Camchain and tensioner seen up close in a cutaway bike engine

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Carburettor Or Fuel Injection?

Blog Date - 28 July 2015

This question keeps on cropping up on this website and it is a question that I keep on pondering over myself. 

Which is better, Carburettor or Fuel Injection?

The biggest "advantage" most mechanical people perceive with carburettors is that of being able to understand them and fix them. The carburettor is a physical, mechanical item that can be taken apart, looked at and even adjusted or altered. Fuel injection systems on the other hand are seen as being somewhat akin to computers. While a home mechanic may be able to work out what should be connected to where there is nothing but electrical signals from sensors and black boxes that cannot be fixed, merely replaced in the hope that it will all come to life again.

But lets compare at a modern carburetted motorcycle, such as my gf's Keeway RKS 125 with a modern fuel injected motorcycle, my own Honda CBF 125.

Sharon's carb is a CV (Constant Velocity) model. As such it contains an idle jet and circuit, the main jet and venturi, a choke system and various passages to allow the diaphragm and spring to control air flow and pressure to ensure something like the correct fuel mixture under a variety of engine conditions. My CBF has a simple tube with a flap and a squirty bit on the top, the injector. I used to think the throttle body was complex until I purchased one to see what it's all about. It really is a simple thing, much simpler than a carbuerttor.

A fuel injection throttle body from a CBF 125
Throttle bodies are in fact quite a simple tube with some bits attached.

What makes fuel injection seem complex are the sensors required to enable the computer to control the amount of fuel injected into the engine. My CBF is about as simple as it can be, even so there are - ahem - Manifold Absolute Pressure, Air Intake Temperature, Engine Oil Temperature, Throttle Position, Engine Position and Lambda Sensor. The information these sensors send to the computer are used to calculate how hard the injector works at squirting in fuel. If any sensor fails then the computer will either default to fail safe values of the engine will simply not run. 

What I and most mechanics hate about sensors and electronics in general is that you can't SEE the problem. If the engine is rattling and you tear it down you'll find a loose bearing, a broken spring or a worn out guide. Electrically if you're lucky you'll find a broken wire but otherwise the part looks fine and shows no sign of damage. There are tests that can be made with cheap multimeters but you have to know what the correct values are in the first place and these results are often less than decisive. Otherwise you are dependant on some means of communicating with the computer to enquire what it is not happy about.

My Honda has a wire you can short out and the computer will flash the engine warning light to signal a number. This number can be referenced against a chart in my Haynes manual and the poorly sensor identified. Can the sensor be fixed? Nope, just replaced. What if the computer is dead? Nope just replaced. Simply put if any of the electrical components fail you are sure to be left requiring a lift home and ordering some parts from Honda.

More complex and powerful fuel injected motorcycles have even more sensors to go wrong and the computer cannot be talked to by shorting out a wire. These bikes need to be returned to a main dealer with the correct computers, programs, leads and charts to work out what the issue MAY be, and even then the experts are known to get it wrong all too often. So it seems fuel injection is complicated, can only be diagnosed by expensive main dealers, can only be fixed by expensive main dealers and any home mechanic can't do a thing about it.

Are carburettors any better?

I certainly used to think so until I owned my CBF 250, a carby machine. It refuses to tick over properly, details are available in other blog posts. Now I CAN tinker with this. I can alter jets, clean parts, adjust settings and play with things all from my scruffy little shed. But...and it is a very big BUT...I have neither the expertise or experience to fix the issue. I have a myriad of suggestions from friends, none of which helped. I have read around the internet and tried ever more solutions but to no avail. Carburettor tuning is a dark art and much like electronics you still can't actually see what the problem is. You end up guessing and making educated alterations that make things better or worse, or in my case no difference at all. 

A carburettor in bits on a table
Of course carburettors are so SIMPLE! Pfffft.

So I may decide to replace the whole carburettor. This I can do at home but it is certainly not a cheap option. The cheapest second hand item of Fleabay is £75, compared to the £15 I paid for the injection throttle body for the 125. A brand new one is far in excess of £300, I can replace every sensor on my 125 for less than that. 

Many argue that carbs can be fixed. They can be cleaned. They can be adjusted. But unless you are a tuning god then getting them set up right is nearly impossible. If the carb is physically broken they cannot be fixed either, they can only be replaced. I suspect because any backstreet spanner monkey can remove and replace a carb it makes them feel in control. I doubt there's many DIY mechanics out there with the level of expertise required to actually tune or fix one though. It's the sensation of doing something that makes carbs feel better, but the end result is still the same, bikey no worky.

So I am not convinced that carbs are any easier to fix than injection systems. What about the act of using them?

I'm sorry to the old school readers but here injection wins hands down. Early fuel injection systems were sometimes blooming awful but modern ones are sorted. Because of the sensors the computer knows the load, the temperature, the current engine state and the required response from the rider it can precisely control the fuel entering the engine. Depending on the type of bike this can be optimised for performance or economy. Sharon's carby 125 returns around 120 mpg, my fuel injected 125 returns 140 mpg due to the more accurate metering. Some machines can have different modes, crazy mental sports bikes can become gentle and safe when "rain mode" is selected, none of this can be done with a carburettor.

