This article originally appeared in the October 1990 Issue of Cycle. 


Building a furious 400 in three easy steps.

By Ken Vreeke

 From the outside it doesn't look any different. Keen eyes might pick up the aluminum exhaust canister, but that hardly betrays the screaming little beast lurking within the compact cases of our project CB-1. Roll on the throttle at 8000 rpm, however, and the engine responds instantly, accelerating hard enough to make you forget its only a 400, the high-pitched wail of the tiny inline-four screaming towards its 13500 rpm redline. This is a motorcycle that looks like the quintessential Q-ship, and runs like a serious finger-in-the-socket nickle rocket.

 Perhaps best of all, the modifications required to make the CB-1 capable of surprising FZR400s and bigger guns on backroads are both simple and far more affordable than you might expect. Affordability helps make the CB-1 a prime hop-up candidate. In addition to reasonably priced hot-rod components, Honda recently knocked $600 off the price of a 1990 CB-1 (from $4298 to $3698), and dropped the price of leftover 1989 models from $4063 to $3498. That makes the CB-1 $1200 to $1400 cheaper than Yamaha's FZR400 and that difference can buy a lot of hop-up parts.

 Second, the CB-1 engine--with its downdraft induction, compact 4-valve head, short stroke dimensions (55.0 by 42.0mm), and 13.500-rpm redline--has all the right stuff for more power. Liquid cooling, gear- driven camshafts, with shim-under-bucket valve tappets fro high rpm reliability, and a strong clutch make the CB-1 engine a natural to hop-up. In Japan, the same basic engine powers the domestic CB400RR, and that bike makes considerably more power.

 Last, the CB-1 chassis is a paragon of stability, able to handle more horsepower without modification. The stiff perimeter-style, tubular steel frame uses the engine as a stressed member, rolls on wide 17-inch wheels front and back, and steers through a stout, 41 mm fork. With no fairing, the CB-1 hangs its engine out in the open, tempting even the most passive shade-tree hacker.

 Hop-up projects are risky business. Unless you know what modifications an engine needs, you can spend a bundle just discovering what doesn't work. To avoid this, we enlisted the services of Ontario Moto Tech owner Kaz Yoshima, purveyor of 400-four power since the mid-seventies. Yoshima provided the pieces and performed the modifications. To more accurately measure the effects of his engine changes, we structured this project in three stages, ranging from a simple pipe-swap to cams, carbs, and head work. To keep costs down we limited modifications to the top-end only.

 We began by establishing a base line for stock performance, running a bone-stock CB-1 on the Kerker dyno and at the drag strip. The stocker pumped out 44.8 rear-wheel horsepower at 11,500 rpm, and posted a 13.28-second, 96.54-mph quarter-mile run. For perspective, a stock 49 state FZR400 makes 50.8 horsepower, and runs a quarter in 12.43 seconds at 103 mph.


Stage 1 involved minor jetting modification to the carburetors, and a straightforward replacement of the stock four-into-one exhaust system with an Ontario Moto Tech street pipe.

 Because the stock CB-1 is a cold-blooded beast, thanks to EPA-lean jetting, Yoshima replace the standard 2.51 mm-diameter needles with 2.49 mm needles to enrich the mixture at part-throttle openings. He then replaced the stock number-35 pilot jets with richer number-38 jets. Located next to the main jets and accessible through the float bowls, the pilot jets are active at all throttle settings and engine speeds, and can thus have a considerable effect on throttle response and power output.

 Unlike the double-wall stock pipe, the three-piece OMT street system uses single-skin headers, painted Kal- Gard black. That, plus a brushed aluminum canister, shaves 4 pounds--23 percent--off the stock system. To boost flow without greatly increasing noise, the pipe uses a two-stage muffler with a louvered core and perforated outer tube wrapped in replaceable fiberglass. Installation of the pipe is a cinch: The individual sections fit together perfectly, and the exhaust flanges line up directly with the easily accessible ports.

 To our delight, the OMT pipe is surprisingly quiet. We performed an unofficial SAE drive-by sound test, and our GenRad sound meter recorded noise emissions for the OMT pipe only 3 db greater than the stock exhaust system--well within the realm of good neighbor relations on the street.

 Next came a trip to the Kerker dyno, where the results of Stage 1 were recorded in rear-wheel horsepower. Below 4500 rpm, the stock engine has a slight advantage over Stage 1, but from there the project bike edges ahead, building to a three horsepower advantage from 8000 to 9500, spiking upward to best the stocker by 8 horsepower at 10,000 rpm, and peaking at 11,500 with 50.8 horsepower--a gain of 6 ponies or 12 percent.

