Hey people,

See this question come up quite frequently, regarding methods of how to fuel bike carbs.

This is a bit dependent on the model of the car you have, as they come with different fuel pumps standard.

Firstly, it helps to understand the basics of the carbs as to how the fuel gets into them.

Bike carbs have a 'float bowl' this is a small reservoir on the bottom of each carb (which you have 4 of) this fills up with fuel, which the main/pilot/idle jets are submerged into (different carbs vary slightly). This is where the carbs get their fuel from when they are operating.

Within this float bowl, there is a float, this pivots, and attached to it is a needle with a tapered seat (please dont confuse this with the tapered needle in the throttle slide). This needle seals against the carb body when the float is pushed up inside the carb (forced up by the level of fuel in the carb, like a toilet system) this will then stop fuel flowing into the carbs from the supply, until some of the fuel in the float bowl is used, allowing the float to fall, which breaks the seal on the tapered needle.

Most bike carbs have a single fuel inlet connection, which is usually a 10mm fitting (seems common on 600cc carbs anyway) which then bridges all 4 float bowls together.

You are now in a position where you need to get fuel from your tank on the car, to this single fitting on the carbs.

You may have noticed, or read about many methods, I guess these can be summarised as follows:

Standard fuel pump - T piece method

Standard fuel pump - Regulator method

After market pump (eg a Facet pump) - T piece method

After market pump - Regulator method

Bike carb fuel pump

Obviously there are a variety of model polos, commonly mk2-4 seem to be having carbs fitted.

Early mk2's have a mechanical pump driven off the cam, however, most people do an engine conversion, so I'm not going to explain this one as the pump is lost.

Mk3's have both a lift pump and a high pressure pump for the injection system, according to Haynes, the lift pump has a higher flow rate than the pressure pump, however I am unsure of what pressure its capable of.

Mk4's have an in tank sender unit, which is the only pump on the car, SPI models have a lower pressure pump than MPI models.

Its worth adding at this point, that to get pressure, you must have a restriction, no restriction will allow flow.

So if you connected a high pressure pump directly to the carbs, the needles in the float bowls would be your restriction, the fuel pump would over power this, and fuel would leak from the carbs, hence why a method of controlling the fuel is required.

So.

T piece method.This is where you use both the cars feed and return fuel lines.

You create a loop, joining the feed and return line together, so fuel can simply be pumped from the tank, around this loop, back to the tank, this means there would be no resitriction (unless your return line was a smaller diameter etc)

Where you join the feed and return line (in the engine bay) you use a T piece fitting, the spare fitting on the T piece will then be connected to the fitting on the carbs, this allows the fuel to also run to the carbs, and the fuel will flow into the float bowls when required, if there is a restriction there, the fuel will simply continue in a loop down the return line back to the tank.

There are arguments as to which way to fit the tpiece, I have never ran a T peice so I wont comment.

This method can be used with the standard or aftermarket pumps, you just need to ensure there is enough flow (whether the standard lift pump alone is enough, I dont know, hopefully somone can add).

This is a good method, and has been tried and tested successfully, as there is no pressure build up causing leaks at the carbs, and the 'loop' almost acts as a fuel cooler.

You will have to mess around with wiring to get standard pumps to work without the ECU (mk4's ecu uses the fuel pump as an Immob, and a mk3's ecu controls the power to the pump when it knows the engine is running)

After market pump the same T piece method can be used, its worth adding:

A mk3 fuel sender with lift pump, fuel can be dragged through this, whether the lift pump is running or not - no need to modify

A mk4 fuel sender will not allow fuel to be dragged through it without it running, I modified mine in the 6n so it was just a pickup, I will look for pics.

Summary:

Easy method

Cheap (will just need fuel hose, clips and a T piece, maybe some wire and conncetors)

No pressure build up (unless a restriction was added or the return line was squashed etc)

Tried and tested successfully with a variety of pumps.

Regulator method. There is more than one type of regulator.

Ok so you may want to run a fuel pressure regulator.

Its worth noting now, there is more than one way of regulating the fuel pressure.

You can buy an 'inline' reguatlor, which is virtually just a restrictor - I am not convinced by these.

A decent regulator has 3 fittings, one feed in, one feed to the carbs, and one is a return.

This type bleeds the fuel off down the return line.

You can also buy a 'turbo' regulator if youre running boost. This will have a vacuum nipple to be connected from the inlet manifold, the variance in inlet vacuum due to boost will adjust the fuel pressure to suit the conditions.

If youre running a regulator, youll need a pressure gauge too. The fuel should be regulated to 3psi for bike carbs (as per Bogg Bros instruction).

If you intend on running an injection pump, youll likely need two regulators, due to the massive change of fuel pressure from what the pumps designed for.

You will be able to get hold of one, to take it from say 3 bar to 1 bar, then another which will regulate 0-1bar of pressure (1bar = 14.7psi)

IMO this method isnt the best, pretty expensive and hassle, but others run it with no issues. However some have tried just T piece and get leaky carbs, but I dont know what state their carbs are in, and how free their return line is etc.

Bike fuel pump method. A pump designed for the job...to an extent.

A bike fuel pump, and bike carbs..sounds a good idea hey?

These are a clever pump, virtually the same on all carb'd bikes.

These 'self regulate' and do not need a return fuel line.

Simply a fuel feed to the pump, then a line from the pump to your carbs, power and ground, and done.

The pump turns on and off with fuel demand (see video).

This sounds a good idea, however, its worth remembering, the tank on a bike is mounted above the pump, so effectively you have a gravity fuel feed to the pump, however on your car the tank is far away and the fuel is going to be dragged upwards out of the tank.

This said, I have ran one in both my Carb'd cars with no issues, or leaks.

You can pick these up for around £30-£40.