That's my tuppence worth...I'm sure you'll have your own opinions...bring it on!

Reader's Comments

DAVID C said :-
Worra loada tosh! Fuel injection systems are fixed and set by the factory and allow no alterations, modifications or improvements from the owner. I currently run a XJ600 with a stage 2 Dynojet kit fitted and K&N filters. It made a huge difference to the running of the bike and eliminated the built in flat spots for emissions. On the modern version if you don't fit an original Yamaha air filter it will send all the mappings to pot and possibly ruin your bike. It's Yamaha's and all the other maker's way of getting you to buy only from them.
1/1/2000 12:00:00 AM UTC
Ren - The Ed said :-
Hi David. I am told the Dynojet stuff is very good at sorting out the "compromises" manufacturers have to make to meet emission regulations. Of course this applies to fuel injection systems too. As such Power Commander seem to have cornered the market here. As well as sorting out the original settings the Power Commander is also programmable which allows a rider to make changes to his fuelling with a few clicks rather than a strip down.
1/1/2000 12:00:00 AM UTC
Henrik said :-
Thanks to Ren for giving this "injection thing" it's own sepperate topic, a big and very interesting subject for several reasons. I see good point's in both Ren's opening post, and Davids answer. I will re-read both this one, and
the orther article about the injector, plus I am doing some googles also to get better down into it. I will likely return later when more substance and structure arrives in my brain, also I am hit by pre-marriage stress :-)

For now I will just repost my contribution the "Consumer Adventure Motorcycles" topic yesterday, as it realy belongs more here:


My own "feeling", coming ages back from a electronic background/education,
and the word from a few MC mechanics that I speak to now, seems to confirm that basic components for injection systems is not someting you repair, but something you are left to replace only, even as a pro that is, and only OEM parts are available typically, thus they get away with the extreme overpricing. So seccond hand shops, and ebay is only option. As for the chinese, the keeway maxi scooters sold in DK are injected, with parts a little more reasonable, but still,.. Yes I might eventually try a injected MC, and in that case likely resell while still fresh :-) I have just turned 50, and with bikes like KLE still with lot of reliability and km to run, after 24+ years, guess I am covered, needed be :-) could be interesting to see you take on how, if, and to what degree, typical injection component could be repaired, maintained, and simply supplied long term,.. with availability and prices that makes sense and keeps a beloved bike rolling on like the previous generations can.
1/1/2000 12:00:00 AM UTC
Ren - The Ed said :-
Thanks Henrik. It was your input that guided me to write this. I knew it would be devisive but as long as everyone is mature then it could lead to an interesting debate.

If you have much to say on the matter I'd gladly take it as a guest blog post rather than just a comment.

As for getting married! I've managed to dodge that so far. I hope you have a fab day and an even better life :)
1/1/2000 12:00:00 AM UTC
Henrik said :-
I will try to get back to it, I am sure we will have a mature and interesting debate, with good contributins form all corners, case
is that I needto rest it for a little while, and my reason for the
absence is not only the wedding, but also my roof, a little bit to
open, a fresh pic here, dam, its a late work, when I need a little
rest I take the XRX out in the woods for an hour or so, some small
probs with that also, but keept it going so far, and enjoy it, (to
be continued also), planning a new super light luggage set-up for
small-touring in the countryside and wilderness,.. Reading up on
injection is interesting, as we speak I have the yellow engine-lamp
turning on and of in the Corsa, only becourse of my "op-com" I can
enter this hell with a computer-program, foremost wrong codes like
"primary lambda heat element failure", you cant even count on these
messages, in this case it is just the connector to the heater that
is failing, I clean it then, and reset the computer, until the lamp
turns on next time, its all just to sensitive, and complicated. One
from my old kayakclub had to sell his car when this circus arrived.
Ordinary people, they get their wallet sucked flat, any explonation,
or bill, they just have to eat it, no way to be in control, its sad
if we soon need to lease everything becourse of this, I hear that in
the USA its common now that people don´t whant to own a car anylonger,
the latest motorcycles is just as complicated.

1/1/2000 12:00:00 AM UTC
Ren - The Ed said :-
Henrik - yeah that's a fair sized hole in your roof. I'd get that fixed if I were you.

Perhaps the problem is the complex systems on the complex cars and motorcycles. If the manufacturers kept these systems simple like on the CBF 125 then we ordinary people would have a chance of being able to fix them?
1/1/2000 12:00:00 AM UTC
Bob said :-
Having owned many bikes of both sorts my experience is generally this:

Carbs - lower fuel economy, requires occasional strip and clean and there tends to be a bit of fettling required now and again to keep things going well.

FI - works perfectly, easy to start the engine every time, excellent fuel economy but often snatchy throttle response and sometimes you get that annoyin hunting effect on a steady throttle.

