Is Honda Building An EFI-Controlled, Direct Injected Two-Stroke?

Patent Applications, Premix, & Plenty Of Rumors!

Social media sites, message boards, and tailgates everywhere have been buzzing in the last few days about the news that Honda has filed a patent application for an all-new, direct-injected two-stroke engine. Some die-hards seem sure that this is the second coming of two-strokes, while purists are already complaining about having to plug a laptop into their two-stroke in order to tune it. But before you go clearing out a spot in your garage for a revolutionary new machine, let’s take a look at some of the facts, and see if we can’t answer for ourselves what this technology may mean to the off-road community.

First, full disclosure: Dirt Rider helped contribute to some of the misplaced enthusiasm surrounding this engine by way of an April Fool's post that we tossed up on April 1st of 2014. If you revisit this post at, you'll clearly see that we tried to make the description of this bike so outlandish that only the most naive readers would believe it was true. Unfortunately (albeit somewhat amusingly), a huge number of enthusiasts convinced themselves that a sub 200-pound, EFI-controlled two-stroke was on the way for less than $5,000, and we ended up with a lot of good-natured hate mail on April 2nd of that year! 99% of those who saw the post laughed it off, but the ruse left everyone thinking about the possibility of an updated two-stroke from one of the Japanese manufacturers. Oh, and we also received one seemingly legitimate death threat—that was fun!

As you may recall, most of the initial justifications for manufacturers to turn to four-strokes (and, eventually, electronic fuel injection) were environmental (although not related to sound—remember, two-strokes get less noise complaints than thumpers because the sound does not travel as far). Anyhow, part of the emissions allegations against two-strokes is that back when these bikes were regulated off-road, the oils used were primarily petroleum based lubricants with extremely rich oil ratios, and the gasoline back then was simply not as clean. Even with carburetors, today’s two-strokes are much less “emissions dirty” than they used to be, mainly due to decreased particulate matter (carbon) and much leaner oil ratios. In other words, there is no reason that current two-strokes can’t be made to comply with EPA and National Parks regulations, just like snowmobile and marine two-strokes have. This isn’t just a pipe dream; various marine and diesel engines have the blessing of the EPA and the air board, and a ‘Certificate Of Compliance’ would say a lot about a two-stroke motorcycle!

On the tech front, electronic fuel injection would be a fun innovation to see in production on a major two-stroke, primarily because carburetors do not provide a reliable shut-off and unburned fuel is always present. EFI will ostensibly minimize many of the pollution complaints about carbureted two-strokes, but it won’t eliminate the unburned fuel passing through the cylinder and out the exhaust before the compression and ignition occurs, which is the biggest allegation against two-strokes. To significantly clean two-strokes up will require direct injection, which would occur after the exhaust port is closed. DI could allow the addition of an oil ring that would lubricate the motor like a four-stroke, since the gas would always be above the piston. Translation? No more need for premix, no unburned gas emissions and possibly a lower compression ratio for less overall stress on the engine. What’s not to like about that?

As you may have already seen, Beta’s new two-stroke RR models feature an oil-injection system. While this is a neat innovation to see, it’s NOT a fuel injection system; there’s still a Keihin PXK 36mm carburetor that feeds the cylinder, and you still have to tune the jetting of this bike, but you no longer have to mix oil into the fuel. The Electronic Oil Injection System basically only takes care of lubrication and measuring how much oil is truly required at various RPM. Benefits include the fact that riders no longer have to pre-mix fuel, as well as the claim that 50% of the inefficient combustion/unburned oil from the exhaust is reduced.

On the Japanese front, Yamaha turned some heads earlier this summer with a very unexpected new model announcement, one that could signal a coming resurgence in two-stroke technology. The new bike is called the YZ250X, and the short version is that it is an off-road variation of the Yamaha’s YZ250 two-stroke, much like the YZ250FX is an off-road twist of the MX-specific thumper. In terms of differences from the motocross model, the YZ250X is fairly simple; I can already hear the critics lamenting Yamaha’s decision not to include electric start, a heavyweight flywheel, or handguards (actually, I agree pretty strongly with that last point). But if the YZ250FX four-stroke taught us anything, it’s that minimal modifications to an already great motocross bike make for a great off-road bike, and I think the YZ250X will be a great step in the right direction. That said, it will be equipped with a boring old carburetor—no fancy computer-controlled tuning system here (yet).

