Grinding while coasting

Ok so I have a 1980 maxim 650. That I recently modified quite a bit lowered about 3-4 inches in the rear. So now when riding and I let off the throttle it makes a grinding sound and it seems to be worse in lower gears for instance if I am in 5th it will grind from 75mph down to 50mph 4th grinds until you get below 30 3rd grinds down to about 20 2nd down to around 10the and 1stI barely ever downfall grind unless your below about 2mph it doesn't do it when the bike is in neutral but it does still grind even when the clutch is disengaged I haven't opened it up yet mainly because i have no idea where to start also it never did this before the bike was modified so soothing happened during the build process the only thing that was done to the motor is I removed all the covers for paint and powdercoating any input would extremely helpful

 

Caliper rubbing on disc?

 

I don't think that's it it feels as though its coming from under my seat and you would think if the caliper is rubbing it would do it constantly. I did forget to mention that I put a suicide shifter on the bike but I don't think that the extra weight would cause it to do this.

 

Haven't done this sort of mod, but 4 inches sounds like a big change. Maybe something is rubbing, or your shaft drive is binding up? Based on your symptoms I'd guess it's drive-line related.

 

The U-joint is very likely operating beyond the limit of it's normal range of travel, or just plain worn out, or both.

 

Mine does the same thing in first. Its my u-joint causing it. Does the grinding increase and decrease in a rhythm?

 

A 3 or 4 inch drop will put the driveshaft at too harsh of an angle for sure. The stock rear shocks had 3" of travel; you're killing your u-joint.

 

One "Modder" dropped the rear until the tire hit that frame loop - a 5 inch drop, then welded the swingarm solid, then rode the bike thousands of miles (don't remember who).

The "grinding" changes gear-to-gear, which puts it on the clutch side of the transmission. (my opinion).

 

Ok so ideally it shouldn't be the u-joint because I had tweaked the motor mounts to get a better angle for the drive shaft not to mention that a ujoint problem should cause a harsh vibration at highway speed not strictly while coasting in gear. That's my opinion anyway