 This dyno performance was backed up at the strip where the project bike ran a 12.70-second, 103.18-mph quarter mile--half a second quicker and 6.6 mph faster than stock. In 45-to-70-mph roll-on acceleration test, the project bike continued to impress, outrunning the stocker by over half a second in the top three gears. As these roll-ons indicate, the project bike's slight power loss below 4500 rpm is inconsequential: The CB-1's gearing is so short (6540 rpm at 60 mph in top gear) that you have to bog the engine down to a bout 40 mph in sixth to fell the loss. At any speed above that, the project bike stomps the stocker, exiting corners and launching away from traffic with new-found authority.

 Compared to the benchmark FZR400, the Stage 1 CB-1 makes more midrange power and absolutely flattens the Yamaha in roll-on acceleration. At the upper end of the performance spectrum, the project CB-1 is dead even in peak power output and quarter-mile speed compared to a 49-state FZR400. And the cost of this bolt-on performance? Including the OMT exhaust system ($320), and jetting ($38), our stage 1 CB-1 modifications totaled $358.


For Stage 2, Yoshima noodled around inside the cylinder head in search of more power, but found no obvious problems. The CB-1's intake ports are remarkably straight and clean for a stock head, with few obvious intrusions from the airbox bells to valves. Yoshima opted to concentrate on the valve seat area, first smoothing the sharp edges where factory machine work meets poor casting, then enlarging the valve seat area itself. Honda gives the CB-1 generous valve/seat purchase, and increasing seat diameter effectively increases port area at the head. It also allows the valves--protruding into the combustion chamber--to sink deeper into the head, decreasing combustion chamber obstructions. The tiny valves (22mm intake, 19.5mm exhaust) were then reshaped to match the seats.

 This kind of exacting head work is arduous and--at $560--expensive. In the case of the CB-1, no radical head work was necessary, but the results are nontheless easy to see on the dyno. Through low-end and midrange. Stage 1 and 2 power curves are essentially interchangeable, but from 9500 rpm up the Stage 2 engine pulls three horsepower ahead, peaking at 12,00 rpm with 54.4 horsepower--4 hp up on Stage 1, a whopping 10 horsepower stronger than the stocker.

 At the drag strip, the Stage 2 engine produced marginally quicker times than Stage 1--12.67 seconds versus 12.70--but top speed was down to 101 mph. Why the poky performance? We suspect the freer breathing CB-1 box was starving for air. the standard airbox draws air from just behind the steering head, an area that's almost sealed off from the atmosphere: Two cosmetic allow plates block air passages from the sides, the cylinder head and radiator cut off air from the bottom, and the tank forms the roof. The main air inlet comes from the small gap between the tank and the frame rails. Moreover, the engine superheats this trapped air from below, creating a hot box from which the engine must breath. To clear the air passage, we unbolted the two cosmetic side plates. With these plate sin place, the Stage 2 CB-1 would barely edge ahead of the stocker in side-by-side roll-on tests. After removing the plates, we again ran the CB-1s, and this time the Stage 2 bike opened a gap more indicative of its 10-horsepower advantage. Back at the drag strip with the plates removed, the Stage 2 CB-1 ran a 12.62-second quarter mile--just a tick quicker, and over 2 mph faster than before, but still no improvement over Stage 1 quarter- mile performance. With an additional 4 horsepower, the Stage 2 bike should have gone faster. With more time to find the missing speed, we would have continued to experiment with the CB-1's airbox, which we suspect is still too restrictive for the Stage 2 engine. As it was, we left Stage 2 puzzled by the drag strip numbers, but impressed by the fact that this engine could be coaxed to produce 54 horsepower with few modifications, and remain just as well mannered as the stocker.


For the third and final stage, we wanted maximum horsepower without busting the bank, so Yoshima worked from the head gasket up. He replaced the stock 32mm Keihin carbs with 32mm Keihin flat-slide CR downdraft racing pieces, and swapped the street-quiet muffler with a louder, less restrictive OMT racing canister. The stock camshafts were also supplanted by hotter lobes.

 The CB-1's stock camshafts provide conservative valve timing, and any hope of ramming a bigger fuel charge through the cylinder began with new cams. Yoshima swapped the U.S.-spec cams with those found in the Japanese-market CB400RR. The RR cams ($360) have much steeper ramps for increases valve acceleration, and offer 10 degrees more exhaust duration, slightly more intake lift and considerably more overlap--that period when intake and exhaust valves are open simultaneously, allowing negative pressure waves from the exhaust side to help pull fresh intake charge into the cylinder.

 With the RR cams, the CB-1's valves open sooner, quicker and stay open longer, but valve timing is still on the conservative side. Stock FZR400 cams have considerably more valve lift. In Japan, where 400s form the popular Formula 3 class, more radical cams are available for the CB-1 engine through HRC, but these high- profile cams require pistons with more valve clearance, stronger valves, stiffer springs, etc. The RR cams installed in our CB-1 are designed to work with the stock valve gear and pistons. Because they are gear- driven and easily accessible, these cams drop into place quickly and easily: An experienced mechanic can perform the swap in about an hour.

