Is this exhaust pinched?

Recently got a Vespa Bravo with the Polini exhaust attached. Something's inhibiting the ped from making use of the exhaust, as when we remove the pipe it goes about 5 mph faster. I'm new to peds, but commonsense tells me this looks like a pinched pipe (I'll post a second view shortly). Can anybody give me their opinions?

My related question is this... the previous owner bumped up the carb and gave it this new exhaust, but it doesn't appear he made any extra holes in the air filter for increased intake. Is this something I should do to make the bike "breathe" a little more efficiently so it gets some extra rpms? I have been making the ped eat its wheaties, but the tailpipe just isn't doing it's work and I need to know why.

thanks for anything you can tell me.


Re: Is this exhaust pinched?

Here's view 2 of the exhaust.


Re: Is this exhaust pinched?

yeah.. that's exactly the type of damage that would render the pipe useless..

Sound waves and gas pulses must be free to move back and forth in an expansion chamber, while maintaining their strength.. This requires smooth interior shapes that gradually blend into one another ..

take it to a muffler shop.. Midas, for instance.. Call first .. Some muffler/exhaust repair chains do all their bending jobs in one particulal shop that is set up to bend tubing.

Get an estimate for them to bend you a new header pipe (the part with the crease) ..

find someone who welds.. Cut and preserve the first couple inches of the header-to-cylinder joint from the old pipe and weld it back onto the new (after all the parts are bolted onto the bike and are tack-welded in the exact correct position).


Do proper plug chops on the tuned bike before drilling holes or messing with anything.

Re: Is this exhaust pinched?

Troy Weisenmiller /

actually, thats the way it is made. i got my polini pipe and it was brand new and it suppsoed to be liek that. i think it helps is fit in between the kick stand and to install it. but mine was bent the same way. looks fine to me

Re: Is this exhaust pinched?

come on.. there's no way in hell the pipe was crushed with a sharp crease as is plainly PICTURED here..

if it needed to be crushed like that in order to clear something, Polini doesn't know wtf they are doing, which i doubt.

Could something else be causing additional exhaust restriction? Sure ..

Is this pipe in good shape? no.

Re: Is this exhaust pinched?

double up on the wheaties! :P

Re: Is this exhaust pinched?

I really don'y know what plug chops are--as I said I'm new to peds, so my lingo is restricted to what little I know and commonsense mechanics. Could you explain it to me?

Re: Is this exhaust pinched?

One objective of proper tuning is to insure the air:fuel mixture ratio that the carburetor delivers is correct.. another is to select the proper heat-range spark plug .. another is to set ignition timing for optimum performance.

All of these things leave tell-tale indications on the spark plug. Accurate spark plug "reading" can let you know what's happening inside the engine. You basically get a brand new plug, run it, remove it and examine it. The plug condition is like a photo snap shot of engine conditions at some speed.


You mentioned drilling holes in the air box which would allow more air into the carburetor. More air mixed with some set amount of fuel results in a "leaner" mixture. (also, more fuel mixed with some set amount of air would be a "richer" mix).

If a plug chop shows that the mixture is too rich, it might be proper to increase the amount of intake air (or to somehow provide more fuel.. same difference) thus leaning the mixture to the proper ratio.

Since engine damage can result from running too "lean" at top speed / top RPM, plug chops are most often used to determine proper fuel mix ratio at top speed, wide open throttle.


Plug Chop:

Get a brand new plug. Find a long flat road about half a mile or more long.

Thoroughly warm up the engine.

Run wide open throttle , top speed for about half a mile.

Kill (chop) the engine with the kill switch or ignition switch and coast to a stop. Do not let the engine idle.

Remove the new plug, install the old plug and ride home.

Examine the new test plug under good lighting and magnification .. look for signs of overheating, lean or rich air:fuel mixture, etc ..


If the new plug comes out looking just like it did when it came out of the box, the engine is in good tune and the proper heat range plug was selected.

Black soot on the new plug means the mixture is rich.

Signs of overheating (melted electrode edges or grey, pitted porcelain insulator etc.) could mean the mix is lean.. Or it could mean other things, like too-high compression ratio, too-advanced ignition timing, and other heat related engine problems.

slight correction..

_If a plug chop shows that the mixture is too rich, it might be proper to increase the amount of intake air (or to somehow provide LESS fuel.. same difference) thus leaning the mixture to the proper ratio_

Re: slight correction..

The pipe may have been made that way- if it was from a fall, there would be scrapes and dents on the back portion of the pipe too.

It should line up with the pedals and crank arms, or the kick stand if it is intensional. The stock pipe on a grande does the same thing.

It does look like it's seen a lot of use, and heat. Yo may want to clean out the pipe by burning it, or using sea foam (a cleaner available at auto parts stores)

Once you learn to master plug chops, your moped will run excellent!

Re: slight correction..

Thanks very much for all this information on tuning--it must have taken you a long time to write this out and I really appreciate it. I have several new spark plugs so I'll give plug chopping a try. I'm printing out your suggestions to take along.

We were planning to look at the timing but hadn't gotten the chance to yet, so we'll start fresh tomorrow, trying to get that ratio correct. I'll keep you posted. And, it's almost inevitable I'll have questions, too.

Re: slight correction..

since it's a modified bike and you're getting into the finer points of tuning, try the Performance Tuning forum.. might get more relevant info there.

Re: slight correction..

Troy Weisenmiller /

dude, im seriously telling you thats the way it comes. if you look at both pics, yes it gets narrower, but it doesnt get that narrow to pinch, but it also get wiser if you look at it thus still allowing as much air to pass as a round pipe. but mine was new and i thought it was bent also, but after examing it, it was jsut the way its made.

although she might have pinched it more, the pipe does come narrow in that region of the pipe.

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