How The Yamaha YZ400F Brought Four-Strokes Back To Motocross

When Four-Strokes Ruled Motocross

The first motocross world champions rode four-strokes. Bill Nilsson won the first 500 cc world title in 1957 on an AJS he had converted himself, and big British singles stayed competitive in the premier class into the mid-1960s. Jeff Smith won the 500 world title on a BSA four-stroke in 1964 and 1965. It would be 1993 before a four-stroke won the premier world title again.

Why Four-Strokes Lost To Two-Strokes

A four-stroke engine fires once every two turns of the crankshaft. A two-stroke fires every turn. The four-stroke also has to carry and spin a camshaft, valves and springs. At the same rpm, a two-stroke of the same size gets twice as many power strokes from a lighter, simpler engine. Run the two side by side in the figure below.

The results showed up on the track. From 1966 to 1968 Paul Friedrichs won three straight 500 world titles on a CZ two-stroke of only about 360 to 380 cc, beating 500 cc four-strokes on power to weight. By 1973 even Honda, the company built on four-strokes, sold a two-stroke motocrosser, the CR250M Elsinore. Our guides on how a two-stroke works and how a four-stroke works show both cycles step by step.

Europe Keeps The Four-Stroke Alive

Four-strokes never fully left the woods or Europe. KTM's LC4, a liquid-cooled four-stroke single, arrived in 1987, and liquid cooling made a competitive off-road four-stroke possible again. Husaberg was founded around 1988 to 1989 by former Husqvarna engineers to build an ultra-light four-stroke. Jacky Martens won the 1993 500 world title on a Husqvarna four-stroke, and Joel Smets won it on a Husaberg in 1995, then again in 1997 and 1998.

The aftermarket saw it coming too. Carson Brown told the Gypsy Tales podcast that his family's company, BBR, built aluminum frames for Honda's XR400 trail engine using 1996 CR250 geometry, chasing a race-ready four-stroke before the factories delivered one.

The AMA Rule That Changed The Math

The opening came from the rule book. The AMA allowed four-strokes up to 550 cc to race the 250 class, and four-strokes up to 250 cc in the 125 class. It also gave a one-season exemption from the production rule, so a manufacturer could race a prototype before it was on sale. Cycle World reported the 550 cc limit in its 1997 YZ400F preview. Sources disagree on whether the rule dates from 1996 or 1997, so we do not pin a year on it.

The logic was simple. A four-stroke gets half as many power strokes per revolution, so roughly double the displacement should roughly even out the power. Davey Coombs, who sometimes sat in on AMA advisory board meetings for his father, told Gypsy Tales that when the board heard "four-stroke" in the mid-1990s they pictured enduro bikes like Husabergs and big XRs. Yamaha said it needed 400 cc, the rule passed, and he says Yamaha later came back saying it was thinking 450.

Yamaha YZ400F Specs

Yamaha's answer was a liquid-cooled single with a compact five-valve, twin-cam head. Cycle World's 1997 preview says the head was developed with help from the engineers behind Yamaha's YZF sport bikes. The prototype details below come from that preview; the production figures come from Motorcycle.com's YZ400F spec sheet.

Prototype and production engines differed. Cycle World lists a 95.0 mm bore for the prototype, while the production spec sheet lists 92 mm. You will see a 12.5:1 compression ratio quoted on spec sites, but neither of the sources above states it, so we leave it out.

Las Vegas 1997: The First Four-Stroke Supercross Win

Doug Henry had been leading the 1997 supercross championship before a crash at Houston with Jimmy Button left him out of the title fight with a broken wrist, Coombs recalls. At the Las Vegas finale Henry raced Yamaha's four-stroke prototype. He took the holeshot and led every lap, the first AMA Supercross win on a four-stroke, according to Cycle News. Coombs says the track was dry and treated, slick hard-pack that suited a four-stroke's smooth pull.

Coombs's view, years later, is blunt. "At that moment we should have stopped," he told Gypsy Tales, meaning the displacement number should have been changed as soon as the four-stroke was already winning. Coombs remembers the Vegas bike as a 426; Cycle News calls it the YZM400 prototype, so treat the exact displacement as unsettled.

1998: The First Four-Stroke Motocross Title

The production YZ400F went on sale for 1998, the first modern production four-stroke that could run with the 250 two-strokes. Doug Henry won the 1998 AMA 250 National Motocross Championship on it, the first AMA motocross title on a four-stroke.

