Pumping it up!

[The Pastor]

anyone added a fuel pump to a gravity feed carbed bike before?

[Ixion]

Um, why would you? Or do you mean a pump to pump fuel from an auxiliary tank to the main tank?

A pump trying to pump fuel to a conventional carb wont work very well. You'll have to somehow convert the float valve into a bypass valve.

And again, why? Gravity is about the best pump there is.

[reofix]

hell....get a 4 inch pump and really ram some gasoline into that baby!!

[Maha]

anyone added a fuel pump to a gravity feed carbed bike before?

Yes I have, but it fell off.

[The Pastor]

Um, why would you? Or do you mean a pump to pump fuel from an auxiliary tank to the main tank?

A pump trying to pump fuel to a conventional carb wont work very well. You'll have to somehow convert the float valve into a bypass valve.

And again, why? Gravity is about the best pump there is.
my float bowls are empty at high revs - somthing about no airbox and large as hell jets :S

the bypass valve, would that do the same as a bypass return pipe to the tank?

[engineer002]

Hi

Ive had the same issue in the past and one thing most people forget when they tune a bike is the fuel petcock is designed to flow so many litres per hour for a normal engine in a normal state of tune.

I ended up having to change the plumbing from the tank to the carbs to a much bigger bore to get the fuel flow at high rpm's this included the fuel on/reserve petcock.
:whistle:

[The Pastor]

there is a fourm (albeit a bit dead) on cbr250.com that people have done what I was thinking - but no good search feature or much detail on how they did it. but they have had some success with the fuel pump.

I guess im just wanting to know if its as simple as slicing the fuel line and putting in a pump. or a little bit more involved.

[Reckless]

We used fuel pumps on our karts. Two types, a mechanical pump off the rear axle or a most commonly a pulse pump off crankcase pressure. I 'd guess it must be a 2smoker for the pulse pump type(i have no experience with 4 strokes). The trick is to run a return line back to the tank via a y juction. You have to have the lines the correct pressure so you don't force open the float valve with the fuel pressure and flood the carb. We used this system of because of coarse our tanks weren't above our engines. In our road racing set ups we used hollowed out car fuel filters as small reserve tanks just above the carb. As flat out down pukekohe back straight the pulse pump couldn't keep up especially with an adjustable power valve taking methanol from the fuel bowl and stuffing it directly into the back of the carb. But this system basically amounted back to a small reserve gravity feed tank for the end of the straight??

So I think we've come full circle I agree with the above comments you should be able to get enough flow from your tank with gravity as long as the petcock and lines are of sufficient size. I know my ZIR has a different size petcock than the 900 of similar years for this very reason.
Secondly one thing people always forget to check is the breather. Make sure your breather hole is large enough to let the air in at the same rate (or higher) than you expect to let the fuel out or it won't happen. Fuel breather holes are often in the cap and are very small. If the air isn't getting in to let the fuel out you will starve your carbs especially at sustained high revs.
Just a few hints??

[Katman]

A vacuum operated fuel pump as is on the Suzuki LTF300s might be all you need.

[The Pastor]

We used fuel pumps on our karts. Two types, a mechanical pump off the rear axle or a most commonly a pulse pump off crankcase pressure. I 'd guess it must be a 2smoker for the pulse pump type(i have no experience with 4 strokes). The trick is to run a return line back to the tank via a y juction. You have to have the lines the correct pressure so you don't force open the float valve with the fuel pressure and flood the carb. We used this system of because of coarse our tanks weren't above our engines. In our road racing set ups we used hollowed out car fuel filters as small reserve tanks just above the carb. As flat out down pukekohe back straight the pulse pump couldn't keep up especially with an adjustable power valve taking methanol from the fuel bowl and stuffing it directly into the back of the carb. But this system basically amounted back to a small reserve gravity feed tank for the end of the straight??

