So today on my 09 250SF I've gotten some pretty annoying sounding pre-ignition I believe... while idling at lights and giving a little crack of throttle.

It's 15c today, so not really that warm... but I've gotten used to driving in -10 - +5 recently.

I've got some 87 in the tank right now... have tried higher in the past without much change.

Only changes to the bike since I've noticed this problem is more apparent would be the Krieger Cam Chain Adjuster. Which shouldn't affect pre-ignition or timing at all correct?

This ONLY happens when I'm idling, and I give it a little crack of throttle, maybe 1/16th-1/8th a turn. Quick snaps... If I roll on, it's fine.

Should I be concerned?

In my driveway I managed to actually stall the bike when I got home.. just raised the idle a hair to see if it would prevent it...

If I hold 3k RPM and do the same, it doesn't knock at all, ever. It's only at idle.. around 1200-1300rpm.

Powerband feels great... engine pulls as good as ever.

The carb was rejetted by the previous owner, with stock pipe... snorkel is removed with some duct tape to seal up the airbox a bit. I believe he said there is a 124 main jet in it. Is that too rich for a stock exhaust? Is my piston going to look like a burnt chocolate cake like that one in the 351 thread? Haha.

If I missed any critical information, let me know.

 

Does your bike pop on deceleration as well? You might try turning out the fuel screw a bit, that has an effect at the throttle position you describe.

 

Are you sure it's not your cam chain tapping a little? I believe Kreiger said they can do that with his adjuster, especially when the bike is cold and not warmed up yet. Or was your bike good and hot when its doing this? Or both hot and cold?

Are you laughing at my chocolate covered piston? Ha.

Do you know if your 124 jet is Keihin (Kaw) or dynojet or other? Is the duct tape covering part of the snorkel hole? What's your elevation? Have you or the PO made any changes to the idle mix screw?

 

It does it when it's hot.

No idea what jet, probably Keihin, as the PO thought the DJ kit was a waste of money, and he already had some jets that worked in his tool box.

I don't have any popping on decel, just a good grumble tone of engine braking, at any RPM...

I don't even know what the Snorkel hole looks like, but there is still a good sized hole in the airbox lid.

It's definitely not the cam chain tapping, it's a knock, hard to describe... kind of like 2 bowling ***** hitting each other, but instead of whatever they're made out of, add 50% aluminum?

I know very little about engines, but I'm not afraid to dive in when I have access to the proper tools, and do plan on one day getting a 330cc kit for it, unless the 351 proves to be just as reliable.

Yes IDRIDR I am laughing at your burnt up piston, that's incredibly nasty looking. I hope mines still shiny when I take it out Haha.

I don't even know where the fuel screw is, I've rebuilt a carb on a Suzuki LS650 about 5 years ago... that's about where my real world engine experience ends. But that carb is eons different than this carb appears to be.

Also, I do have a slight ticking sound when idling, which I think is the KACR. That's just a light tapping type sound though. Maybe valves? Not sure.

 

If you're getting that noise at idle, I'd find it very strange to be preignition/detonation.

 

Preignition is pretty common at idle, I know my BSA did the same on the hot summer days. While riding there is a load on the engine, it will be running richer, it will be less likely for pre-ignition.

If you want to richen the fuel a little, just take of the seat and add another row a duct tape or put the snorkle back in. Removing the air box cover will make the fuel very lean to the point it will not run over 5000, from my experience.

 

Yes, detonation can occur at idle, but on this engine under these described conditions, I think it would be unlikely. There doesn't appear to be an overheating issue. There doesn't appear to be any effort to manipulate timing on this engine, and of course the timing is normally non-adjustable. I'm more in the camp of it being something else mechanical. Still, trying to listen to and determine another's engine noises over the internet is rather difficult...LOL!

 

wow, would you by chance have a sound clip of what you are referring too?

how does the plug read?

 

I suppose I can envision what you describe. This isn't a BSA. Could the fuel screw/idle circuit be so tight that it detonates at idle? Hmmm. Perhaps.

regarding the airbox lid, the way mine was jetted, it would not run over 6000 rpm with the lid on. It all depends....one size does not fit all.

As those above said...can you record some sound and post? Text just does not do it.

 

Aside from the assumed detonation... it runs great all the way till redline.

I'll see if I can get a sound clip after work today.