Modern Vespa

I had a local mechanic clean out the carb and adjust the valves on my Fly 150 and now it won't get above 35 ish mph on flat ground, he's looked at it a total of 3 times but says hes done all he can and doesn't know what's wrong with it, verified that the valves were set to spec and carb was clean and assembled properly on the inside according to him, everything seems to be assembled correctly on the outside. The scooter starts and idles fine but the rpms don't rev up like they used to, lacking low end torque and top end speed.

My father, who used to be an auto tech and has worked on his Harley, and I are going to look at it later today to attempt to trouble shoot/repair, any ideas where to start?

Check the rubber boot or membrane on the "back" of the carb, the side opposite the bowl/jets.

If that is cracked, or crimped, or assembled crookedly underneath the plate, the engine will behave exactly as you've described. And it is easy to forget about or overlook!

I'll bet #5 on the diagram is not seated. It can look fine, but still not be correct.

It looks like the accelerator pump may have gone bad on the carburetor, there was a small leak from that area. you can't buy just the accelerator pump as far as I know but I did find a float bowl assembly with gasket and the accelerator pump from scooterwest.com for $67 so I went ahead and ordered that and see if that fixes it, I also adjusted the valves, which was a good learning experience and not as hard as I thought, although it did make it run better it still had the same issue of hesitation, my father seems pretty certain the accelerator pump is the issue the symptoms match up, we'll find out in about 7-10 days.

I'd pull the top off the carb and reseat part #5 while I wait.

And I'd also inspect it carefully (hold it up against a bright light) for tears or cracks.

Replacing the bowl was no help still topping out at 30 mph took diaphragm out no obvious cracks or holes, does this spring look normal is it always this long? Reseated diaphragm with no improvement. A new one would be $50 with shipping should I order a new one? Any ideas? Could it be the petcock or something else? I hate to have to take it to the dealer they charge an arm and a leg ($359+$150/hr) and are far away

Should I just order a new carb? What should my next step be? We don't see any obvious vacuum leaks were even testing it with a gauge and stethoscope.

I took a video of what it sounds like, it's bogged down when u give it gas and backfiring that was there before replacing the bowl and diaphragm and wasn't remedied by either, it was not bogged down before the mechanic worked on it, I took video

https://youtube.com/shorts/zocxm0kZLcQ?feature=share

Why would this vacuum gauge be jumping around like this does that tell you anything

https://youtube.com/shorts/9rqsKyWymxM?feature=share

Video jumps around too much for me to see the engine bay. What's up with the pink hose connected to the inlet manifold?

Rubber diaphragm looks decent. Did you happen to mess with the needle? Move the clip position any?

What is this and what does it do

The pink hose is a vacuum gauge we're using for testing

We had to, to inspect the diaphragm and we put it back together and it still has the bog off idle.

Mechanic said he adjusted the valves so we looked at the valve adjustment and we set it at 2v .004 and .006, to adjust the valves is that where you were supposed to be to be cuz it had 4V also?

We checked technicians work just to see if they were adjusted right they were close but we reset them anyway.

Could there be some issue with the exhaust? Would it be worth removing the exhaust and inspecting it and running it with it removed seeing if that has any effect?

Looks like the carb warmer. Certain models got a warmer for cold climates.

Did you change the C clip position on the needle?

I doubt your exhaust is causing problems. They don't magically start causing speed loss.

As far as I know we put the clip back properly, I don't have the bike with me it's at my dad's house and he's not home.

So we don't think it's the exhaust, but the $64,000 question remains, what IS causing the power loss? Any takers?

Considering ordering a new carb but not even sure that will fix it? Running short on ideas here, need to get this thing running again. It's starting to become a time/money pit.

There's a few different spots for the clip. If you're bored, pull the needle and raise the clip (which lowers the needle). That should lean out the bogging. If that doesn't work, put it back to where it was.

Another stab in the dark would be the evap system. YouTube has some tutorials on how to disable it/remove it. It won't hurt anything but could eliminate some variables such as cracked vacuum hoses/charcoal canister.

I can almost guarantee that it's your carb diaphragm.

pull the carb, take it all down and clean and check all the components. then reassemble. be sure, absolutely sure that everything is squeaky clean. then be 1000% sure that the diaphragm is sealed properly-- you can do this by blowing compressed air into the intake side of the carb and watch the slide raise. check that the spring isn't binding. lift the slide with your fingers, it should go all the way up smoothly with slight resistance.

while you've got that off, check in intake manifold. actually pull it and look at it. it may be cracked underneath or under the clamp.

while you're there, check the air filter. make sure the box is sealed properly and the filter is installed properly and not totally wasted.

hth

Did he take out the jets?? There's a chance the needle guide above the main jet fell out. Without it the engine runs like crap…

Use waterproof plumber's grease around the lip the diaphragm rests in. Put a light coating on your thumb, and transfer it around the lip. Gently, but firmly work the diaphragm into the groove. Place the cap on without the spring, and apply firm even pressure downward. Then gently lift the cap and reinstall with the spring. Hold the cap firmly in place while you bring it home with the screws.

Well the diaphragm definitely was A culprit, if not THE culprit, put some Vaseline on the lip and reseated it and most if not all of the power came back, got it up to 50 mph on the short stretch of straight flat road I had to work with.

The engine has picked up a bit of a back fire if I roll on throttle hard then immediately off, so some further fine tuning seems to be needed, tried adjusting the fuel/air mix on the carb a little bit with the limited time I had, but didn't seem to make a difference with the backfiring. Bike is still parked at the rents, plan to retrieve it on Wed and do any more fine tuning if possible. Any suggestions on dealing with a back fire when rolling off the throttle when bike is at rest?

Thanks to all here for the help with this!

What's the idle jet?

(You'll want a 38)

Remove the evap line to the carb and run the air fuel mix screw out until it's flush with the body of the carb. Roughly 2.5~3 from base.

The idle will pick up, so knock that down with a half turn or so on the speed.