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.

[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

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

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.