Engine Trouble Knocking at Higher RPMs

[crassius]

the tube the needle hangs in is a jet and the needle varies the orifice of that jet

 

[KCvale]

Dual spark plug head... That is funnier than Boot Bottles as a waste of money!
Spend away, but at least try the NGK plug first as those 3 prong plugs are really junk, I have a whole box of the useless plugs and I put NGK 5944's in everything I build.

 

[Steve Best]

My bike is actually doing the same thing but my plug looks good to me, I like to run just a tad rich to keep my stuff together

That plug does not look good. It is much more than a "tad" rich my friend.
Note the black colour and heavy sooting. It is extremely rich and cold.

It should look more like this:

Note the shaded ground electrode, the insulator tan coloured to the bottom.

Here is normal, too rich and too lean (circled) on well used plugs:

Steve

 

[HeadSmess]

i would throw the 3 electrodes in the bin. the fact that they are cheap nasty things... thats enough reason. they may look ok, but i have found some china plugs to be deceptive.

there appears to be a crack in the centre electrode. if any part of it starts glowing which it undoubtedly is, it does exactly what you describe...nasty noises, an engine that feels like it is seizing... and is fine in just a few minutes, if not seconds after dying.

the only plug chop that works in this case is with an angle grinder.

ngk all the way! wouldnt bother with the three terminal types too much though. plain old b6hs is fine, iridium if you feeling the urge to splurge.

3 electrodes...hard to regap, and do you really expect 100,000km from one plug?

personally i prefer the bR series...just have a resistor in the plug.

why cant you run two plugs?

electricity follows the path of least resistance (usually, ha ha)

three prong plugs "work" on this... the smallest gap sparks with the least voltage. it wears out, gap gets bigger, until all gaps equal. then spark jumps whichever one it likes. you do NOT get three simultaneous sparks

two plugs...same ignition coil... same deal. just more electrodes to choose from.

if you have a coil with both ends accessible...that can work in a twin plug situation. you then need a coil with twice the voltage rating...has to fire across two plugs.

run two CDI's in parallel? the magneto cant supply enough juice to charge two.

and, of course, the benefits are marginal in a small engine. twin spark only starts to work with pistons about 4 inches across or more...

 

[KCvale]

Nicely explained bud ;-}

i would throw the 3 electrodes in the bin. the fact that they are cheap nasty things... thats enough reason.
ngk all the way! plain old b6hs is fine, iridium if you feeling the urge to splurge.

...personally i prefer the bR series...just have a resistor in the plug.

Here is why a Resistor plug (designated as a R in the model #) from the NGK web site:

Resistor Spark Plugs
Resistor type spark plug’s incorporate an internal resistor to suppress ignition noise generated during sparking. The ignition noise which is often known as either electromagnetic interference (EMI) or radio frequency interference (RFI) can disrupt electronic communication signals, effecting the performance of sound systems, cellular phones, two-way radios and even engine management systems.
As EMI or RFI can also cause premature failure to other electronic components in a modern vehicle, it is important that resistor spark plugs are used to prevent this possibility.
Put simply, when a spark is formed it creates a mini-earthquake in the electronic world; the internal resistor in a spark plug filters out the noise created that may affect electrical components.

​

Even with a Resistor plug there is enough RF/EMF for most wireless speedometers to not work.

...why cant you run two plugs?

electricity follows the path of least resistance (usually, ha ha)

three prong plugs "work" on this... the smallest gap sparks with the least voltage. it wears out, gap gets bigger, until all gaps equal. then spark jumps whichever one it likes. you do NOT get three simultaneous sparks

two plugs...same ignition coil... same deal. just more electrodes to choose from.

if you have a coil with both ends accessible...that can work in a twin plug situation. you then need a coil with twice the voltage rating...has to fire across two plugs.

run two CDI's in parallel? the magneto cant supply enough juice to charge two.

and, of course, the benefits are marginal in a small engine. twin spark only starts to work with pistons about 4 inches across or more...

Exactly.
All you need is sufficient spark to completely ignite all the fuel mixture.

