Right,

You've got MikeyMK telling us they are MEGA on fuel, as in 50 mpg! But spoke to Mk3 Rob the other night, and he tells me that the are AWFUL on petrol, as in 15 mpg when ragged! (I fully accept that they generally make cars RAPID though lol)

Discuss please.

Joe

Depends on how they are jetted.. eg, they can be jetted for maximum power/economy/power at low,mid,or high revs etc....

I'd say as a general rule though, better cruising, but worse when you boot it.....

No carb is ever going to give you the spread EFI does. Carbs can be set up to run quite lean and give quite good economy with a lower power output or quite rich with a high power output. A carb shouldnt theoretically be able to fuel better than EFI pretty much anywhere, so you are unlikely to see big economy improvements however its jetted. You'll gain power because the engine wont be breathing through a daft single throttle plate. You also need to consider that there isnt such a thing as "bike carbs" there are loads of different carbs off loads of different bikes, look at the bike's power outputs and try to match them to that of your car roughly, (because power output is directly related to airflow, as is throat size so these are an important link) For a GT I would personally use carbs off a 600 sportsbike, because most sports 600's make about 100-110BHP which is relatively close and mean that by the time you get some mods in they might be about spot on. Use carbs that are too big and they will perfom badly. It might be helpful to find carbs fitted with a dynojet kit too, as these are far more variable than standard bike ones (adjustable needle clips etc.)

Right,

You've got MikeyMK telling us they are MEGA on fuel, as in 50 mpg! But spoke to Mk3 Rob the other night, and he tells me that the are AWFUL on petrol, as in 15 mpg when ragged! (I fully accept that they generally make cars RAPID though lol)

Discuss please.

Joe

I fail to see your point

Mikey is saying he could get 40-50 mpg cruising along, 60 in 5th sort of thing. Ragging economy is something completely different, I can get my 1.0 Mk3 to drop down to below 20 mpg if I want with the 1.4 throttle body.

I dont think i got my point over very well lol

Rob told me his VR6 is better on fuel than the numerous polos hes had with bike carbs. I think he meant on the whole.

Plus contrary to what i have read, he told me you have to get them re-set up every 6 months.

Would probably make it easier if someone with bike carbs gave us the low-down lol.

I think the simple answer is, you're never going to see better economy figures on an engine with carbs over a fuel injection system. It's not rocket science, modern EFI allows you accurate control of fuelling and ignition across the rev/load range - carbs allow you to get the fuelling there or thereabouts ish.

If you could really tinker with bike carbs to get 50mpg+ on a 60mph cruise on a 1.6 8v engine running safely why the hell did VW choose to use an injection system?

I dont really see the issue here, carbs are just not as good at delivering fuel as EFI is. I have bike carbs on all 3 of my bikes (i dont have them on a polo because I dont think its a very good idea), and they work fine, however they suffer from carb icing in the winter, they dont take air temperature into account, they cant give better economy than EFI. A carb has a few fuel circuits and tries to fuel your whole engine on these few across different loads, throttle positions and RPM's. It cant ever do this as well as EFI because EFI has a huge number of fuel circuits and can look at each part of the predefined map and match that to the current conditions. It can also adjust fuelling to match air temp, make adjustments from your lambda sensor etc.

Ask yourself a question, why do you want to fit bike carbs?

If its for more power, you will have to make an inlet manifold to fit them, so just make a inlet that has injector slots in it. Drill some holes, weld on some cylinders, and bolt on your standard fuel rail. Now fit your bike carbs which will just be used as throttle plates. You could really do with running a megasquirt with this because the fuelling will need to be wildly different from stock. But there you go, you will have the advantage of bike carbs (Individual throttle plates) and the advantages of EFI (it will fuel properly)

Both systems will need setting up, though it will be easier to map the MS than set up the carbs because you have a lambda to go by, and clicking a few buttons is easier than removing carbs, draining float bowls, changing main jets, float height, or needle clip position.

You shouldnt have to get bike carbs set up every 6 months, the fuelling will be slightly different in the winter because they dont account for air temp. They occasionally need balancing, but this takes 20 mins on a bike including removing the tank. Should take 10 on a car tops. Should need doing more than once every couple of years.

