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Re: Vespa Si Problems  Peter Staal
 Oct 12, 2001 17:41 PDT 
Hi Curtis,
If your airfilter came off from the 'backfire' it sound more like frontfire
to me ;)
Can it be that a vespa Si has e valve regulated inlet ? (carbon valve
between carb and cylinder?) if so this could also be the pain.
Replacing the sparkplug and cap can never harm you (beneith money wise;)) be
sure to take the old with you as spares..
more comments between the following lines
Subject: RE: Vespa Si Problems
 Here is the update on the Vespa Si. I got it back fixed from the shop as
mentioned earlier and now it's dead again (Thomas predicted this).
Backfire causing all loss of power. When this happened before, the shop
said the air filter had blown off from the backfire. So among making
sure that everything was free from clogging (which it was) they replaced
the air filter to it's proper place. So they fixed the damage, but not
the source of the problem. Now I pedaled straight to the shop as soon as
it blew again. The shop will replace the spark plug cap (can't hurt
right???) and check to make sure the compression release is not
sticking. They say there was no scoring or build up anywhere. Maybe
change the spark plug itself.
I use the moped to get to work. About a 30-35 minute drive depending on
which direction. Full throttle.
* I consider this as good use ;)
I weigh about 155
* So your moped doesn't have to work like a horse.
It get about 120 miles per tank and I use the correct ratio of 2% or
**and one tank contains? 4 liters / 1 gallon or so? gives (metric)
180km/4liter 1:45 is very lean gas use but in flat road no exeption.
 did you respect the 'brake-in' period since it was very new when you
bought it..
I kept the break-in period in mind, but did not really know what to do
during that time. I simply did not push the moped to it limits, and
still don't.
* could have used a richer oil mix but isn't harmfull if you didn't push it
too hard. and i expect you did so. no problem here.
According to the shop, everything looks to be in good condition.
*That is what i expected too, and is also the worrying part of it...
since it is not running now(?) it is hard to find out where the banging
sound came from, the exhaust or the inlet.. if the air filter blew out it
had to be the inlet, then the question rises what source could give a spark
if it all was generated at the exhaust, it was much simpler. had to be hot
combustion gas leaking in the exhaust or unburned mix coming out with a
detonation front following it (ignition of mechanicaly triggerd) (making a
spectacular flame coming out in the dark some times)
the only way a gas burst could enter the cranckcase / carburator (apart form
heavy construcion failures) is by passing the transfer ports (when exhaust
/port is too narrow or clogged up) and the gas is 'looking' for a way out,
although the inlet port must be shut in this position.... or by passing the
piston rings at top position. when running at high revs the working pressure
is at peak level and is the most likely time to give this effect, so far the
theory..in both case giving very bad driving performance, and i mean very
bad. this all counting with a conservative piston design.
If you have a 'new' design piston (with holes aside, and a inlet valve) this
could cause blowbacks when the reedvalve isn't working fine too.(non
detonating blows )
If a high pressure front made your air filter came off it is very likely
that this also blew back the gas in the direction of the float chamber
(possibly damaging things there too)
Could you describe the type of banging noise? like a firecracker (like using
decompression valve when driving) or more a loud puffing sound ?
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