So I think we've come full circle I agree with the above comments you should be able to get enough flow from your tank with gravity as long as the petcock and lines are of sufficient size. I know my ZIR has a different size petcock than the 900 of similar years for this very reason.
Secondly one thing people always forget to check is the breather. Make sure your breather hole is large enough to let the air in at the same rate (or higher) than you expect to let the fuel out or it won't happen. Fuel breather holes are often in the cap and are very small. If the air isn't getting in to let the fuel out you will starve your carbs especially at sustained high revs.
Just a few hints??

are you suggesting that if i make a container in front of the carbs as an extra tank it might be enough to deliver the fuel when its running a bit dry? because that sounds a lot cheaper to me :niceone:


A vacuum operated fuel pump as is on the Suzuki LTF300s might be all you need.

I've seen those around - how do they work?

Also how does a fuel relay work? Could I put one on my bike or will i have to run the return pipe?

[Reckless]

are you suggesting that if i make a container in front of the carbs as an extra tank it might be enough to deliver the fuel when its running a bit dry? because that sounds a lot cheaper to me :niceone:

No! I'm more suggesting that Gravity feed should be enough! Check your Petcock, fuel lines, Test all with the fuel filter out (if you have one), dirty fuel filters where a major part of a lot of problems people had, check to make sure air can get in to the tank at a faster rate than you expect the fuel to get out (or leave the cap ajar for a lap or to), make sure your float levels are correct so there is enough fuel in the carbs in the first place, and the float vavle is working and big enough.

If all the above is A+ I have trouble seeing any set of carbs can outrun the fuel supply! But I am far from an expert here and some others may have helpful suggestions.

BTW:The extra tank thing was only because the kart pulse pump couldn't keep up with the flow at full acceleration on the main straights and needed the rest of the track to top up the little tank to give that bit extra in reserve. You shouldn't have this problem with gravity feed as you already have that big steel tank you fill at the gas station (Same thing).

[The Pastor]

cheers mate,

let me clear a few thing up.

My bike has modified jetting - stock is 105/110 size main jets - now it has 145 mains.

it only empties at full tit at 14-15000 rpm - when it would be useing the most i imagine.

My bike does not have adjustable floats.

But what you are saying is that the fuel tap inlet diameter and the connection to the carbs are too small to allow the ammount of fuel to get in? In which case a pump would be needed to force it through the smaller diameters at a higher veloisity?

[pete376403]

The bottleneck is the needle valve in the carb. Look at the size of the hole the fuel has to get through - it's many times smaller than the fuel tap or fule lines.
When I was riding speedway the 40mm Mikuni I had on the Jawa had the same problem. the fix is to cut a slot through the side of the needle valve holder just below the point where the tapered needle seats. The slot was cut using a milling machine and a cutter about 1mm wide.

[Reckless]

Yeh good point pete! But still cannot think that going from 110 to 145 main would drain the carbs, you'de have to hold it full throttle for heck of a long time for the flow out to exceed whats coming in given that all the other factors above are A1. But you could well be correct. Not to up on anything but 2stroke but MX engines but isn't that a big jump for just taking the airbox out? I think there may be more to this?

Further to your suggestion if the float level is set far to low as well it could have the same effect. Secondly if the float needle is sitting quite high in the valve you could get a jet drill and drill it out clean up the seat a bit and still have it seal properly.

I just want to say again the smallest hole in the system is often the breather in the tank lid it could be as simple as the tank is pressuring at full tit and with holding the fuel up in the tank! Simple to check!


My bike does not have adjustable floats.
I think you'll find it does you just have to know what to bend a little.

Secondly I think your not quite getting the jist of what we are saying prob need to get someone to look at it mate, the answer to your problem should be fairly simple.

Thats it for me, getting out of my small area of knowledge ( the mechanics types here are already probably having a laugh)

Merry Xmas

[roadracingoldfart]

I had a similar issue with a big bore 400 years ago. I took a leaf from the sidecar boys of the day and installed a small tube rail above the carbs to allow a small capacity tank that was unafected by lean angel and sloshing. It sounds a bit rough but i assure you it worked a treat with std tank tap etc.
I had gone from std 19 mm carbs to 26 mm sliders lol . The poor thing would hold top gear full noise for only about half the length of Manfeild back straight then cough and splutter till the fuel caught up.
After the tube modification i did a sprint over the Wairarapa and in the flying 1/4 mile (1/2 mile run up) it never had an issue, my problem was solved.

Cheers Paul.

[The Pastor]

to check the breather hole, i could just ride with the filler cap open?

I just tried that, and it didnt change a thing.