I'm still unsure of the question exactly, but properly set up EFI gives better fuel economy than carbs, hands down, and bike carbs shouldnt need setting up every 6 months.

I don't think mikey was meanign that the carbs alone gave him better fuel consumption. He was meaning that the engine produces enough torque that he can put it in 5th and allow it to almost idle along at 50/60mph, hardly needing any throttle at all. Bcause the engine is hardly more than idling, it doesnt use much more fuel than at idle.

I was talking to a chap who owned a TVR sagaris, he used to drive from Harrogate to York everyday along 50mph roads, and still averaged around 35mpg. Which is impressive, considering that it was designed to be a road-legal Le Mans car. Its not about the carbs being more fuel efficient, the point he was making was that he didnt need to put his foot to the floor to keep up with traffic.

Ask yourself a question, why do you want to fit bike carbs?

If its for more power, you will have to make an inlet manifold to fit them, so just make a inlet that has injector slots in it. Drill some holes, weld on some cylinders, and bolt on your standard fuel rail. Now fit your bike carbs which will just be used as throttle plates. You could really do with running a megasquirt with this because the fuelling will need to be wildly different from stock. But there you go, you will have the advantage of bike carbs (Individual throttle plates) and the advantages of EFI (it will fuel properly)

isnt that basically what the webber alpha system is?!which if i remember is hugely expensive, whereas bike carbs can be botched up for next to nothing and give you a few smiles with a battered £50 shitter before the engine goes bang! i know i wouldnt want to spend weber alpha money on a polo, a raddo maybe but not a polo!!lets face it if you start tuning an engine mpg prob isnt top of the list of priorities, however its a fact that bike carbs are far better mpg wise than twin 40s. carbs will never be as good mpg wise as injection, thats the whole reason injection is used on new cars these days!

Bike carbs are nice, simple, cheap way of of getting individual cylinder fueling, I'm sure everyone in an ideal world prefer and EFI system, but in most ways its a bigger pain in the arse than using bike carbs

Guys you are missing the point terribly

Bike carbs are meant for bikes!!

Anyone who fancies a pillion ride on an FZ1S with me is welcome & you will see 100mph in second gear. Without carbs....

they arent missing the point, they mean its a cheap way to get some nice induciton roar, better power and a bit more fun outta a car, bike carbs are just the same principle as any carbs so fitting them to a car isnt that different. You could also say twin 40s are meant for lotus's cos some of the older ones had them as standard!

I have bike carbs on my bandit too, it came with them and a dynojet kit and i know what id rather set up on a car, most people can do a bit of reading, scrounge some jets and needles and get bike carbs near enough. I wouldnt know where to start programming an ecu although it could also be read up on I'm sure but whats the point when the ecu alone prob costs near the price of most polos that would be getting bike carbs!

I'm building a polo with bike carbs just for fun, if i wanted somethin cheap and really fast id build a 2 litre nova or corsa, but i dont like them and id be happy to hit the ton and have a 1.3 screaming!

Bike carbs all the way for me in this case!

^^ Just wait everyone till the good old bike-carbed Landy racer with a 4.2 Jag straight 6 comes out the workshop next summer...

Ask yourself a question, why do you want to fit bike carbs?

If its for more power, you will have to make an inlet manifold to fit them, so just make a inlet that has injector slots in it. Drill some holes, weld on some cylinders, and bolt on your standard fuel rail. Now fit your bike carbs which will just be used as throttle plates. You could really do with running a megasquirt with this because the fuelling will need to be wildly different from stock. But there you go, you will have the advantage of bike carbs (Individual throttle plates) and the advantages of EFI (it will fuel properly)

isnt that basically what the webber alpha system is?!which if i remember is hugely expensive, whereas bike carbs can be botched up for next to nothing and give you a few smiles with a battered £50 shitter before the engine goes bang! i know i wouldnt want to spend weber alpha money on a polo, a raddo maybe but not a polo!!lets face it if you start tuning an engine mpg prob isnt top of the list of priorities, however its a fact that bike carbs are far better mpg wise than twin 40s. carbs will never be as good mpg wise as injection, thats the whole reason injection is used on new cars these days!

