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Fuel Pressure/engine stavation issues


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roscobbc

Committee Member

Wed 20 Sep 17 22:30

Fuel Pressure/engine stavation issues


Puzzler for the week. Haven’t even looked at the car yet. I'm posting this up on here as some of you fellow petrol-heads (like myself) have been around a 'few' years and have a wealth of experience, so this is bit like a crossword with me giving the clues and you looking for the right answer, with me answering when I’ve finally found the cause.
Car is my 489 cu in Vette. Carb is Proform 950 cfm double pumper. Fitted with Holley high volume mechanical fuel pump. Pump was only replaced last year - not that there was anything wrong with stock fuel pump (it was 10 years old - I just thought it was time to replace it) the stock fuel pump never seemed to have a issue 'feeding' the carb.
I had a major fuel starvation problem a couple of weeks ago when accelerating hard through the gears it lost power in the upper speed band.
It did the same thing on my first run up Kop Hill this weekend – so, full bore up the hill in 1st and 2nd and it lost power for a seconds or so, only regaining it when I backed-off the gas and allowing fuel bowls to re-fill.
Got back to the paddock, left the engine running and opened the hood. The in-line fuel pressure gauge was showing zero, yet engine was still running. Felt rubber fuel line and couldn’t feel it pulsing (indicating fuel flow). Stopped engine and removed the rubber fuel pipe to fuel rail feeding both fuel bowls (which has the pressure gauge) and fuel squirted all over the place – so there was residual pressure between the fuel rail and the on-line canister fuel filter (which is between the fuel pump and fuel rail).
Re-connected, started the engine and fuel pressure was back to normal 4.5 lbs approx. Blasted up the hill on my second run without an issue – no fueling issues.
Cruised the 65 miles or so home on the M25 without a hitch. Last bit before I got hone and I hit the gas for a few hundred yards or so – and the starvation issue was back again.
Is it crap in the fuel line between cartridge fuel filter and fuel rail.
Cartridge filter has a 1/4” diameter spigot for the fuel return back to the tank.
Is there crap in the fuel line between tank and fuel filter.
Fuel rail ends are connected by a 6” length of braided hose. Its not unknown for this stuff to deteriorate with modern fuel – perhaps this restricted internally caused a pressure differential between primary and secondary float bowls.
Perhaps its the Holley fuel pump that is breaking down internally (after only a year)
Suggestions are welcomed – I’ll publish the results of my investigation in a couple of days.



Last edited by roscobbc on Wed 20 Sep 17 22:34; edited 3 times in total. [3 %]
--------------------
CCCUK Chairman
'68 coupe - 4 speed - 3.08 - 489 cu in - 570 bhp @ 5850 rpm - 606 ft/lbs torque (821 Newton Metres) @ 3850 rpm
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roppa440

Club Member

Thu 21 Sep 17 08:56

Re: Fuel Pressure/engine stavation issues


Cheap braided hose can come apart internally.

Only way is to start at the tank and work your way along systematically.

Apart from possible hose problems I will list what comes to my mind as I write to give you ideas.

Sock filter on the fuel pick-up in the tank blocked.
Tank or tank cap not vented so causing suction.

Fuel filter blocked.

Restriction in fuel line as already said but also on a engine that size you need 3/8" line ( or -8AN) all the way from the fuel in the tank to the fuel rail at the carbs.

On my Challenger I decided a big mechanical Carter pump was big enough for my 440 engine. But I modified the in/out ports to take -8AN fittings and lines. Actually I ran 1/2" line from the 3/8" pickup to the pump but only because I got it for nothing.

From the pump I went -8AN to a big high flow filter. Then -8 to the regulator in and out.

Only when at the carbs I reduced the line to -6AN to feed each carb. 6.5 to 7psi pressure set at that point with gauge on that line.

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roscobbc

Committee Member

Thu 21 Sep 17 10:10

Re: Fuel Pressure/engine stavation issues


Thanks for advice - I think fuel lines are stock 5/16" - its the return that is either 1/4" or 7/32". Thing is..........its been OK with the same set-up for quite a while - even stock type fuel pump was OK. Oddly enough the thought of an airlock between fuel pump and in-line filter and fuel rail seemed only seemed like a possibility after I had removed and replaced the fuel line and it ran perfectly afterwards for a while. The idea of percolation crossed my mind.......if it was a really hot day and I was caught in traffic perhaps..........but never affecting high speed running.


