Scooterotica

Hi all, my Vespa pinks at certain rpm and at partial throttle.

A few questions,

What is pinking and what causes it?

How do you fix it?

Cheers :0) :fb:

My experiance of a pinking engine is generally your engine over heating and can be caused from the carb not jetted corectly (not getting enough fuel) Also if the timing is set wrong and the compression ratio of the head is wrong

Like LI 150 says

Post a few more details - barrel/kit, timing , carb and jetting

Details of the engine are:

Mallosi 210, ported cases, reed valve, JL RH Exhaust.

30mm Delorto, jetting is spot on as its just been dyno'd

Timing is as far as it can go round on the stator.

Could the squish have a part to play?

Pinking or detonation occurs when trapped "end gas" spontaneously ignites.
The piston rises and closes off the exhaust port, compression commences. As the piston approaches TDC the ignition system fires and the fuel air gas starts to burn in a controlled and progressive way away from the plug. This is called the flame front, burning, expanding gases one side - increasingly compressed and therefore heated fuel mixture on the other. The pressure rise is quite steep from 1 atmo at beginning to 100 ish.
Detonation occurs when the remaining fuel mix is unable to resist blowing up. This is what the octane numbers represent - the ability to resist spontaneous combustion. The noise you hear is not burning but an explosion when all the remaining mixture instantly turns into heat and pressure. The more rapid the burn the more violent the heat and pressure rise.
And as we know it can be sufficient to cause mechanical damage and even destruction to the combustion chamber, cylinder head/piston crown. The control of end gas by the use of squish band has been around a long time. The theory is that as the piston approaches closely to tdc, gas trapped in the squish area will be ejected in a controlled way to promote turbulence. Generally this is thought to be beneficial and a detonation inhibitor by instigating tumble in the gases, re mixing the burnt/unburnt gases so combustion can proceed in a more stable form.
The removal of TEL from petrol has lowered the knock (AKA pinking) resistance, other compounds have been used to try to regain knock resistance but most are, like benzine, even more toxic than the lead compounds, or only marginally effective.

Thanks Warts.... that has explained loads. So are we saying that it is best to use super unleaded due to its higher octane content to help eliminate the possibility of pinking/detonation?

It is best... Higher octane fuel burns 50 f cooler than 91.

Your bike shouldn't be that close to the limit though. You need to check for an air leak somewhere. Inlet gaskets etc

Also just because it's been dynoed doesn't make it perfect necessarily. Also check the heat range of your plug. Ideally you want a b8 or b9 plug

You say timing is as far as it can go round on the stator? I have found on PX200's that the stator slots need to be filed to get to 18 or 19 degrees before top dead centre which is where i think most kits should run on them.
Get a timing disc and strobe gun and check your timing properly as well because guessing will get you nowhere.
Hope this helps.

dopeshots wrote:Thanks Warts.... that has explained loads. So are we saying that it is best to use super unleaded due to its higher octane content to help eliminate the possibility of pinking/detonation?

This is not always the answer as Super Unleaded fuel has enhancements in the mix which increase the burn effect of the fuel & if the mixture is not set rich enough you will end up running with a leaner mixture than when using 95 octane Unleaded.

Also petrol companies change the blends of their fuels as the seasons change. In winter there are anti icing addatives etc.

both Lambretta's & Vespa's do not benifit from modern electronic sensors which monitor ambient air temp', cylinder temp', exhaust gas Co, Co2 & HC then alter the carb fuel mixture for you!

It is proberbly the carb jetting that is lean at a particular throttle opening?
I set up the carb mixture to run around 5-6% Co in the dynojet room as it is an artificial inviroment, warmer & more humid.
This allows for when running at night there is a safety margin to compensate for the colder air temp etc, but this means it is slightly rich during daytime riding!

If you are able to get to Newport, South Wales I can chech the mixtures etc.
Out of interest you should be getting 18-19bhp @ rear wheel about 6750rpm with the RHS JL Exhaust & 30mm Dellorto Reed setup.

Ian Frankland
Taffspeed

dodgy subject
im running a standerd p2 with simmo pipe 120 main jet pre mix when at 3/4 throttle after 50 mph i get the pinking sound checked the plug and its a tad ruich not a lot tho and no visable signs of anything on the crown infact once clean it looks brand new?
no dyno close by unfortunatly