Souping the 650 Seca Part Ii

Souping the 650 Seca Part II

A new camshaft grind solves our CV-carburetor problems.

John Ulrich

As seen in the March Cycle World, this project started out simply enough: make a 1982 Yamaha Seca 650 fast enough to run with the newer 650 Fours. Getting good dragstrip times was easy enough, but retaining streetable power throughout the power band proved more difficult. Before we were done, we had replaced the Yamaha's stock CV carbs with slidethrottle 28mm Keihins off a 1978 CB750F, which finally eliminated serious hesitation and flat spots below 6000 rpm when aftermarket exhaust systems were used.

While the replacement carbs worked well enough, they added to the project's cost and made the twist grip significantly harder to turn. We wondered what it would take to make the Seca fly without changing the carbs.

But first, to recap, we had already added 68mm Yoshimura pressure-cast pistons to increase the Seca’s displacement to 761cc; done a racing valve job;

installed Yoshimura heavy-duty valve springs and Yoshimura racing camshafts; and tried Yoshimura, Maxi-X and stock exhaust systems.

With those parts, and with the Keihin carbs fitted, the Seca turned 1 1.98 sec. and 110.15 mph with the Maxi-X exhaust; 11.99 sec. and 110.97 mph with the Yoshimura exhaust; and 12.18 sec. and 108.69 mph with the stock exhaust.

The Keihin carbs added cost, and the cams, designed for a Seca 750, didn't have a tach drive gear, forcing us to use an electronic tach, running the tab up higher still.

Yoshimura has since introduced new camshafts designed for use in both Seca 750s and Seca 650s. The new cams come with a tach drive gear, and haye less duration.

Less duration normally means less peak power and better low-speed power. In this case, the original Yoshimura camshafts may have had more duration than the bike could handle. The first set

of cams had 257° of duration on intake, and 255° exhaust, with 9.0 and 8.7 mm of lift, respectively. The new cams both have 245° of duration and 9.0mm of lift. All the timing points are measured at l mm lift.

With the new cams installed, the Seca ran perfectly with the Yoshimura exhaust system and the stock Hitachi CV carbs. In the quarter-mile it ran 11.96 sec. at 110.97 mph, our best time yet. This comes with a savings of $340 in project cost, because the $200 carburetors and $140 tachometer and drive plug aren't needed.

So. there is such a thing as too much of a good thing. We had more cam than we needed. How satisfying it was, when it was made right. It makes a person want to go out and . . . put in a hotter cam . . . put in big pistons ...