“hi nick, thanks for your reply in my thread can i ask you a couple of questions?
if the pinging/rattle isn’t from fuel what causes it? what can i do to fix this problem? i never heard it with the stock piston and carb so assumed it had to do with running lean from pumper and hi-comp je piston
can you point me to good info on fitting a fan? i think i have found a 2nd ktm 640 adv setup,
i am looking into fluidyne radiators the pwr’s look awesome but my budget doesn’t
any advice or info is greatly appreciated i love my brp and would like to have it issue free.”
Detonation can be caused by lots of things. In short it’s the early spontaneous combustion of the fuel/air mix. Now it can be caused by too much ignition advance and other tuning related issues like hot spots in the cylinder head and combustion chamber designs that are prone to pre ignition but your engine is pretty stock so lets stick to what’s making yours ping/rattle/detonate.
If it runs fine at 95-100 degrees and only starts rattling at elevated temperature id suggest there are a few things going on.
1. The piston you now have is a flat top i’m assuming. the stock piston is dished. The dished piston for the same given compression ratio is better able to control the burn ( it’s actually a chemical reaction but lets not split hairs here) then the flat top. My experience is if i can run a slightly dished piston over a flat top, even if i lose a point or two of compression, i will have a better end result in terms of power and resistance to detonation. the flat top does little to centralize the combustion and usually the valve pockets are quite sharp that can become heat risers if we are running on the edge of detonation. These heat risers or hot sports act like glow plugs in a diesel and cause combustion to occur before the plug fires.
I can’t emphasize enough how damaging detonation is. The piston and rod are having to compress the ignited mixture, placing massive stresses on the rod/piston/crank. This actually compounds the hot spot problem so the problem gets worse.
It’s for this reason i don’t use a flat top if i can avoid it, in the safari engines where i knew we would get awful fuel we still ran 10.8:1 but did it with barrel machining and running a 1.0mm squish band to cool the outside of the piston.
2. Heat. If it’s OK at 95-100 why not just keep it there? most road bikes use thermo fans. A quick look on ebay will get you a little fan that is more waterproof than a fan computer. The KTM one will be fine. You will need a switch in the cooling system to turn it on, i’d tap a hole in the coolant elbow that runs out of the XR650 Head, that way of you stuff it there are no tears before bedtime on a replacement. You can always have a little boss welded onto it for added thread depth.
Below is a crap wiring diagram that will get you going. note i switch on the negative as it’s safer for shorts (fan just turns on, no melted wires) For power, the XR650 Runs the headlight and tail light on AC and the blinkers and horn on DC ( all XR’s do) as the rectification losses are too much for the pissant 70watt stator. So take power from the horn circuit, chances are it’s not being used anyway.
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