It maybe worth having a spare, as asking the extra amount of work from it may reduce its life - who knows.

Video to explain kinda.

http://s963.photobucket.com/albums/ae114/h...2DA8F2141E7.mp4

If youre going to wire any fuel pump up, Id recommend fitting an inertia switch. This is a switch which when shocked hard (crash) it will break the feed to the fuel pump, turning it off.

So if you crash, and a fuel pipe was broken etc, and you were unconcious, you could end up with fuel spraying everywhere, this will prevent it.

I crashed and the inertia switch turned my fuel pump off, so thats tried and tested too!

Little info on the carbs:

You may buy a set of used carbs, throw them on, and get fuel leaking from around the T pieces between the float bowls (where the fuel feed goes in) this is often due to the O rings on these going hard as the carbs havnt been used, or thembeing perished and broken.

I stripped my carbs when I got them and had this issue, they were in good condition aesthetically, so I soaked them in neat petrol for around an hour and were fine after that.

Didnt use the engine for 2 months, and the same thing happened again, just the fuel spraying out of them for 5 mins, then being left overnight fixed that one.

There are two pipe connections on top of the carbs, these are vents/overflows from the float bowls, these MUST vent to atmostphere (unless youre boosting the carbs) a restriction on these will make your car run stupidly rich.

Mine are connected to a catch bottle in the engine bay, away from hot things.

Hopefully this helps some people.

I stand by the bike pump method, however others have ran the other methods and will stand by them, so it depends what you want, kinda depends on what car you have and what sort of money you want to spend and such!

Nick

Edited 19 August, 201214 yr by harris.racing

all mk2,s have a mechanical fuel pump bro, unless its a german NZ or G40

The standard lift pump was enough to fill up a set of DCOE float bowls, on its own, so i'd image it will be enough to do the 600cc carbs.

I'd have though it would be, not sure about with a reg though?

Do mk2f's even have a inertia switch as standerd :S lmao. Where is it ?

Not sure. In newer cars they can be built into the ECU I think.

On Cymro's build thread he explains about the mk2f wiring.

Wasnt to sure as i've never seen one on the car . There only a 5ver so theres no exuses in not getting one tbf lol ..

So if i put it on the Igntion feed side it shall cut the power to pump and coil aswell. Less chance of fire .

Nah they don't.

Sticky, Pin ? mods?

I found it useful and I'm sure alot of others will too :)

Current..

Tank to filter to pump to swirl pot to..

Mid outlet - carbs

Upper outlet - tank

All previous..

Tank to filter to pump to t-piece to tank, with the carbs on the t-piece.

Swirl pot replaced t-piece just to give a more reliable feed, after a couple of lean moments on the bypass

Not sure. In newer cars they can be built into the ECU I think.

On Cyrmo's build thread he explains about the mk2f wiring.

6N2's onwards use the airbag ecu to kill the fuel pump relay in the event of a bump!

bump. Do you have to split the carbs if you want to replace the Fuel t piece oring ?

bump. Do you have to split the carbs if you want to replace the Fuel t piece oring ?

Yes. Use a little grease on them, too (the o-rings that is - though there's no harm on the carb join tubes too, because they can seize the carbs together if moisture gets to them).

Once put together again, they'll need fine-tune level of re-syncronizing.

you dont have to completely split them. just undo the bolts holding them together, and slowly prize apart the carbs just enough to wiggle the t piece out. tis a bit easier that way as you dont have to take them off the car.

Yeah man agree with mikey, if your carbs have been sat they'll likely leak too!

So replacing the orings/soaking in neat fuel is a good idea

Ill see how they are .. Were ran on a car about 3 months ago so should be ok tbf :)

Mine leaked after being off my 6n for a month or two lol.

Can this be FAQ'd please!

From personal experience i think the bike pump is by far the best option, i used the t piece with a facet pump to begin with and ran this for 18 months, however the thing that really got on my nerves was the constant noise from it which was especially loud if the engine was off but the ignition was on for radio etc

I know this could have been solved with a switch but i also thought it looked very messy, so i switched to a bike pump and it is much better, it only primes for a couple of seconds and then no more noise, and due it self regulating i never have a problem with it leaning out or running rich due to the pump.

Its a winner everytime for me, i dont see why anyone would use anything else to be fair

Edited 9 September, 201214 yr by dubownerbilco

From personal experience i think the bike pump is by far the best option, i used the t piece with a facet pump to begin with and ran this for 18 months, however the thing that really got on my nerves was the constant noise from it which was especially loud if the engine was off but the ignition was on for radio etc

I know this could have been solved with a switch but i also thought it looked very messy, so i switched to a bike pump and it is much better, it only primes for a couple of seconds and then no more noise, and due it self regulating i never have a problem with it leaning out or running rich due to the pump.

Its a winner everytime for me, i dont see why anyone would use anything else to be fair

This.

It's made for the job so can't get much better .. be better if you could get the outlets to be at the bottem of the tank lol

My first post in the FAQ topic has a list of links to various other useful bike carb threads, i've added this one to it.

A little late but HÜCO suction pumps are self-regulating low-pressure high-volume pumps that should be good for bike carbs.

I'm using a 133010 in the engine compartment.

Good link that!

Ideal to buy a new one instead of used bike one. New bike ones are ££

To clarify are you using the bike pump to pull fuel right from the tank? Or are you using another pump to feed the bike pump? thanks

Hi, can i fit bike carbs to my 1.3 litre polo? Are they a straight fit? where do i drill my distributor to fit my new jets?

Can i mount my carbs upside down so they look better?

LOL

Seriously though, has anyone tried fuelling them off of a GT fuel pump + T-peice?

I don't want to be changing the pumps over for one go at the pod so would rather just T peice the exisiting stuff and run a line to the carbs, the GT pump is 3 bar iirc.