Why sudden urge to fit bike carbs ?

[composimmonite]

I keep reading on this, and other, forums about people fitting bike carbs to their vehicles and can't understand why !
They remove perfectly good injection systems which to me should be superior and easier to modify / tune. Traditional Weber and such twin choke or 2 twin choke carbs are ignored and it's straight to a secondhand set of bike carbs.

If you go back to basics the engine is a machine for converting the energy held in the liquid we call petrol into motive power. It needs to burn with the right ratio of air in a confined place (combustion chamber) to do this. Carbs are a way of getting the liquid fuel mixed with air and into an engine. Accepted more fuel / air mixture you can get into the combustion chamber then the bigger "bang" it will create.
I'm all for doing things in different ways but this current trend for bike carbs is beyond my understanding when there are easier alternatives like modified injection, different CAR carbs, turbo, supercharger etc.
Anyone tell me why the sudden urge for bike carbs on vehicles - is it because they are cheap to buy ?
What do insurers say when you tell them you've fitted bike carbs to your Micra, Fiesta, and such ?
Do they pass MOT emmissions ?

I'm not knocking bike carbs but and purely trying to understand why they have become so popular ?

Nice STANDARD carb set up on a Honda S800



Paul H

[welshpug]

sudden urge? its been a fad for bodging them on for years!

its probably popular because its cheap to bodge them on, which is where I totally agree that its just not the way forward especially with a modern engine that probably had decent management on it already.

however to craft a well executed properly designed setup is far from cheap, though in most instances no more expensive than an off the shelf ITB setup, many will outperform a stock jenvey (i'm comparing a non hose joinered bike ITB not carb here btw, carbs just cannot compare whether it be bike or car type)



have an example of a well engineered solution using very high quality o.e japanese paired throttle bodies used on a psa tu5J4 race engine



and one using the ITB type



and have a 1600 sigma

[Dez]

Multiple reasons, usually with cost factoring quite high. They're supply/demand carbs, like an SU, but made with modern manufacturing processes and tolerances, so realistically they're as efficient as carbs can ever get. Webers or dells are massively expensive in comparison, and usually old and knackered requiring lots of expensive new parts. Plus they're rubbish anyway, they never stay set up for long before requiring re-adjustment.

I was dubious at first, but now there's loads of jets available for em (there wasn't at first) so you don't have to drill them, and I've done a few setups for people that have workwd really well, I'd choose them over anything else (except injection).

[EmDee]

Jul 10, 2013 10:17:00 GMT Dez said:

Because reasons.

This is why I'm using them. I've not had to alter my standard ignition, they were cheap and hold their state of tune for faaar longer than more expensive ones designed for cars. Also they've given me 30 bhp over standard.

[bortaf]

Fitting one carb per cylinder is about air flow, more air = more petrol = more power, bike carbs are the cheapest way to do it with reliability is all.

[adam73bgt]

I dunno about bike carbs, but I have had an urge to fit 4 SU's to a 4 Cylinder engine in a similar fashion to most bike carb setups.. small SU's are very cheap too

[optima21]

think bike carbs are a cheap performance option for many people, and they then have the bragging rights to say they've been fitted. I've got a couple of sets of crb600 carbs for my polo GT and one set has been modified so that the centres between the carbs is the same as the cylinder bore centres, so if they ever get onto the polo, they'll be on more downdraught ports (aluminium tube bonded into the head with JB weld). If you're fitting them for improved performance you may as well go the whole hog. if all the claims you see for improved bhp are true with just the carbs bolted on, just think what they would produce with suitable headwork etc.

you also alot of posts on forums with people fitting bike carbs and having issues with them, and in some cases worse that standard performance and horrendous fuel ecconomy because tey havnt been set up, but they gain an instant 20bhp due to the induction roar.

think I'll stick with injection on my polo GT with injection and other induction mods though ( headwork, modified inlet manifold with redesigned plenum, bored out throttlebody etc, as its very nice to drive as it is.

[Bozwell]

Jul 10, 2013 11:55:37 GMT bortaf said:

Fitting one carb per cylinder is about air flow, more air = more petrol = more power.

not quite true. you can fit one big carb on a log manifold and get the same airflow. it is to do with charge robbing and pulse tuning. with a log manifold and one carb the valve opening on one cylinder can pull the fuel/air mix from another runner for another cylinder so that one then has a low volume of fuel air mix.

this is worse with high lift cams and some engines will run really rough until the engine rpm is increased quite substantionally. individual runners/throttles really help a lot with low end drivibility with high lift/long duration cams.

the length of the runners can easily be altered to take advantage of pulse/ram tuning.

if we take an American V8 for example a four barrle Holly will allow more airflow because there are four butterflys feeding all cylinders through a common plenum individually (remember the valves are not open all at the same time).

a Holly fed V8 can usually have more horsepower than a four carbd Weber V8. so why fit Webers? it is for drivability. thats why you see drag cars with Holleys because they are just flat out where as a circuit race car has Webers because it needs the engine to pull from low rpm for the corners.

going to why fit carbs instead of injection.


when the air is pulled through a carb the fuel needs to vapourise and in doing so it will pull some of the heat from the inlet charge cooling it down. also there is more time for the fuel to properly mix with the air. most injection cars have the injector right at the head to aid slow running but this position hurts top end power as it doesn't give any time for the fuel to mix with the air. look at any full race injection system and they will have standoff injectors outside the trumpet giving maximum time for fuel to mix with the air.


what ever system is chosen to fuel the engine they will all have an advantage and disadvantage over each other. bike carbs are a cheap way to have your fuel mixed well with the air.

[roccoguy]

Bike carbs are cheap to buy, easy to install, easy to tune and hold their tune much better than more expensive car carbs... whats not to like?


