I have 52K miles on a 2013 S40.
all but the first 1100 are mine. It is flawlessly maintained and runs like new, except very recently.

Recently - perhaps since the rectifier quit? - burning out the headlight.. and was immediately replaced - the engine knocks and pings badly at Wide Open Throttle, like while climbing a mountain grade at 65, or accelerating hard out of a curve.

Yesterday I was forced to buy 87 octane fuel in the middle of nowhere  (I otherwise always run 92 ethanol free) and the knocking/pinging was awful.  

Symptoms quickly resolve when throttle is reduced.

Where should I start?  Is spark advance somehow vulnerable?  Might the main jet be clogged?

Congrats on 52k miles!  At that mileage, and it only occurring at higher rpms under a load, I’d guess carbon buildup in the cylinder.  I’d inspect the spark plug and look for clues.  If you have an endoscopic camera, try to get a look inside the cylinder through the spark plug hole.

Since you’ve been doing regular maintenance and using quality fuel, and you haven’t had this issue before, I doubt it’s a dirty main jet.  The primary jet would clog and show issues before the main jet would (usually).  But…check the air filter….maybe it could be that simple.  Or your bike has become spoiled on high octane 92 and let you know it when you filled up with 87 octane. I’ve always just used 87 top tier gas and haven’t had any knocking/pinging (stock engine, no high comp piston)

I’m no mechanical guru, but a fresh spark plug and new air filter have done wonders for me in the past.  And it’s cheap and easy…and it never hurts to have a fresh plug and air filter if you have to dig into in further (at least you can rule those out).  Hopefully you don’t have to pull the head and clean it and the valves and piston.  Because I’m lazy and not a gear head, I’d be inclined to try some Seafoam or Berryman’s Chem Tool (high mileage) before opening up the engine.

Summary - New spark plug, air filter, and Berryman’s for starters.

Berryman’s comes in 2 flavors; B12 and B60 (high mileage).  I’d go with B60 in the gas tank.  B12 did a really good job on a piston I cleaned once removed from the engine. I haven’t tried the “Total Fuel System Treatment” product (maybe that’s flavor #3), but it specifically lists combustion chamber.

From Berryman’s website… —>
https://www.berrymanproducts.com/question/can-i-use-b60-like-b12/#:~:text=It's%20true%20that%20B%2D60,B%2D12%20for%20your%20soaks.

Hey Kramer, you have a unique problem.  Never seen any posts regarding knock/ping since I joined back in 2018.

Can you clarify a few things.

Did the knock/ping just start suddenly, or was it something that you noticed occasionally and then it degraded to the point where it's a constant problem?

By any chance did the onset of the knock/ping occur shortly after filling your fuel tank?

Is your engine stock or has the compression been raised?


Generally, knock/ping is associated with too much ignition advance, low octane fuel, too much compression, extremely lean air/fuel ratio, or a hotspot in the combustion chamber.

For now, I will assume you have a stock compression ratio.  The fastest way to rule out any issues with carbon accumulation would be to simply check your compression.  Screw in a compression gage, hold the throttle wide open, and crank the beast.  You should have around 150 to 160 psi.  The stock engine runs fine on 87 octane, so if you have 160 psi or less it most certainly should run fine on 92 octane.   If the squeeze is 160 psi or less, you can rule out heavy carbon accumulation bumping up your compression ratio.

If you took in a slug of water during a refuel or heavy rain, the water can sit in the bottom of your float bowl.  Many times, the slug of water won't affect idle or cruise, but at wide throttle openings it will get picked up at the main jet and cause all sorts of trouble.  When I suspect problems with water, I very carefully remove the float bowl with the carburetor in place.  Take great pains not to tip the float bowl and dump the contents.  Pull the float bowl straight down so you can inspect the fuel in the bowl for droplets of water.  It will be very evident.

You stated that this problem reared it's ugly head shortly after a voltage regulator failure.  The solid-state ignition system on the S40 features an electronic ignition advance.  Without special tooling it's impossible to check ignition timing or verify proper ignition advance.  I'm not an ET, but seems to me if you had issues with other electrical components failing due to the regulator, then the ignition unit may have also been damaged.  It could be stuck in an advanced condition.  As I recall, it's supposed to have about 5 degrees BTDC at idle and somewhere around 28 degrees BTDC at 4K.  I use this special tool along with a strobe light to fiddle around with mine.  If you can come up with something similar you could verify that you aren't lighting the fire too early.  You could try just shooting the strobe on the timing marks under the timing plug, but the oil blast will be off the hook (like a real mess).  I suspect it would be impossible to see the marks.  A tool with a decent degree wheel and an oil barrier is a better way to go.  Maybe some of the other members have a good way to verify proper ignition advance.

The petcock is such a weak spot. Lookin at the possibilities and necessary skills to solve them, the easiest ones to start with, IMO,would be drain the bowl and see if it's got water in it, and the petcock, which is a recommended mod anyway.

My carburetor has a drain on the bowl. When I was checking for crud or water I just drained it into a clear glass catch .

If you drain the bowl and have a stock petcock, ya gotta put it on prime to start it.


You Can try running it on Prime. If that makes it run right, ya found it.