New air filter; now top speed is 15 mph

I got my 1978 Vespa Bravo off of ebay in good working condition break>

So I put the thing on, and now the moped starts and runs better (I had trouble before) but its maximum speed is now 15 mph where before it was 25 mph with a makeshift filter. What could be wrong? Could I have the wrong air filter? Could I have installed it wrong? To tell the truth, I can't figure out why the whole plastic thing is attached, it only seems to restrict the air to three small holes. Can I punch more holes into the plastic housing without negative consequences?

I should add, we took the carburetor off and cleaned it before I put in the filter, so that's not the problem.

Re: New air filter; now top speed is 15 mph

Ben Van Zoest /

In general filter intake holes are 50% over intake manifold square area. Maybe try adding a hole, how big are the existing holes?

Re: New air filter; now top speed is 15 mph

Ben Van Zoest /

By the way I have calculated from 3 different mfg's examples that 50cc engines need to have 50 sq mm air intakes.

Incidentally the air to fuel ratio is 50:1

Re: New air filter; now top speed is 15 mph

That sounds kind of small.

That would be like an 8mm bore.

Re: New air filter; now top speed is 15 mph

You can see the holes in the picture of the auction I posted. they are on the side, where the metal flat piece is. They look like panpipes. There are three of them.

Re: New air filter; now top speed is 15 mph

ಠ_ಠ Jon Folks ಠ_ಠ /

If you take off the filter and go faster, then just drill holes or something in your plastic filter until its running best. Do one at a time.


Re: New air filter; now top speed is 15 mph


I own a bravo myself and I fiddled with the air filter box myself recently, so I wanted to share my experiences.

What I think happened is the following: the person you bought your bravo from changed the jet size (put in a higher number jet) in the carburetor with a result that more air was needed than the air box would supply, so he probably took that off. (with the possibility of dust/mud/water getting sucked into your engine)

So now you have two options:

putting smaller jets in your carburetor until your air box supplies the right amount of air. (check your spark plug color) As a result, your moped will make more miles per gallon of fuel, perhaps at the cost of a few mph. (do you have a variator? I don't and my top speed is governed by the rear pulley more than by jet size/air supply)

increasing the amount of air that the air box can supply.

if you do this, I'd start with drilling more holes behind the metal gauze filter. There now seems to be only two holes there, my original stock filter had three holes there. These newly drilled holes will not increase the noise your moped makes. (i drilled those holes in a hexagonal pattern)

If there's no improvement in speed, you can drill an extra set of holes in the outer shell but this will increase the noise if you don't try to mimic the current setup. so if you drill a hole, you should glue a pipe onto it. (I made them from a few inches of a rubber fuel tube) Then the engine sounds stock and you have more air intake.

My air box looked a little like the attachment and it was very easy to drill three more holes and put on three pipes on the other side. (mirror image)

I can't see if your pipes run through the center or not so in your case it might be more difficult.

best regards.


Re: New air filter; now top speed is 15 mph

Thanks for your reply!

Can it be noisier than it was without the filter?

To be honest, I didn't detect much of a difference between the noise before I had a filter and the noise after I had a filter.

Why would someone put in a bigger jet? I doubt this is what happened since it's been sitting in someone's garage for years, (had 200 some miles on it) but you never know...

Where would I get these "pipes" you're referring to?


Re: New air filter; now top speed is 15 mph

Sorry I missed your comment about the rubber fuel tube.

Also, there are NO holes behind the metal mesh filter in the plastic casing. All of the air seems to come from the three small pipes on the side of the outer casing.

Re: New air filter; now top speed is 15 mph

sorry for this late reply.

Once the air gets through those three holes (normally they are on top when the filter is installed) it has to get to the metal gauze. To get there, it has to pass those holes that are in the inner shell, behind the metal gauze. The picture of the auction that you refer to shows two holes there...

good luck

Re: New air filter; now top speed is 15 mph

Before you go drilling holes in anything, you should clean the metal screen in your air filter. You should also take the spark plug out of your moped. Is it black? thats too much fuel and not enough air, is it white too much air not enough fuel. I doubt that the problem is jets that are too large, I wuld be willing to bet that the screen is dirty or clogged. Clean it with carb cleaner or fresh clean gas.

Re: New air filter; now top speed is 15 mph

It's not the plug, we checked that.

I didn't think the screen might be dirty. It didn't look dirty but that doesn't preclude some transparent film being on it that I didn't see.

I'll check that out, thanks.

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