What does knock sound like?

[Minimichael]

My Briggs 8B let's out a distinct clink or clank or ping or pang or spank now and then. It happens when I'm laying into the throttle from a low speed and comes right about the time I am about to ease off on the throttle. This part seems important because it seems to be around the height of torquing. One loud "sprack" and that's all, until the next time it happens. Sounds like metal on metal.

 

[Studeman68]

"spark knock".. I'll bet...what fuel are you using? It's pre-ignition, literally the mixture is firing off early and when it does it expands the cylinder and on the contraction it "slaps" the piston...
Caused by high tempurature in the combustion chamber and low octane fuel igniting early. Can also be caused by timing advanced too far...

 

[Minimichael]

Okay thanks. That's how I'd diagnosed it, too. And I imagine that's hard on the rod. I'm thinking about shaving a flywheel key back a smidge could help if it's a timing thing. Otherwise high octane, maybe a colder plug, and probably add back the small section of blower cover across the top of the head. I'd left that off for vanity purposes.

 

[BrownStainRacing]

Okay thanks. That's how I'd diagnosed it, too. And I imagine that's hard on the rod. I'm thinking about shaving a flywheel key back a smidge could help if it's a timing thing. Otherwise high octane, maybe a colder plug, and probably add back the small section of blower cover across the top of the head. I'd left that off for vanity purposes.

For vanity purposes,...... jus run it while you are wearing your high heels and lipstick. Lol

 

[Minimichael]

Might help me if I break down on the side of the road

 

[Minimichael]

"spark knock".. I'll bet...what fuel are you using? It's pre-ignition, literally the mixture is firing off early and when it does it expands the cylinder and on the contraction it "slaps" the piston...
Caused by high tempurature in the combustion chamber and low octane fuel igniting early. Can also be caused by timing advanced too far...

Btw, I had been using the cheap stuff, but now have high octane ethanol-free to try.

 

[Minimichael]

For vanity purposes,...... jus run it while you are wearing your high heels and lipstick. Lol

Yeah I thought the head looked cooler without the little extra band of shroud over the top. Cool in a hot way I guess. So now I'm lukewarm about it.

 

[Minimichael]

And what if that 1 inch strip doesn't make that much of a difference? What if I really need better cooling for this guy? Maybe float a small louver panel across the head to direct airflow downward, too?

Might pick up some hot points around the neighborhood that way..

 

[sparkwizard]

Get a quart of Tru-Fuel from Lowe's or any car parts store.

 

[Minimichael]

Get a quart of Tru-Fuel from Lowe's or any car parts store.

Well, I bought $6/gal 91 octane ethanol-free. So we'll see if that helps, as I'm probably riding it today. Tru-fuel is gonna have to wait til I win the lotto or pump prices hit $20/gal!

 

[Minimichael]

91 octane ethanol-free appears to do the trick. I rode it for about 40 mins, with all the usual stop signs, cul-de-sacs and other reasons to slow and then accelerate hard, and ...no knocks!!!

So if the better gas is the answer, all I think I'll do in addition is try and upgrade to a colder plug. Not gonna go digging for that extra little toupee...

 

[sparkwizard]

Now you can keep advancing your timing until it starts to knock again.

 

[Minimichael]

Took it out again and got a couple knocks later in the ride.

 

[BrownStainRacing]

Took it out again and got a couple knocks later in the ride.

Is there alot of carbon built up in the combustion chamber???

I think I would pull the head and clean it up, looking for a hot spot somewhere. Maybe even smooth every thing out.

I would definitely find out where the ignition timing is set at. It'll give you a starting point anyway.

 

[Minimichael]

I hadn't thought of it being an actual spot, but yeah that's a great point. I'll post if or when I do open it up. Here's the plug..

And my timing is stock, whatever that might be. (I know, I know...I could look it up)

A big thing I like about this bike is it has good low rpm grunt. It's only 2.2hp but it does a good job of pulling my butt up the road, from a stop. So I think I like the timing where it is already because you can really feel those combustion strokes making torque. Feels like they're landing just right. ...right up until one goes off early!