It’s at this point that the four-stroke elite begin asking, “What’s the big deal with two-strokes, anyways?” I think that for a lot of us, it’s about nostalgia. Many of us have years of experience mixing gas, and no matter how good four-strokes get, the feeling of accelerating on a thumper just doesn’t compare to the crisp, barky acceleration of a perfectly tuned two-stroke. Another advantage is the simplicity of these bikes; four-strokes have chased away more than a few riders, primarily due to their high cost but also because they are somewhat complex and can be intimidating to maintain and rebuild. On the other hand, even a blown-up two-stroke can be rebuilt in an afternoon. And again, two-strokes gather far fewer noise complaints, making them a go-to trail maintenance machine and the engine of choice for riders who recreate in close proximity to people who love to hate dirt bikes. Overall, there are numerous compelling reasons to love these bikes, and I think many of us can agree that the manufacturers (and we consumers, to an extent) gave up on them too quickly in the early 2000s.

There’s a strong precedent for bringing back two-strokes, and it’s based upon the tried-and-true KISS principle: Keep It Simple, Stupid. We all know that these bikes are easy to maintain and modify, whereas putting a rod through the cases on a thumper effectively turns the motorcycle into a very expensive downhill mountain bike. Riders miss the days when a top end change could be completed between motos and post-race maintenance didn’t include a valve check. Yet it should be remembered that if the manufacturers begin reinvesting in two-stroke technology, the end result may not be as simple as we think. Although, with advancements in EFI being so rapid, there’s a good chance that any new technology would have enough sensors within the system to self-adjust on the fly, meaning that you might not need to hook that bike up to a laptop after all.

Now, about that new Honda patent: like many of its Japanese competitors, Honda has found incredible success in the production of general use utility engines. In all likelihood, this could be a boat, generator, lawn mower, or some other type of general-purpose appliance engine. Yes, the patent drawings appear to show that the fuel would be injected after the exhaust port has closed, which would minimize unburned fuel. But this doesn’t look like a motorcycle engine, and there’s also a big difference between a patent application drawing and a blueprint; what appeared online in the last week is the former, as a blueprint would be the technical rendering of the exact application that Honda was constructing. In other words, this drawing simply shows a concept that Honda intends to build, and is in no way proof that there’s an all-new enduro bike on the way. Yes, we’ve heard credible rumors that several manufacturers already have direct-injection/EFI two-stroke prototypes, and if you do some digging you’ll find that Honda has been experimenting with alternative two-stroke engines for over two decades (Activated Radical Combustion, anyone?). But until you see a legitimate press announcement from Honda stating that the new two-stroke will be a reality (and believe me, we’ll tell you right away if that happens), don’t get too hopeful—and don’t believe everything that you read on Facebook.

This concept rendering (it’s not real, people!) was put together by Dirt Rider Art Director Joe McKimmy to show what a new Honda CR250R may actually look like. We can dream, can’t we?
Here’s one of Honda’s patent application drawings that appeared online recently and sparked a landslide of interest in an EFI-controlled two-stroke engine. Many off-road enthusiasts concluded that this must be for a new motorcycle engine, but it’s more likely a general-purpose utility application (think generator or boat).
Looking at this drawing, it would appear that Honda’s design has fuel spraying in from the back (lower right hand) side of the engine, directly into the cylinder. This could seemingly eliminate the unburned fuel passing through the cylinder and out the exhaust before the compression and ignition occurs.
EFI could potentially minimize many of the pollution complaints about carbureted two-stroke motorcycles, but to significantly clean two-strokes up will require direct injection, which would occur after the exhaust port is closed.
Beta’s new two-stroke RR models feature oil-injection, which is essentially a computer-controlled system that feeds a Keihin PXK 36mm carburetor.
Beta’s new two-strokes feature an oil tank that accepts 650cc of premix oil (the very same stuff you currently use to mix your fuel, although Beta strongly recommends going with the highest quality oil you can find). Filling this tank full of oil will reportedly last for several tanks of straight gasoline that go through the bike via the fuel tank (of course, the mileage you get will depend upon the rider and the conditions). The way the system works is that oil is drawn from the under-seat tank and then fed through the oil injection system as required by the engine’s revs and load, both of which are measured by a Throttle Position System (TPS).
Yamaha’s upcoming two-stroke off-road model, the YZ250X, is essentially an off-road variant of the MX bike (much like the YZ250FX is a spin-off of the YZ250F motocrosser).
As you’ve read already, the new YZ250X is carbureted; there’s nothing groundbreaking about the engine design, although the wide-ratio transmission is a pretty cool feature.
Even though it’s lacking in major innovations (let’s be honest, it doesn’t even have handguards), the YZ250X shows a renewed interest in the two-stroke market by a Japanese manufacturer. Don’t like the new blue bike? Don’t buy one, but don’t be surprised if two-strokes go away completely due to lack of demand.