Modern Vespa

so just when i thought i was getting used to this spark knock thing i have

you know the one that only happens at a certain rpm(around 20 in 2nd gear)

i thought i would drop a shorter plug in there, one of the ones i have around for my smallframe

guess what, its worse

so with a shorter plug(less compression)knock is worse

what does this mean

cause i don't have a clue

hp,greasy,oops,corsa,monza,eric,ect....

the crank is tight(no slop)
the fly(lightened) and head(high comp)hot rod al
the barrel is stock with work done, the piston is cut(fullerton)

Can't recall your set up at the moment but you need the correct length plug in there. I don't think the shorter one will lower compression. Has to be the right length to ignite the fuel correctly.

Are you running a gasket between the head and cylinder?

no gasket between the head and cylinder

i know thats not the correct plug

i tried it because what you are doing is lowering the compression

it was suggested by geasy to throw a gasket in there to bleed off a bit of the compression, i didn't have a gasket so i thought i would try a shorter plug instead

it ran fine with the shorter plug, until i get to that rpm where it knocks and sure enough, instead of getting better it got worse

The plug is a certain length because it has to be to fire the juice at the right place to ensure uniform explosion.

Put the right plug in, throw in a copper gasket in the cylinder head (which also lowers the compression) and then see what happens.

If it's knocking at a certain RPM you're too lean there. Need to figure out what throttle position you are in (mark your throttle with white out and mark a spot on the headset to comare it to - do it in 1/4, 1/2, 3/4 increments). When you hear the knock, look to see where the throttle is corresponding to the mark on the headset. Then you will know which jet is affecting you at that throttle position.

Jetting is throttle slide position, not RPM.

will do

i will order a copper gasket from motorsport and try that

i already replaced the plug with a proper e8

i played with the jetting a couple weeks ago

it has a 124

i tried a 125 and a 128 and still the same

Cool. And when you mark your throttle positions make sure you twist it a tiny bit to compensate for the slack. Once you have the cable taut and account for that 1-2mm of slack, mark that as 0 throttle, then twist it all the way open and mark that as FULL, then go back to your 0 and half the distance, that's 1/2, etc.

AFAIK, The sparkplug electrode/insulator length often affects the knock situation by heat, in that a thinner tip will often heat quicker and cause preignition, but the engine will heat up faster. Often, stouter fatter electrodes are use when the engine is expected to run for a long time, and a thinner one is for short trips in cold weather.

Before I wonder about the volume of the interior volume of the sparkplug electrode, I'd check other things such as fuel quality, timing, head gaskets/shims, piston/conrod/crank compatibility, and perhaps do a compression test and calculate the effective compression ratio.

Also, a loose sparkplug can get quite hot as there is little mass and it needs to be heatsunk by the head or it to prevent it from offering a hotspot for preignition to start. It would seem that anti-seize does two jobs, heat sink compound as well.

i am no good in explain thing like this but without gasket ur compresion is way high and its seem oke for me to use a shorter spark plug (i always do that).. for me seem that ur too lean, once again since u didnt mention my name so u probaly know i am not that expert and i might be wrong

Been a machanic all my lif on small engines and high preformance stuff. First the length of the plug is for cooling of it . Spark knocks are caused by the burning of 2 fuels , one that is ignited buy a hot item in the cyc,(maybe spark plug tip) or somthing else and by the spark from your ign system. Wrong ign timing will cause engines to run to hot, wrong heat range will do the same thing but if everything is perfect first then it's the shitty fuel we are getting at the pump. Now clean your cyc cooling fins and fan put the right heat range in make sure your timing is perfect or even 2or 3' less then what is called for. My 1966 180SS timing is 26' I have it at 24 with no spark knock. Good luck in your quest but what I have read your barking up the wrong tree with trying to lower your compression

EDM

It's pretty much been covered by the previous posts. Detonation from a glowing thing or too low octane. A small burr on anything in the combustion chamber that gets red hot acts like a glow plug similar to the little RC engines. Look for little rough things. Carbon chunks glow pretty good. "Hey baby, rub it down and make it smooth like lotion."

Buy some octane booster, racing fuel, or whatever and you can eliminate that probability. Get that octane over 100 and it won't spark knock because of too high compression on anything you can kick start without grunting. Unless maybe you've got some one-off really weird combustion chamber shape/piston mismatch or something.

Sometimes, the glow plug thing can cause weirdness when you kill the ignition. It will really get your attention.

A series inline spark tester that flashes could help identify any extra ignition pulses if there are possibly any for some weird reason. A timing light should freeze cooling fins with a steady ignition system.

It's now time to pull the spark plug or remove the head, clamp down the jug with with a bunch of washers, and either get someone to pull you or go down a hill at a high rate of speed to check for mechanical noise. A video would be nice.

Yep. Has happened to me twice on the RD.

Nothing like a twin cylinder 2-stroke SCREAMING at 10,000 RPMs.

