I have this raping noise coming from what seems to be the top end of my engine (difficult to pin point as I'm going down the road) when the engine lugs a bit. It is not a regular raping but more of a random one. It seems to go away when I rev the engine to higher RPM's. I an even run the bike like a "RAPED APE" going up through the gears, and as long as the rpms are fast I don't hear any raping.
Could this noise be coming from weak or broken Valve springs?

 

Best thing to do with engine noises is to get a mechanics stethoscope and listen to each of the various parts. Listen at each of the rocker ends, 3 points on the pushrod covers, the lifter blocks, each of the cams, other side of the engine too, etc etc.

Each of the spots should sound the same as the other like spots. The sound is mostly like rotating metal parts.

Some guys use a screwdriver or a length of oil line, but the steth gives a clearer sound. Inexpensive at Princess Auto or Harbor Freight.

Check the cylinder compression.

These are noisey engines. Probably there is nothing wrong; it is just doing what it is supposed to do. But you do have to check it out for peace of mind.

 

is the only time you hear it when you lug the engine down while driving? could it be pinging because you are trying to run at too low of rpm's?

 

I have to say, I think my '73 XLCH is probably doing the same thing yours is. Kinda hard to pinpoint where its coming from. but it does seem to be from the front cylinder somewhere. Its intermittent. I was wondering if it was a valve tapping the piston or something.

 

Somebody is raping you cylinders? I wouldn't know if that is a crime but it sounds painful and it must be obvious to diagnose. I mean wouldn't it be easy to see someone raping your cylinder??

 

We always think these noises are something terrible, perhaps terminal. They rarely are. These are just ironhead engine noises. These are very noisey machines.

This is the obvious answer. If running at too low RPM the engine will make various clunking sounds as well as pinging. Both are bad for the engine. Gear down and you are good to go.

However, as i pointed out before, for your own peace of mind it is wise to check these things out. A mechanics stethoscope is a useful and inexpensive addition to your tool kit.

 

There are good ideas here already. Start by looking at any adjustments that could cause the noise....

 

Well ... I Did sleep with the bike but I did respect her in the morning...Really! LOL

 

I rebuilt mine from the crankpin up looking for a noise like that.... still had it when I was done.... until I changed the plugs predetonation

 

When I saw the OP describe a "raping noise coming from what seems to be the top end of my engine when the engine lugs a bit lug", my first thought was detonation.

You could try higher octane gas to see if that makes it any better. Retarding the timing will have an effect as well, if that's the source of the noise.

It's very difficult to diagnose noises over the internet...