An NGK plug does a fantastic job of making a big spark so long as you give it enough juice from the CDI.
Stock systems us a pair of wimpy small failure prone wires to get the juice from the Mageneto Coil to the CDI.
That's why I don't use those stock wires or the stock plug in my 2-stroke builds.

I replace the stock wire with some good 16g double insulated wire..

...and connect the CDI directly to the Mag with that.

...specifically the mag end

...to the CDI

I don't know how much that helps, but two things are for sure, I get optimum spark and it looks darn good ;-}

 

[Jeremy/Bicycle Engines]

Awesome and extremely accurate information KC! I am glad I found you!

 

[GreenMantis]

Hello all and great site yall have. Long time lurker, first time poster.
I recently bought my first 66/80 cc motorized bike, it came with 1 spare engine and most of the parts for another kit. The bike had a large crack in the frame near the bottom where you pedal but I already had a better bike for the transplant. Anyhow. The motor runs fine and starts up without a hiccup except when its cold (I have to pedal a little longer, no big deal) but my big problem is that it knocks on the high end. I have a 36T sprocket and I just put a speedometer that is calibrated for my 29" rims and I am hitting about 20-25 normal with a top speed of 29.1 registered on it. I don't know what kind of RPM's I am actually running because I don't have a tachometer yet. I have the stock NT carb it looks like. When the knocking starts it robs a LOT of power and it stops accelerating and actually decelerates a little. I can tell that it really wants to give more but its just cant for some reason. Under reasonable load it seems to work much better but once I hit a downhill or get up enough speed on a flat stretch is when it really happens. The chain isn't hitting anything, I have a fairly heavy duty spring tensioner on it. I don't ride the bike for long periods of time, typically 20 minutes or less.

Wondering if it is because the ignition is firing too soon and I need to get a CDI with a delay at the top end or maybe its not balanced and there is too much play in the bearing at the piston, but if it was the bearing wouldn't it do it all the time?

Keep in mind that I have no idea how old this motor is, what brand it is, or what the old owner put it through before he sold it. He probably weighed about 250 but we live near the beach so its pretty flat.
Trying to include everything I think might help but I'm sure that I left something out. Any help would be Greatly appreciated. Thank you and Semper Fi.

 

[GreenMantis]

The number one cause of pre-ignition, or "knock", is low octane fuel. Buy the highest octane you can get, and if possible, find a service station that will sell you pure gasoline, and not gas that's been watered down with ethanol, these engines weren't designed to run on it. And most gas stations sell E10 only, now, and they don't have to tell you.
I usually put napthalene moth balls in my gas, because those will boost the octane up considerably. The secret with those is to read the label, most moth balls sold as such now days are made from paradichlorobenzine, not napthalene. Paradichlorobenzine is urinal cake, don't put that stuff in your fuel. I'll probably take all kinds of flack for saying it, but the fact remains, I always got a 50% fuel economy improvement over regular octane, undiluted pump gas. The results will be considerably less with E10 fuel, but still better than straight from the pump. I noticed they now sell little cans of gasoline at the hardware store, next to the 2 stroke line trimmers, that will be undiluted, high octane gas for the exact same reason, these engines don't care for what they sell at the gas pump these days.

 

[HeadSmess]

um...honda found they could get away with using 70 octane with high compression engines simply by letting the things rev to 16,000rpm.

use the lowest octane fuel possible, it develops more power.

if youre getting detonation, drop a gear and let the poor engine REV. its the low speed, full throttle, uphill lugging that primarily causes detonation.
high octane simply burns SLOWER.

anyways. this isnt detonation caused by fuel issues. its pre-ignition caused by a faulty spark plug.


you may have to tell my current boss to stop getting pump gas...as a business, we go through 100 litres of fuel a week. three of the shindaiwa brushcutters are over four years old now, with no troubles. other than gearbox failures, shaft bearings wearing out, clutches burning out, and blunt blades on the hedgetrimmers. i guess all those issues are due to using pump gas. he will be glad to know

 

[HeadSmess]

the tube the needle hangs in is a jet and the needle varies the orifice of that jet

not just A jet, but the NEEDLE jet main jet is the screw in replaceable bit. needle jet can also be called a spraybar... then you have the jet needle. terminology can be so confusing at times...