1: Turn off ignition. Still screaming.
2: Remove key. Still screaming.
3: Remove both spark plug wires. Still screaming.
4: Jerk fuel lines out of carbs, wait for it to run out of fuel.

The second time I just jerked the fuel lines.

Strange shit watching a motor rev out with no spark plug wires attached to the plugs.

One time i was riding with a girl next to me (somewhat new ) on a smallframe.

Her spark plug kinda worked loose a little, and started sucking in air around the spark plug hole, leaning the mix. After the engine warmed up, it started screaming ...10,000rpm-ish. She didn't know what to do. Kill switch didn't work, etc. I was able to reach over to grab the spark plug wire and pull it free, but the voltage / current was enough to blow my arm backward away from the bike like something you'd see out of the movies. I had my hand on the wire at the time, so my hand clenched down on the wire as my shoulder flew back, and it ripped the plug wire clean away and i threw it about 30 yards accidentally. ( that was the last time i ever touched a spark plug wire when the bike was at that high an rpm )

We were in the middle of an intersection, and i rode my bike over to the sidewalk, parked it, then told her to put her bike in neutral, put it up on the stand and get off it.

I got on it, took it over to the sidewalk, put it in 4th, and let out the clutch while holding in the brake to stall it.

My shoulder ached for like 3 days.

great stories

this is where i am right now

corsa suggested marking throttle positions

the spark knock is at 1/8 to 1/4 throttle

i turned the mix screw out(left)1/4 turn, still knock-1/2 turn,still knock-3/4,still knock

my mix screw is now at 2 3/4 out
main jet is 125
timing is at 19
ngk e8

i guess now i will go play with the timing again
if you remember i already retarded it 2 degrees
i will now try 3 degrees

this is a tight motor with great performance mods done by really good mechanics

metal burs are more than likely out of the question

With a mixture screw that far out I'd increase the size of the pilot jet.

I havn't seen you mention the rest of your jet stack.
the main jet is only one part of the voodoo that is jetting.
EG:
My setup needs a BE4 mix tube, BE3 causes pinging and stuttering at higher rpm

sean.

i have a be3

i will get a 4 and give it a go

Looks like you have a few different things to try.

Reminder: Change ONE thing at a time, not two, not three. One.

I keep forgetting about mixers since I switched to a mixer-less carb but that is a good point.

Also your idle/pilot jet could be clogged. I just pulled the two on the RD last night because I was having very lean conditions at idle - 1/4 throttle, and again at 3/4 throttle if I let off to coast - it would buck up and down.

Pulled both of them and both of them were blocked with shit. Worked the dookie out with a thin wire and it screams again.

i have a be4 and a 120 stack and a gasket on there way to my house from motorsport today

i will keep posting

the first thing i will try is swapping the 160/be3 for a 120/be4

if that does,nt work then i will go back to the 160/be3 and then try the gasket

No....
Start with the 160/BE4 and try your 125 jet.
The be4 is richer than the be3 ...ie, the be4 allows LESS air in to be mixed with the fuel.
If your too rich, you can start droping the main jet.
If you still cant get it dialed in, its time to play with the air corrector
eg: 160 = lean....120 = rich

The main jet regulates the fuel into the mixture chamber (the void arround the jet stack)
The air corrector regulates the total ammount of air into the circuit.
The mixture tube determines how the AIR interacts with the fuel in the mixture chamber....the mixture tube is an air nozzle, it allows more or less, faster or slower air into the mixture chamber.

hope that makes sense
Now you can look at how air velocity/vacume/back pressure/filter restriction/primary compression/secondary compression and back charging from the exhaust pipe effects all of the above.

-change one thing at a time and check the results-

I need a beer
sean s

update

the stack is now a 160/be4/130

runs great

really good

still knocks though

i have included throttle positions

Have you tried running in the knocking throttle position and then messing with the enricher to see if anything changes?

What he said.
also,
What year is the bike and is it the stock carb?
Which idle jet do you have?

sean s

update

bad clutch side seal

that is what caused my last seize and on top of that the two seizes together cooked my crank

and since i was running with low oil level i was cookin cork as well

so new mazzi crank, cork for the clutch, seals and bearings, cross while i'm there, engine mounts and hopfully this will be the end of knock

Bummer Jimmy James...

When you comn' to SD?!

Double bummer.

Thanks for the update. Being left hanging with no feedback is not cool when someone is trying to learn something from a thread, as I am.

jimmy, sounds like you had an expensive day. the mazzi crank alone is spendy.

To get the old mounts out I've got a trick that's much better than the old drill them out nonsense.

Put a long handled (and I mean one of those big fuckers) screwdriver in through the end of the swingarm, now move the screwdriver in a circle like you're stirring batter. Go faster and faster in that big circle (you should have 2-3 inches of screwdriver down in the hole).

That mount will work itself right out of the swingarm.