Boost Ports and Cruise Control | Answers

Ask The Pro

By Karel Kramer, Photography by Motorcyclist Archives

Sorts of Ports
I am installing new pistons in a 1972 Yamaha CS5 electric-start 200. On the intake side of the pistons, there are two oblong ports that line up with two “troughs” cut into the cylinder wall. These troughs aren’t tied to the transfer ports, or anything else, so what are they for: to create turbulence in the air/fuel mixture, or to relieve pressure above/below the piston somehow? The ports in the new pistons are measurably smaller than the ports in the old pistons. I don’t know if enlarging the ports is beneficial or necessary. I’d like to know what they do first.

Jeff Poulton
Via E-mail

We hadn’t heard of those sorts of ports either, but we have heard of Harry Klemm (www.klemmvintage.com). Harry has heard of them, and says they are "boost ports." The air/fuel charge in the crankcase transfers from the underside of the piston to the top side through those ports. While the ports contribute to cylinder filling and total port area, perhaps the biggest advantage is having cool gas under the piston for additional cooling.

Are these ports worth enlarging? Possibly: They were bigger on the stock pistons, after all. We'd suggest running the motor first to see how it performs before doing any such "porting."


Cruise Control
Why is it that sport-touring bikes like the Honda ST1300, Kawasaki Concours14 and Yamaha FJR1300 do not have cruise control? Even though sport-tourers seek out twisty backroads, we still spend many hours on the interstate getting there. Are you aware of any aftermarket companies that make cruise control for these bikes?

One might suspect that lawyers are reluctant to let the companies they represent put cruise control on such powerful and sporty motorcycles, but the primary reason is probably cost. To wit, BMW offers cruise control on its K1600GT, which costs thousands more than the Japanese competition.

There are, however, aftermarket options. We don’t have any hands-on experience, but we hear that an Australian company called Motorcycle Cruise (www.mccruise.com) offers machine-specific cruise control that works very well. Unfortunately, those systems cost upwards of $1000.

Some owners have had luck with a unit from Audiovox that uses engine vacuum, and another from Julianos (a hot-rod parts company) that is all-electronic. Simpler mechanical options abound, from the $10.95 Crampbuster (www.crampbuster.com) to the $200 Brakeaway (www.brakeawayproducts.com) that engages with the push of a button and disengages when you squeeze the front brake lever.

Got Questions for Answers?
Send it to mcmail@sorc.com

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