Also would the float level change when you play around with the jetting?

[Motu]

I doubt if a 250 would suck enough fuel to flow faster than the fuel lines and tap...I think you have another problem.

To run alcohol on standard carbs you needed bigger hoses and fittings to cope with the flow - maybe you should look into an alky set up.My Blue Magnum's can run petrol or Alky,they have 10mm hose fittings.

[The Pastor]

if anyone is wondering, i pulled the plugs and they are ok

[Squiggles]

Does it still bog mid revs?

[The Pastor]

Does it still bog mid revs?
it only bogs on low revs - 7000 and under

actually mid revs are all ive got

7-14000rpm is my usable range - which is good for road riding. But obviously I want the top end :)

[The Pastor]

ive been having a chat to a mate who knows everything there is to know about carbs and cbr250's

he reckons i need trumpets on my carbs to reduce the pressure at the top end.

I will have a go and report back! - he said the reason he needed the fuel pump was for long periods of time at wot in 6th.

[pete376403]

Wow, thats a theory I haven't heard before.

I always thought that bell mouths (trumpets) on carbs were to provide a smooth transition for air entering the carb, to avoid the shock wave that a sharp edged step (ie the carb opening) would cause, which effectively reduces the throat size.

When running extreme overlap cams there will often be blow back through the carb and the bell mouth assists in capture of the fuel/air "fog" so it can be drawn into the engine, rather than wasted. The length of the bell mouth is also used to assist in the ram effect, but this tends to have greatest effect at one particular rev range. Variable lengths can be used to broaden the effective rev range.

Thats what I thought anyway

[The Pastor]

sorry i wasnt clear on my post before

he said the trumpets reduce pressure inside the carb (lower than atsomopheric pressure) therefore sucking in more petrol.

Time will tell how it works!

[motorbyclist]

are your 250 honda electrics going to be reliably up to the task?

are you sure this isn't a kink in the fuel line, blocked filter or petcock issue - my pocketrocket died at high revs simply because the fuel line went uphill at one point and was holding an air bubble!

the issue could be a carburation one - you've removed the airbox and opened the jets to compensate... unfortunately you've gone and undone what honda engineers spent a lot of time getting right. perhaps the intake cam timing etc requires a certain vacuum at the intake end to function smoothly, as does the exhaust end need the right backpressure and pipe length to work at it's optimum for the exhaust timing


if all that is ok and the petcock will still work, then the pump will need to have a means of pumping fuel back into the tank (via a relief/bypass valve), or be able to reliably stop/start and maintain flow while it isn't running

most importantly you must be sure that the bike will be able to start!


my suggestion would be to run the bike on a 600ml coke bottle of petrol or similar to ensure the problem is not the float valve (by eliminating the petcock/filter) - then fill the tank to the brim to ensure that an increase in fuel pressure will infact provide a solution


then as a final solution, go nick the working fuel system from another honda or whatever fits rather than pissing around designing a fuel pumping system yourself



he said the trumpets reduce pressure inside the carb (lower than atsomopheric pressure) therefore sucking in more petrol.


how exactly?

they improve the flow rate by the methods described above - which would indicate a pressure closer to atmospheric than without them

fuel is (primarily) drawn into the carburetor from the bowls by the venturi effect

[pete376403]

sorry i wasnt clear on my post before

he said the trumpets reduce pressure inside the carb (lower than atsomopheric pressure) therefore sucking in more petrol.

Time will tell how it works!

Well thats not quite how it works either. The only thing that makes air (and with it the fuel) enter the carb is atmospheric pressure, assuming a non turbo/super charged engine. What happens in a carb depends a lot on Bernoullis Principle (Google it) Basically this says when air has to increase speed, it will reduce pressure.* The carb has a restriction (venturi) near or at the point fuel is introduced to the air stream. The restriction will cause the airspeed to accelerate. Now the fuel is at atmospheric pressure also - the air pressing down on the top of the fuel in the float bowl provides this. The pressure in the fuel system up to the needle valve is irrelevant, because the needle valve is maintaining a level of fuel in the bowl,then closing when this level is achieved. So you have fuel at atmospheric pressure, and air, because it is speeding up, at less than atmo pressure. This causes fuel to rise up the jet holder and into the venturi, where is mixes with the air, to pass into the engine.