BUT - I've never had a catastrophic carb failure (is it even possible?), yes I've had overflowing float bowls and blocked jets but I've always got home. With FI you are quite literally dead in the water. The built in diagnostics are pretty rudimentary at best, so it tells you the O2 sensor is out of range, OK then £80 for a new one? What if you fit it and it wasn't that?
On my car (Vitara) I use a bluetooth OBDII adaptor and the "Torque" app on my android, which gives full real time information on all the FI systems, with this it's possible to diagnose most problems but it's still problematic.
It depends on what sort of person you are, if you drive a newish car and are happy to take it to the dealer's for service then why not do the same with your bike? If however you like to be self-sufficient then choose your next FI bike carefully, some manufacturers are better than others with the on board diagnostics. Yamaha tend to be very good, using the clocks as the diagnostic terminal and there's a lot of information available. KTM are dealer plug-in only.
I had a BMW G650X Country and purchase the GS911 BMW specific bluetooth interface, which I used with my phone - it was actually pretty good and allowed me to fix a problem with the idle actuator stepper motor.

So it's a mixed bag, but if I was setting off across the wilderness I'd want a carb, no question about it.

My FX650 has no fuel pump, no sensors, no water cooling, no ABS and that's the way I like it!
1/1/2000 12:00:00 AM UTC
Ren - The Ed said :-
Cheers Bob.

As for snatchy throttles on FI, I thought that was a thing of the past? The modern bikes I've ridden are all perfectly fine, but then I usually end up on smaller machines where the power is lower anyhow.

Now you mention it I can't think of an example of catastrophic carb failure. I've had problems that have made the bike a real bitch to ride but yes, somehow, some way it got me home. FI has never let me down far...but I can already see it won't be a struggle, it'll be a breakdown truck.

I would hope that diagnosis would improve. I fear though that most marques will make it almost essential to take the bike to an expensive main dealer simply to appease the dealers. I understand that the act of swapping out is usually the main method of diagnosis but again some machines require talking to the computer to advise it that it has had a replacement!! I'm glad my CBF is not that complex.

As the third world countries start to receive FI motorcycle I wonder what they'll make of them. More interestingly will be how they learn to bodge, fix, bypass and repair them in back street garages. We may learn a lot from the ingenuity of those who's pockets are very shallow.
1/1/2000 12:00:00 AM UTC
Bob said :-
Let's not forget that the only reason we are having to think about FI at all is because of EU emissions targets. Honda still manufacture and sell the old XR650L in the USA for example
1/1/2000 12:00:00 AM UTC
Ren - The Ed said :-
Do you think we wouldn't bother with fuel injection if it weren't for emissions? I'd never considered that. I need to research when FI was first invented, before or after we started to worry about the air we breath. Interesting point Bob.
1/1/2000 12:00:00 AM UTC
Pegmonkey said :-
With the addition of ethanol to many fuels, carbs suffer the most with their their dissimilar metals producing electrolytic currents that corrode jets and passageways. But, if I'm stuck in the woods on my enduro when it won't start, I'd much rather clean a gravity fed carb than have to try and trouble shoot electronics with my primitive cave man style tool kit.
1/1/2000 12:00:00 AM UTC
Ren - The Ed said :-
Ah pegmonkey - you mean the hammer! Maybe you're over-prepared and also take a chisel, you know, just in case.
1/1/2000 12:00:00 AM UTC
Jay said :-
Hello, I have a keeway RK 125.
Finding out that fuel infection are faster than carb despite the failure you get when you can't find the problem. I want to convert carb into fuel injection on this bike. Is there a kit available?
Thank you
15/1/2016 11:22:11 PM UTC
Ren - The Ed said :-
Hi Jay. I am not aware of a "bolt on" kit. I am sure it can be done but you'd have to get very technical and learn a great deal. In all honesty the difference between a carby bike and an injected model is nominal at best.

If you want your RK to go faster then a simple tune up on the carb would be a lot easier than trying to tune an injector anyhow.

Of course any such tuning would...
Invalidate any warranty
Invalidate your insurance unless you tell them about it
Possibly invalidate your license unless you've passed your test

Tuning can be a great way to learn about how bikes work and how to get the best from them. And for a guy starting out carbs are the best place to start.
16/1/2016 7:05:27 PM UTC
Paul said :-
Can you transfer back to a carb and what does it take to do it if so.
12/4/2018 11:04:34 PM UTC
Ren - The Ed said :-
Hi Paul.

Firstly you'd need to find a suitable carb. It needs to have the correct bore, flow rates and mountings. Then you'll need to sort out the ECU. The computer will be expecting various inputs like air flow and air temperature from sensors in the FI throttle body. I imagine you'd need a different ignition system with all the associated parts to make that work.

My CBF125 here in the UK is injected. I am informed that in India there is a version of my bike that has a carburettor. To install a carb on my bike I'd need the carb, the ECU (more likely a CDI) and the wiring harness to match.

It is possible - anything is possible. But it's not a simple task.
13/4/2018 8:03:22 AM UTC

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