Loss of power and weird noise at full throttle?

"Spark knock" typically comes from too much ignition advance. Higher intake temps make it worse, but it is not wholly dependent on temperature. Spark knock will usually happen when cold as well unless it is only a marginal problem.

Higher temps REALLY aggaravate compression/octane knock.

My bike only starts rattling after it is fully warmed up. Still does it after miles of 45+mph light throttle cool-off riding also. :applause: I still have to try some good premium pump gas....

when i first get on the bike, i can hit full throttle without knock, after a bit the knock starts at high throttle and increases as it gets hotter. i bought some different jets today and hope they will correct the problem.

Spark knock, compression knock, octane knock, pinging, knocking, and probably a bunch of other names are all the same phenomenon: detonation.

Conditions that allow deto to take place can be created by different things (too much ignition advance, too low octane fuel, too much dynamic compression for the fuel, too much heat, and more) but the phenomenon itself is all the same thing regadless of the aggravating factor or factors.

could you explain ignition advance and what i can do to correct it?


could you explain ignition advance and what i can do to correct it?


Although we sometimes think of the spark plug firing when the piston reaches Top Dead Center (TDC), engines are designed to fire the spark plug many degrees of crankshaft rotation before TDC. Somewhere between 15-30 degrees might be typical, depending on the engine.

The reasoning for the timing of the spark is related to the time required for the air/fuel mixture to burn. Sometimes we imagine that the a/f mix inside the combustion chamber explodes when the plug sparks. This isn't what happens. The a/f mixture burns in a predictable, controlled manner. Under normal circumstances the a/f mixture combusts by way of a process called deflagration (a fancy name for burn by way of thermal conductivity). As deflagration takes place cylinder pressures raise incrimentally until all the a/f mixture has been consumed. The goal is to achieve peak cylinder pressures somewhere around 15 or 20 degrees of crank rotation After Top Dead Center (ATDC). This timing of combustion events has been shown to be the most efficient.

The way in which this all can be related to detonation goes back to time. Deto is when heat and pressure combine to break down the gasoline into new molecules by way of processes called pre-flame reactions. Basically, if you put gasoline under enough heat and pressure it will change into NEW chemicals that are highly unstable. So unstable that they can ignite spontantiously without waiting for the advancing flame front to get there and consume them normally. The only time it's possible for the gasoline to experience these extreme conditions is after the spark plug has fired and normal combustion (deflagration) has begun. The gasoline that resides around the edges of the combustion chamber is the gasoline that is suceptable to deto simply because it is the last part of the mixture to burn. This so called "end-gas" has to sit there and endure the ever increasing heat and pressure caused by the advancing flame front (the normal burn started by the spark plug) AND the still raising piston (remember the normal burn starts while the piston is still on its way up). If these extreme conditions are severe enough and last long enough, some of the gasoline breaks down and ignites all by itself. That is detonation.

So, the sooner you start the normal burn with the spark plug, the longer the gasoline around the edges of the combustion chamber is going to be subjected to the conditions that will allow deto to take place. It all has to do with time.

The only way I can think of that your timing could be too far advanced is if you sheared your timing key (the thing that orientes the flywheel on the crankshaft). You can check the timing with a timing light to see if anything is amiss.

What kind of gas are you running? with a 650 bigbore you need at least 100 octane. 91 can get you by but you may still get detonation issues. Be careful with the bigbore conversions even when done right they can easily start to pull themselves apart fairly easily.

wow, thank you for the response although i have nearly no idea what half of it means.

i will work on getting the timing checked, in the mean time i have put some 91octane in it.

thnx again

I definetely see a huge difference with the 91 octane fuel (that's the highest octane I could find, and afford), it only knocks when I have it wide open and going up hill.

I also upped my jetting, I think this helped as well.

Thanks for all the help!:)

I think at least part of the pinging with my bike was bad gas. It had been in there since last season. I drained it out and put it in my turbo 2.3L Mustang.....it REALLY didn't like that! I put some fresh Chevron 91 in the bike, but I haven't had time to ride.

I took the head cover and head off recently and looked at the top of the piston. It was COVERED with carbon soot (more towards the front of the piston)...in some places 1/8th of an inch thick.

I was thinking I was running lean with the jetting I had and this DETO problem, but with the carbon soot does this say that I am running rich...I AM SO CONFUSED!

whats your jetting? you were prob. having a "carbon knock".

52/158 jetting...thanks martinfan

thats good jetting. you about 3000 ft?

Not sure if you mentioned it, but check you air filter and valve clearances.

Leave the jetting alone for now and just try adding more octane booooster in steps.

If the noise goes away then you know you have a ignition problem.

If the mxture is rich or lean, either can fuel detonation.

If you dont have access to better fuel then maybe try fitting a hotcam stg 1 race cam - this'll help reduce the compression, and youll be riding that much faster and cooling the engine more! :thumbsup:

thats good jetting. you about 3000 ft?

Yeah 2,880 to be specific!


thats my jetting here at 4300 ft and its pretty close. a bit rich but starts cold easily.

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