Using ITBs and making a maifold to house injectors would deffinately be the best idea. As said above and by others injection gives you better everything. carbs are always a compromise due to the very very simple mechanical design of fuel being drawn in through a jet.

Megasquirting it would mean fully mappable ignition so if you want fuel economy you've got it and if you want to squeeze every last pound of torque out of it you can.

isnt that basically what the webber alpha system is?!which if i remember is hugely expensive, whereas bike carbs can be botched up for next to nothing and give you a few smiles with a battered £50 shitter before the engine goes bang! i know i wouldnt want to spend weber alpha money on a polo, a raddo maybe but not a polo!!lets face it if you start tuning an engine mpg prob isnt top of the list of priorities, however its a fact that bike carbs are far better mpg wise than twin 40s. carbs will never be as good mpg wise as injection, thats the whole reason injection is used on new cars these days!

Did anyone say get a weber alpha system?

I said since you are making a manifold anyway, add your existing injectors to it by adding some injector tubes.

That will cost nothing except a little more time making the manifold. You can then use knackered carbs (bent needles, snapped jets, ovalled emulsion tubes, split diaphragms, worn slides etc.) Which can be picked up for 99p on ebay rather than £30 odd quid for some decent bike carbs. Use the knackered carbs as throttle plates and by the time you have picked up a carb pressure fuel pump (between £5 and £40 depending what you get) and the price you will pay for jets when you are setting it up you could easily spend £60 on a bike carb setup. Given that you can pick up a partial megasquirt kit from diyautotune.com for $68 (£33 due the very weak dollar) add £20 for the resistors and transistors you will need from farnell.com and make your own case and you have a good quality fuelling system that will return better fuel economy and a better power output with less headaches for similar money to bike carbs. Add to this that you can remove the megasquirt when you are finished and either sell it or add it to your next project, running bike carbs seems like a worse proposal.

^^ That actually seems like a very good idea

Sounds like hassle to me.

You dont need a manifold for bike carbs,i used four universal flanges and some pvc vacuum tubing.

Bike carbs are named as such,simply because bike manufacturers used them - they were designed for the four-stroke petrol engine. They're not better than efi throttle bodies and I've said that before. But they're a darn sight better than a single carb,easy to fit,cheap as chips,simple and reliable.

They were used this century on some fine machinery,are far more advanced than carbs developed decades ago for cars,and having a little squirter on each cylinder instead of a clumsy flush into a snatchy multi-junction manifold is obviously gonna make very accurate distribution of the fuel.

Injectors however,spray the fuel as a fine spray,and that theoretically gives the fuel more surface area,thus helping it burn - though much of this is lost in re-clotting on compression and through the valves.

The reason car manufacturers didnt use them is due to catalyst laws enforced on cars over fourteen years ago - cats require electronically adjustable fuelling to prevent over-fuelling,which damages cats,for those who cant control their right foot.

So for efficiency,if bike carbs are set up correctly,the engine will run at the correct mixture throughout the rev range. Rob's,it seems,were not - but they're not shy on full throttle,and you can only expect them to drink if you want max power from the engine.

But in any engine,with any fuel system,higher revs gives the fuel less time to burn - so unburnt fuel gets chucked out the exhaust at very high revs. The lower the revs,the cleaner and more thorough the burn.

So whilst its obvious that an engine running at 6k uses three times the fuel as the same engine running at 2k (as 3x2=6 lol),even more fuel is needed at 6k to compensate for poor burn. But its not really an issue at car-engine rev speeds.

My 1.6 MK2,which some of you may have heard is fitted with four siamesed Keihin carbs from a Honda CBR900F,is far more economical than a 1.3i Fiesta i also drive - as the Polo's torque on light throttle means i can travel about at the same pace at only half the revs.

Bike carbs are the most advanced carbs available,more efficient than spi and are more responsive than mpi thats running a poor flow induction system. Only multipoint injection with clean flow bodies (needed to reduce intake vacuum) can beat them - and such a system is very expensive and/or hassle compared to bike carbs.