Holed piston!!

[Lamaddict]

I've only holed a piston once and that was on an iron barrel. The hole was caused by a too low main jet, simple as that. However it's worth strobing your timing. I'd love to have a scoot dynod but don't have that option so it's down to plug chops, trail and error but but get there in the end.

[Muppet]

im liking all this info; if i want to hole a piston on my fast; expensive; all singing an dancin; 35bhp+; you cant catch me on this scooter; all i have to do is have an engine profesionaly ported £,000 ; using the best parts available £,000 ; run it on the best oil money can buy £,000 ; dyno run it £,000 to make sure its working perfectly; then it will explode pretty much on demand; this will be when i am half way to or half way back from a rally riding at the same speed as a standard 150 PX can do; you can count me in now wheres my credit card :fb: ,

muppet,

[rosscla]

:biggrin: ^^^^^^^

[nelson pk]

Great post from Rich T. Couldn't agree more on most points. Backing off from full throttle to cruise in the mid is where 90% of all tuned lammies go pop. Your scoot should be set up for this if your doing rallies and long journeys.
The bad fuel thing i think is a bit of a red herring in most cases. Agreed we all have to lower compression, retard timing for lower octane unleaded and ethanol isn't great but i think most scoots go pop due to lean jetting or jetting without enough safety margin , that can turn lean with change in air density, get hot when under load etc.
Rather than looking at the fuel, i would be looking for possible small air leaks as well, as i carefully dismantle the top end. Checking my timing is spot on etc.
I'm not saying that bad fuel is a complete myth but i dont think its very common.

Super unleaded for me too.....always!

Just thought of one other possible contributing factor...........

The Lambretta design of how the exhaust is bolted on is pretty shite. If i had a pound for every lambretta exhaust that leaked around manifold area (stub fittings as well) i'd be a millionaire! Well, ok, i may have a few hundred quid.
Anyway my point is that Lambretta exhausts can come loose and have small leaks very easily but quite a few people dont realise that this will make the scoot run leaner.
If your jetting is not errring on the side of caution this could be the time when it goes pop!

All in all there are loads of things that can go wrong or cause a 2 stroke to go pop before looking at bad fuel in my opinion.

[Steve S2]

In my case, there was no leaks the timing was spot on and on the dyno it was below 12s all the way through the revs, and I had covered over 2000 miles in the last 12 months with not a single problem.
And no tuning work was carried out on the dyno all we did was push them on and pushed them off.

I am not saying that all holed pistons are down to petrol. But why would 2 scooters hole the piston within 5 mins of each other, after filling up from the same with the same fuel.

[Muppet]

^^^maybe it was sympathy pains^^^

muppet,

[Muttley McLadd]

Isn't this where the experience of the dyno operator comes into its own?

[Darrell Taylor]

i find most common causes of holed pistons to be spark plug grades in use,then blown head gaskets,then split inlet rubbers but i find the 2 scoots at shell story very concerning.
i always ask to run on a 10 plug and with some running advise dont suffer from oiling up
as a dyno operator and tuner with an excellent low failure rate i find many of the comments incorrect,seen 5 holed piston in last year and easily found the causes despite there owners not spotting them 4 were head gaskets that when shown in detail soon spotted them 2 due to a pulled stud,one due to a degraded paper base gasket(never use them! use ally ) and 1 loose head nuts then 1 with a splt inlet rubber that you could put your finger through and the owner said the tickover rose to 4k when he stopped at a junction but still thrashed it home(for about 200 metres at least)
the scenario of the 2 scoots at the shell station has got to be the fuel ,the odds are stacked against it being anything else
the port widths on tuned cylinders are accepted when there sold on a production cylinders ,the latest kits from a certain brand are now out to nearly 70% ex port width and started at 60% in there first forms ,the ring only sees the width at closing point so shaped appropriately doesnt need to be any harder on a ring than a standard port but cant speak of how other porters cut there top radius only my own
the most regular situation i find when dynoing scoots is there regularly far too lean on pilot and needle this is generally the case in an attemp to clean up too rich main jet settings and everything you do at the bottom end carries thru to the top
without the dyno scoots are set up by feel of what feels best from road testing and lean feels absolutely fantastic but is short lived
an important point when on the dyno is to check throttle positions at all rpms 1/8th throttle at 2000 rpm may be lean but at 8000 may be rich ,ride up to the back of a wagon at 8000 and roll off and thats the fuelling your running on and at great risk of seizing
compression and jetting can be safely optimised but ignition timing is the invisible killer ,compression needs to be tuned to ignition timing and fueling and experience is often the only way to know what works
so ultimately 2 choices exist working to a set of industry based calculations/testing regimes or a blast down the road and pot luck
i do truly believe a 1/4 mile strip and a race track is a great dyno as performance can be measured against lap time or 1/4 mile time but unlike a dyno the times keep improving until you go pop as you dont know when youve gone too far ,costly but then you know never to go that lean or advanced or as high on compression again.

[Rich_T]

In my case, there was no leaks the timing was spot on and on the dyno it was below 12s all the way through the revs, and I had covered over 2000 miles in the last 12 months with not a single problem.
And no tuning work was carried out on the dyno all we did was push them on and pushed them off.

I am not saying that all holed pistons are down to petrol. But why would 2 scooters hole the piston within 5 mins of each other, after filling up from the same with the same fuel.

Because they are two kitted scooters where, quite possibly, the overall engine configuration does not allow for the tolerance in fuel supply and ambient conditions to conspire. Fuel is produced to a standard and it will vary, as do the other factors.

My point is that if the piston gets holed this is a "real world" occurance, dyno's are a simulation of real world events where some basic elements have a degree monitoring. If it does not happen on the dyno then this only goes to underline that it isn't an accurate model and exactly the reason why I am very cautious of what dyno information to use and what not (time in the saddle is worth a lot more).

If it were soley fuel from that specific station there would be an ring of dead RS125's and all other manor of two strokes littered about in a radius of 6 miles. This does not mean that fuel was not the perverbial straw, it could have been. If no tuning was done on the dyno this is not a conclusively vindication that everything in the garden is rosey, and to be honest 2000 miles is not a huge distance to sign off on.

The fact of the matter (for road going scooters) is that you can not control the ambient conditions (weather) and you can not control the exact specification of the fuel you put in the tank (you buy it and get going-you don't stand there with test tubes, and a lab coat). The things we can control are how we build the engine, what compressions, ignition timings and jets we use. The logical process, to me, is to configure the settings we can control to counter balance the ones we can not. This builds in a certain amount of system redundancy and this is what should make a Lambretta engine reliable.

If the AF ratio is flat-ish between mid to full rpm on WOT then it would be no surprise to me that you have a lean AF ratio if you rolled off the throttle when you were up to speed (or exactly the wagon example DT mentions).

[drunkmunkey6969]

Rich......when was the last time you saw a thousand RS125s doing the knaresbourough to York ride out? Chances are, if you did.....you'd see quite a few stranded roadside.....especially if they had stopped production decades ago and had all been rebuilt a dozen times by 'my mate who knows what he's doing'.