 Installing the race canister took advantage of the increased exhaust flow, but made the CB-1 too loud for the street. On the intake side, the flat-slide Keihins bolted easily into place once the stock carbs and airbox were removed. The advantage of the CR carb is easy to see: While the stock CV carb has a butterfly plate and pivot shaft in the middle of the venturi, the CR's slide permits a smooth venturi free of obstruction for maximum throat and intake flow. Equipped with an accelerator pump the princely ($800) CRs feature a unique two-slide design: the main slide is supported by four rollers for smooth throttle action, while a second, parallel free-plate located between the main slide and cylinder head improves sealing for more accurate fuel metering and throttle closure. The result is a smooth action downdraft flat-slide that provides a steady idle. The parallel design is loud, however; the secondary plate banging against the main slide and carb body sounds a bit like the rattling of dry-clutch plates. Our CRs, run at the racetrack with velocity stacks and no filters, also created intake honk that rivaled the exhaust noise. These carbs are available with individual K&N filters for street use, but intake noise will undoubtedly remain high.

 Back on the dyno, the Keihins ran too rich to accept full throttle at low engine speeds; Our CRs were the first set fitted to a CB-1, and Keihin is still experimenting with jetting. Forced to run part throttle below 8500 rpm, the Stage 3 engine output was erratic, spiking up and down, but never logging a full power reading. From 9000 rpm to 11,500, where the engine pulled full-throttle, the Stage 3 engine ran roughly even with the Stage 2. At 12,000 rpm, the Stage 3 power curve began to spike upward, hitting 56.2 and climbing to 58.6 at 12,500 -- 4 horsepower up from Stage 2. and 14 stronger than stock.

 But that's as far as our Stage 3 engine could go: Just past 12,500 rpm, the electric rev-limiter kicked in, effectively snuffing the light in two cylinders well below the bike's 13,500-rpm redline, and further below 14,300 rpm where the rev limiter is supposed to kick in. While the CB-1's electronic tachometer indeed reads 14,300 when the rev-limiter comes in, actual engine speed--measured on the Kerker dyno and later calculated by us during top-speed testing--is significantly lower: 12,727 rpm, to be exact. Because the rev- limiter is controlled by computer chips sealed inside the impenetrable black box, we couldn't simply adjust or disconnect the limiter. Our second, stone-stock CB-1 had a premature rev-limiter as well, a discovery that surprised engineers at American Honda as much as it surprised us, and triggering a flurry of telexes to Japan requesting an explanation and a replacement black box.

 Unfortunately for our project CB-1, the faulty rev-limiter ended the power party just when the engine seemed to be gathering steam for a peak rush. At the drag strip, the bike peeled off a 12.31-second, 105.04- mph run--a full second and 9 mph faster than stock. On a miserably hot day at Willow Springs International Raceway, our stock CB-1 crept up to a top speed of 106 mph on the front straight; the Stage 3 bike accelerated smack into its rev-limiter in top gear--116 mph on the radar gun--with barely half the straightaway behind it.

 What lies on the far side of the project bike's rev-limiter remains a mystery. Though we suspect the Stage 3 bike will crank out at least 60 horsepower and top out in the area of 130 mph, confirmation will have to wait for a black box with a correct rev-limiter.

Despite the problems, we are impressed with the bike's durability--even the stock clutch, repeatedly hammered at the drag strip during the course of this project, never once slipped or groaned--and we are pleased with the CB-1's punch. Stage 1 was the simplest, most effective, and least expensive modification, producing performance increases that brought the CB-1 on par with the FZR400 top end, while further enhancing the Honda's low-end and midrange advantage.

 In pursuit of power, the climb always gets steeper and more costly as you go: Though more extensive and expensive, Yoshima's Stage 2 cylinder head work also produced impressive gains at reasonable cost, and, like Stage 1 modifications, enhanced rather than compromised the CB-1's rider-friendly character on the street. At Stage 3, the cost of a single horsepower rises sharply--too high for all but serious racers. Street riders looking to go beyond Stage 2, however, should consider the RR cams. Even with stock carbs and the Ontario Moto Tech street pipe, these cams can further boost performance for reasonable cost.

 In the end, the project CB-1 has one overwhelming advantage: With the exception of the exhaust pipe, the bike retains its stock veneer, a sleeper in street duds. Your average street squid can't tell the difference between a standard version and the hot rod. And nothing ends a skirmish more quickly and conclusively than the element of surprise.

Ontario Moto Tech
P.O. Box 79
Reseda, CA 91337
(818) 342-8456
Parts List: