Carb conversion for TBI

You forgot drum brakes.

Seen a few 125/300 KTM‘s with Electronic Problems on Race Day , don’t need something like that, even when it’s only 1 or 2 out of 100 bikes 

I came from the Last Generation Carburated Husquarna straight to the New Beta 2027 RX 350. 

That 350  runs so clean with this old simple Keihin PWK on Stock Jetting , Never had a TBI , and i  think that I don’t miss anything . 

 

Corporate marketing guys just love guys like you. Did you know there are instances where drum brakes can perform better? A drum brake can exert more braking force than a disc brake, but what it cannot do is handle repeated loading on the drum without getting very hot and fading. That is what I meant by "fit for purpose" above.

I asked AI if there are any issues related to developing EFI for 2 stroke engines compared to its long-established use on 4 stroke engines. Apparently there are issues with trying to make EFI work on a 2 stroke, that is not faced when working with 4 stroke engines.  It is a good reason for "keeping it simple-stupid on a 2 stroke.  Here is what it had to say;

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Engine-development experts have long recognized that electronic fuel injection is more difficult to engineer and calibrate on a conventional two-stroke engine than on a four-stroke. However, that doesn't mean EFI is inherently inferior to carburetion on a two-stroke.  The distinction is important: four-stroke EFI benefited from decades of automotive development, while high-performance two-stroke motorcycle EFI must contend with some fundamentally different combustion and gas-flow characteristics.  In fact, there are good technical reasons why a properly jetted Keihin PWK carburetor can still deliver excellent throttle response and power.

1. The fundamental difference is how the engines move air and fuel

In a four-stroke engine, the intake valves and exhaust valves largely separate the incoming charge from the outgoing exhaust. Although valve overlap creates some interaction, fuel delivery is comparatively predictable.

In a crankcase-scavenged two-stroke, the situation is more complicated.  As the piston descends, the exhaust port opens, followed by the transfer ports. Fresh charge enters the cylinder while exhaust gases are still leaving. The expansion chamber's pressure waves strongly influence how much fresh charge remains trapped.  Consequently, the amount of air entering the engine is not necessarily the amount that remains in the cylinder to be burned.  That creates a substantial challenge for fuel-injection calibration.

2. Two-stroke engines have much more complicated pressure waves

On a 2 stroke engine, the expansion chamber is not simply an exhaust pipe. It's an essential part of the engine's gas-exchange system.  Pressure waves traveling through the chamber can pull fresh mixture toward the exhaust port and then push some of it back into the cylinder before the port closes.  These effects vary with engine RPM, throttle opening, exhaust temperature, power-valve position and engine load.

A carburetor responds mechanically to airflow and pressure differences through its venturi. That doesn't mean it automatically delivers the ideal mixture, but its fuel delivery is naturally coupled to the engine's breathing.

An EFI system must estimate how much fuel to inject using sensor readings and calibrated maps.  KTM's 2023 TBI system, for example, uses information including throttle position, RPM, crankcase pressure, atmospheric pressure and temperature. Its electronic power valve is also coordinated with the ECU.  That is considerably more complicated than selecting a pilot jet, needle and main jet.

3. Exhaust oxygen sensors can be misleading on two-strokes

Four-stroke EFI systems often use oxygen sensors to correct fueling through closed-loop feedback.

On a conventional two-stroke, some fresh charge can escape through the exhaust port without being burned. This means an exhaust oxygen measurement doesn't necessarily represent the actual air-fuel ratio of the mixture that burned inside the cylinder.  SAE research has specifically documented that the trapped air-fuel ratio cannot reliably be determined from the total incoming air and fuel or overall exhaust composition in a scavenged two-stroke.

This makes closed-loop fuel control harder, especially across rapid changes in throttle and engine speed.

4. Fuel-injection location matters much more

There are three principal approaches used on modern two-strokes.

Research published through SAE and Applied Thermal Engineering confirms that direct injection can reduce fuel short-circuiting, whereas ordinary upstream electronic injection cannot eliminate that fundamental problem.

Interestingly, direct injection is not automatically the ideal motocross solution. At high RPM, there is very little time between exhaust-port closure and ignition for the fuel to atomize, evaporate and mix thoroughly.

5. Why a carburetor can still work exceptionally well on a motocross two-stroke

A Keihin PWK carb has several characteristics that remain attractive for racing:

• Fuel delivery responds directly to the pressure difference through the carburetor.

• The pilot, needle and main circuits overlap to provide progressive metering.

• Changes can be made independently without ECU access.

• There is no dependence on fuel pumps, electronic sensors or injector control.

But carburetors also have limitations. Fuel can temporarily accumulate on intake surfaces, mixture delivery can lag airflow changes, and changes in altitude and temperature affect calibration.

EFI can compensate for many of these effects, and a well-developed EFI system can provide excellent transient throttle response.

In fact, Cycle News praised the immediate throttle response of KTM's 2023 250 SX TBI system, while also observing that both maps felt somewhat lean at higher RPM.  That illustrates the distinction between the potential of EFI and the quality of a particular manufacturer's calibration.

I dont know how to respond to this.

I have 20 bikes in my garage and 13 of them have drum brakes front and back, so bring on the marketing guys.

People need to use their brains and think things through rather than go with the simple, easy answer that doesn't require any thinking or research whatsoever. The easy answer for too many people is that newer technology is always better in all circumstances and older technology is inferior in all respects. They fail to even listen to the reasons why, even when the explanations are coming from experts, because it is just easy to follow the simplest thinking.

As for the drum brake example, for a long time after rear disc brake kits were available, drag racers in many classes still continued to prefer drum brakes in the rear because of lower residual brake drag.  This is because drum brakes use return springs to pull the shoes away from the drum when the driver releases the pedal. Disc brakes rely on piston-seal elasticity and slight rotor movement to release their pads, and this design feature results in some brake drag which can impact acceleration slightly and their resulting ET.  The rear drum brakes also provide considerable braking torque to haul a race car down from high speed because of what is called "self-energizing action" (shoes dig into the drum). This benefit of drum brakes is well known amongst drag racers and you can easily fact check, if you want to learn something. It is still a very good technology with some advantages when it is "fit for purpose". Naturally you would not want drums brakes on an autocross car because they retain heat and you would lose braking power after repeated braking.

Does that help?

Modern boat engines are running Fuel Injection (2 strokes), without issue.  The technology is well tested.  Seems KTM did it as cheap as could be.  Others don't seem to be having so many issues.  TM has been offering it for 5+ years now, haven't heard any issues, like the Austrian stuff.   Jetsurf?  Fuel Injected 2 stroke...   Jet Skis, no problems.  The technology is tested, and proven.  Unless you cheap out in manufacture, it's reliable. 

On my billy cart, when I was a kid, we had a wooden handle, with a pivot in the middle.  Pull the handle, the other end of the wood would contact the ground.  The harder you pull, the higher the stopping force.  Why don't cars use this technology today?  

Thank you for the lecture.  The only drag racers who use drum breaks now are vintage or budget builds.  Serious racers use 4 rotors and dual calipers.  Drum breaks also work well on the rear of a nice sedan to get around town.

Every manufacturer sands beta who are pushing development further in 2 strokes are using FI.  I would consider them "experts".  

But im sure you can find some dudes on Facebook and use some AI search to prove to me why current technology development in 2strokes is inferior.  

Remember this conversation in lets say 5 more years.  Im sure no one will be asking for a carb 2 stroke the same way no one wants to go back on the 4 stroke.

Glad I didnt throw away that old roost boost .

Regarding - new technology like fuel injection etc.

Some of it doesn't get introduced for performance or reliability reasons, sometimes it's for emissions regulations, noise regulations, safety regulations that sort of thing. 

If there were no emissions regs. in the world , I doubt any manufacturer would voluntarily fit a catalytic converter or DPF to any vehicle, and carbs would still be fitted to many basic vehicles. 

The new tech isn't necessarily "better" it just complies with modern rules. But marketing will rarely say this.

Drum brakes - I see some  VW  EV cars (rear wheel drive) now have these on the rear brakes , they claim they are a better solution with the regen going through the back axle ?

You bought that 350 beta? Isn't it down like 10 hp compared to a KTM 300?

In reality, tpi, tbi, doesn't add that much more in performance over the same bike with a carburetor.

Yes, there might be a little something hear and there, but basically both have about the same performance characteristics.

Now, look at what had to be done to get a efi to run as good or better than a carb, extra components, cost, weight, complexity....etc.

Let's start:

fuel pump, baro sensor, ccp sensor, injectors-2, temp sensor, added high pressure fuel lines, tps sensor, added wiring and connectors, electronic oil injection...am I missing anything?

All this to make an engine run marginally better than a proven carburetor and hopefully you get a bike that runs as intended, there are those bikes that have run issues right off the showroom floor.... Now sorting out those issues are much more complex.

Not an easy task for the average guy.

Now don't get me wrong, I'm not a efi hater, infact I just picked up a 27' 300 xcw.

I'm just pointing out the facts, from my perspective. 

Average guy loves to walk into the garage, press a button and have the bike work. That’s what KTM has. January or July, bike starts and runs relatively the same

Unless the battery is dead.  My carbureted, kickstart 2 strokes start in January or July just the same.

My 2025 150sx has been crisp from day 1 with no issues at 14 hours. 

Yeah, I’ve kicked over a carb 2t in January 100x. That sucks. Rough even trying to get the kids bike started during indoors season 

Telling people to use their brains after copy / pasting an ai prompt on efi 2 strokes is perfect.

Huh?????  Using Chat-GPT to summarize what some experts are saying about why it is more difficult to adapt EFI to 2 strokes has nothing to do with thinking through the issue.

I never comment on here because it always seems to evolve into name-calling, but I found this post interesting because I’m a two-stroke guy with both type  bikes in the stable. I’ve experienced some of the issues you can have with both.

I live in North Texas and we are only a few hundred feet above sea level with no major elevation changes. It  is a set it and forget it type of situation when it comes to carbs here, other than maybe some temperature changes when you can play with the air screw. I have a 2019 Beta XTrainer and it is jetted perfectly here. Other than making sure the pump gas I use doesn’t sit in the carb for too long, the bike runs perfect all the time. On the flipside, I took it up to Colorado this summer, and was staying at 8800 feet and rode up from there.  I used the beta jetting app, scoured the Internet and used AI for jetting advise before driving up, and they all gave me the same main jet and needle clip position instructions. So I jetted it at home, which is not totally easy by the way, because you have to get the carb out of the boot to change the needle clip, and getting it back into the boots is as a complete PIA. So… I got up there and ran like crap…basically Super lean. I made sure to bring several main jets, and after changing to the next main jet size up, and putting the needle and clip it back to the 2nd from top (my Texas setting), it ran great. That said, it’s still a carb and i could still feel/hear slight differences with the big elevation gains and losses. I at least understand jetting and could fix it. Someone with no mechanical ability would have ridden a bike that was too lean the entire time which would have sucked. I wouldn’t have dealt with any of that if I had taken my Huskey TBI, but couldn’t because its set up for moto.

On the flip side, I was just at the mx track a few weeks ago with the Husky mx bike. I started it before the first session and while it was idling, I blipped the throttle, and as soon as it went past quarter throttle, it would totally bog to the point that if I didn’t roll the throttle back, it would just die. Tried it over and over with the same result. I knew it was FI related and there was nothing i could do about it. Had to ride the nephews 250F instead. It turned out to be a bad TPS sensor. So that was an example of when 2-stroke FI does go bad, you are pretty much effed. 

As for how FI 2-strokes run, I love the smoother powerband of my Husky TC250. I’m a 56 yr old ex A class offroad racer and B class mx racer and between age and three broken wrists (and tons of other injuries), I dont need a firebreather that hits incredibly hard. I ride on the softer map so I can use less energy and it’s still fast. The aggro map is too much for me! I understand a truly fast young guy may want more, but I can still go pretty good for shorter bursts, and I personally like what FI has done to 2-sroke power, especially in the off-road world where it smooths out the power and self adjusts for temp and elevation. Then again, my carb Beta runs amazing (here anyway), is reliable, and is a total billygoat with smooth power also!

I actually enjoyed reading that AI summary on the issues making FI work on a 2-stroke more complicated than a 4-stroke. Makes sense after reading it. I too have wondered why FI on 4-strokes is basically bulletproof and yet 2-stroke FI seems to have issues. I definitely know because I have recent real world experience with the issue side. I’ll say on balance I want to see FI on 2-strokes become very reliable and ubiquitous because I’d rather not fiddle with carbs, but since it has not become as reliable as 4-stroke FI, I do understand the pushback from some people. It will be VERY interesting to see how this new KX327 runs. The Japanese don’t cut corners so I’m guessing it may be sorted out better than the Austrian bikes. We’ll know soon!

I had a 25’ TBI 300 and sold it after 20 frustrating hours. Great when it ran right (it hardly did), terrifying death trap when it didn’t. Problems after hour 1 on the meter. Cuts off and bogs at dangerous times. Dealer was useless and said everything’s fine. Tried all the, “forum fixes”, no luck, just more money wasted. I’m an engineer so a terrible trait I have is over analyzing everything, and reading hours upon hours of information. I have came to two personal conclusions on these bikes. 

1. They are extremely over complicated. Many parameters have to be met via the electronic power valve, the TPS, the CCPS, various temp sensors, and more, for the ECU to, “hopefully” do what it needs to do correctly. 

2. More of a theory, but I don’t think it’s that far fetched. I have read that this was a rushed project during Covid and the issues people have now are the bi- product. 

I truly do think I personally got a lemon and not all of TBI’s are this shitty. But I did end up getting an ole trusty YZ250 and haven’t had 1 single issue so far .