Shop Talk | Tech Questions & Answers

See No Evil

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Tire Tricks
Why do Yamahas have a bigger, 16-inch tire in the front and a 15-inch one in the back, whereas most other motorcycles have the same sizes, both front and rear?

Via email

_Like everything else in motorcycling, tire sizes tend to follow certain fads. At one time, the majority of motorcycles had the same diameter wheels fitted to both ends. That eventually changed, and for many years the most commonly used configuration was a 19-inch front combined with an 18-inch rear. As tire technology advanced and tastes changed, sizes went up and down like the zipper on a hooker’s dress, so the only hard and fast rule regarding tire sizes is that there’s no hard and fast rule. Currently there’s probably just as many bikes running on dissimilar-sized tires as there are on tires of the same diameter. _

Typically a larger front gives a lighter feel and helps the bike steer over obstacles (which is why off-road bikes use a larger narrow front tire) while providing a bit of high speed stability, though that is subject to the bike’s overall steering geometry.  

Speaking broadly, the most likely explanation is that during testing the bike just worked better with the larger front wheel. Assuming the bike was a cruiser of a given frame geometry, the larger wheel most likely made the bike easier to turn, and provided more stability. It may also have been done for styling reasons, or to allow clearance for the brake components or just because they got a hell of a deal on 16-inch front wheels.

**Pull the Plug
What's the best way to remove the mixture screw EPA plug without ruining the mixture screw beneath it? **

Dave C.
Via email

_To remove the emissions plug, I use either of two methods. If you’re lucky you can dislodge the bugger by using a spring-loaded center punch—when the punch fires, the plug will tip or deform slightly, and then you can use the punch to work one edge of the plug until the silly thing pops out. If that doesn’t work, I just wrap a lot of tape around a small drill bit (1⁄8 inch or smaller works fine), maybe an 1⁄8 of an inch or less from the tip. The tape acts as a stop to prevent the bit from plunging through the plug and into the screw when it breaks through. BTW it’s better to use too much tape here than not enough, and it’s better to start out close to the tip of the bit. If the bit doesn’t cut through you can always move the tape up the bit. Once the hole is cut you can thread a small sheet metal screw into the cap then pull it out with a vise grip. Occasionally the drill bit will even walk the cap out for you and save you the trouble of pulling it. _

**Cam Conundrum **

**I’m writing to ask about Screamin’ Eagle cams. I have a 2010 Harley Street Glide with a D&D slip-on muffler for the 2-in-to 1 exhaust, and an Arlen Ness Big Sucker air filter. I want something that will get me going good in 1st and 2nd gears, but I don’t want to lose power at higher speeds. My service adviser says the best bolt-in cam to use is the 255 all around. He knows that I want my bike to have better sound too! (I’m having trouble finding a pipe that produces that deep thunder like my Heritage has.) So he thinks the 204 cam might be better; the power range is a little weaker but he thinks the 204 produces a better sound. What do you suggest? Is there that big of a difference between the 255 and 204? Please help, ‘cause I know you know what you’re talking about!  **

Mark G.
Via email

_I’ve never selected a cam based on sound, and despite your accolades, I’m not all that conversant with H-D cams and what works best (or in this case will provide what you’re looking for in an aural experience), so the only thing I can tell you is that the 255 grind is used to provide big bottom-end power, and works best with 103- and 110-inch motors with 10–1 compression ratios. It tends to run out of breath around 5000 rpm, but pulls like a mule going up a ladder under that. On the other hand, the 204 produces strong midrange-to-top end power. It peaks at 5800 rpm, but bolts right into the stock motor. _

_So which is best? If you want a big bang with monster bottom end, build a high compression, big-inch motor and go with the 255 grind. I promise you it’ll make plenty of noise and run like Jack the Bear. _ _On the other hand, if you’ll be happy with a hot rod touring motor based on a stock power plant, the 204 is the way to go. As to which sounds the best, I’m going to go out on a limb and say that you’ll probably be happy with the 204, but you’ll be thrilled with the 255, provided you increase the size of the engine and compression to suit. _