Modern Vespa

I just found out my pm pipe shoots out blue flames when it backfires...oh how I love this pipe!

Anyone know the best way to make it happen on command?

And how bad is it to rev the engine so the clutch engages and keep the brakes on?

bad. that's a good way to ruin transmission parts, belt clutch and pulleys.

best,
-greasy

I find that riding past the local police station is the most reliable way to make the scooter backfire.

"Hello boys!"

I remember that from billboards a few years back...

we are just coming out of winter here. The girls are still covered up.

excellent, that leaves something to the imagination...


best,
-greasy

Be careful - Little Miss Lollipop might come out again...

i don't think that you'll get any complaints. it seems that those who missed the first round have been chomping at the bit ever since!

keep it PG13 and, dare i say it, titillating...

oh, and include a picture of your pipe. lest we go off topic.

best,
-greasy

That would involve writhing around on a shagpile rug with christmas lights in my driveway. Hmmmm.

at first thought i took this to be quite suggestive...

then i realised that in aussie speak, this meant rolling about on a bath-mat with a flash light.

*laughs*

best,
-greasy

Dare I say ROFL?

ROF in Christmas lights...

PM porn merchant

Backfire (technically an afterfire) indicates a lean condition in your fuel circuit.

Running lean for an extended period of time will raise the operating temperature of the engine (which is air cooled to begin with) leading to premature component failure.

Additionally, you will need to accelerate your oil-change interval to accomodate the higher operating temps which will lead to faster oil break-down.

Unless you upjet the carb and retune the bike, all you've done is make it louder.

At least on my GT, the PM back/afterfire condition cannot be "tuned out" and is due to the freer-breathing exhaust. Mine pops when letting off the gas after high/higher revs, such as decelerating or engine braking (if there is such a thing with t'n'g scooters). My engine is tuned to PM's specs - factory Piaggio specs - and not running lean or rich, and I still get pops from the PM pipe.

To get the pop: rev up and let go of the accelerator!

The backfire/popping is a result of the air/fuel mixture becoming very lean when the throttle is closed and the engine is rotating well above idle speed. This is often caused by an open exhaust or air-box mod.

When the throttle is closed (in the idle position), fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system. The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, combined with the higher than normal air-flow rate (caused by the open exhaust or air-box mod) force fuel through the small gap formed by the throttle valve and carburetor throat. Under these conditions the pilot jet cannot deliver enough fuel to create a normal air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber so it gets sent into the exhaust system unburned and collects there.

Eventually, enough raw fuel accumulates to create the proper air/fuel mixutre. That then ignites in the exhaust causing the pop or backfire.

Putting a 'free flow' exhaust on an otherwise stock engine will lean the fuel circuit WAAAAY out.

Running for an extended period of time with this condition ( backfires included) CAN lead to exhaust valve failure (burnt valves) as well as cylinder/head temperature issues.

The only solution is to retune the carb with larger pilot and main jets as well as adjust the needle and slide valves.

I agree with you about running lean etc. with the PM exhaust, however even GTS's that have been set on a Dyno with a PM box and are definitely not running lean still pop on the over run! (as do alot of other correctly tuned vehicles with free flow exhausts). If it was popping every time you closed the throttle I would be a little concerned but a little pop here and there, I personally wouldn't lose any sleep over it.

The GTS engine in "stock" form runs very lean anyway to meet Euro3 emissions, that's why (even in stock form) the engine / exhaust get very hot.

Also in losing the catalyst etc I think you get an acceleration gain with just the exhaust. My GTS with exhaust fitted would out accelerate Suit Scoots before he got his PM pipe.

Our mechanic had a scooter that could backfire on the press of a button. It had something to do with either a fuel line, a spark plug, or possibly both going to the exhaust. Scared the bejesus out of me. I get the logisitics confused with the backfire button and the smoke screen button. I know the smoke screen button routed oil into the exhaust.

To get it to happen on a 4-stroke with an exhaust, you could lean out the air/fuel mixture screw to create a lean idle condition, but I wouldn't do it on a bike I cared about. Especially considering the fragile nature of aftermarket exhausts on the GTSs.

hey max, hows the deal with PM UK going? you NEED to be the one to provide their products to the US.


oh, and about the pm pipe - there is definitely a performance boost...noticeable in the midrange. top speed isn't affected.

Here ya go
http://www.thehoffmangroup.com/Autoloc/Products/Products_Flame.lasso

HA! their site says, "the exhaust flame thrower kit is a must for all vehicles".
imagine a world where all cars have flames spitting outta their exhaust....that'd be nice

Well Suitscoot, thank god you've now got a PM. You couldn't be having that!

As a side note. When using oil on the exhaust to create smoke, make sure it's not used oil. We almost burned up my friends van trying to save a buck by using used oil. I guess it wouldn't be as much of a problem on a bike with the exhaust hanging out there. But when that oil was burning under the car things got exciting quick

its ok, he weighs more than me

The smoke screen routed oil IN to the exhaust. And I think it came directly from the two-stroke oil tank.