I've seen badly fitted/setup ones that have actually decreased performance as well as giving terrible mpg, but as with anything like this they are only as good as the person fitting/setting them up.


people trying to compare bike carb setups to full blown ITB setups are totally missing the point. Bike carbs are a cheap DIY tuning solution, ITB's are not.

As for the standard injection system they are replacing being easier to modify/tune, how many people have the knowledge and tools to remap a standard efi management?

I don't see how fitting bike carbs to a car is any more of a "bodge" than fitting twin webbers to one. After all they are both carb setups designed to work on internal combustion engines

[jonw]

Bike carbs are generally simpler, cheaper and generally easier to get hold of.

Fitting car carbs to bikes does happen, especially in the US.  SU's and occasionally webers appear on harleys!

[jordanisace]

As for the standard injection system they are replacing being easier to modify/tune, how many people have the knowledge and tools to remap a standard efi management?


reality is alot of newer engines (zetecs) need a programed ecu/megajolt setup if they're run on carbs to play around with the timing advance anyway.

claims of 30bhp on a smallish engine without any extra work seem optimistc at best to me!

[prey]

no ones mentioned that the sound is more addictive than crack compared to an oem injection setup lol.

[madmog]

The ECUs of modern engines often expect signals from other things that won't be in the older car that the engine is going into. So you may have to have some kind of aftermarket programmible ECU + a session at the rollingroad to run F.I.

A Fiat engine I'm researching for example 'needs' a signal from the ABS sensor or it goes into limp mode. Perhaps that can be bypassed and perhaps not. So, even transferring an F.I. engine without uprating might throw up odd problems.

I think the feeling with bike carbs is they'll work without wires and sensors so less to go wrong. They're quite simple to understand, you can dismantle, see what is going wrong and reassemble easily. Blast out jets in your shed/kitchen with carb cleaner rather then taking injectors somewhere to be ultrasonically cleaned

While distributerless cars need Megajolt or equivalent, it's much simpler to get in the ballpark than programming injectors as well as sparks.

Perhaps also, people that are into older cars are already more familiar with carbs than computers.

So, while F.I. done right would give best performance, bike carbs would be simpler to get close and cheaper

[Kieran]

Further to most of the reasons above, bike carbs are about as 'state of the art' as carbs got.

SU and webers / dellortos were born in the 50's and 60's, bike used them up until the early 2000's.

Being cv carbs mean they are tractable and are a large carb that has the ability to be a small carb (like an su).

Modern bike carbs come with tps fitted and can be used with mega jolt etc.

Emissions wise on a classic no problems at all. Modern car you will struggle with a cat test.

Aftermarket efi is great but the dump to e to set it up can be massively expensive. Mega jolt and bike carbs, I can set up myself on a straight bit if road.

Fitted a few sets and had nothing but joy from them.

[popuptoaster]

Mostly it's because many modders are frightened of electrickery and don't want to mess around with high pressure fuel lines and ECU chips, maps and sensors.

Even if they cost more people would still fit them despite Efi being a better option.

[roccoguy]

Jul 12, 2013 20:00:08 GMT popuptoaster said:

Mostly it's because many modders are frightened of electrickery and don't want to mess around with high pressure fuel lines and ECU chips, maps and sensors.

Even if they cost more people would still fit them despite Efi being a better option.

What about the many people that run bike carbs with aftermarket management like mega jolt, or along side the standard ecu using only the ignition map. Are these many modders also afraid of electricity, ecu's, maps and chips?

Some people prefer to run carbs, some people prefer to run injection, its personal preference at the end of the day. I don't see why people feel they need to try and some how belittle someone because they prefer to play with carbs. In this thread there's already been someone labelling all bike carb setups as a bodge and now someone trying make out people only doing it because they are scared of electricity lol

[popuptoaster]

Chill out dude, i know actual real life people who have fitted bike carbs because they don't understand injection and prefer to deal with carbs, totally not interested in doing any wiring at all, you point out some guys use mega jolt, i pointed out SOME cars guys don't like electricity, wont touch a modern car but will happily do a total resto on something with no electrical gadgets..

It's not unusual for posts in the tech sections to appear from guys asking for help converting to electronic ignition (for instance) from guys who have fitted a complete drive train.

No need to get all defensive.

[willowandmikey]

Bike carbs look kinda sexy , I know nothing on the practicalities I just think there pretty.

[roccoguy]

Jul 12, 2013 22:53:09 GMT popuptoaster said:

Chill out dude, I know actual real life people who have fitted bike carbs because they don't understand injection and prefer to deal with carbs, totally not interested in doing any wiring at all, you point out some guys use mega jolt, I pointed out SOME cars guys don't like electricity, wont touch a modern car but will happily do a total resto on something with no electrical gadgets..

It's not unusual for posts in the tech sections to appear from guys asking for help converting to electronic ignition (for instance) from guys who have fitted a complete drive train.

No need to get all defensive.

You didn't point out you knew some people, you made a sweeping statement implying it was MOSTLY because modders were scared, which is nonsense.

Here's an unbodged polo mk2f running bike carbs seens as there is a lack of bike carb pics in this thread



using the carbs, cam and megajolt he's upped power from the original 75bhp (on efi) to 115bhp (consistent on 3 rolling roads) and consistently runs low 15's and has even broken into the high 14's which is the same sort of times as you'd expect from mildly tuned G40's, which are efi and supercharged. All setting up/mapping was DIY.

Some people may not like the idea, but bike carbs are a tuning mod and are capable of giving very good results over standard efi

[pork]

I fitted bike carbs purely coz I have no idea on efi, they super cheap, sound awesome.......and turbo quite well
All tuning done by me, an utter novice, and an O2 sensor

Here's mine