2007 Suzuki Boulevard C90 when the engine is revved the speedometer gauge acts like a tac

SOURCE: where is the air filter on a 2007 c90 1500 suzuki boulevard?

The gas tank is under the seat. To get to the air filter, you have to remove the speedometer chrome cover and the upper side covers.
I can do mine in about 10 minutes. I also have a '07 with 17000 miles on it now.

SOURCE: Boulevard C90 Having trouble w/keeping battery charged.

to determine that, you will need a voltmeter and a battery charger. first, put battery on level surface and check to see if the acid is at a good level, if not, you will have to get some at a parts store and refill it to level indicated on the side. fully charge the battery over night with the charger taking the battery out first. the next day, disconnect charger and write down voltage obtained from voltmeter. next, install battery on motorcycle and try to start it. if it's starts, use voltmeter to check voltage at battery while running engine, write it down. let it run a few minutes like 20 and shut it off. check voltage, if it droped, your alternator is not charging. if it's the same as when you first tested it at 12 volts, you might be good only if you had to refill it, if not, the battery is defective and must be replaced.

SOURCE: Viewing oil indicator Suzuki Boulevard C90 (2008)

I used a mirror.... leaned it at a slight angle against the garage wall next to the bike and viewed the level window perfectly while sitting on the bike.