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Old 06-23-2003, 03:36 PM   #1
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Default Carburetion 101, gravity feed to fuel pump....

Ok, I'm working on my project and I need to convert this motor (bike engine) from gravity feed to fuel pump driven.

I'm using a mechanical mikuni fuel pump. My idea is to run the fuel feed from the tank to the pump, then run the pump to a tee, one side goes directly to the carb, and the other side goes to some sort of restrictor and then back to the tank as a return line.

Since I'm sure with gravity feed the "pressure" is very low, it seems as if I'd need some sort of pressure regulator as the restrictor on the return line. Probably something around 1psi. I guess I could just find some fitting, drill a hole in it, and shove it inline... but that seems really hokey.

Anyone know of an available part that can regulate 1psi or so? This is very low flow, using 1/4" fuel line. I've seen some adjustable regulators that can go as low as 1psi, but they sort of seem like overkill for my situation.
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Old 06-23-2003, 03:39 PM   #2
 
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Default Re: Carburetion 101, gravity feed to fuel pump.... (dustin)

I think you would need to look for special fuel pumps, something of the high volume / low pressure variety. Then get yourseld a matching regulator.

EDIT. take a look into ATV's as they use fuel pumps.
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Old 06-23-2003, 03:40 PM   #3
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Default Re: Carburetion 101, gravity feed to fuel pump.... (tonyXcom)

Well I already have a mikuni mechanical fuel pump that will work fine. I can't just force feed the carb or I'm sure fuel will spray everywhere, but if I don't restrict the return line some how I don't see how any fuel will get to the carb when it would be easier to just return to the tank.
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Old 06-23-2003, 03:43 PM   #4
 
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Default Re: Carburetion 101, gravity feed to fuel pump.... (dustin)

how bout if you adjust whatever drives the fuel pump to control the output.
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Old 06-23-2003, 03:43 PM   #5
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Default Re: Carburetion 101, gravity feed to fuel pump.... (tonyXcom)

Yeah I found a part that would work on a '85 or so Honda Odyssey ATV... and I even have the part number. But I can't find anyone that carries the part. Hehe.

It's part # 16701-VMO-770
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Old 06-23-2003, 03:44 PM   #6
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Default Re: Carburetion 101, gravity feed to fuel pump.... (tonyXcom)

Quote:
Originally Posted by tonyXcom
how bout if you adjust whatever drives the fuel pump to control the output.
Well I need active regulation of pressure. Pressure is created by restriction in the return line....

The fuel pump is a pulse pump, it is driven off crankcase vacuum.

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Old 06-23-2003, 03:48 PM   #7
 
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Default Re: Carburetion 101, gravity feed to fuel pump.... (dustin)

Try these guys
http://www.mach1motorsports.com/

They have been around for a long time and they may have access to weird **** like that. They are in vallejo.

How would you control the FPR? Vaccum? Or do carbs need a constant pressure?
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Old 06-23-2003, 03:51 PM   #8
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Default Re: Carburetion 101, gravity feed to fuel pump.... (tonyXcom)

They just need constant pressure to keep the float bowl full. And the fuel pump is powered by crankcase pressure, so lower rpm = lower fuel flow.
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Old 06-23-2003, 03:54 PM   #9
 
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Default Re: Carburetion 101, gravity feed to fuel pump.... (dustin)

Then why do you need a regulator and a return line? maybe you need to regulate it at the vaccum line from the crankcase to the fuel pump and regulate the amount of vaccum which would regulate the output to 1psi at idle or whatever you need.
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Old 06-23-2003, 03:56 PM   #10
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Default Re: Carburetion 101, gravity feed to fuel pump.... (tonyXcom)

Quote:
Originally Posted by tonyXcom
Then why do you need a regulator and a return line? maybe you need to regulate it at the vacuum line from the crankcase to the fuel pump and regulate the amount of vacuum which would regulate the output to 1psi at idle or whatever you need.
Kinda what I was thinking, it might be best to control the vacuum rather than the fuel.
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Old 06-23-2003, 03:57 PM   #11
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Default Re: Carburetion 101, gravity feed to fuel pump.... (tonyXcom)

But I need 1psi all the time, these carbs were designed to run off gravity feed.

The pump works by pulse... i.e. pressure then vacuum of the changing crankcase volume (pistons going up and down). How would you regulate pressure/vacuum in both directions?

Placing a restrictor in the fuel line with a 1psi spring will regulate pressure on the in side to exactly 1psi regardless of flow. Hence, at higher engine RPM I will need more fuel flow, but still only 1psi of pressure. I just need to keep the float bowls full... like gravity would.
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Old 06-23-2003, 04:00 PM   #12
 
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Default Re: Carburetion 101, gravity feed to fuel pump.... (dustin)

Quote:
Originally Posted by dustin
Placing a restrictor in the fuel line with a 1psi spring will regulate pressure on the in side to exactly 1psi regardless of flow. Hence, at higher engine RPM I will need more fuel flow, but still only 1psi of pressure. I just need to keep the float bowls full... like gravity would.
Is that what the ATV part you were referring to is?
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Old 06-23-2003, 04:00 PM   #13
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Default Re: Carburetion 101, gravity feed to fuel pump.... (dustin)

