Help. Royal Star mileage and power loss. Your thoughts please.

The Story: I have an original '98 Royal Star with only 11K original miles. And have just experienced loss of power and gas mileage after servicing carbs and an oil change.

I tried to address most questions right out of the gate....so be patient with me.

1. Stored in air conditioned garage with Yamaha floor length cover.

2. Change oil (Rotella 15W-40), oil filter and gear lube yearly, and Sta-Bil for winter nap.

3. Dealer rejetted in 2002 with Dynojet 4148.001 and installed K&N filter at that time.

4. 2014 I cleaned carbs thoroughly:

a. Replaced all 4 valve seats, 4 needle valves and 4 o-rings.

b. Inspected and cleaned diaphrams, slides, jets and carb bodies.

c. Set float heights at 8.5mm with 0.1mm tolerance (carbs up side down) required only slight adjustments.

d. Per Dynojet: (No changes.) Fuel screws out 3.5 and circlip at middle (#3 position) on jet needle.

5. Synchronized #1-#4 carb vacuum while at 1050 rpm.

a. Original exhaust.

b. Premium gas.

c. Starts instantly - no choke required at any time.

6. Idles smooth, but since carb clean:

a. Bike has no guts from any speed.

b. Top speed 73 mph (down from 105 mph).

c. 20-25 mpg (down from 30 mpg).

d. At approximately 2,800 rpm bike begins to pull much better in 1st-4th, but not in overdrive.

e. No exhaust smoke. No fuel smell. No popping or backfires.

7. Checked all 4 spark plugs (DRP7EA-9) after ride and idle, and again after ride and throttle chop at 70 mph. All spark plugs are identical with:

a. Totally white insulator (no deposits) along its full length.

b. Light tan electrodes with ground electrode showing heat range coloration to 60% along its length toward shell rim.

c. Inside of the shell (and its rim) is black but NOT sooty.

d. All plugs show equal spark when grounded externally.

8. Fuel pump tested and working properly. I always shut off gas petcock when engine is off.

9. After removing gas tank and airbox, I reconnected the gas tank along side and started engine:

a. Started and idled well.

b. Attempted throttling caused engine bog and backfires at nearly all rpm's, which occurred with fuel pump feed and also with straight gravity feed from gas tank. (Apparently too lean without airbox restriction.)

10. See 4c. above. Pulled carbs (carefully leveled the quad) and checked float heights with external fuel level gauge and gravity fed gas source: (From center mark of diaphram cover to 25.4mm below - the Service Manual specification.)

a. #1 - #4 are all exactly at the Manual specification.

b. No apparent gas leaks anywhere, so I'm assuming the the valve jet(s) and valve seat(s) are sealing properly.

11. Oil level through site glass is 1/8" below upper mark when cold.

a. No smell of gas from the oil fill opening.

b. Could my crankcase oil volume be too high causing loss of power by increased friction?

Review 6a-e and 11b above.

Does this make sense...? Due to spark plug condition, bog and oil level, I'm thinking of:

1. Lowering each jet needle's circlip one notch to see if more fuel will improve performance and consequently mileage.

2. Adjusting the fuel mixture screws 4 turns out.

3. Draining engine oil to 1/8" above lower mark.

Any comments, questions or wisecracks are appreciated.

Otherwise, if I need to change internal carb components, then I might as well do it now since the carbs are off. Thanks to all.