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Old 02-08-2016, 08:53 AM
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69ISH 69ISH is offline
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Name: Rodney
fuel aerating in fuel bowls

I have had my car back on the dyno for the second time in the last month.
1st time I was told that the fuel was boiling and the car had breathing issues, It was recommended to re route my fuel hose which runs from the pump from the carb which was running up and across my motor to where it now is which it runs out to the front of the car and tgen across and under the battery and then up to the carb. I also wrapped the fuel hose in insulation.
I also modified my shaker so thatt there is more room for airflow, sits lower so that I was able to fit a 1/2" wood spacer and fitted the air filter with the lid that is also a filter.
I also fitted a new 110gph mechanical fuel pump as to make sure tgere were nomissues there.

After this the car still ran like **** and I lost 3/10ths of a second and 4Mph.

It went back on the dyno yesterday and when it gets up over 5000rpm approx you can see the fuel in the bowls starting to aerate that gets worse the more you accelerate and eventually fuel starts flowing out of the fuel bowl vents, THIS WAS ORIGINALLY BELIEVED TO BE THE FUEL BOILING. The fuel now is definitely cold and I did have my finger in tge fuel as it was pissing out to confirm it definitely was not heat related.

We removed both fuel filters, swapped carbies with a known trouble free one, swapped needle and seats with brand new ones tried different float levels and still had the same issue every time.

I am thinking it is either the fuel pump is making more than its 8psi limit or there is air being sucked in between the pump and the tank.

Has anybody got any other suggestions as to what may cause this issue or how to resolve it.

On a good note it has now made 391rwhp without still being tuned properly which I am very happy with considering it only made 348rwhp with the speed demon 750 (also not tuned). I am confident it will easily make 400 once this issue is resolved and if I had 5k to spend I would swap the heads (they were off the 347 that I killed) cam and intake.
Cheers Rodney
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  #2  
Old 02-08-2016, 12:45 PM
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D4RKSIDE D4RKSIDE is offline
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I had no idea but you sparked my interest.
good discussion here:
http://speedtalk.com/forum/viewtopic.php?f=1&t=43020

And an article:
http://www.prosystemsracing.com/010211scoop.html

Seeing as you swapped carb and still had the issue. Look at the regulator (or lack off) and fuel lines.
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Old 02-08-2016, 04:41 PM
Intruder Intruder is offline
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Are you running a return line and if you are where from?
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Old 02-08-2016, 05:17 PM
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You will need to run a return line back to the fuel tank
ask me how I know
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Old 02-08-2016, 06:25 PM
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BlackoutSteve BlackoutSteve is offline
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Fuel boiling in the carb is called percolation and even fuel that was bought in Winter and used in warmer months can cause this.
Assuming the 1/2" "wood" spacer is a phenolic spacer between the carb and intake? If so, yes you need this to help keep the carb cool.
Air-gap manifolds like the Air Gap and Victor Jnr also help keep the carb cooler than low or high-rise "solid" dual plane styles.

Make sure your fuel pressure is not too high (6 psi is plenty) and I'd suggest fitting a regulator even with fuel pumps that "self regulate" like the Holley Red for example.
If you do, or already have a reg, make sure it is mounted as close to the carb as possible.
A reg that is mounted in a cooler position like on an inner fender, will set the pressure based on the temperature of fuel at that location. As fuel heats up at the engine, it naturally expands and therefore the pressure rises above the reg's set pressure.

As others have suggested, a return line is a good way of keeping the fuel cool by circulating it back to the tank.
The problem with dead-head set ups can be that the fuel will remain stagnant and absorb heat at times of low fuel demand.

If you buy a reg and run a return line, make sure it is a bypassing type like the Holley 12-803BP and not the standard 12-803.

With the fuel coming out of the vents when cold, I say this could be a needle & seat hanging open from perhaps too much pressure, dirt, wear on the needle & seat or even a binding float hinge. I have also know brass floats to get leaks and lose their buoyancy.

Hope something here helps.

391 RWHP..
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Old 02-08-2016, 07:25 PM
Geoff 4 Geoff 4 is offline
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At 5000 rpm where you observed the fuel coming out of the vents, the engine will be consuming about 37 US gph. That is 1/3 of a litre every 30 seconds. A lot of fuel. [ I am assuming the 391 rwp translates to about 440 engine hp.]

So two things happening

[1] Engine is consuming a lot of fuel @ 5k
[2] Still enough fuel left in the bowl to exit through the vents.

I do not think the problem is due to fuel boiling or heat related, it is getting consumed by the engine too quickly to get hot/boil.

