's why you can't start her up.

You might be able to solve this just by draining the carb through the drain bolt. It'll take some cranking to refill the bowl, but maybe the subsequent jostling of the needle valve as that happens will put things right again.

Took it all apart and everything (needle, float and seat) looked good and was functioning properly. Blew everything out with compressed air for good measure. Reassembled and installed and still just cranking without starting. I'm stumped. I'm about ready to buy a new carb, but I'd rather not if I can avoid it.

To run you need a spark at the right time and fuel. You have fuel, try checking the spark plug.

I pulled the plug. It's sparking like a champ. It's definitely not spark, it's got to be the carb.

So what's left is air and compression. I suspect there's air available - so check compression, and if no tester then at least check the valve clearances.

You're sure you've cranked enough to allow the bowl to refill, and give those clean jets a chance to do their jobs?

The battery has enough power in it to both crank the engine and provide the spark?

Next step: starting fluid sprayed into the air intake to confirm it's really the carb and not spark or timing. (I think you're right, it's the carb.)

I've cranked it more than enough times to fill the bowl. There's definitely gas in there. I've got the battery on a charger now because I cranked it so much. The battery had plenty of power before I wore it down. It's definitely not spark. It was sparking perfectly when i pulled the plug and grounded it to the head. I doubt it's timing. The bike only has 148 miles on it. I think I might just bite the bullet and order a new carb. I've done everything I can think of and she still won't start. Thanks for all your help! Much appreciated.

There's an electrical connection towards the bottom of my carb and it's not the bystarter. Does anyone know what that's for?

That's the carb heater…

What does it do? I dont think the replacement carb has it.

Broken Carb Heater

None of my carbs have this fancypants feature. Schwanky!

Thank you!

Thanks!

Thanks!

I went ahead and bought a new carb from scooterpartsco. Hopefully, this will take care of the problem. I'll post an update after I get the new carb installed. Thanks!

Do not hold the carb heater while it is getting power. It gets very hot very fast.

Gotcha! Thanks!

Can I bypass the carb and spray some starting fluid right down the intake boot to try to start this bike? I don't see why not, but i just want to be sure before i do it. Thanks!

Nevermind. I sprayed some carby cleaner down the intake and she fired right up. The carb has got to be bad.

I opened up the airbox and a lot of gas drained out. Why would there be any gas in there? Is it because the motor was flooded from cranking it so much? Thanks!

Yes. That happens when the needle valve isn't sealing, the bowl fills up with fuel, and fuel keeps on coming in until it flows downhill through the carb intake and into the air filter. That is why the condition is called a "flooded" engine.

Your starter fluid experiment confirms that fuel delivery is the problem, BUT, if so much fuel has gotten into the airbox it is also possible that the filter is saturated and isn't letting in air. Once you clean that up you might want to try to start her again.

That points to thee needle valve in the carb sticking open - so the bowl overfills and the gas just keeps coming while you're cranking and producing vacuum.

I once had the opposite problem of the needle valve stuck closed. Opened it all up and cleaned the layer of gum, all good after that.

Will do! Thank you! I really appreciate your responses. You've been very helpful and patient with me. I have a feeling she's gonna fire up once I get her back together. The air box had a lot of fuel in it and the foam element was pretty damp with gas. I ordered a new air filter element. I should have it by Saturday. I'll report back once I get her put back together, cleaned up and then I'll try firing her up. Thanks again!

I'll take another look at the tip of the needle valve and the seat. I didn't see anything amiss with either of them this morning. When I pushed the float in, I couldn't blow any air into the carb. The needle was definitely sealing. I'll take another look at both of them after lunch. Thanks for the help, Jim!

It's always a good idea to have an extra filter on hand, but unless the current filter is showing signs of decomposition, there's no reason not to put it back just because it came into contact with gasoline. In fact gasoline is what one MIGHT use to clean a dirty filter (although I use water and dish soap). Clean it out, add a few milliliters of oil to it and squeeze it throughout the sponge, put it back, and see if she runs!

I put her all back together and she's running like a Vespa with 148 original miles on her ticker! I went through the carb with a fine tooth comb and put her back together. She couldn't run any better than she is right now. Thank you so much Juan (and Jim). I couldn't have done it without you guys. You both rock! I'm going out for another spin. Thanks again!