Also, I'm checking out that site you listed. All the ATVs look to be gravity feed. Do you know of a model on their site that has a pump/regulator?
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Old 06-23-2003, 04:01 PM   #14
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Default Re: Carburetion 101, gravity feed to fuel pump.... (tonyXcom)

Quote:
Originally Posted by tonyXcom

Is that what the ATV part you were referring to is?
Yeah. It's just a very small and simple fuel pressure regulator. But I can't find a source for it, so I can't even confirm that it exists. So I was hoping someone else had a comparable idea.
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Old 06-23-2003, 04:01 PM   #15
 
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Default Re: Carburetion 101, gravity feed to fuel pump.... (dustin)

I suggested them so that you can call them and feed them that part number. They are a local honda motorcycles dealership and have been around for a long time, so they might have a lead to old parts like that.
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Old 06-23-2003, 04:04 PM   #16
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Default Re: Carburetion 101, gravity feed to fuel pump.... (dustin)


dustin:

That's the same setup we use on karts. The dirt bike motors are designed to be gravity fed by the fuel tank (above the engine). Once the engine is on a kart, it needs a pump.

The "T" and restrictor setup will not work, IMO. That was one of the biggest problems when shifter karts initially gained popularity.

What does work is a thing called the "Magic Can".

The magic can is a small cylinder. It has two 1/4" barbed fittings on the bottom. The fuel from the pump goes into one of the fittings and the other fitting connects the can to the carb. At the top of the cylinder is a 3/8" barbed fitting. You connect a line here and run it back to the tank.

So, the can acts as a small reservoir to simulate gravity feed. In certain conditions, the pump will exceed the demands of the engine. In this case, the excess fuel takes the path of least resistance (the 3/8" fitting) and goes back to the tank.

Here's a pic of my setup without the can:
Click the image to open in full size.

Here is a diagram of how it works:
Click the image to open in full size.

Very simple and works like a champ. I've tried many things, but nothing works as well as this.

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Old 06-23-2003, 04:07 PM   #17
 
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Default Re: Carburetion 101, gravity feed to fuel pump.... (Sonny)

that looks like the answer right there Click the image to open in full size.

I have messed with shifter carts, but never knew how the fuel setup worked. thanks for the good info.
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Old 06-23-2003, 04:11 PM   #18
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Default Re: Carburetion 101, gravity feed to fuel pump.... (tonyXcom)

now the important question: what is this 'project'?
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Old 06-23-2003, 04:11 PM   #19
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Default Re: Carburetion 101, gravity feed to fuel pump.... (Sonny)

Hmm that looks like a good idea. The can needs to be above carb level obviously.

Am I not thinking straight, or are there no other restrictions on piping locations?

The project is actually, and ironically, a trackmagic shifter kart with a 500cc kawasaki engine. The carbs are really high already relative to the rest of the kart, about as high as the top of the seat.


Where can I source such a can? I guess some sort of catch can would work with modification. Any suggestions as to size? I'm trying to feed two keihin carbs.
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Old 06-23-2003, 04:20 PM   #20
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Default Re: Carburetion 101, gravity feed to fuel pump.... (dustin)

Also Sonny, do you really think the world would end if I used 1/4" fuel line everywhere? That's all the bike used when this motor was in it.

Or is the flow differential what makes this entire system work?

I might be able to fabricate a "magic can" out of some sheetmetal stainless. My aluminum welding skills aren't very good.
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Old 06-23-2003, 04:27 PM   #21
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Default Re: Carburetion 101, gravity feed to fuel pump.... (dustin)


500cc? LOL...are you crazy? Click the image to open in full size. They don't even race 250's on the sprint tracks any more because you can't keep the front tires on the ground.

I think 1/4" line will work fine. Some of the fast 250's that run at Sears use this setup with no problems.

It is the pressure differential that makes this setup work. If the return line to the fuel tank was the same size as everything else, it wouldn't work since the fuel has no "easy" path.

What engine is this? A 500cc 2 stroke or a 500cc 4 stroke?

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Old 06-23-2003, 04:31 PM   #22
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Default Re: Carburetion 101, gravity feed to fuel pump.... (Sonny)

4 stroke

it's a '94 500EX (ninja) motor.
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Old 06-23-2003, 04:42 PM   #23
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Default Re: Carburetion 101, gravity feed to fuel pump.... (dustin)


I'm not sure what kind of power that setup makes, but a strong 125cc 2 stroke makes about 45hp and a stroke 250cc makes 80-90 hp.

I think your fuel setup will be fine, but you are gonna have to add a lot of weight to the left side of your seat to balance out that tank motor. Click the image to open in full size.

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Old 06-23-2003, 06:56 PM   #24
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Default Re: Carburetion 101, gravity feed to fuel pump.... (Sonny)

I'm just going to eat a lot of nachos.
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Old 06-24-2003, 12:01 AM   #25
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Default Re: Carburetion 101, gravity feed to fuel pump.... (Sonny)

Can I use any source for the pulse port on the pump Sonny?

I.e, would a port on the valve cover work fine... or do I literally need to tap the crank case? How much "pulse" does it really take to make this pump work right.
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