I think your pump pressure is overcoming the n&s in the carb. Do you have a fuel pressure gauge?
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Old 02-08-2016, 08:28 PM
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69ISH 69ISH is offline
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Thanks for the replies all, I have watched the videos and even though the fuel in the bowls are both aerating/frothing the fuel in my bowls has definitive small bubbles and the fuel rises that much that the fuel pisses out of the vent tubes and I had fuel spots that had made it to the roof of my car.
I am not running a return line, regulator or fuel pressure gauge.
I did have a fuel pressure gauge fitted before the carb with the previous pump and with no load upto 4500rpm it never got above 6psi. I never revved it above this.
I am using this pump and it is the same as I had in it.
https://www.holley.com/products/fuel...arts/12-289-11
At this stage I am going to run the car low on fuel and replace all the fuel hose between the tank and the pump, I will also fit 2 hose clamps at each end of hose and try that first if no joy I will refit the fuel pressure gauge and run the car up to 6000-6300rpm with my go pro set up to video the gauge to see what it gets up to, if it is high I will fit a regulator (I have one kicking around somewhere).
Then if this fails I will have to make more decisions on what will be the next step.
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Old 03-08-2016, 06:58 AM
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BlackoutSteve BlackoutSteve is offline
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I'd strongly be considering a rear mounted electric pump.
So many advantages over a mechanical.
Next time you shut off your engine, reach down and put your hand on it and see how hot it is..
I've never known hot fuel to be good in any situation.
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Last edited by BlackoutSteve; 03-08-2016 at 08:10 AM..
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Old 03-08-2016, 01:59 PM
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69ISH 69ISH is offline
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Quote:
Originally Posted by BlackoutSteve View Post
I'd strongly be considering a rear mounted electric pump.
So many advantages over a mechanical.
Next time you shut off your engine, reach down and put your hand on it and see how hot it is..
I've never known hot fuel to be good in any situation.
I really don't want to run a return line or have to add any more electrical components.
When the fuel was coming out of the vent tubes it was definitely cooler than room temperature. I had only filled up at the end of the street hence why I believe it was colder than room temperature.
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Old 03-08-2016, 05:10 PM
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BlackoutSteve BlackoutSteve is offline
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Then if it isn't a percolation issue from heat, fuel spilling from the vent tubes is a pressure and/or needle & seat issue due to causes already mentioned.
Fuel doesn't flow up hill unless something is pushing it.
Fit a gauge to see, and I'll always suggest fitting a regulator to know.

Also, bear in mind that petrol has a very low boiling point. Often less than 60C.
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Last edited by BlackoutSteve; 03-08-2016 at 05:13 PM..
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Old 03-08-2016, 05:44 PM
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Fit a reg(6psi) and check needles and seats, and above all continue to use your mechanical pump and don't get caught up in the hype that they're rubbish

Mechanical fuel pumps.......Remember back in 60 and 70's before the almighty Electric fuel pump, when it got to 40 degrees did every car on the road just stop working? I think not
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Old 03-08-2016, 10:48 PM
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bonnevista bonnevista is offline
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I'm thinking return line as well. They use them in many factory applications. If the factory didn't think it was necessary, they wouldn't have done it.
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Old 04-08-2016, 05:19 PM
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69ISH 69ISH is offline
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My theory is as the fuel aerates there is not enough fuel to float the float and close the needle and seat, now because the fuel pump is working so fast at 5500rpm and the fuel is flowing so fast with the needle and seat not closing the jets and engine can not consume the fuel so it spits out the vent tubes.
Sound feasible?
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Old 04-08-2016, 06:03 PM
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BlackoutSteve BlackoutSteve is offline
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Quote:
Originally Posted by 69ISH View Post
My theory is as the fuel aerates there is not enough fuel to float the float and close the needle and seat, now because the fuel pump is working so fast at 5500rpm and the fuel is flowing so fast with the needle and seat not closing the jets and engine can not consume the fuel so it spits out the vent tubes.
Sound feasible?
Not that theory, no.
How are you assuming the fuel is becoming aerated in the first place?
The pump is easily within it's normal speed and would not be creating this situation though cavitation if that's what you're thinking.

I do know one way that fuel can become aerated within the line, but Jim doesn't believe it exists and I'm too afraid to mention it...

..but that's usually a supply issue, and not a fuel pouring out of the vent tubes issue.
The fuel bowls by design are generally pretty good at decanting aerated fuel.

Reg and gauge.
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Old 04-08-2016, 06:27 PM
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69ISH 69ISH is offline
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Quote:
Originally Posted by BlackoutSteve View Post
Not that theory, no.
How are you assuming the fuel is becoming aerated in the first place?
The pump is easily within it's normal speed and would not be creating this situation though cavitation if that's what you're thinking.

I do know one way that fuel can become aerated within the line, but Jim doesn't believe it exists and I'm too afraid to mention it...

..but that's usually a supply issue, and not a fuel pouring out of the vent tubes issue.
The fuel bowls by design are generally pretty good at decanting aerated fuel.

Reg and gauge.

I am running a Quickfuel black diamond 830 with the glass sights on the fuel bowls and you can see the bubbles through the sights, the bubbles start appearing at approx 5000 rpm and the more the rpm increases the more bubbles until all you can see in the sight bowl is bubbles and eventually fuel spits out of the vents.
This is the best thing I can find to explain what the bubbles look like, there is no frothing or foaming in the fuel.http://previews.123rf.com/images/lia...tock-Photo.jpg
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