You don't want the reduction in pressure to occur anywhere but in the venturi. Bell mouth/trumpets are not going to achieve this, but they will smooth the airflow into the carb, and they can be used to alter the tuned length of the intake.
And they look quite cool.
*the opposite is true, when air speed slows,it increases in pressure. this occurs in, for example, turbocharger compressors - the air comes off the turbine blades at high speed, enters the scroll housing where it is slowed down and the rise in pressure results

http://en.wikipedia.org/wiki/Bernoulli's_principle#Real_world_application

[The Pastor]

yeah thats pretty much what he said. trumpets increase the speed (by reducing the diameter) which lowers the pressure....

or something!

- andrew, i was going to nick one of a mc19 cbr250 but they are prone to death. I might not need a fuel pump but we will see how it goes, i'll re asemble the bike today, sunburn willing!

[Ixion]

You could of course quite easily check this theory about the float bowls running dry.

Take the bike to whatever revs it is it dies at, let the engine die, then (if your tap is not vac operated) turn off the petrol , pull in the clutch hit the kill switch . Roll to a stop, hop off and undo the carb drain plugs. If the bowls are empty, no petrol comes out. Theory proven. Otherwise, otherwise.

[Squiggles]

You could of course quite easily check this theory about the float bowls running dry.

Take the bike to whatever revs it is it dies at, let the engine die, then (if your tap is not vac operated) turn off the petrol , pull in the clutch hit the kill switch . Roll to a stop, hop off and undo the carb drain plugs. If the bowls are empty, no petrol comes out. Theory proven. Otherwise, otherwise.

There be no need for sense in this thread! The man wants to burn money so lets just support his efforts rather than talk of floats and adjusting their levels... :msn-wink:

[The Pastor]

I've attached the trumpets and went for a ride, its pretty sweet now.

[The Pastor]

so i guess in the end it wasnt draining the carbs, just running really lean.

[The Stranger]

My bike has modified jetting - stock is 105/110 size main jets - now it has 145 mains.

it only empties at full tit at 14-15000 rpm - when it would be useing the most i imagine.



You need more head.



My 750 was sweet so long as the tank was over half.

[Mrs Busa Pete]

[QUOTE=The Stranger;1871996]You need more head.



What are going to give it to him :bleh:

[Squiggles]

just running really lean.

Careful, That'd make me right a couple of months ago :buggerd:

[motorbyclist]

so i guess in the end it wasnt draining the carbs, just running really lean.

so perhaps an air filter would be a good solution to this problem, along with ensuring you bike survives the next 10000km?

[The Pastor]

Careful, That'd make me right a couple of months ago :buggerd:
nah you'd be right a few months ago - but it wasnt running as lean as it was with bigger jets in it!

[The Pastor]

so perhaps an air filter would be a good solution to this problem, along with ensuring you bike survives the next 10000km?
thats just silly talk!

[motorbyclist]

thats just silly talk!

of course what was i thinking:rolleyes:



perhaps just adapt a pod filter to the end of all the trumpets then? sounds much more difficult than simply chucking a standard airbox back and still sounds like a performance mod:niceone:

[The Pastor]

problem with that is the carbs are too close together for pod filters - oval ones would work but @ $120 each - and I need 4, its way to expensive.

But on the cbr250, sweet FA can get into the air intake area, no masive gaping holes etc.

Im looking at making a filter of some sorts, just enough to stop big stones / washers and bolts from going in etc.

If it was a naked bike or had some big air intakes that collected bugs etc i'd be more worried, but it doesnt, so im not :)

[motorbyclist]

problem with that is the carbs are too close together for pod filters - oval ones would work but @ $120 each - and I need 4, its way to expensive.

that's half the challenge mate!

where's your sense of adventure?:eek:

[The Pastor]

ive had pods on it before, they didnt give me the power i needed:devil2:

[motorbyclist]

right, so cutting out at 11 is the power you need?


doesn't matter too much, but when blue smoke comes out and you find new rings hard to find, remember - i told you so :bleh:

i learnt the hard way too

[The Pastor]

its running fine now with the trumpets mate

dont worry a rebuild is all part of the plan ;)