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Discussion Starter · #1 ·
I have a nice hollow knock in my #4 cylinder. It only does it briefly after you let up on the gas, and sometimes when I am taking off. Anyone got any ideas????

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Liquid, no I don't know what is causing the problem, however, I am curious to learn how you isolated the problem to the #4 cylinder?

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If it is a pronounced knock, and it gets worse when the engine gets warmer, it is probably caused by bearing clearances being too loose (read worn out). I wouldn't rule out fuel quality (too low octane) or bad spark plug/wire/coil or plugged jet, etc.. but it sounds to me like a bad bearing.

Have it checked out by a professional though.

Good luck

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Discussion Starter · #4 ·
I isolated it by listening.... #1-#3 do not knock, and it is a rather mild knock. I have also had a couple of other people ask me at stops why they can hear it on the right side at idle, but not the left. As for the bearing issue, is there 4 bearings... 1/cylinder? I visualy checked the spark plug, and it looks okay... I do have a shitty #4 wire coming from the coil. (but wouldn't it be a lot worse if it wasn't fireing, kinda like not want to run well at all?) I just bought two new coils off ebay for $34 not a bad deal if you ask me since local shops want about $175 a piece, which I think is a little steep. I guess I'll put the coils on anyways, and I could always try new plugs.

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Yeah, there are four sets of rod bearings in a four cylinder engine, If for instance, a bike was run low on oil (not saying you, just making an example
) and the #4 cylinder is the last to get oil, the bearing is starved and wears thinner than the others. This causes excessive clearance on the big end of the rod and a subsequent knock when the piston is thrust down against the crank. Oil gets thinner (lower viscosity) when hot so that is why it gets worse with heat, because the oil can no longer cushion the impact of the rod and the noise you hear is the two meeting and clunking together.

Conversely, forged pistons generally do not have an Iron expansion ring inside so they "slap" the cylinder wall when cold, but go away when warmed up. In this case, the noise is caused by the piston "rocking" in it's bore because the bore (cylinder) has enlarged with wear. Cast aluminum pistons usually have an iron ring cast in to them to limit the amount of thermal expansion.

One more thing to check is cam bearing clearance. I had a knock in my old nighthawk and I got to looking at it and the cam bearings were wore out (and the cam was almost flat) and it was making a racket.

Plugs, coils, fuel and wires are cheap remedies that anyone can do themselves. An engine teardown is necessary to inspect the bearings. Good luck and Hope I helped you!

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Discussion Starter · #7 ·
Upon futher inspection last night, the sound, sounds like it is coming from the top end. Maybe a valve? I know it has overhead cams, so are we talking about cam bearings correct? I also noticed that it doesn't do it all the time wether hot or cold, it is really weird I am leaning towards the ignition more than a internal problem. But I'll throw the new plugs in tonight, and I should have the coils by monday. Thanks for you help guys!


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