Everyone Follows

• 2000 and 2001: Yamaha's YZ426F, then the YZ250F, the first 250 four-stroke motocrosser, so four-strokes covered both pro classes.
• 2002: Honda's CRF450R, with a Unicam head and an aluminum frame. The 450 became the premier engine.
• Late 2003: Ricky Carmichael says Honda flew a senior engineer from Japan to discuss his CR250 two-stroke. The engineer told him they could not give the two-stroke the low-rpm pickup he wanted and he should race the four-stroke. Carmichael says he was about a second and a half a lap faster on the 450 that afternoon.
• 2004: Carmichael's perfect outdoor season on the CRF450R, the first perfect season on a four-stroke.
• 2005: Carmichael won AMA Supercross on a Suzuki RM250, the last premier title on a two-stroke.

By about 2006 the AMA had settled on 450 cc four-strokes against 250 two-strokes in the premier class, and 250 four-strokes against 125 two-strokes in the small class. Emissions rules pushed in the same direction off the track. California adopted off-road emissions rules in 1994, and the EPA's off-highway motorcycle standard of 2.0 g/km of hydrocarbons plus NOx phased in during 2006 and 2007, with an exemption for competition bikes. Two-strokes lose some fresh charge out the exhaust, which is why those rules hit them hardest.

Did They Get The Math Wrong?

Coombs thinks so. "We got the math wrong," he said on Gypsy Tales, meaning the industry in general. He argues the small class should have been something like 150 and 300 cc rather than 250 four-strokes against 125 two-strokes, and he links the result to louder bikes at local tracks and more pressure on riding areas. He also believes two-strokes would have faced environmental pressure anyway, just not as severe.

There is another side. Coombs relays that Ryan Dungey said he probably would not have won races on two-strokes, and four-strokes made strong low-rpm torque and traction available to every rider. Clutch technique changed with them: Gary Bailey says riders learned to slip the clutch constantly on two-stroke 125s and leaned on it less once four-stroke torque arrived.

Two-Strokes Never Died

Two-strokes were not banned. They were pushed out of the premier class by the displacement formula and out of US trail sales by emissions rules, and they kept racing in two-stroke classes and amateur 125 racing. KTM's 250 and 300 EXC TPI, the 2018 models, were the first production fuel-injected two-stroke enduros, injecting into the transfer ports to cut fuel lost out the exhaust. In 2023 KTM's SX two-strokes switched to throttle body injection. Our 2-stroke vs 4-stroke vs electric guide covers which to buy today.

Owning A Four-Stroke Today

A modern four-stroke trades the two-stroke's frequent top ends for valve clearance checks and more attention to engine oil and oil filters. The intervals are in your owner's manual and in our 4-stroke dirt bike maintenance guide, and the valve clearance check walks through the job. MotoMind tracks those tasks by engine hours and shows each one as upcoming, due or overdue in the app, so you can see what is coming due before a race weekend.

FAQ

What was the first modern four-stroke motocross bike?

The 1998 Yamaha YZ400F, the first modern production four-stroke that could run with 250 two-strokes. Earlier four-strokes from Husqvarna, Husaberg and KTM won in Europe, but the YZ400F brought them back to mainstream motocross.

What are the YZ400F specs?

A liquid-cooled 399 cc single with a 92 x 60.1 mm bore and stroke, a five-valve head and two overhead camshafts. The prototype had a dry sump that kept most of its oil in the frame backbone and a close-ratio five-speed.

Who won the first supercross on a four-stroke?

Doug Henry, at the 1997 Las Vegas Supercross finale on Yamaha’s four-stroke prototype. He took the holeshot and led every lap.

Who won the first motocross championship on a four-stroke?

Doug Henry won the 1998 AMA 250 National Motocross Championship on the production Yamaha YZ400F.

Why does a 450 four-stroke race against a 250 two-stroke?

A four-stroke fires half as often, so the AMA let four-strokes run up to about double the displacement of the two-strokes. The class formula settled at 450 vs 250 around 2006.

What was the last two-stroke to win a premier AMA title?

Ricky Carmichael’s Suzuki RM250 won the 2005 AMA Supercross championship, the last premier title on a two-stroke.

Did the AMA ban two-strokes?

No. The displacement formula pushed them out of the premier class and emissions rules pushed them out of US trail sales, but they still race and KTM sells fuel-injected two-strokes.

When did the YZ400F become the YZ426F?

Yamaha followed the YZ400F with the YZ426F for 2000, then added the YZ250F in 2001.