Making the BMC Mightier
After riding over to Bennett’s Performance in Signal Hill, California, our BMC’s 100-inch RevTech was in stock trim sans the modified air intake that sports a wild-looking skull cover.
Once the carburetor and exhaust system were removed, Eric Bennett removed the non-adjustable push rods by cutting them with bolt cutters. Note the covers were pushed out of the way since they were to be reused.
With the cam position sensor out of the way, Bennett used a socket and ratchet to remove the rotor. From there, a Trock puller was used to remove the gear cover.
A pair of magnetic lifter holders were used to secure the hydraulic lifters in a position above the cam lobes to free up the cam for removal.
Bennett measured the length of the old cam and compared it to the length of the new cam to determine whether any shimming would be needed. The measurements between the two indicated a difference of only 0.002 inch, so shimming would probably not be necessary.
We chose a Headquarters HQ-0026 Thunder Plus cam. This little bumpstick provides a 0.600-inch valve lift.
Bennett applied some assembly lube to the Headquarters cam before he slid it into position.
With the lifters still out of the way, Bennett snugged down the gear cover before…
…he checked the end play of the new cam. He used this homemade tool, which threads into the end of the cam and allows him to feel how much the cam moves in and out, since he has done this enough to have the touch.
Mikuni supplies everything needed to install the 45mm flatslide carb, right down to the Prolong precision oil to lubricate the throttle cables.
Just one look and it’s obvious the new 45mm Mikuni on the right will pass far more air than its 42mm sibling.
Bennett used a specially designed hex wrench to remove the allen bolts that secured the intake manifold to the heads. Anyone who has ever tried removing these bolts knows just how hard it can be with an ordinary key.
Next, the pushrods were slipped into the pushrod tubes, the upper and lower O-rings were installed, and the pushrods were set in place.
With the tranny in neutral, Bennett bumped the starter to rotate the engine until the front intake pushrods were all the way up. That left the rear intake pushrod all the way down and ready to be adjusted.
No expensive tools needed here, just a simple wooden clothespin holding the pushrod tube up and out of the way as Bennett made the adjustment. To do this properly, follow the instructions included in your particular pushrod set.
The same procedure was followed for the remaining three pushrods, after allowing bleed-down time for the lifters.
Back to cool tools — a Sputhe pushrod cover installation tool made quick work of buttoning things up.
Mikuni’s intake manifold replaced the stock unit.
After the breather assemblies and mounting brackets were in place, Bennett installed the new 45mm carb on the mounting flange and connected the breather hoses as he supported the air cleaner.
The RevTech exhaust pipe was bolted to the heads, then the collector bracket bolted in place…
…before the collector was slipped onto the head pipe.
Lastly, the adjustable end cap and discs were secured to the outlet of the collector. By adding or removing discs, or opening or closing the end cap, the characteristics of the pipe can be adjusted to allow for maximum performance.
We dropped by Lifestyle Cycles in Anaheim, California, and put the bike on its dyno. All the parts worked quite well together. We realized a 7.6lb-ft gain in torque and a 5.5 gain in horsepower across the entire revolutions per minute range, which is what we were looking for.
One of the bikes that rotates from staffer to staffer at our office is an ’02 BMC 918ST. The 918 came to us equipped with a potent and hard running, stock 100ci RevTech engine, but after piling up about 10,000 miles, we decided it was time to open the motor and shake things up a bit. The goal for the changes we had in mind was additional torque, especially in the low-to-mid rpm range.
Components for this hop-up would consist of the following: a RevTech 2-into-1 tunable exhaust system designed to boost the low-end torque, a Headquarters HQ-0026 camshaft, Andrews adjustable pushrods, and a Mikuni HRS 45 carburetor designed to supply the motor with all the fuel it might need.