--------------------
CCCUK Chairman
'68 coupe - 4 speed - 3.08 - 489 cu in - 570 bhp @ 5850 rpm - 606 ft/lbs torque (821 Newton Metres) @ 3850 rpm
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roppa440

Club Member

Thu 21 Sep 17 10:22

Re: Fuel Pressure/engine stavation issues


I had problems with fuel boiling in the lines and even in the fuel bowls years ago. So did a lot to insulate all fuel feeds from heat.

Notice on my carb set up I have a heat shield under the carbs? Front edge raised to defect heat from the radiator fan as that front fuel bowl was getting very hot!
Under that plate is a 1/4" thick insulating Cometic gasket.

Exhaust passage through the intake blocked off. Underside of the intake insulated above the large air gap.

Fuel and oil lines insulated where they pass the headers. Fuel line from the tank ran inside the chassis connectors and behind an ally plate where it snakes past the headers (as well as the insulation).

As I said in the other post I fitted the large fuel filter and regulator mounted behind the grille.

That fuel was as cool as could be by the time it got to the carbs. Yes

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Daytona Vette

Club Member

Thu 21 Sep 17 12:12

Re: Fuel Pressure/engine stavation issues


Many years ago I had a fuel starvation problem with an MGB just after the car had had an SU Tune up at an SU agent - took it back several times carbs stripped and rebuilt prob was intermittent but re-occurred many many times - took it to SU themselves they checked it over and said it was a coil prob - changed coil still the same - one day it cut out again I coasted to the kerb in utter despair, I sat for a bit and then lifted the bonnet removed the fuel filter and blew out backwards the fuel blew out onto the pavement it was orange - eureka the tank had rusted causing the micro filter blockage.

Check the filter reverse blow on to white paper for debris

One very hot summer my 454 suffered fuel starvation when the fuel was vapourizing due to the heat on the day - cured with the addition of a heat shield plate under the Holley.

The ambient temp on your hill climb day would not have been high, but if you are running with some of the rubber / sponge insulation missing from around the radiator surround you will also be suffering from a higher under bonnet temperature (the water temp gauge could still show normal temp)

Best of luck


--------------------
Barry - Daytona Vette

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Redlevel

Club Member

Thu 21 Sep 17 15:36

Re: Fuel Pressure/engine stavation issues


Put a fuel pressure gauge in the line. If the needle is all over the place you really need a regulator to keep the pressure steady and present if you haven't already fitted one.

4.5psi doesn't sound like a lot for what sounds like a thirsty bb with a huge carb.

If you have any fuel pipe immersed in the tank, make sure it is fuel rated on the outside as well as for the normal inside rating to guard against deterioration.

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Redlevel

Club Member

Thu 21 Sep 17 15:38

Re: Fuel Pressure/engine stavation issues


Put a fuel pressure gauge in the line. If the needle is all over the place you really need a regulator to keep the pressure steady and present if you haven't already fitted one.

4.5psi doesn't sound like a lot for what sounds like a thirsty bb with a huge carb.

If you have any fuel pipe immersed in the tank, make sure it is fuel rated on the outside as well as for the normal inside rating to guard against deterioration.

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roppa440

Club Member

Thu 21 Sep 17 15:40

Re: Fuel Pressure/engine stavation issues


Holleys are best run at 6.5psi. I have ran as high as 7psi but it wasn't needed so backed off a bit.

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roppa440

Club Member

Thu 21 Sep 17 15:48

Re: Fuel Pressure/engine stavation issues


The old Carter and Edelbrock Performer carbs run at something like 4.5psi from memory.

But anything based on a Holley should be 6.5 or slightly more.



Last edited by roppa440 on Thu 21 Sep 17 15:54; edited 1 time in total. [6 %]

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roppa440

Club Member

Thu 21 Sep 17 15:53

Re: Fuel Pressure/engine stavation issues


Another thought. The Proform carbs need the fuel bowl level set half way up the sight plugs. Unlike the holleys where it is just trickling out of the hole.

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